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 Saccharomyces cerevisiae mitogen-activated protein (MAP) kinase homolog, that constitutively activate the yeast mating signal transduction pathway and confer hypersensitivity to mating pheromone. Surprisingly, the phenotypes of dominant FUS3(gf) mutations require the two protein kinases, STE7 and STE11. FUS3(gf) kinases are hyperphosphorylated in yeast independently of STE7. Consistent with this, FUS3(gf) kinases expressed in Escherichia coli exhibit an increased ability to autophosphorylate on tyrosine in vivo. FUS3(gf) mutations suppress the signal transduction defect of a severely catalytically impaired allele of STE7. This 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 determines its threshold of sensitivity to upstream signals.