page 63, Walther et al. overturn this idea, demonstrating that cytoplasmic filaments have no essential function in the nuclear import of bulk cargos. The results also provide new insights on the composition of the filaments.
Using the Xenopus egg extract system, the authors analyzed two cytoplasmically oriented nucleoporins, CAN/Nup214 and RanBP2/Nup358. CAN/Nup214 was previously thought to be a component of the cytoplasmic filaments, but in the new work it appears to be located near the entrance to the translocation channel in the NPC. RanBP2/Nup358, however, is an essential component of the cytoplasmic filaments: depleting this protein causes NPCs to form without cytoplasmic filaments. Surprisingly, NPCs lacking cytoplasmic filaments show no deficiency in nuclear import, indicating that the filaments are dispensable for this function.
Walther et al. propose that instead of being an obligatory stop on the nuclear import pathway, NPC cytoplasmic filaments of RanBP2/Nup358 may arrest import complexes to allow specific modifications before they enter the nucleus.
For a nuclear pore to form, the inner and outer membranes of the nucleus must fuse. This process is thought to involve the integral membrane protein gp210, which has a short COOH-terminal tail extending into the cytoplasm. Drummond and Wilson, reporting on page 53, now provide strong evidence to support this idea and present a new model to help explain nuclear pore formation.
Recombinant gp210 tail polypeptides, or antibodies against the tail, stop nuclear pore formation at an early stage. The polypeptides also cause the accumulation of “mini-pores,” which appear to represent an intermediate that is unable to dilate into a full-size pore. The authors suggest that specific nucleoporins may bind to the gp210 tails around these mini-pores, forming spokes or struts that dilate the pore. ▪