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.