2HYE , 4A0K
111-395: 8451 99375 ENSG00000139842 ENSMUSG00000031446 Q13619 Q3TCH7 NM_001008895 NM_001278513 NM_001278514 NM_003589 NM_146207 NM_001363448 NM_001363450 NP_001341868 NP_001341869 NP_001341870 NP_001341871 NP_001341872 NP_001341873 NP_666319 NP_001350377 NP_001350379 Cullin-4A is a protein that in humans
222-516: A carboxyl group, and a variable side chain are bonded . Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids , have a great variety of chemical structures and properties; it is the combined effect of all of
333-470: A gene may be duplicated before it can mutate freely. However, this can also lead to complete loss of gene function and thus pseudo-genes . More commonly, single amino acid changes have limited consequences although some can change protein function substantially, especially in enzymes . For instance, many enzymes can change their substrate specificity by one or a few mutations. Changes in substrate specificity are facilitated by substrate promiscuity , i.e.
444-556: A CUL4A complex, Cereblon, was discovered to be a major target of the teratogenic agent thalidomide . CUL4A protein is 759 amino acids long and forms an extended, rigid structure primarily consisting of alpha-helices . At the N-terminus , CUL4A binds to the beta-propeller of the DDB1 adaptor protein which interacts with numerous DDB1-CUL4-Associated Factors (DCAFs). As a result, the N-terminus
555-653: A PCNA-dependent manner. During unperturbed cell cycle progression, ubiquitination and downregulation of these proteins by CRL4A occurs at the onset of DNA replication. DNA damage such as UV irradiation also induces CRL4A-mediated destruction of those proteins. Both substrates are also regulated by the SCF complex . CRL4-mediated destruction of p21 relieves cyclin E - Cdk2 inhibition and promotes S phase entry. Loss of Cdt2 expression increases p21 expression in cells and stabilizes p21 following UV-irradiation. CUL4A deletion results in delayed S phase entry in mouse embryonic fibroblasts, which
666-552: A combination of sequence, structure and function, and they can be combined in many different ways. In an early study of 170,000 proteins, about two-thirds were assigned at least one domain, with larger proteins containing more domains (e.g. proteins larger than 600 amino acids having an average of more than 5 domains). Most proteins consist of linear polymers built from series of up to 20 different L -α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group,
777-403: A defined conformation . Proteins can interact with many types of molecules, including with other proteins , with lipids , with carbohydrates , and with DNA . It has been estimated that average-sized bacteria contain about 2 million proteins per cell (e.g. E. coli and Staphylococcus aureus ). Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on
888-834: A detailed review of the vegetable proteins at the Connecticut Agricultural Experiment Station . Then, working with Lafayette Mendel and applying Liebig's law of the minimum , which states that growth is limited by the scarcest resource, to the feeding of laboratory rats, the nutritionally essential amino acids were established. The work was continued and communicated by William Cumming Rose . The difficulty in purifying proteins in large quantities made them very difficult for early protein biochemists to study. Hence, early studies focused on proteins that could be purified in large quantities, including those of blood, egg whites, and various toxins, as well as digestive and metabolic enzymes obtained from slaughterhouses. In
999-616: A hereditary cancer, xeroderma pigmentosum has helped identify several genes which encode proteins in the NER pathway, two of which are XPC and XPD. XP is caused by a homozygous deficiency in UV DNA damage repair (GG-NER) which increases the patients' risk of skin cancer by 1000-fold. In heterozygous patients, the risk of cancer is sporadic but can be predicted based on analytical assessment of polymorphisms in XP related DNA repair genes purified from lymphocytes . In
1110-510: A junction between the double-stranded and single-stranded DNA around the transcription bubble . In addition to stabilizing TFIIH, XPG also has endonuclease activity; it cuts DNA damage on the 3' side while the XPF – ERCC1 heterodimeric protein cuts on the 5' side. The dual incision leads to the removal of a ssDNA with a single strand gap of 25~30 nucleotides. The small, excised, damage-containing DNA (sedDNA) oligonucleotides are initially released from
1221-585: A lesion in DNA: the blocked RNA polymerase serves as a damage recognition signal, which replaces the need for the distortion recognition properties of the XPC-RAD23B and DDB complexes. CS proteins (CSA and CSB) bind some types of DNA damage instead of XPC-Rad23B. Other repair mechanisms are possible but less accurate and efficient. TC-NER initiates when RNA polymerase stalls at a lesion in DNA, whereupon protein complexes help move
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#17328551491091332-763: A limited lifespan. Accelerated aging in the mutant involves numerous organs. Mutations in the ERCC2 (XPD) gene can lead to various syndromes, either xeroderma pigmentosum (XP), trichothiodystrophy (TTD) or a combination of XP and TTD (XPTTD), or a combination of XP and Cockayne syndrome (XPCS). TTD and CS both display features of premature aging. These features may include sensorineural deafness , retinal degeneration, white matter hypomethylation, central nervous system calcification, reduced stature, and cachexia (loss of subcutaneous fat tissue). XPCS and TTD fibroblasts from ERCC2 (XPD) mutant human and mouse show evidence of defective repair of oxidative DNA damages that may underlie
1443-478: A little ambiguous and can overlap in meaning. Protein is generally used to refer to the complete biological molecule in a stable conformation , whereas peptide is generally reserved for a short amino acid oligomers often lacking a stable 3D structure. But the boundary between the two is not well defined and usually lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids, usually regardless of length, but often implies an absence of
1554-634: A multi-system premature aging degenerative phenotype that appears to strengthen the link between DNA damage and aging . (see DNA damage theory of aging ). Cockayne syndrome (CS) arises from germline mutations in either of two genes ERCC8 (CSA) or ERCC6 (CSB). ERCC8 (CSA) mutations generally give rise to a more moderate form of CS than ERCC6 (CSB) mutations. Mutations in the CSA gene account for about 20% of CS cases. Individuals with CSA and CSB are characterised by severe postnatal growth and mental retardation and accelerated aging leading to premature death at
1665-540: A negative regulator of NER activity. In addition to the "global" type of NER, the CRL4A complex also appears to play a role in "transcription-coupled" NER in conjunction with the Cockayne Syndrome A protein. CRL4A complexes appear to be activated by certain types of DNA damage (most notably, UV-irradiation) and several substrates are preferentially ubiquitinated after DNA damage induction. CUL4A's role in modifying chromatin
1776-410: A particular cell or cell type is known as its proteome . The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly. The region of the protein responsible for binding another molecule is known as the binding site and is often a depression or "pocket" on the molecular surface. This binding ability is mediated by
1887-500: A protein carries out its function: for example, enzyme kinetics studies explore the chemical mechanism of an enzyme's catalytic activity and its relative affinity for various possible substrate molecules. By contrast, in vivo experiments can provide information about the physiological role of a protein in the context of a cell or even a whole organism . In silico studies use computational methods to study proteins. Proteins may be purified from other cellular components using
1998-411: A protein is defined by the sequence of a gene, which is encoded in the genetic code . In general, the genetic code specifies 20 standard amino acids; but in certain organisms the genetic code can include selenocysteine and—in certain archaea — pyrrolysine . Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification , which alters
2109-539: A protein that fold into distinct structural units. Domains usually also have specific functions, such as enzymatic activities (e.g. kinase ) or they serve as binding modules (e.g. the SH3 domain binds to proline-rich sequences in other proteins). Short amino acid sequences within proteins often act as recognition sites for other proteins. For instance, SH3 domains typically bind to short PxxP motifs (i.e. 2 prolines [P], separated by two unspecified amino acids [x], although
2220-490: A protein, named "V", which acts as a substrate receptor and bridges an interaction between DDB1 and STAT proteins (the structure of the CRL4A complex is pictured in the inset) - thus inducing STAT1 ubiquitination and degradation DCAF1 is also named VPRBP due to its interaction with HIV-1 protein Vpr . Although DCAF1/VPRBP appears to have a crucial function in tumor suppression, DNA replication and embryonic development, HIV-1 "hijacks"
2331-486: A role in biological recognition phenomena involving cells and proteins. Receptors and hormones are highly specific binding proteins. Transmembrane proteins can also serve as ligand transport proteins that alter the permeability of the cell membrane to small molecules and ions. The membrane alone has a hydrophobic core through which polar or charged molecules cannot diffuse . Membrane proteins contain internal channels that allow such molecules to enter and exit
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#17328551491092442-406: A series of purification steps may be necessary to obtain protein sufficiently pure for laboratory applications. To simplify this process, genetic engineering is often used to add chemical features to proteins that make them easier to purify without affecting their structure or activity. Here, a "tag" consisting of a specific amino acid sequence, often a series of histidine residues (a " His-tag "),
2553-432: A solution known as a crude lysate . The resulting mixture can be purified using ultracentrifugation , which fractionates the various cellular components into fractions containing soluble proteins; membrane lipids and proteins; cellular organelles , and nucleic acids . Precipitation by a method known as salting out can concentrate the proteins from this lysate. Various types of chromatography are then used to isolate
2664-698: A study relapse rates of high-risk stage II and III colorectal cancers, XPD (ERCC2) polymorphism 2251A>C was significantly correlated with early relapse after chemotherapeutic treatment. Studies have indicated that the effects of polymorphic NER genes is additive, with greater frequency of variants, greater cancer risk presents. In humans and mice, germline mutation in genes employed in NER cause features of premature aging. These genes and their corresponding proteins include ERCC1 ( ERCC1 ), ERCC2 (XPD), ERCC3 ( XPB ), ERCC4 (XPF), ERCC5 (XPG), ERCC6 (CSB) and ERCC8 (CSA). DNA repair-deficient ERCC1 mutant mice show features of accelerated aging, and have
2775-441: A variety of techniques such as ultracentrifugation , precipitation , electrophoresis , and chromatography ; the advent of genetic engineering has made possible a number of methods to facilitate purification. To perform in vitro analysis, a protein must be purified away from other cellular components. This process usually begins with cell lysis , in which a cell's membrane is disrupted and its internal contents released into
2886-527: Is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents ), radiation and other mutagens . Three excision repair pathways exist to repair single stranded DNA damage: Nucleotide excision repair (NER), base excision repair (BER), and DNA mismatch repair (MMR). While the BER pathway can recognize specific non-bulky lesions in DNA, it can correct only damaged bases that are removed by specific glycosylases . Similarly,
2997-492: Is a highly conserved "LEXE" motif within helix one of the homeodomain. When multiple amino acids within this motif were mutated, HOXB4 became resistant to CRL4A-mediated degradation. The substrate receptor, or DCAF, required for HOX protein degradation remains unknown. The Cul4a gene is required for normal spermatogenesis and meiosis in male germ cells of mice. Cul4a males produce abnormal sperm and are infertile. While both CUL4A and CUL4B are expressed in male gametes, CUL4A
3108-552: Is also modified by covalent attachment of a NEDD8 molecule at a highly conserved lysine residue in the C-terminal region. This modification appears to induce conformational changes which promotes flexibility in the RING domain of cullin proteins and enhanced ubiquitin ligase activity. Overall, CRL4A complexes have a modular structure which allows for sophisticated regulation by the cell and influence over numerous substrates and processes in
3219-399: Is also mutated or amplified in about 4% of melanomas (although the mutations are dispersed and individual mutations occur sporadically). In mouse models, Cul4a knockout resulted in pronounced resistance to UV-induced skin carcinogenesis. Cre -induced Cul4a overexpression in mouse lung tissue promoted hyperplasia . Due to the observed amplification of CUL4A in several carcinomas and
3330-443: Is attached to one terminus of the protein. As a result, when the lysate is passed over a chromatography column containing nickel , the histidine residues ligate the nickel and attach to the column while the untagged components of the lysate pass unimpeded. A number of different tags have been developed to help researchers purify specific proteins from complex mixtures. Nucleotide excision repair Nucleotide excision repair
3441-590: Is crucial for the recruitment of substrates for the ubiquitin ligase complex. At the C-terminal end, CUL4A interacts with the RBX1 /ROC1 protein via its RING domain . RBX1 is a core component of Cullin-RING ubiquitin ligase (CRL) complexes and functions to recruit E2 ubiquitin conjugating enzymes . Therefore, the C-terminus of CUL4A - along with RBX1 and activated E2 enzymes - compose the catalytic core of CRL4 complexes. CUL4A
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3552-562: Is dictated by the nucleotide sequence of their genes , and which usually results in protein folding into a specific 3D structure that determines its activity. A linear chain of amino acid residues is called a polypeptide . A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are rarely considered to be proteins and are commonly called peptides . The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in
3663-525: Is encoded by the CUL4A gene . CUL4A belongs to the cullin family of ubiquitin ligase proteins and is highly homologous to the CUL4B protein. CUL4A regulates numerous key processes such as DNA repair, chromatin remodeling , spermatogenesis , haematopoiesis and the mitotic cell cycle . As a result, CUL4A has been implicated in several cancers and the pathogenesis of certain viruses including HIV . A component of
3774-628: Is found in hard or filamentous structures such as hair , nails , feathers , hooves , and some animal shells . Some globular proteins can also play structural functions, for example, actin and tubulin are globular and soluble as monomers, but polymerize to form long, stiff fibers that make up the cytoskeleton , which allows the cell to maintain its shape and size. Other proteins that serve structural functions are motor proteins such as myosin , kinesin , and dynein , which are capable of generating mechanical forces. These proteins are crucial for cellular motility of single celled organisms and
3885-469: Is higher in prokaryotes than eukaryotes and can reach up to 20 amino acids per second. The process of synthesizing a protein from an mRNA template is known as translation . The mRNA is loaded onto the ribosome and is read three nucleotides at a time by matching each codon to its base pairing anticodon located on a transfer RNA molecule, which carries the amino acid corresponding to the codon it recognizes. The enzyme aminoacyl tRNA synthetase "charges"
3996-445: Is highly expressed in pachytenes and diplotenes . It is at these stages that CUL4A-deficient male germ cells exhibit high levels of apoptosis , improper DNA repair and accumulation of the CRL4 substrate Cdt1 . The chromosomal region ch13q34 which contains the CUL4A gene is amplified in 3-6% of certain carcinomas including: breast, uterine, lung, stomach and colorectal cancers. CUL4A
4107-461: Is inefficient for polypeptides longer than about 300 amino acids, and the synthesized proteins may not readily assume their native tertiary structure . Most chemical synthesis methods proceed from C-terminus to N-terminus, opposite the biological reaction. Most proteins fold into unique 3D structures. The shape into which a protein naturally folds is known as its native conformation . Although many proteins can fold unassisted, simply through
4218-407: Is known as DDB2 and is able to directly bind DNA lesions associated with UV-irradiation. DDB2 is a DCAF protein and is both a ubiquitination substrate of the CRL4 complex and also serves as an E3 ligase protein for other substrates such as XPC and histones (see next section) near the damage site. Due to its ubiquitination of DNA damage-recognizing proteins DDB2 and XPC, CUL4A has been described as
4329-504: Is largely related to DNA repair activities and occurs after DNA damage induction. Both CUL4A and its closely related homolog CUL4B may ubiquitinate histones H2A, H3 and H4. The yeast homolog of CUL4A, Rtt101, ubiquitinates histone H3 and promotes nucleosome assembly and CRL4A complexes perform similar functions in human cells. CRL4 complexes also affect histone methylation events and chromatin structure through regulation of histone methyltransferases . The histone H4 monomethylase PR-Set7/SET8
4440-404: Is often enormous—as much as 10 -fold increase in rate over the uncatalysed reaction in the case of orotate decarboxylase (78 million years without the enzyme, 18 milliseconds with the enzyme). The molecules bound and acted upon by enzymes are called substrates . Although enzymes can consist of hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with
4551-498: Is rescued by deletion of p21. In human retinal pigment epithelial cells, loss of Cdt2 expression also result in p21 dependent delayed S-phase entry, and re-expression of p21 in S-phase, which results cycles of incomplete replication, long term accumulation of p21, and in some cases induction of apoptosis. After promoting initiation of eukaryotic DNA replication at the origin , Cdt1 is inactivated by Geminin and targeted for degradation by
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4662-593: Is responsible for a protein which recognizes DNA during the early portion of the NER pathway. This gene can have polymorphisms at Intron 9 and SNPs in Exon 15 which have been correlated with cancer risk as well. Research has shown that a biallelic poly (AT) insertion/deletion polymorphism in Intron 9 of XPC is associated with increased risk for skin, breast and prostate cancers, especially in North Indian populations. The study of
4773-486: Is the code for methionine . Because DNA contains four nucleotides, the total number of possible codons is 64; hence, there is some redundancy in the genetic code, with some amino acids specified by more than one codon. Genes encoded in DNA are first transcribed into pre- messenger RNA (mRNA) by proteins such as RNA polymerase . Most organisms then process the pre-mRNA (also known as a primary transcript ) using various forms of post-transcriptional modification to form
4884-526: Is ubiquitinated on chromatin by CRL4(Cdt2) complexes during S phase and following DNA damage in a PCNA -dependent manner. CRL4A complexes regulate entry into the DNA synthesis phase, or S phase , of the mitotic cycle by regulating protein expression levels of the replication licensing factor protein Cdt1 and cyclin-dependent kinase inhibitor p21 . In both cases, CRL4A utilizes Cdt2 as the DCAF to bind both substrates in
4995-613: The Ligase-III-XRCC1 complex seal the nicks to complete NER. The process of nucleotide excision repair is controlled in Escherichia coli by the UvrABC endonuclease enzyme complex, which consists of four Uvr proteins: UvrA, UvrB, UvrC, and DNA helicase II (sometimes also known as UvrD in this complex). First, a UvrA-UvrB complex scans the DNA, with the UvrA subunit recognizing distortions in
5106-486: The amino acid leucine for which he found a (nearly correct) molecular weight of 131 Da . Early nutritional scientists such as the German Carl von Voit believed that protein was the most important nutrient for maintaining the structure of the body, because it was generally believed that "flesh makes flesh." Around 1862, Karl Heinrich Ritthausen isolated the amino acid glutamic acid . Thomas Burr Osborne compiled
5217-644: The muscle sarcomere , with a molecular mass of almost 3,000 kDa and a total length of almost 27,000 amino acids. Short proteins can also be synthesized chemically by a family of methods known as peptide synthesis , which rely on organic synthesis techniques such as chemical ligation to produce peptides in high yield. Chemical synthesis allows for the introduction of non-natural amino acids into polypeptide chains, such as attachment of fluorescent probes to amino acid side chains. These methods are useful in laboratory biochemistry and cell biology , though generally not for commercial applications. Chemical synthesis
5328-645: The sperm of many multicellular organisms which reproduce sexually . They also generate the forces exerted by contracting muscles and play essential roles in intracellular transport. A key question in molecular biology is how proteins evolve, i.e. how can mutations (or rather changes in amino acid sequence) lead to new structures and functions? Most amino acids in a protein can be changed without disrupting activity or function, as can be seen from numerous homologous proteins across species (as collected in specialized databases for protein families , e.g. PFAM ). In order to prevent dramatic consequences of mutations,
5439-493: The 1700s by Antoine Fourcroy and others, who often collectively called them " albumins ", or "albuminous materials" ( Eiweisskörper , in German). Gluten , for example, was first separated from wheat in published research around 1747, and later determined to exist in many plants. In 1789, Antoine Fourcroy recognized three distinct varieties of animal proteins: albumin , fibrin , and gelatin . Vegetable (plant) proteins studied in
5550-562: The 1950s, the Armour Hot Dog Company purified 1 kg of pure bovine pancreatic ribonuclease A and made it freely available to scientists; this gesture helped ribonuclease A become a major target for biochemical study for the following decades. The understanding of proteins as polypeptides , or chains of amino acids, came through the work of Franz Hofmeister and Hermann Emil Fischer in 1902. The central role of proteins as enzymes in living organisms that catalyzed reactions
5661-498: The 20,000 or so proteins encoded by the human genome, only 6,000 are detected in lymphoblastoid cells. Proteins are assembled from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of the gene encoding this protein. The genetic code is a set of three-nucleotide sets called codons and each three-nucleotide combination designates an amino acid, for example AUG ( adenine – uracil – guanine )
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#17328551491095772-491: The 5' side incision is made and DNA repair begins before the 3' side incision. This helps reduce exposed single stranded DNA during the repair process. Replication factor C ( RFC ) loads the Proliferating Cell Nuclear Antigen (PCNA) onto the DNA strand. This allows DNA polymerases implicated in repair (δ, ε and/or κ) to copy the undamaged strand via translocation. DNA ligase I and Flap endonuclease 1 or
5883-516: The EC number system provides a functional classification scheme. Similarly, the gene ontology classifies both genes and proteins by their biological and biochemical function, but also by their intracellular location. Sequence similarity is used to classify proteins both in terms of evolutionary and functional similarity. This may use either whole proteins or protein domains , especially in multi-domain proteins . Protein domains allow protein classification by
5994-463: The MMR pathway only targets mismatched Watson-Crick base pairs . Nucleotide excision repair (NER) is a particularly important excision mechanism that removes DNA damage induced by ultraviolet light (UV). UV DNA damage results in bulky DNA adducts — these adducts are mostly thymine dimers and 6,4-photoproducts. Recognition of the damage leads to removal of a short single-stranded DNA segment that contains
6105-470: The NER pathway for which polymorphism has shown functional and phenotypic impact are the XPD and XPC genes. XPD, also known as ERCC2, serves to open DNA around the site of damage during NER, in addition to other transcriptional activities. Studies have shown that polymorphisms at Exon 10 (G>A)(Asp312Asn) and Exon 23 (A>T)(Lys751Gln) are linked with genetic predisposition to several cancer types. The XPC gene
6216-578: The SCF and CRL4 complexes. Cdt1 expression is stabilized by RNAi-mediated knockdown of DDB1 or both CUL4A and CUL4B, which suggests redundant or overlapping function of the two CUL4 proteins for Cdt1 regulation. Only reduction of Geminin expression seems to induce re-replication in Cdt1-overexpressing cells. CRL4s also utilize Cdt2 and PCNA to degrade the p12 subunit of DNA polymerase δ during S phase and after UV irradiation. CRL4A complexes appear to induce
6327-709: The ability of many enzymes to bind and process multiple substrates . When mutations occur, the specificity of an enzyme can increase (or decrease) and thus its enzymatic activity. Thus, bacteria (or other organisms) can adapt to different food sources, including unnatural substrates such as plastic. Methods commonly used to study protein structure and function include immunohistochemistry , site-directed mutagenesis , X-ray crystallography , nuclear magnetic resonance and mass spectrometry . The activities and structures of proteins may be examined in vitro , in vivo , and in silico . In vitro studies of purified proteins in controlled environments are useful for learning how
6438-405: The addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding; for example, the aminoacyl tRNA synthetase specific to the amino acid valine discriminates against the very similar side chain of the amino acid isoleucine . Proteins can bind to other proteins as well as to small-molecule substrates. When proteins bind specifically to other copies of
6549-543: The alpha carbons are roughly coplanar . The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone. The end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus (the sequence of the protein is written from N-terminus to C-terminus, from left to right). The words protein , polypeptide, and peptide are
6660-531: The amino acid side chains in a protein that ultimately determines its three-dimensional structure and its chemical reactivity. The amino acids in a polypeptide chain are linked by peptide bonds . Once linked in the protein chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen, and oxygen atoms are known as the main chain or protein backbone. The peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that
6771-574: The binding of a substrate molecule to an enzyme's active site , or the physical region of the protein that participates in chemical catalysis. In solution, proteins also undergo variation in structure through thermal vibration and the collision with other molecules. Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: globular proteins , fibrous proteins , and membrane proteins . Almost all globular proteins are soluble and many are enzymes. Fibrous proteins are often structural, such as collagen ,
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#17328551491096882-570: The body of a multicellular organism. These proteins must have a high binding affinity when their ligand is present in high concentrations, but must also release the ligand when it is present at low concentrations in the target tissues. The canonical example of a ligand-binding protein is haemoglobin , which transports oxygen from the lungs to other organs and tissues in all vertebrates and has close homologs in every biological kingdom . Lectins are sugar-binding proteins which are highly specific for their sugar moieties. Lectins typically play
6993-417: The cancer-prone condition xeroderma pigmentosum (XP) alone, or in combination with the severe neurodevelopmental disorder Cockayne syndrome (CS) or the infantile lethal cerebro-oculo-facio-skeletal syndrome. An ERCC5 (XPG) mutant mouse model presents features of premature aging including cachexia and osteoporosis with pronounced degenerative phenotypes in both liver and brain. These mutant mice develop
7104-558: The cell is as enzymes , which catalyse chemical reactions. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the reactions involved in metabolism , as well as manipulating DNA in processes such as DNA replication , DNA repair , and transcription . Some enzymes act on other proteins to add or remove chemical groups in a process known as posttranslational modification. About 4,000 reactions are known to be catalysed by enzymes. The rate acceleration conferred by enzymatic catalysis
7215-436: The cell surface and an effector domain within the cell, which may have enzymatic activity or may undergo a conformational change detected by other proteins within the cell. Antibodies are protein components of an adaptive immune system whose main function is to bind antigens , or foreign substances in the body, and target them for destruction. Antibodies can be secreted into the extracellular environment or anchored in
7326-752: The cell's machinery through the process of protein turnover . A protein's lifespan is measured in terms of its half-life and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids , proteins are essential parts of organisms and participate in virtually every process within cells . Many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism . Proteins also have structural or mechanical functions, such as actin and myosin in muscle and
7437-408: The cell. Although the individual parts vary, all cullin-based ubiquitin ligases exhibit these characteristics. The DDB1 adaptor protein was initially characterized as the large subunit of a heterodimeric complex (UV-DDB) that was found to recognize damaged DNA and participate in a form of repair known as nucleotide excision repair (NER). The smaller subunit of this Damaged DNA Binding protein complex
7548-450: The cell. Many ion channel proteins are specialized to select for only a particular ion; for example, potassium and sodium channels often discriminate for only one of the two ions. Structural proteins confer stiffness and rigidity to otherwise-fluid biological components. Most structural proteins are fibrous proteins ; for example, collagen and elastin are critical components of connective tissue such as cartilage , and keratin
7659-621: The chemical properties of their amino acids, others require the aid of molecular chaperones to fold into their native states. Biochemists often refer to four distinct aspects of a protein's structure: Proteins are not entirely rigid molecules. In addition to these levels of structure, proteins may shift between several related structures while they perform their functions. In the context of these functional rearrangements, these tertiary or quaternary structures are usually referred to as " conformations ", and transitions between them are called conformational changes. Such changes are often induced by
7770-441: The chief actors within the cell, said to be carrying out the duties specified by the information encoded in genes. With the exception of certain types of RNA , most other biological molecules are relatively inert elements upon which proteins act. Proteins make up half the dry weight of an Escherichia coli cell, whereas other macromolecules such as DNA and RNA make up only 3% and 20%, respectively. The set of proteins expressed in
7881-490: The construction of enormously complex signaling networks. As interactions between proteins are reversible, and depend heavily on the availability of different groups of partner proteins to form aggregates that are capable to carry out discrete sets of function, study of the interactions between specific proteins is a key to understand important aspects of cellular function, and ultimately the properties that distinguish particular cell types. The best-known role of proteins in
7992-460: The damaged DNA surrounding the lesion then fill in the repair patch. Mutations in GG-NER machinery are responsible for multiple genetic disorders including: At any given time, most of the genome in an organism is not undergoing transcription; there is a difference in NER efficiency between transcriptionally silent and transcriptionally active regions of the genome. For many types of lesions, NER repairs
8103-560: The degradation of numerous members of the HOX transcription family, which are essential regulators of haematopoiesis. The first member of the HOX family identified as a target of CRL4A-mediated degradation is HOXA9 , which is essential for haematopoietic stem cell maintenance and has been implicated in a subset of myeloid leukemias . The HOXA9 degron lies within the homeodomain , which is crucial for DNA binding. Sequence alignment studies showed that there
8214-408: The derivative unit kilodalton (kDa). The average size of a protein increases from Archaea to Bacteria to Eukaryote (283, 311, 438 residues and 31, 34, 49 kDa respectively) due to a bigger number of protein domains constituting proteins in higher organisms. For instance, yeast proteins are on average 466 amino acids long and 53 kDa in mass. The largest known proteins are the titins , a component of
8325-621: The disease. XPA , XPB , XPC , XPD, XPE , XPF, and XPG all derive from хeroderma pigmentosum and CSA and CSB represent proteins linked to Cockayne syndrome. Additionally, the proteins ERCC1 , RPA , RAD23A , RAD23B , and others also participate in nucleotide excision repair. A more complete list of proteins involved in NER is found below . Eukaryotic nucleotide excision repair can be divided into two subpathways: global genomic NER (GG-NER) and transcription coupled NER (TC-NER). Three different sets of proteins are involved in recognizing DNA damage for each subpathway. After damage recognition,
8436-521: The duplex in complex with TFIIH but then dissociate in an ATP-dependent manner and become bound to replication protein A (RPA). Inhibition of gap filling DNA synthesis and ligation results in an accumulation of RPA-bound sedDNAs in the cell. Replication protein A (RPA) and XPA are the last two proteins associated with the main NER repair complex. These two proteins are present prior to TFIIH binding since they are involved with verifying DNA damage. They may also protect single-stranded DNA. After verification,
8547-447: The erroneous conclusion that they might be composed of a single type of (very large) molecule. The term "protein" to describe these molecules was proposed by Mulder's associate Berzelius; protein is derived from the Greek word πρώτειος ( proteios ), meaning "primary", "in the lead", or "standing in front", + -in . Mulder went on to identify the products of protein degradation such as
8658-521: The excised segment by actively breaking the hydrogen bonds between the complementary bases. The resultant gap is then filled in using DNA polymerase I and DNA ligase. The basic excision process is very similar in higher cells, but these cells usually involve many more proteins – E.coli is a simple example. TC-NER also exists in bacteria, and is mediated by the TRCF (Mfd) protein. TRCF is an SF2 ATPase that uses ATP hydrolysis to translocate on dsDNA upstream of
8769-509: The fact that CRL4 complexes target multiple DNA repair and tumor suppressor genes , CUL4A can be considered an oncogene in certain contexts. Due to its robust expression (particularly during DNA replication) and modular nature, CRL4A complexes can be co-opted or "hijacked" to promote viral proliferation in mammalian cells. Certain paramyxoviruses avoid the interferon response in cells by targeting STAT1 and disrupting signaling. Simian virus 5 and type II human parainfluenza virus express
8880-480: The genome and recognize helix distortions: the XPC -Rad23B complex is responsible for distortion recognition, while DDB1 and DDB2 ( XPE ) can also recognize some types of damage caused by UV light. Additionally, XPA performs a function in damage recognition that is as yet poorly defined. Upon identification of a damaged site, subsequent repair proteins are then recruited to the damaged DNA to verify presence of DNA damage, excise
8991-590: The helix, caused for example by pyrimidine dimers . When the complex recognizes such a distortion, the UvrA subunit leaves and an UvrC protein comes in and binds to the UvrB monomer and, hence, forms a new UvrBC dimer . UvrB cleaves a phosphodiester bond 4 nucleotides downstream of the DNA damage, and the UvrC cleaves a phosphodiester bond 8 nucleotides upstream of the DNA damage and created 12 nucleotide excised segment. DNA helicase II (sometimes called UvrD) then comes in and removes
9102-711: The human population. If located in NER genes or regulatory sequences, such mutations can negatively affect DNA repair capacity resulting in an increase likelihood of cancer development. While the functional impact of all polymorphisms has not been characterized, some polymorphisms in DNA repair genes or their regulatory sequences do induce phenotypical changes and are involved in cancer development. A study of lung cancer cases found modest association between NER specific SNP polymorphisms and lung cancer risk. The results indicate that some inherited polymorphic variations in NER genes may result in predisposition to lung cancer, and potentially other cancer states. Two important genes in
9213-525: The late 1700s and early 1800s included gluten , plant albumin , gliadin , and legumin . Proteins were first described by the Dutch chemist Gerardus Johannes Mulder and named by the Swedish chemist Jöns Jacob Berzelius in 1838. Mulder carried out elemental analysis of common proteins and found that nearly all proteins had the same empirical formula , C 400 H 620 N 100 O 120 P 1 S 1 . He came to
9324-436: The lesion. The undamaged single-stranded DNA remains and DNA polymerase uses it as a template to synthesize a short complementary sequence . Final ligation to complete NER and form a double stranded DNA is carried out by DNA ligase . NER can be divided into two subpathways: global genomic NER (GG-NER or GGR) and transcription coupled NER (TC-NER or TCR). The two subpathways differ in how they recognize DNA damage but they share
9435-478: The major component of connective tissue, or keratin , the protein component of hair and nails. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane . A special case of intramolecular hydrogen bonds within proteins, poorly shielded from water attack and hence promoting their own dehydration , are called dehydrons . Many proteins are composed of several protein domains , i.e. segments of
9546-443: The mature mRNA, which is then used as a template for protein synthesis by the ribosome . In prokaryotes the mRNA may either be used as soon as it is produced, or be bound by a ribosome after having moved away from the nucleoid . In contrast, eukaryotes make mRNA in the cell nucleus and then translocate it across the nuclear membrane into the cytoplasm , where protein synthesis then takes place. The rate of protein synthesis
9657-405: The membranes of specialized B cells known as plasma cells . Whereas enzymes are limited in their binding affinity for their substrates by the necessity of conducting their reaction, antibodies have no such constraints. An antibody's binding affinity to its target is extraordinarily high. Many ligand transport proteins bind particular small biomolecules and transport them to other locations in
9768-496: The nobel prize in 1972, solidified the thermodynamic hypothesis of protein folding, according to which the folded form of a protein represents its free energy minimum. With the development of X-ray crystallography , it became possible to determine protein structures as well as their sequences. The first protein structures to be solved were hemoglobin by Max Perutz and myoglobin by John Kendrew , in 1958. The use of computers and increasing computing power also supported
9879-500: The order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more protein. For instance, yeast cells have been estimated to contain about 50 million proteins and human cells on the order of 1 to 3 billion. The concentration of individual protein copies ranges from a few molecules per cell up to 20 million. Not all genes coding proteins are expressed in most cells and their number depends on, for example, cell type and external stimuli. For instance, of
9990-440: The physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Some proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors . Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes . Once formed, proteins only exist for a certain period and are then degraded and recycled by
10101-453: The polymerase backwards. Mutations in TC-NER machinery are responsible for multiple genetic disorders including: Transcription factor II H (TFIIH) is the key enzyme involved in dual excision. TFIIH and XPG are first recruited to the site of DNA damage (XPG stabilizes TFIIH). The TFIIH subunits of XPD and XPB act as a 5'-3' and 3'-5' helicase, respectively — they help unwind DNA and generate
10212-424: The process of cell signaling and signal transduction . Some proteins, such as insulin , are extracellular proteins that transmit a signal from the cell in which they were synthesized to other cells in distant tissues . Others are membrane proteins that act as receptors whose main function is to bind a signaling molecule and induce a biochemical response in the cell. Many receptors have a binding site exposed on
10323-534: The protein or proteins of interest based on properties such as molecular weight, net charge and binding affinity. The level of purification can be monitored using various types of gel electrophoresis if the desired protein's molecular weight and isoelectric point are known, by spectroscopy if the protein has distinguishable spectroscopic features, or by enzyme assays if the protein has enzymatic activity. Additionally, proteins can be isolated according to their charge using electrofocusing . For natural proteins,
10434-427: The proteins in the cytoskeleton , which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses , cell adhesion , and the cell cycle . In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized . Digestion breaks the proteins down for metabolic use. Proteins have been studied and recognized since
10545-582: The same molecule, they can oligomerize to form fibrils; this process occurs often in structural proteins that consist of globular monomers that self-associate to form rigid fibers. Protein–protein interactions also regulate enzymatic activity, control progression through the cell cycle , and allow the assembly of large protein complexes that carry out many closely related reactions with a common biological function. Proteins can also bind to, or even be integrated into, cell membranes. The ability of binding partners to induce conformational changes in proteins allows
10656-576: The same process for lesion incision, repair, and ligation. The importance of NER is evidenced by the severe human diseases that result from in-born genetic mutations of NER proteins. Xeroderma pigmentosum and Cockayne's syndrome are two examples of NER associated diseases. Nucleotide excision repair is more complex in eukaryotes than prokaryotes , which express enzymes like the photolyase . In humans and other placental animals , there are 9 major proteins involved in NER. Deficiencies in certain proteins leads to disease; protein names are associated with
10767-573: The sample, allowing scientists to obtain more information and analyze larger structures. Computational protein structure prediction of small protein structural domains has also helped researchers to approach atomic-level resolution of protein structures. As of April 2024 , the Protein Data Bank contains 181,018 X-ray, 19,809 EM and 12,697 NMR protein structures. Proteins are primarily classified by sequence and structure, although other classifications are commonly used. Especially for enzymes
10878-498: The segmental progeroid (premature aging) symptoms (see DNA damage theory of aging ). Mutations in the ERCC3 (XPB) gene can lead, in humans, to xeroderma pigmentosum (XP) or XP combined with Cockayne syndrome (XPCS). Deficiency of ERCC4 (XPF) in humans results in a variety of conditions including accelerated aging. In humans, mutational defects in the ERCC5 (XPG) gene can cause either
10989-430: The sequencing of complex proteins. In 1999, Roger Kornberg succeeded in sequencing the highly complex structure of RNA polymerase using high intensity X-rays from synchrotrons . Since then, cryo-electron microscopy (cryo-EM) of large macromolecular assemblies has been developed. Cryo-EM uses protein samples that are frozen rather than crystals, and beams of electrons rather than X-rays. It causes less damage to
11100-405: The substrate, and an even smaller fraction—three to four residues on average—that are directly involved in catalysis. The region of the enzyme that binds the substrate and contains the catalytic residues is known as the active site . Dirigent proteins are members of a class of proteins that dictate the stereochemistry of a compound synthesized by other enzymes. Many proteins are involved in
11211-706: The surrounding amino acids may determine the exact binding specificity). Many such motifs has been collected in the Eukaryotic Linear Motif (ELM) database. Topology of a protein describes the entanglement of the backbone and the arrangement of contacts within the folded chain. Two theoretical frameworks of knot theory and Circuit topology have been applied to characterise protein topology. Being able to describe protein topology opens up new pathways for protein engineering and pharmaceutical development, and adds to our understanding of protein misfolding diseases such as neuromuscular disorders and cancer. Proteins are
11322-400: The tRNA molecules with the correct amino acids. The growing polypeptide is often termed the nascent chain . Proteins are always biosynthesized from N-terminus to C-terminus . The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass , which is normally reported in units of daltons (synonymous with atomic mass units ), or
11433-535: The teratogenic compound thalidomide. Thalidomide and other derivatives such as pomalidomide and lenalidomide are known as immunomodulatory drugs (or IMiDs) and have been investigated as therapeutic agents for autoimmune diseases and several cancers - particularly myelomas. Recent reports show that IMiDs bind to CRL4 and promote the degradation of IKZF1 and IKZF3 transcription factors, which are not normally targeted by CRL4 complexes. Human CUL4A forms direct interactions with: Human CUL4A-DDB1-RBX1 complexes promote
11544-472: The tertiary structure of the protein, which defines the binding site pocket, and by the chemical properties of the surrounding amino acids' side chains. Protein binding can be extraordinarily tight and specific; for example, the ribonuclease inhibitor protein binds to human angiogenin with a sub-femtomolar dissociation constant (<10 M) but does not bind at all to its amphibian homolog onconase (> 1 M). Extremely minor chemical changes such as
11655-463: The three subpathways converge for the steps of dual incision, repair, and ligation. Global genomic NER repairs damage in both transcribed and untranscribed DNA strands in active and inactive genes throughout the genome. This process is not dependent on transcription. This pathway employs several "damage sensing" proteins including the DNA-damage binding (DDB) and XPC-Rad23B complexes that constantly scan
11766-424: The transcribed strands of transcriptionally active genes faster than it repairs nontranscribed strands and transcriptionally silent DNA. TC-NER and GG-NER differ only in the initial steps of DNA damage recognition. The principal difference between TC-NER and GG-NER is that TC-NER does not require XPC or DDB proteins for distortion recognition in mammalian cells. Instead TC-NER initiates when RNA polymerase stalls at
11877-680: The transcription bubble and forward translocate RNA polymerase, thus initiating dissociation of the RNA Polymerase ternary elongation complex. TRCF also recruits the Uvr(A)BC nucleotide excision repair machinery by direct physical interaction with the UvrA subunit. Though historical studies have shown inconsistent results, genetic variation or mutation to nucleotide excision repair genes can impact cancer risk by affecting repair efficacy. Single-nucleotide polymorphisms (SNPs) and nonsynonymous coding SNPs (nsSNPs) are present at very low levels (>1%) in
11988-446: The ubiquitin ligase complex to induce arrest of the cell cycle in G2 phase . The CRL4A induces ubiquitination of the nuclear isoform of uracil-DNA glycosylase . HIV-2 also appears to utilize CRL4A via Vpx protein-induced destruction of a lentivirus -inhibiting deoxynucleoside triphosphohydrolase named SAMHD1 . In 2010, Ito et al. reported that Cereblon, a DCAF protein, was a major target of
12099-690: The ubiquitination of: protein is a CRL4A substrate only when directed by viral proteins protein is a CRL4A substrate only when directed by IMiDs Protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues . Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions , DNA replication , responding to stimuli , providing structure to cells and organisms , and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which
12210-466: Was insulin , by Frederick Sanger , in 1949. Sanger correctly determined the amino acid sequence of insulin, thus conclusively demonstrating that proteins consisted of linear polymers of amino acids rather than branched chains, colloids , or cyclols . He won the Nobel Prize for this achievement in 1958. Christian Anfinsen 's studies of the oxidative folding process of ribonuclease A, for which he won
12321-581: Was not fully appreciated until 1926, when James B. Sumner showed that the enzyme urease was in fact a protein. Linus Pauling is credited with the successful prediction of regular protein secondary structures based on hydrogen bonding , an idea first put forth by William Astbury in 1933. Later work by Walter Kauzmann on denaturation , based partly on previous studies by Kaj Linderstrøm-Lang , contributed an understanding of protein folding and structure mediated by hydrophobic interactions . The first protein to have its amino acid chain sequenced
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