New motifs in Ndc1 mediating interaction with the Nup84 complex and nuclear membranes
The nuclear pore complex (NPC) embedded in the double nuclear membrane is built from ~30 different nucleoporins (Nups) in multiple copies, of which a few are integral nuclear membrane proteins. One of these transmembrane Nups is Ndc1, which is thought to play a role in interphase NPC assembly at the fused inner and outer nuclear membrane. In this study, we discovered a direct interaction of Ndc1s transmembrane domain with Nup120 and Nup133, members of the Y-complex that coats the nuclear pore membrane. In addition, we identified a so far unrecognized amphipathic helix (AH) in the C-terminal domain of Ndc1, which can bind to high curvature liposomes. When overexpressed in yeast this amphipathic motif is toxic and dramatically alters the intracellular membrane organization. Further genetic investigations revealed that Ndc1-AH functionally interacts with related motifs in the C-terminus of Nups Nup53 and Nup59, known to serve in nuclear pore membrane binding and link between NPC modules. This relationship could explain why the essential function of Ndc1 can be suppressed by deleting the amphipathic helix from Nup53. Our data indicate that nuclear membrane biogenesis dependent on a balanced ratio between amphipathic motifs in diverse nucleoporins is essential for interphase NPC biogenesis.