Functional analysis of plp1 and plp2, two homologues of phosducin in yeast

Mammalian phosducins are known to bind G protein beta gamma subunits in vit ro, and are postulated to regulate their signaling function in vivo. Here w e describe two homologues of phosducin in yeast, called PLP1 and PLP2. Both gene products were cloned, expressed, and purified as glutathione S-transf erase fusions. Of the two isoforms, Plp1 bound most preferentially to G bet a gamma. Binding was enhanced by pheromone stimulation and by the addition of GTP gamma S, conditions that favor dissociation of G beta gamma from G a lpha. Gene disruption mutants and gene over expression plasmids were prepar ed and analyzed for changes in signaling and non-signaling phenotypes. Hapl oid spore products bearing the plp2 Delta mutant failed to grow, suggesting that PLP2 is an essential gene. Cell viability was not restored bg a mutat ion in STE7 that blocks signaling downstream of the G protein. Haploid prod ucts bearing the plp1 Delta mutant were viable and exhibited a 6-7% increas e in pheromone-mediated gene induction. Cells overexpressing PLP1 or PLP2 e xhibited a 70-80% decrease in gene induction but no change in pheromone-med iated growth arrest. These data indicate that phosducin can selectively reg ulate early signaling events following pheromone stimulation and has an ess ential role in cell growth independent of its regulatory role in cell signa ling.