Mutations in the SAM domain of STE50 differentially influence the MAPH-mediated pathways for mating, filamentous growth and osmotolerance in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the MAPKKK Ste11p is involved in three mitogen -activated protein kinase (MAPK) pathways required for mating, filamentous growth and the SHO1-dependent response to hyperosmolarity. All three pathwa ys are also dependent on Ste50p. Ste50p and Ste11p interact constitutively via their N-terminal regions, which include putative SAM domains. Here we s how that the interaction of Ste50p and Ste11p is differentially required fo r modulation of Ste11p function during mating, filamentous growth and the S HO1-dependent response to hyperosmolarity. Two derivatives of Ste50p with m utations in the SAM domain were isolated and characterised. The mutant Ste5 0 proteins showed reduced binding to Ste11p and a tendency to form homodime rs in two-hybrid and in vitro binding assays. Interestingly, these two Ste5 0p-SAM mutants were associated with increased activation of the mating and filamentous-growth pathways, but a reduction in the SHO1-dependent growth r esponse to hyperosmolarity. relative to the wild-type Ste50p. Moreover. whe n exposed to hyperosmolarity, these Ste50p-SAM mutants activate genes in th e mating (FUS1) and filamentous-growth (FLO11)pathways to higher levels tha n does the wild type. Thus the Ste50p-Ste11p interaction may differentially modulate the flow of information through the various MAPK-mediated pathway s.