Homeostatic Regulation of Nucleoporins is the Primary Driver of Nuclear Pore Biogenesis

Nuclear pore complexes (NPCs) are the multiprotein channels that control the access to the genome. Because of their critical functions, the number of NPCs that cells assemble is tightly controlled and varies between different cell types, cellular states, and in diseases. However, the mechanisms that regulate NPC formation and numbers in mammalian cells remain largely unclear. Using a genome-wide siRNA screen for modulators of NPC assembly, we identified translation-related factors, proteasome components, and the CCR4-NOT mRNA deadenylase complex as the top regulators of NPC numbers. We found that stabilization of nucleoporin mRNAs by inhibition of the CCR4-NOT stoichiometrically increases nucleoporin protein levels and enhances NPC assembly in normal and cancer cells. Inhibiting protein translation in CCR4-NOT defective cells blocks the increase in NPC numbers, indicating that nucleoporin mRNA levels are rate-limiting for NPC biogenesis. We found that inhibiting proteasome activity also promotes NPC biogenesis, indicating that nucleoporins required for nuclear pore formation are normally produced in excess. Our findings show that nucleoporin mRNA and protein degradation restrict NPC assembly and indicate nucleoporin homeostasis is the main mechanism regulating NPC numbers in mammalian cells.