Saccharomyces cerevisiae Ndc1p: A novel role in nuclear pore assembly and an interaction with a new spindle pole body component, Nbp1p

Both the spindle pole body (SPB) and the nuclear pore complex (NPC) are multisubunit organelles embedded in the nuclear envelope throughout the life cycle of the budding yeast Saccharomyces cerevisiae. However, the mechanism by which these two organelles are inserted into the nuclear envelope during their biogenesis is not well understood. Previous studies have shown that Ndc1p, the only known integral membrane protein that localized to both the SPBs and the NPCs, is required for SPB duplication. In this study, I generated a novel temperature-sensitive (ts) allele of NDC1 to investigate the role of Ndc1p at the NPCs. Yeast cells carrying this allele (ndc1-39) fail to insert the SPB into the nuclear envelope at the restrictive temperature. While nuclear protein import and NPC distribution in the nuclear envelope are unaffected, ndc1-39 mutants fail to properly incorporate a nuclear pore protein into NPCs. These results suggest that Ndc1p is also required for NPC assembly, in addition to its role in SPB duplication. Ndc1p may be crucial in the biogenesis of both the SPBs and the NPCs at the insertion step into the nuclear envelope.; In addition, I identified NBP1 as a dosage suppressor of the ndcl-39 allele in a genetic screen. I showed that Nbp1p is a new SPB component that localizes at or near the central plaque of the SPB. Besides Ndc1p, Nbp1p also shows genetic interaction with two other SPB components, Mps2p and Spc42p. These results suggest that Nbp1p plays a role during SPB duplication, and it may provide a molecular link between Ndc1p and other SPB components.