Esportazione nucleare

Nuclear export signals (NES) are short amino acid sequences on cargo proteins that direct their transport from the nucleus to the cytosol. Three types exist: the classical 10-residue leucine-rich signal characterized by stretches of leucine and isoleucine residues, the 38-residue M9 signal found in hnRNP A1, and the 24-residue KNS signal found in hnRNP K. Each NES type is recognized by specific nuclear exporters that facilitate cargo movement through nuclear pore complexes.

Exportin 1, also called CRM1, recognizes cargo proteins bearing leucine-rich NES sequences. Exportin 1 first binds to Ran-GTP, forming a complex that undergoes conformational change. This activated exportin-Ran-GTP complex then binds the cargo protein, creating a trimeric export complex ready for translocation through the nuclear pore complex.

Ran-GTP is a small GTPase that acts as a critical cofactor in nuclear export. It binds to exportin 1 and enables the exporter to recognize and bind cargo proteins. Ran-GTP maintains the stability of the export complex during translocation through the nuclear pore complex. When Ran-GTP is hydrolyzed to Ran-GDP in the cytosol, it triggers conformational changes that weaken cargo binding and disassemble the export complex.

The exportin-Ran-GTP-cargo complex interacts with FG repeats lining the inner channel of the nuclear pore complex. These interactions allow the complex to disrupt the gel-like selective barrier of the NPC and diffuse into the cytosol. The FG repeats facilitate passage while maintaining the barrier's selectivity for properly recognized cargo molecules.

Ran-GAP, a GTPase-activating protein on the cytoplasmic fibrils of the nuclear pore complex, initiates hydrolysis of Ran-GTP to Ran-GDP. This hydrolysis triggers a conformational change in exportin 1 that weakens its binding affinity for the cargo. The weakened interactions cause the export complex to disassemble, releasing the cargo protein into the cytosol for its functional role.

After cargo release in the cytosol, exportin 1 and Ran-GDP are recycled back to the nucleus through nuclear import mechanisms. This recycling allows exportin 1 to bind fresh Ran-GTP and participate in subsequent rounds of nuclear export. The recycling process ensures continuous availability of export machinery and maintains the directionality of nucleocytoplasmic transport.

Unlike leucine-rich NES, the M9 and KNS signals promote bidirectional nuclear trafficking of proteins and RNA. The M9 signal exhibits temperature-sensitive behavior, allowing nuclear-restricted proteins to move to the cytosol at high temperatures while restricting them inside the nucleus at low temperatures. This dynamic regulation contrasts with the constitutive, unidirectional export mediated by leucine-rich signals.