Nuclear pore targeting of the yeast Pom33 nucleoporin depends on karyopherin- and lipid-binding

Pom33 is an integral membrane protein of the yeast nuclear pore complex (NPC), required for proper NPC distribution and assembly. To characterize Pom33 NPC-targeting determinants, we performed immunoprecipitation experiments followed by mass spectrometry analyses. This identified a novel Pom33 partner, the nuclear import factor Kap123. In vitro experiments revealed a direct interaction between Pom33 C-terminal domain (CTD) and Kap123. In silico analysis predicted the presence of two amphipathic α-helices within Pom33-CTD. Circular dichroism and liposome co-flotation assays showed that this domain is able to fold into α-helices in the presence of liposomes and preferentially binds to highly curved lipid membranes. When expressed in yeast, under conditions abolishing Pom33-CTD membrane association, this domain behaves as a Kap123-dependent nuclear localization signal (NLS). While deletion of Pom33 C-terminal domain (Pom33ΔCTD-GFP) impairs Pom33 stability and NPC targeting, mutants affecting either Kap123 binding or the amphipathic properties of the α-helices do not display any detectable defect. However, combined impairment of lipid and Kap123 binding affects Pom33 targeting to NPCs. These data highlight the requirement of multiple determinants and mechanisms for proper NPC localization of Pom33.