Nuclear export signal
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A nuclear export signal (NES) is a short amino acid sequence of 5-6 hydrophobic residues in a protein that targets it for export from the cell nucleus to the cytoplasm through the nuclear pore complex. 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. Few sequence commonalities have been observed in the NESes that have been identified. Ribonucleic acid (RNA) are composed of nucleotides, and thus, lack 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.
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