STE5 RING-H2 DOMAIN - ROLE IN STE4-PROMOTED OLIGOMERIZATION FOR YEASTPHEROMONE SIGNALING

Ste5 is a scaffold for the mitogen-activated protein kinase (MAPK) cas cade components in a yeast pheromone response pathway. Ste5 also assoc iates with Ste4, the beta subunit of a heterotrimeric guanine nucleoti de-binding protein, potentially linking receptor activation to stimula tion of the MAPK cascade, A RING-H2 motif at the Ste5 amino terminus i s apparently essential for function because Ste5(C177S) and Ste5(C177A C180A) mutants did not rescue the mating defect of a ste5 Delta cell. In vitro Ste5(C177A C180A) bound each component of the MAPK cascade, but not Ste4, Unlike wild-type Ste5, the mutant did not appear to olig omerize; however, when fused to a heterologous dimerization domain (gl utathione S-transferase), the chimeric protein restored mating in an s te5 Delta cell and an ste4 Delta ste5 Delta double mutant. Thus, the R ING-H2 domain mediates Ste4-Ste5 interaction, which is a prerequisite for Ste5-Ste5 self-association and signaling.