KIR, A NOVEL RAS-FAMILY G-PROTEIN, INDUCES INVASIVE PSEUDOHYPHAL GROWTH IN SACCHAROMYCES-CEREVISIAE

Kir belongs to a novel class of Ras-family G-proteins which includes G em and Rad, These proteins are unique among Ras super-family G-protein s since their expression is under transcriptional regulation in mammal ian cells, To gain insight into the function of Kir, we took advantage of the well-defined signal transduction pathways of yeast, When kir i s expressed in Saccharomyces cerevisiae, the transformants form pseudo hyphae and exhibit invasive properties characteristic of yeast cells a re undergoing a developmental transition induced by GTPase nitrogen st arvation, Analysis of pseudohyphal signaling pathway mutants suggests that the Kir-induced pseudohyphae formation requires a MAP kinase casc ade involving ste20, ste11, ste7 but not ste5 gene products, Furthermo re, our results are consistent with the idea that Kir functions upstre am of the STE20 kinase which plays a critical role in two distinct MAP kinase cascades.