Transport across the nuclear envelope (NE) is mediated by nuclear pore complexes (NPCs). These structures are composed of various subcomplexes of proteins that are each present in multiple copies and together establish the eightfold symmetry of the NPC. One evolutionarily conserved subcomplex of the NPC contains the nucleoporins Nup53 and Nup155. Using truncation analysis, we have defined regions of Nup53 that bind to neighboring nucleoporins as well as those domains that target Nup53 to the NPC in vivo. Using this information, we investigated the role of Nup53 in NE and NPC assembly using Xenopus egg extracts. We show that both events require Nup53. Importantly, the analysis of Nup53 fragments revealed that the assembly activity of Nup53 depleted extracts could be reconstituted using a region of Nup53 that binds specifically to its interacting partner Nup155. On the basis of these results, we propose that the formation of a Nup53-Nup155 complex plays a critical role in the processes of NPC and NE assembly.
INTRODUCTIONThe nuclear envelope (NE) provides a physical barrier between the nucleus and cytoplasm and their unique metabolic tasks. The NE is defined by three morphologically distinct regions. The outer nuclear membrane (ONM) is continuous with the rough endoplasmic reticulum (ER) and contains a similar set of proteins (reviewed in Mattaj, 2004). The inner nuclear membrane (INM) lies adjacent to the nucleoplasm and contains a unique repertoire of proteins that, in part, mediate its interactions with the nuclear lamina and chromatin. Finally, at numerous locations along the NE, the ONM and INM are interrupted by pores where the INM and the ONM are bridged by a connecting membrane termed the pore membrane (POM) domain. Within these pores reside the nuclear pore complexes (NPCs), aqueous channels that provide portals for both passive and active transport of macromolecules.An NPC contains ϳ30 nucleoporins (Nups), many of which are evolutionarily conserved in structure and function (Tran and Wente, 2006). In many cases these Nups are organized into subcomplexes that are present in multiple copies, and they are distributed around the central axis of the NPC, contributing to its characteristic eightfold symmetry. Different groups of Nups and their respective subcomplexes contribute to distinct structural components of the NPC. For example, Nup214/Nup84 are components of the cytoplasmic fibrils (Kraemer et al., 1994;Bastos et al., 1997), the Nup53/Nup155 complex is part of the central core (Marelli et al., 1998), and Nup153 and Tpr contribute to the nucleoplasmic ring and fibrils (Krull et al., 2004).When metazoan cells enter mitosis, their NE is disassembled allowing for the mitotic spindle to access the condensed chromatin. Several models based on previous findings have been proposed to explain the mechanism by which this occurs (reviewed in Hetzer et al., 2005). NE disassembly is accompanied by the phosphorylation of multiple NE proteins including the lamins and several Nups (Burke and Ellenberg, 2002, and refer...