Disassembly and reassembly of the nuclear pore complex

Abstract

The nuclear pore complexes (NPCs) are multiprotein communicative channels spanning the nuclear envelope. In higher eukaryotes NPCs reversibly disassemble during mitosis into distinct nucleoporin subcomplexes. Some cell types (e.g. oocytes and early embryonic cells) also contain mimics of NPCs of unknown function, which are located in cytoplasmic membranes. They are termed annulate lamellae pore complexes (ALPC).

This study was aimed at understanding the process of mitotic disassembly and reassembly of the NPC and at elucidating the function of ALPCs. Using syncytial Drosophila embryos as a model we have tested the proposed function of ALPCs as a storage compartment for nucleoporins fueling assembly of new NPCs in rapidly proliferating cells. Surprisingly, we found that ALPCs are not depleted during assembly of new NPCs and that they represent only a minor fraction of the total embryonic nucleoporins while the major fraction is persistently soluble. We conclude that in Drosophila, ALPCs play only a minor role as a storage compartment for nucleoporins.

We developed a novel in vivo model system based on syncytial Drosophila embryos to study mitotic disassembly/reassembly of the NPC. We found that the major mitotic kinase Cdk1 is the key regulator of both NPC and ALPC disassembly/reassembly in vivo and that Cdk1 activity is able to phosphorylate and solubilize nucleoporins in vitro. We also found that phosphatase activity, sensitive to okadaic acid (OA), is required for reassembly of both NPCs and ALPCs in vivo. Additionally, we showed that the Ran GTPase system, that drives active nucleocytoplasmic transport during intephase, is selectively required for post-mitotic reassembly of NPCs but not ALPCs in vivo. Our findings suggest that in live cells NPC assembly is regulated by a dynamic equilibrium between kinase (Cdk1) and phospahatase (sensitive to OA) activity and that it is spatially coordinated by the Ran GTPase system.

Finally. using the nucleoporin gp210 as a model. we have tested a role of mitotic phosphorylation of nucleoporins in disassembly of the NPC. We present evidence that a single mitotic phosphorylation of gp210 weakens its binding to the NPC and interferes with its postmitotic recruitment to the newly formed NE. These findings represent the first direct evidence that mitotic nucleoporin phosphorylation functions in disassembly of the NPC.