A link between the synthesis of nucleoporins and the biogenesis of the nuclear envelope

The nuclear pore complex (NPC) is a multicomponent structure containing a s ubset of proteins that bind nuclear transport factors or karyopherins and m ediate their movement across the nuclear envelope. By altering the expressi on of a single nucleoporin gene, NUP53, we showed that the overproduction o f Nup53p altered nuclear transport and had a profound effect on the structu re of the nuclear membrane. Strikingly, conventional and immunoelectron mic roscopy analysis revealed that excess Nup53p entered the nucleus and associ ated with the nuclear membrane. Here, Nup53p induced the formation of intra nuclear, tubular membranes that later formed flattened, double membrane lam ellae structurally similar to the nuclear envelope. Like the nuclear envelo pe, the intranuclear double membrane lamellae enclosed a defined cisterna t hat was interrupted by pores but, unlike the nuclear envelope pores, they l acked NPCs. Consistent with this observation, we detected only two NPC prot eins, the pore membrane proteins Pom152p and Ndc1p, in association with the se membrane structures. Thus, these pores likely represent an intermediate in NPC assembly. We also demonstrated that the targeting of excess Nup53p t o the NPC and its specific association with intranuclear membranes were dep endent on the karyopherin Kap121p and the nucleoporin Nup170p. At the nucle ar envelope, the abilities of Nup53p to associate with the membrane and dri ve membrane proliferation were dependent on a COOH-terminal segment contain ing a potential amphipathic or-helix. The implications of these results wit h regards to the biogenesis of the nuclear envelope are discussed.