The Nsp1p Carboxy-Terminal Domain Is Organized into Functionally Distinct Coiled-Coil Regions Required for Assembly of Nucleoporin Subcomplexes and Nucleocytoplasmic Transport

Nucleoporin Nsp1p, which has four predicted coiled-coil regions (coils 1 to 4) in the essential carboxy-terminal domain, is unique in that it is part of two distinct nuclear pore complex (NPC) subcomplexes, Nsp1p-Nup57p-Nup49p-Nic96p and Nsp1p-Nup82p-Nup159p. As shown by in vitro reconstitution, coiled-coil region 2 (residues 673 to 738) is sufficient to form heterotrimeric core complexes and can bind either Nup57p or Nup82p. Accordingly, interaction of Nup82p with Nsp1p coil 2 is competed by excess Nup57p. Strikingly, coil 3 and 4 mutants are still assembled into the core Nsp1p-Nup57p-Nup49p complex but no longer associate with Nic96p. Consistently, the Nsp1p-Nup57p-Nup49p core complex dissociates from the nuclear pores in nsp1 coil 3 and 4 mutant cells, and as a consequence, defects in nuclear protein import are observed. Finally, the nsp1-L640S temperature-sensitive mutation, which maps in coil 1, leads to a strong nuclear mRNA export defect. Thus, distinct coiled-coil regions within Nsp1p-C have separate functions that are related to the assembly of different NPC subcomplexes, nucleocytoplasmic transport, and incorporation into the nuclear pores.