Nuclear export signal
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A nuclear export signal (NES) is a short amino acid sequence of 4 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex using nuclear transport. It has the opposite effect of a nuclear localization signal, which targets a protein located in the cytoplasm for import to the nucleus. The NES is recognized and bound by exportins. In silico analysis of known NESs found the most common spacing of the hydrophobic residues to be LxxxLxxLxL, where "L" is a hydrophobic residue (often leucine) and "x" is any other amino acid; the spacing of these hydrophobic residues may be explained by examination of known structures that contain an NES, as the critical residues usually lie in the same face of adjacent secondary structures within a protein, which allows them to interact with the exportin. Ribonucleic acid (RNA) is composed of nucleotides, and thus, lacks the nuclear export signal to move out of the nucleus. As a result, most forms of RNA will bind to a protein molecule to form a ribonucleoprotein complex to be exported from the nucleus.
Nuclear export first begins with the binding of Ran-GTP (a G-protein) to exportin. This causes a shape change in exportin, increasing its affinity for the export cargo. Once the cargo is bound, the Ran-exportin-cargo complex moves out of the nucleus through the nuclear pore. GTPase activating proteins (GAPs) then hydrolyze the Ran-GTP to Ran-GDP, and this causes a shape change and subsequent exportin release. Once no longer bound to Ran, the exportin molecule loses affinity for the nuclear cargo as well, and the complex falls apart. Exportin and Ran-GDP are recycled to the nucleus separately, and guanine exchange factor (GEF) in the nucleus switches the GDP for GTP on Ran.
NES signals were first discovered in the human immunodeficiency virus type 1 (HIV-1) Rev protein and cAMP-dependent protein kinase inhibitor (PKI). The karyopherin receptor CRM1 has been identified as the export receptor for leucine-rich NESs in several organisms and is an evolutionarily conserved protein. The export mediated by CRM1 can be effectively inhibited by the fungicide leptomycin B (LMB), providing excellent experimental verification of this pathway.
Other proteins of various functions have also been experimentally inhibited of the NES signal such as the cyto-skeletal protein actin, which functions include cell motility and growth. The use of LBM as a NES inhibitor proved successful for actin resulting in accumulation of the protein within the nucleus, concluding universal functionality of NES throughout various protein functional groups.
Not all NES substrates are constitutively exported from the nucleus, meaning that CRM1-mediated export is a regulated event. Several ways of regulating NES-dependent export have been reported. These include masking/unmasking of NESs, phosphorylation and even disulfide bond formation as a result of oxidation.
The binding of NES to the export receptor of a protein gives the universal export function of NES an individually specified activation of export to each protein. Studies of specified NES amino acid sequences for particular proteins show the possibility of blocking the NES activation of one protein with an inhibitor for that amino acid sequence while other proteins of the same nucleus remain unaffected.
NESbase is a database of proteins, experimentally authenticated leucine-rich nuclear export signals (NES). A study was conducted by Center for Biological Sequence Analysis, Technical University of Denmark; and Department of Protein Chemistry, University of Copenhagen to validate NESbase version 1.0. Every entry in its database includes information whether nuclear export signals were sufficient for export or if it was only mediated transport by CRM1, the export receptor.
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