GENETIC INTERACTIONS INDICATE A ROLE FOR MDG1P AND THE SH3 DOMAIN PROTEIN BEM1P IN LINKING THE G-PROTEIN MEDIATED YEAST PHEROMONE SIGNALINGPATHWAY TO REGULATORS OF CELL POLARITY

The pheromone signal in the yeast Saccharomyces cerevisiae is transmit ted by the beta and gamma subunits of the mating response G-protein. T he STE20 gene, encoding a protein kinase required for pheromone signal transduction, has recently been identified in a genetic screen for hi gh-gene-dosage suppressors of a partly defective G(beta) mutation. The same genetic screen identified BEM1, which encodes an SH3 domain prot ein required for polarized morphogenesis in response to pheromone, and a novel gene, designated MDG1 (multicopy suppressor of defective G-pr otein). The MDG1 gene was independently isolated in a search for multi copy suppressors of a bem1 mutation. The MDG1 gene encodes a predicted hydrophilic protein of 364 amino acids with a molecular weight of 41 kDa that has no homology with known proteins. A fusion of Mdg1p with t he green fluorescent protein from Aequorea victoria localizes to the p lasma membrane, suggesting that Mdg1p is an extrinsically bound membra ne protein. Deletion of MDG1 causes sterility in cells in which the wi ld-type G(beta) has been replaced by partly defective G(beta) derivati ves but does not cause any other obvious phenotypes. The mating defect of cells deleted for STE20 is partially suppressed by multiple copies of BEM1 and CDC42, which encodes a small GTP-binding protein that bin ds to Ste20p and is necessary for the development of cell polarity. El evated levels of STE20 and BEM1 are capable of suppressing a temperatu re-sensitive mutation in CDC42. This complex network of genetic intera ctions points to a role for Bem1p and Mdg1p in G-protein mediated sign al transduction and indicates a functional linkage between components of the pheromone signalling pathway and regulators of cell polarity du ring yeast mating.