Active roles of Gpa1, Scp160, and Apg14 in the yeast pheromone response pathway

In yeast Saccharomyces cerevisiae the G protein betagamma subunits (Ste4/Ste18) have long been known to transmit the signal required for mating. My study demonstrates a positive signaling role for the G protein alpha subunit (Gpa1). GTPase-deficient Gpa1 mutants activate the mating response pathway in the absence of added pheromone, using the same downstream kinase components and through induction of a common set of gene transcripts. Whole genome two-hybrid analysis and copurification from yeast reveal that activated Gpa1 binds to Scp160, an RNA-binding protein associated with polyribosomes.; Scp160 is required to transmit the Gpa1-initiated signal, and appears to function by maintaining Gpa1 expression at levels sufficient to bind Gbetagamma. Scp160's inhibitory effect on pheromone induced transcription activation and its stimulatory effect on mating suggest that mating is a more complicated process than one determined by the MAP kinase cascade through Gbetagamma signaling. Gpa1 appears to have additional functions in mating than those known before.; Apg14, another Gpa1 interacting protein, binds to inactivated Gpa1 and appears to be a Ggamma like protein. It mediates the morphological changes caused by activated Gpa1. Upon pheromone stimulation, Gpa1-GTP leads to reduced expression of Gpa1. Less Gpa1-GDP allows more Apg14 to activate the yeast P13 kinase Vps34 producing more PtdIns(3)P, which alters cell morphology by initiating signal transductions through proteins with PH domains, such as Cla4 and Cdc24.