Nup50, a nucleoplasmically oriented nucleoporin with a role in nuclear protein export

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The nuclear pore complex (NPC) regulates transport between the nucleus and cytoplasm.
• Nup50 (formerly NPAP60) is a polypeptide previously identified as an NPC component.
• The precise function of Nup50 in nuclear transport remains to be fully elucidated.

Purpose of the Study:

• To characterize the localization and function of rat Nup50 within the nuclear pore complex.
• To investigate the role of Nup50 in nuclear protein import and export pathways.

Main Methods:

• Immunofluorescence microscopy to determine Nup50 localization in rat liver nuclei and cultured NRK cells.
• Immunogold electron microscopy to pinpoint Nup50's precise location within the NPC.
• Microinjection of anti-Nup50 antibodies into NRK cells to assess effects on nuclear transport.
• In vitro binding assays to examine interactions between Nup50 fragments and nuclear transport receptors.

Main Results:

• Nup50 is widely expressed in rat cells and tissues, concentrated at the nuclear envelope and intranuclear regions.
• Nup50 localizes to the nucleoplasmic fibrils of the NPC.
• Inhibition of Nup50 function significantly impaired nuclear export of proteins with leucine-rich nuclear export sequences.
• Nup50 directly binds to CRM1, the key export receptor for these sequences, but not other tested receptors.
• Nuclear import of proteins with classical nuclear localization sequences remained unaffected.

Conclusions:

• Nup50 plays a direct and critical role in the nuclear export pathway.
• Nup50 likely functions as a docking site on the nuclear side of the NPC for CRM1-cargo complexes.
• These findings enhance our understanding of the molecular mechanisms governing nucleocytoplasmic transport.