Candida albicans Int1p interacts with the septin ring in yeast and hyphal cells

The ability to switch between yeast and hyphal morphologies is an important virulence factor for the opportunistic pathogen Candida albicans. Although the kinetics of appearance of the filamentous ring that forms at the incip ient septum differ in yeast and cells forming hyphae (germ tubes) (Soll and Mitchell, 1983), the molecular mechanisms that regulate this difference ar e not known. Int1p, a C. albicans gene product with similarity in its C ter minus to Saccharomyces cerevisiae Bud4p, has a role in hyphal morphogenesis . Here we report that in S. cerevisiae, Int1p expression results in the gro wth of highly polarized cells with delocalized chitin and defects in cytoki nesis and bud-site selection patterns, phenotypes that are also seen in S. cerevisiae septin mutant strains. Expression of high levels of Int1p in S. cerevisiae generated elaborate spiral-like structures at the periphery of t he polarized cells that contained septins and Int1p. In addition, Int1p coi mmunoprecipitated with the Cdc11p and Cdc12p septins, and Cdc12p is require d for the establishment and maintenance of these Int1p/septin spirals. Alth ough Swe1p kinase contributes to INT1-induced filamentous growth in S. cere visiae, it is not required for the formation of ectopic Int1p/septin struct ures. La C. albicans, Int1p was important for the axial budding pattern and colocalized with Cdc3p septin in a ring at the mother-bud neck of yeast an d pseudohyphal cells. Under conditions that induce hyphae, both Cdc3p and I nt1p localized to a ring distal to the junction of the mother cell and germ tube. Thus, placement of the Int1p/septin ring with respect to the mother- daughter cell junction distinguishes yeast/pseudohyphal growth from hyphal growth in C. albicans.