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), and it is required for proper NPC distribution and assembly. To characterize the Pom33 NPC-targeting determinants, we performed immunoprecipitation experiments followed by mass spectrometry analyses. This identified a new Pom33 partner, the nuclear import factor Kap123. In vitro experiments revealed a direct interaction between the 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). Although deletion of Pom33 C-terminal domain (Pom33(ΔCTD)-GFP) impaired Pom33 stability and NPC targeting, mutants affecting either Kap123 binding or the amphipathic properties of the α-helices did not display any detectable defect. However, combined impairment of lipid and Kap123 binding affects targeting of Pom33 to NPCs. These data highlight the requirement of multiple determinants and mechanisms for proper NPC localization of Pom33.

Pom33, a novel transmembrane nucleoporin required for proper nuclear pore complex distribution.

The biogenesis of nuclear pore complexes (NPCs) represents a paradigm for the assembly of high-complexity macromolecular structures. So far, only three integral pore membrane proteins are known to function redundantly in NPC anchoring within the nuclear envelope. Here, we describe the identification and functional characterization of Pom33, a novel transmembrane protein dynamically associated with budding yeast NPCs. Pom33 becomes critical for yeast viability in the absence of a functional Nup84 complex or Ndc1 interaction network, which are two core NPC subcomplexes, and associates with the reticulon Rtn1. Moreover, POM33 loss of function impairs NPC distribution, a readout for a subset of genes required for pore biogenesis, including members of the Nup84 complex and RTN1. Consistently, we show that Pom33 is required for normal NPC density in the daughter nucleus and for proper NPC biogenesis and/or stability in the absence of Nup170. We hypothesize that, by modifying or stabilizing the nuclear envelope-NPC interface, Pom33 may contribute to proper distribution and/or efficient assembly of nuclear pores.