Nuclear protein sorting is the selective trafficking of histones, polymerases, gene regulatory proteins into the nucleus and exporting RNAs and ribosomes to the cytosol. It is a tightly controlled process that regulates gene expression within a cell.
Proteins targeted to the nucleus carry nuclear localization signals or NLS recognized by import receptors in the cytosol. Similarly, proteins with nuclear export signals are recognized by export receptors. Import and export receptors are structurally related and freely move across the nucleus by facilitated diffusion. Upon cargo binding, the cargo-receptor complex docks onto the nuclear pore complex or NPC embedded on the nuclear membrane.
Two models describe docking and movement of the cargo-receptor complex across the NPC as follows:
On the nuclear side, a small GTPase Ran, bound to GTP to the incoming cargo-importin complex, induces a conformational change in importin beta and cargo delivery inside the nucleus.
During nuclear export, Ran-GTP inside the nucleus associates with exportin to form the cargo-export complex and move to the cytosol. Ran-GTP hydrolysis on the cytoplasmic fibrils of NPC dissociates the cargo and receptor, releasing them into the cytosol.
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