The Effects of Activated Galpha on MAPK and Formin During the Saccharomyces cerevisiae Pheromone Response

Andrew J. Hart, University of Maine


Humans use G-protein coupled receptors (GPCR) for cell signaling, detection of pathogens, and cancer metastasis. Yeast cells also use GPCR in order to detect their mating partner and nutrients. In order to find their mating partner the yeast use the yeast mating pathway which starts with the binding of pheromone to the GPCR. This results in G protein alpha (G⍺) and beta-gamma (Gβɣ) subunits dissociating from the GPCR, each propagating downstream signaling events. The beta-gamma pathway has been shown to induce cytoskeleton remodeling, resulting in the yeast growing directionally towards pheromone, and gene regulation through the MAPK pathway and the small Rho-like GTPase, Cdc42. The role of the G⍺ pathway is less understood. What is currently known is that G⍺ recruits MAPK to promote actin polymerization and traffics to the endosome where it influences MAPK activation. However, we have seen that the G⍺ pathway affects multiple important factors in the yeast’s mating response; including MAPK, the formin Bni1, and septins, a part of the cytoskeleton. MAPK binds to G⍺ which activates Bni1 to nucleate actin at the site of active signaling. We have seen that when there is an excess of G⍺ signaling the yeast are unable to track a gradient due to changes in the MAPK scaffolding. These changes result in the yeast unable to track a gradient and are unable to properly form a septin structure. Incorporating a deletion of a Rho GTPase activating protein (RhoGAP) used in cytoskeleton remodeling to the yeast cells with excess G⍺ signaling we have seen partial septin formation and recovery of orientation from the excess of G⍺. Finally, we have seen that the excess levels of G⍺ lead to an increase in endocytosis.