A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them ( phosphorylation ) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a functional change of the target protein ( substrate ) by changing enzyme activity , cellular location, or association with other proteins. The human genome contains about 500 protein kinase genes and they constitute about 2% of all human genes. There are two main types of protein kinase. The great majority are serine/threonine kinases , which phosphorylate the hydroxyl groups of serines and threonines in their targets. Most of the others are tyrosine kinases , although additional types exist. Protein kinases are also found in bacteria and plants . Up to 30% of all human proteins may be modified by kinase activity, and kinases are known to regulate the majority of cellular pathways, especially those involved in signal transduction .
34-483: Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe ( S. pombe ). Wee1 has a molecular mass of 96 kDa and is a key regulator of cell cycle progression. It influences cell size by inhibiting the entry into mitosis , through inhibiting Cdk1 . Wee1 has homologues in many other organisms, including mammals. The regulation of cell size
68-414: A dosage-dependent inhibitor of mitosis. Thus, the amount of Wee1 protein correlates with the size of the cells: The fission yeast mutant wee1 , also called wee1 , divides at a significantly smaller cell size than wildtype cells. Since Wee1 inhibits entry into mitosis, its absence will lead to division at a premature stage and sub-normal cell size. Conversely, when Wee1 expression is increased, mitosis
102-496: A limiting factor of drug development. Skp2-targeting anti-sense oligonucleotides and siRNAs are in the drug development pipeline. Preliminary studies have shown that Skp2 downregulation can inhibit the growth of melanomas, lung cancer cells, oral cancer cells, and glioblastoma cells. βTRCP-targeting siRNAs have been shown to sensitize breast cancer cells and cervical cancer cells to existing chemotherapies. The plant hormone auxin binds Tir1 (Transport Inhibitor Response 1). Tir1
136-574: A number of protein phosphatases, which remove the phosphate groups that are added to specific serine or threonine residues of the kinase and are required to maintain the kinase in an active conformation. Tyrosine -specific protein kinases ( EC 2.7.10.1 and EC 2.7.10.2 ) phosphorylate tyrosine amino acid residues, and like serine/threonine-specific kinases are used in signal transduction . They act primarily as growth factor receptors and in downstream signaling from growth factors. Some examples include: These kinases consist of extracellular domains,
170-462: A number of signaling cascades, in particular those involved in cytokine signaling (but also others, including growth hormone ). One such receptor-associated tyrosine kinase is Janus kinase (JAK), many of whose effects are mediated by STAT proteins . ( See JAK-STAT pathway . ) Some kinases have dual-specificity kinase activities. For example, MEK (MAPKK), which is involved in the MAP kinase cascade,
204-698: A recognition for further phosphorylation by Cdc5. The S. cerevisiae protein Swe1 is also regulated by degradation. Swe1 is hyperphosphorylated by Clb2-Cdc28 and Cdc5 which may be a signal for ubiquitination and degradation by SCF E3 ubiquitin ligase complex as in higher eukaryotes. The mitosis promoting factor MPF also regulates DNA-damage induced apoptosis . Negative regulation of MPF by WEE1 causes aberrant mitosis and thus resistance to DNA-damage induced apoptosis. Kruppel-like factor 2 (KLF2) negatively regulates human WEE1, thus increasing sensitivity to DNA-damage induced apoptosis in cancer cells. Wee1 acts as
238-454: A transmembrane spanning alpha helix , and an intracellular tyrosine kinase domain protruding into the cytoplasm . They play important roles in regulating cell division , cellular differentiation , and morphogenesis . More than 50 receptor tyrosine kinases are known in mammals. The extracellular domains serve as the ligand -binding part of the molecule, often inducing the domains to form homo- or heterodimers . The transmembrane element
272-532: A very extensive family of proteins that share a conserved catalytic core. The structures of over 280 human protein kinases have been determined. There are a number of conserved regions in the catalytic domain of protein kinases. In the N-terminal extremity of the catalytic domain there is a glycine -rich stretch of residues in the vicinity of a lysine amino acid, which has been shown to be involved in ATP binding. In
306-419: Is a both a serine/threonine and tyrosine kinase. Histidine kinases are structurally distinct from most other protein kinases and are found mostly in prokaryotes as part of two-component signal transduction mechanisms. A phosphate group from ATP is first added to a histidine residue within the kinase, and later transferred to an aspartate residue on a 'receiver domain' on a different protein, or sometimes on
340-417: Is a frequent cause of disease, in particular cancer, wherein kinases regulate many aspects that control cell growth, movement and death. Drugs that inhibit specific kinases are being developed to treat several diseases, and some are currently in clinical use, including Gleevec ( imatinib ) and Iressa ( gefitinib ). Drug developments for kinase inhibitors are started from kinase assays Archived 2014-11-26 at
374-432: Is a single α helix. The intracellular or cytoplasmic Protein kinase domain is responsible for the (highly conserved) kinase activity, as well as several regulatory functions. Ligand binding causes two reactions: Autophosphorylation stabilizes the active conformation of the kinase domain. When several amino acids suitable for phosphorylation are present in the kinase domain (e.g., the insulin-like growth factor receptor),
SECTION 10
#1732855449626408-600: Is an Auxin Signaling F-box Protein (AFB) that acts as an auxin receptor. Auxin-bound Tir1 stimulates binding of SCF-Tir1 to the AUX/IAA repressor. Subsequent degradation of the repressor results in activation of AUX/IAA (i.e. auxin-responsive) genes. The plant hormone Jasmonate binds Coi1, an FBP. SCF-Coi1 then binds the JAZ transcription factor and targets it for degradation. Degradation of the JAZ transcription factor allows for
442-663: Is an FBP that binds CKIs such as p27 and p21. Skp2 binds p27 only when two conditions are met: p27 is phosphorylated by E/A/CKD2 and bound to Cks1. As a consequence of binding Skp2, p27 is ubiquitinated and targeted for degradation in late G1 and early S. SCF-Skp2 also targets p130 for degradation in a phosphorylation dependent manner. Beta-transducin repeat-containing protein (βTRCP) is an FBP that targets emi1—an APC/C-Cdh1 inhibitor—and wee1 for degradation during early mitosis. βTRCP recognizes these substrates after they are phosphorylated by Polo-like kinase 1 or Cyclin B-CDK1. Fbw7, which
476-435: Is called Swe1. In S. pombe , Wee1 is phosphorylated Cdk1 and cyclin B make up the maturation promoting factor (MPF) which promotes the entry into mitosis. It is inactivated by phosphorylation through Wee1 and activated by the phosphatase Cdc25C . Cdc25C in turn is activated by Polo kinase and inactivated by Chk1 . Thus in S. pombe Wee1 regulation is mainly under the control of phosphorylation through
510-476: Is critical to ensure functionality of a cell. Besides environmental factors such as nutrients, growth factors and functional load, cell size is also controlled by a cellular cell size checkpoint. Wee1 is a component of this checkpoint. It is a kinase determining the timepoint of entry into mitosis, thus influencing the size of the daughter cells. Loss of Wee1 function will produce smaller than normal daughter cell, because cell division occurs prematurely. Its name
544-600: Is delayed and cells grow to a large size before dividing. Protein kinase The chemical activity of a protein kinase involves removing a phosphate group from ATP and covalently attaching it to one of three amino acids that have a free hydroxyl group . Most kinases act on both serine and threonine , others act on tyrosine , and a number ( dual-specificity kinases ) act on all three. There are also protein kinases that phosphorylate other amino acids, including histidine kinases that phosphorylate histidine residues. Eukaryotic protein kinases are enzymes that belong to
578-577: Is derived from the Scottish dialect word wee, meaning small - its discoverer Paul Nurse was working at the University of Edinburgh in Scotland at the time of discovery. Wee1 inhibits Cdk1 by phosphorylating it on two different sites, Tyr15 and Thr14. Cdk1 is crucial for the cyclin-dependent passage of the various cell cycle checkpoints. At least three checkpoints exist for which the inhibition of Cdk1 by Wee1
612-492: Is important: Epigenetic function of Wee1 kinase has also been reported. Wee1 was shown to phosphorylate histone H2B at tyrosine 37 residue which regulated global expression of histones. The WEE1 gene has two known homologues in humans, WEE1 (also known as WEE1A) and WEE2 (WEE1B). The corresponding proteins are Wee1-like protein kinase and Wee1-like protein kinase 2 which act on the human Cdk1 homologue Cdk1 . The homologue to Wee1 in budding yeast Saccharomyces cerevisiae
646-567: Is known to be a haplo-insufficient tumor suppressor gene implicated in several sporadic carcinomas, for which one mutant allele is enough to disturb the wild type phenotype. Fbxo4 is another tumor suppressor FBP that has been implicated in human carcinomas. SCF-fbxo4 plays a role in cell cycle control by targeting cyclin D1 for degradation. Cyclin F is an FBP that is associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Mutations that prevent phosphorylation of Cyclin F alter
680-628: Is not yet understood in detail. Higher eukaryotes regulate Wee1 via phosphorylation and degradation In higher eukaryotes , Wee1 inactivation occurs both by phosphorylation and degradation . The protein complex SCF is an E3 ubiquitin ligase that functions in Wee1A ubiquitination. The M-phase kinases Polo-like kinase (Plk1) and Cdc2 phosphorylate two serine residues in Wee1A which are recognized by SCF. S. cerevisiae homologue Swe1 In S. cerevisiae , cyclin-dependent kinase Cdc28 (Cdk1 homologue)
714-405: Is phosphorylated by Swe1 (Wee1 homologue) and dephosphorylated by Mih1 (Cdc25 homologue). Nim1/Cdr1 homologue in S. cerevisiae , Hsl1, together with its related kinases Gin4 and Kcc4 localize Swe1 to the bud-neck . Bud-neck associating kinases Cla4 and Cdc5 (polo kinase homologue) phosphorylate Swe1 at different stages of the cell cycle. Swe1 is also phosphorylated by Clb2-Cdc28 which serves as
SECTION 20
#1732855449626748-507: Is the human homolog of cdc4 in yeast, is an FBP that targets Cyclin E, Myc, Notch and c-Jun for degradation. Fbw7 is stable throughout the cell cycle and is localized to the nucleus due to the presence of a nuclear localization sequence (NLS). SCF-Fbw7 targets Sic1—when at least six out of nine possible sites are phosphorylated—and Swi5 for degradation. Since Sic1 normally prevents premature entry into S-phase by inhibiting Cyclin B-CDK1, targeting Sic1 for degradation promotes S-phase entry. Fbw7
782-453: The MAP kinases (acronym from: "mitogen-activated protein kinases"). Important subgroups are the kinases of the ERK subfamily, typically activated by mitogenic signals, and the stress-activated protein kinases JNK and p38. While MAP kinases are serine/threonine-specific, they are activated by combined phosphorylation on serine/threonine and tyrosine residues. Activity of MAP kinases is restricted by
816-589: The Wayback Machine , the lead compounds are usually profiled for specificity before moving into further tests. Many profiling services are available from fluorescent-based assays to radioisotope based detections , and competition binding assays . SCF complex Skp, Cullin, F-box containing complex (or SCF complex ) is a multi-protein E3 ubiquitin ligase complex that catalyzes the ubiquitination of proteins destined for 26S proteasomal degradation. Along with
850-672: The anaphase-promoting complex , SCF has important roles in the ubiquitination of proteins involved in the cell cycle. The SCF complex also marks various other cellular proteins for destruction. SCF contains a variable F-box protein and three core subunits: The first hint that led to the discovery of the SCF complex came from genetic screens of Saccharomyces cerevisiae , also known as budding yeast. Temperature-sensitive cell division cycle (Cdc) mutants—such as Cdc4, Cdc34, and Cdc53 —arrested in G1 with unreplicated DNA and multiple elongated buds. The phenotype
884-459: The G1/S transition. SCF activity is largely regulated by post-translational modifications. For instance, ubiquitin-mediated autocatalytic degradation of FBPs is a mechanism of decreasing SCF activity. Well-characterized cell cycle substrates of SCF complexes include: There are approximately seventy human FBPs, several of which are involved in cell cycle control as a component of SCF complexes. Skp2
918-480: The activity of SCF-Cyclin F, which likely affects downstream processes pertinent to neuron degeneration in ALS and FTD. Normally, Cyclin F targets E2f1 for degradation. Recently, SCF complexes have become an attractive anti-cancer target because of their upregulation in some human cancers and their biochemically distinct active sites. Though many of the aforementioned FBPs have been implicated in cancer, cytotoxicity has been
952-476: The activity of the kinase can increase with the number of phosphorylated amino acids; in this case, the first phosphorylation switches the kinase from "off" to "standby". The active tyrosine kinase phosphorylates specific target proteins, which are often enzymes themselves. An important target is the ras protein signal-transduction chain. Tyrosine kinases recruited to a receptor following hormone binding are receptor-associated tyrosine kinases and are involved in
986-570: The central part of the catalytic domain, there is a conserved aspartic acid , which is important for the catalytic activity of the enzyme. Serine/threonine protein kinases ( EC 2.7.11.1 ) phosphorylate the OH group of serine or threonine (which have similar side chains). Activity of these protein kinases can be regulated by specific events (e.g., DNA damage), as well as numerous chemical signals, including cAMP / cGMP , diacylglycerol , and Ca / calmodulin . One very important group of protein kinases are
1020-410: The kinase itself. The aspartyl phosphate residue is then active in signaling. Histidine kinases are found widely in prokaryotes, as well as in plants, fungi and eukaryotes. The pyruvate dehydrogenase family of kinases in animals is structurally related to histidine kinases, but instead phosphorylate serine residues, and probably do not use a phospho-histidine intermediate. Deregulated kinase activity
1054-455: The polarity kinase, Pom1 's, pathway including Cdr2 and Cdr1. At the G2/M transition, Cdk1 is activated by Cdc25 through dephosphorylation of Tyr15. At the same time, Wee1 is inactivated through phosphorylation at its C-terminal catalytic domain by Nim1/Cdr1. Also, the active MPF will promote its own activity by activating Cdc25 and inactivating Wee1, creating a positive feedback loop , though this
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1088-445: The synthesis, degradation, binding interactions, post-translational modifications of regulatory proteins. Of these regulatory proteins, two ubiquitin ligases are crucial for progression through cell cycle checkpoints. The anaphase-promoting complex (APC) controls the metaphase-anaphase transition, while the SCF complex controls G1/S and G2/M transitions. Specifically, SCF has been shown to regulate centriole splitting from late telophase to
1122-469: Was attributed to a failure to degrade Sic1, an inhibitor of S cyclin-CDK complexes. These findings indicated that proteolysis is important in the G1/S transition. Next, biochemical studies revealed that Cdc34 is an E2 enzyme that physically interacts with an E3 ubiquitin ligase complex containing Skp1, Cdc4, and several other proteins. Skp1’s known binding partners—specifically Skp2, Cyclin F, and Cdc4—were found to share an approximately 40 residue motif that
1156-511: Was coined the F-box motif. The F-box hypothesis that followed these discoveries proposed that F-box proteins recruit substrates targeted for degradation, and that Skp1 links the F-box protein to the core ubiquitination complex. Subsequent genetic studies in Caenorhabditis elegans later contributed to the elucidation of other SCF complex components. The eukaryotic cell cycle is regulated through
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