| Description |
Nuclear pore complexes (NPCs), which span the bilayer of the nuclear membrane, are a necessary and prominent feature of the nucleus. The proteins (or nucleoporins) that comprise the NPC participate in diverse cellular processes, from nucleocytoplasmic transport to mitotic spindle assembly to nuclear assembly and disassembly. The nucleoporin Nup153 is an important component of the vertebrate nuclear pore, both structurally and functionally. Its functions include facilitation of nucleocytoplasmic transport during interphase, participation in stepwise recruitment of nucleoporins to reassembling NPCs, and recruitment of the COPI complex to nuclear membranes during disassembly of the nucleus at mitosis. Some functions have been assigned to specific domains of Nup153 and partner proteins associated with individual domains provide added clues about Nup153. Of particular interest is the zinc finger (ZnF) domain of Nup153, which is important for nuclear envelope breakdown (NEBD) at mitosis. To better understand how this domain functions and, more specifically, how Nup153 is regulated during these events, a biochemical approach was taken to further dissect this domain and characterize the interface with partner proteins. Analysis of the Nup153 ZnF by nuclear magnetic resonance (NMR) revealed a structure characteristic of a larger zinc ribbon family with member proteins involved in diverse functions. Through biosensor assays, the single zinc finger motif was found to associate with the small GTPase Ran, and this interaction was minimally influenced by the nucleotide bound to Ran. Further investigation of this interaction by chemical shift and subsequent mutational analysis defined an exposed surface of three amino acids, which conferred the ability to bind Ran. Structural similarity between the GTPases Ran and Arf led to identification of an interaction between the ZnF domain of Nup153 and Arf as well. Functional significance of the partner proteins in a process known to involve the ZnF domain of Nup153 was assayed in an in vitro nuclear disassembly assay. Addition of Arf1 slightly delayed nuclear disassembly, while in contrast, addition of RanGDP caused striking delays in nuclear envelope breakdown. Characterization of the Nup153 ZnF partnership with Ran and Arf1, and their roles in nuclear breakdown, provide insight into possible mechanisms of Nup153 regulation during nuclear disassembly at mitosis. |
