Nuclear transport refers to the mechanisms by which molecules move across the nuclear membrane of a cell. The entry and exit of large molecules from the cell nucleus is tightly controlled by the nuclear pore complexes (NPCs). Although small molecules can enter the nucleus without regulation, macromolecules such as RNA and proteins require association with transport factors known as nuclear transport receptors , like karyopherins called importins to enter the nucleus and exportins to exit.
29-426: 1W9C , 2L1L , 3GB8 , 4BSM , 4BSN , 5DIS 7514 103573 ENSG00000082898 ENSMUSG00000020290 O14980 Q6P5F9 NM_003400 NM_001035226 NM_134014 NP_003391 NP_001030303 NP_598775 Exportin 1 ( XPO1 ), also known as chromosomal region maintenance 1 ( CRM1 ), is a eukaryotic protein that mediates the nuclear export of various proteins and RNAs. XPO1 (CRM1) originally
58-485: A truncated form of importin-α in which the NLS binding domain is missing. In addition, importin-α has been shown to transport the tumour suppressor gene , BRCA1 (breast cancer type 1 susceptibility protein) , into the nucleus . The overexpression of importin-α has also been linked with poor survival rates seen in certain melanoma patients. Importin activity is also associated with some viral pathologies . For instance, in
87-601: A curved-shaped structure, which facilitates binding to the NLS of specific cargo proteins. The major NLS binding site is found towards the N-terminus , with a minor site being found at the C-terminus . As well as the ARM structures, Importin-α also contains a 90 amino acid N-terminal region, responsible for binding to Importin-β, known as the Importin-β binding (IBB)domain. This is also
116-495: A receptor for nuclear localization signals (NLS) , thus allowing transport into the nucleus . Since these initial discoveries in 1994 and 1995, a host of Importin genes, such as IPO4 and IPO7 , have been found that facilitate the import of slightly different cargo proteins, due to their differing structure and locality. A large proportion of the importin-α adaptor protein is made up of several armadillo repeats (ARM) arranged in tandem . These repeats can stack together to form
145-453: A site of autoinhibition, and is implicated in the release of cargo once importin-α reaches the nucleus . Importin-β is the typical structure of a larger superfamily of karyopherins . The basis of their structure is 18-20 tandem repeats of the HEAT motif. Each one of these repeats contains two antiparallel alpha helices linked by a turn , which stack together to form the overall structure of
174-504: Is a stub . You can help Misplaced Pages by expanding it . Nuclear transport Protein that must be imported to the nucleus from the cytoplasm carry nuclear localization signals (NLS) that are bound by importins . An NLS is a sequence of amino acids that acts as a tag. They are most commonly hydrophilic sequences containing lysine and arginine residues, although diverse NLS sequences have been documented. Proteins, transfer RNA , and assembled ribosomal subunits are exported from
203-401: Is capable of binding importins and exportins . Importins release cargo upon binding to RanGTP, while exportins must bind RanGTP to form a ternary complex with their export cargo. The dominant nucleotide binding state of Ran depends on whether it is located in the nucleus (RanGTP) or the cytoplasm (RanGDP). Nuclear export roughly reverses the import process; in the nucleus, the exportin binds
232-511: Is free of importin-β, the cargo protein can be released into the nucleus . The N-terminal importin-β-binding (IBB) domain of importin-α contains an auto-regulatory region that mimics the NLS motif . The release of importin-β frees this region and allows it to loop back and compete for binding with the cargo protein at the major NLS-binding site . This competition leads to the release of the protein . In some cases, specific release factors such as Nup2 and Nup50 can be employed to help release
261-420: Is important due to tRNA's central role in translation, where it is involved in adding amino acids to a growing peptide chain. The tRNA exporter in vertebrates is called exportin-t . Exportin-t binds directly to its tRNA cargo in the nucleus, a process promoted by the presence of RanGTP. Mutations that affect tRNA's structure inhibit its ability to bind to exportin-t, and consequentially, to be exported, providing
290-444: Is to mediate the translocation of proteins with nuclear localization signals into the nucleus , through nuclear pore complexes (NPC) , in a process known as the nuclear protein import cycle. The first step of this cycle is the binding of cargo. Importin can perform this function as a monomeric importin-β protein , but usually requires the presence of importin-α, which acts as an adaptor to cargo proteins (via interactions with
319-648: The Max Delbrück Center for Molecular Medicine . The process of nuclear protein import had already been characterised in previous reviews, but the key proteins involved had not been elucidated up until that point. A 60 kDa cytosolic protein, essential for protein import into the nucleus, and with a 44% sequence identity to SRP1p , was purified from Xenopus eggs. It was cloned, sequenced and expressed in E.coli and in order to completely reconstitute signal dependent transport, had to be combined with Ran (TC4). Other key stimulatory factors were also found in
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#1732884107474348-486: The NLS ). The NLS is a sequence of basic amino acids that tags the protein as cargo destined for the nucleus . A cargo protein can contain either one or two of these motifs , which will bind to the major and/or minor binding sites on importin-α. Once the cargo protein is bound, importin-β interacts with the NPC , and the complex diffuses into the nucleus from the cytoplasm . The rate of diffusion depends on both
377-463: The cell 's cytoplasm to the nucleus . It does so by binding to specific recognition sequences , called nuclear localization sequences (NLS). Importin has two subunits, importin α and importin β. Members of the importin-β family can bind and transport cargo by themselves, or can form heterodimers with importin-α. As part of a heterodimer , importin-β mediates interactions with the pore complex , while importin-α acts as an adaptor protein to bind
406-401: The cytoplasm , Ran - GTP is hydrolysed by Ran GAP , forming Ran - GDP , and releasing the two importins for further activity. It is this hydrolysis of GTP that provides the energy for the cycle as a whole. In the nucleus , a GEF will charge Ran with a GTP molecule, which is then hydrolysed by a GAP in the cytoplasm , as stated above. It is this activity of Ran that allows for
435-419: The nuclear localization signal (NLS) on the cargo. The NLS-Importin α-Importin β trimer dissociates after binding to Ran GTP inside the nucleus , with the two importin proteins being recycled to the cytoplasm for further use. Importin can exist as either a heterodimer of importin-α/β or as a monomer of Importin-β. Importin-α was first isolated in 1994 by a group including Enno Hartmann , based at
464-405: The protein . In order to transport cargo into the nucleus , importin-β must associate with the nuclear pore complexes . It does this by forming weak, transient bonds with nucleoporins at their various F G (Phe-Gly) motifs. Crystallographic analysis has shown that these motifs bind to importin-β at shallow hydrophobic pockets found on its surface. The primary function of importin
493-436: The cargo and Ran-GTP and diffuses through the pore to the cytoplasm, where the complex dissociates. Ran-GTP binds GAP and hydrolyzes GTP, and the resulting Ran-GDP complex is restored to the nucleus where it exchanges its bound ligand for GTP. Hence, whereas importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo. A specialized mRNA exporter protein moves mature mRNA to
522-421: The cargo as well. Finally, in order to return to the cytoplasm , importin-α must associate with a Ran-GTP / CAS (nuclear export factor) complex which facilitates its exit from the nucleus . CAS (cellular apoptosis susceptibility protein) is part of the importin-β superfamily of karyopherins and is defined as a nuclear export factor. Importin-β returns to the cytoplasm , still bound to Ran - GTP . Once in
551-445: The cell with another quality control step. As described above, once the complex has crossed the envelope it dissociates and releases the tRNA cargo into the cytosol. Many proteins are known to have both NESs and NLSs and thus shuttle constantly between the nucleus and the cytosol. In certain cases one of these steps (i.e., nuclear import or nuclear export) is regulated, often by post-translational modifications . Nuclear import limits
580-461: The concentration of importin-α present in the cytoplasm and also the binding affinity of importin-α to the cargo. Once inside the nucleus , the complex interacts with the Ras-family GTPase , Ran-GTP . This leads to the dissociation of the complex by altering the conformation of importin-β. Importin-β is left bound to Ran - GTP , ready to be recycled. Now that the importin-α/cargo complex
609-534: The cytoplasm after post-transcriptional modification is complete. This translocation process is actively dependent on the Ran protein, although the specific mechanism is not yet well understood. Some particularly commonly transcribed genes are physically located near nuclear pores to facilitate the translocation process. Export of tRNA is also dependent on the various modifications it undergoes, thus preventing export of improperly functioning tRNA. This quality control mechanism
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#1732884107474638-673: The infection pathway of the Ebola virus , a key step is the inhibition of the nuclear import of PY-STAT1 . This is achieved by the virus sequestering importin-α in the cytoplasm , meaning it can no longer bind its cargo at the NLS . As a result, importin cannot function and the cargo protein stays in the cytoplasm. Many different cargo proteins can be transported into the nucleus by importin. Often, different proteins will require different combinations of α and β in order to translocate. Some examples of different cargo are listed below. Although importin-α and importin-β are used to describe importin as
667-566: The nucleus due to association with exportins, which bind signaling sequences called nuclear export signals (NES). The ability of both importins and exportins to transport their cargo is regulated by the Ran small G-protein . G-proteins are GTPase enzymes that bind to a molecule called guanosine triphosphate (GTP) which they then hydrolyze to create guanosine diphosphate (GDP) and release energy. The RAN enzymes exist in two nucleotide-bound forms: GDP-bound and GTP-bound. In its GTP-bound state, Ran
696-499: The propagation of large proteins expressed in skeletal muscle fibers and possibly other syncytial tissues, maintaining localized gene expression in certain nuclei. Combining both NESs and NLSs promotes propagation of large proteins to more distant nuclei in muscle fibers. Protein shuttling can be assessed using a heterokaryon fusion assay . Importin Importin is a type of karyopherin that transports protein molecules from
725-407: The study. Importin-β, unlike importin-α, has no direct homologues in yeast, but was purified as a 90-95 kDa protein and found to form a heterodimer with importin-α in a number of different cases. These included a study led by Michael Rexach and further studies by Dirk Görlich . These groups found that importin-α requires another protein, importin-β to function, and that together they form
754-612: The unidirectional transport of proteins . There are several disease states and pathologies that are associated with mutations or changes in expression of importin-α and importin-β. Importins are vital regulatory proteins during the processes of gametogenesis and embryogenesis . As a result, a disruption in the expression patterns of importin-α has been shown to cause fertility defects in Drosophila melanogaster . There have also been studies that link altered importin-α to some cases of cancer . Breast cancer studies have implicated
783-495: The viral protein Rev , an event that is a crucial part of the infection cycle. XPO1 is affected in some cancer types and is therefore viewed as a target for development of anti-cancer drugs. Selinexor , a drug specifically targeting XPO1, was approved by the FDA for treatment of multiple myeloma. XPO1 has been shown to interact with: This article on a gene on human chromosome 2
812-400: Was identified in the fission yeast Schizosaccharomyces pombe in a genetic screen, and investigators determined that it was involved in control of the chromosome structure. It was later shown to be the nuclear transport receptor for cargos with leucine-rich nuclear export signals ( NES ). The structural details of the interaction of XPO1 with its cargos were revealed two decades after the gene
841-445: Was identified. XPO1 mediates NES-dependent protein transport. It exports several hundreds of different proteins from the nucleus. XPO1 is involved in the nuclear export of ribosomal subunits. XPO1 plays a role in export of various RNAs including U snRNAs, rRNAs (as a part of ribosomal subunits), and some mRNAs. XPO1 is involved in various viral infections. For example, it is required for the nuclear export of HIV-1 RNA in complex with
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