A ROLE FOR AUTOPHOSPHORYLATION REVEALED BY ACTIVATED ALLELES OF FUS3,THE YEAST MAP KINASE HOMOLOG

We have isolated dominant gain-of-function (gf) mutations in FUS3, a S accharomyces cerevisiae mitogen-activated protein (MAP) kinase homolog , that constitutively activate the yeast mating signal transduction pa thway and confer hypersensitivity to mating pheromone. Surprisingly, t he phenotypes of dominant FUS3(gf) mutations require the two protein k inases, STE7 and STE11. FUS3(gf) kinases are hyperphosphorylated in ye ast independently of STE7. Consistent with this, FUS3(gf) kinases expr essed in Escherichia coli exhibit an increased ability to autophosphor ylate on tyrosine in vivo. FUS3(gf) mutations suppress the signal tran sduction defect of a severely catalytically impaired allele of STE7. T his finding suggests that the tyrosine-phosphorylated form of FUS3 is a better substrate for activation by STE7. Furthermore, these results imply that the degree of autophosphorylation of a MAP kinase determine s its threshold of sensitivity to upstream signals.