The nucleoporin Nup50 activates the Ran guanine nucleotide exchange factor RCC1 to promote NPC assembly at the end of mitosis

Abstract

During mitotic exit, thousands of nuclear pore complexes (NPCs) assemble concomitant with the nuclear envelope to build a transport‐competent nucleus. Here, we show that Nup50 plays a crucial role in NPC assembly independent of its well‐established function in nuclear transport. RNAi‐mediated downregulation in cells or immunodepletion of Nup50 protein in Xenopus egg extracts interferes with NPC assembly. We define a conserved central region of 46 residues in Nup50 that is crucial for Nup153 and MEL28/ELYS binding, and for NPC interaction. Surprisingly, neither NPC interaction nor binding of Nup50 to importin α/β, the GTPase Ran, or chromatin is crucial for its function in the assembly process. Instead, an N‐terminal fragment of Nup50 can stimulate the Ran GTPase guanine nucleotide exchange factor RCC1 and NPC assembly, indicating that Nup50 acts via the Ran system in NPC reformation at the end of mitosis. In support of this conclusion, Nup50 mutants defective in RCC1 binding and stimulation cannot replace the wild‐type protein in in vitro NPC assembly assays, whereas excess RCC1 can compensate the loss of Nup50. image Nup50, part of the nuclear pore complex basket structure, is known as auxiliary factor for nuclear import. This work shows that it also plays a crucial role in nuclear pore complex (NPC) assembly at the end of mitosis by stimulating the Ran GTPase pathway. Nup50 depletion impairs NPC assembly at the end of mitosis in frog extracts and human cells. Nup50 function in NPC assembly is independent of its interaction with MEL28/ELYS and Nup153 or its NPC localization. Nup50 interacts with the Ran guanine nucleotide exchange factor RCC1 to stimulate RanGTP production critical for NPC assembly.