Genetic interactions indicate a role for Mdg1p and the SH3 domain protein Bem1p in linking the G-protein mediated yeast pheromone signalling pathway to regulators of cell polarity.
The pheromone signal in the yeast Saccharomyces cerevisiae is transmitted by the beta and gamma subunits of the mating response G-protein. The STE20 gene, encoding a protein kinase required for pheromone signal transduction, has recently been identified in a genetic screen for high-gene-dosage suppressors of a partly defective G beta mutation. The same genetic screen identified BEM1, which encodes an SH3 domain protein required for polarized morphogenesis in response to pheromone, and a novel gene, designated MDG1 (multicopy suppressor of defective G-protein). The MDG1 gene was independently isolated in a search for multicopy 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 the green fluorescent protein from Aequorea victoria localizes to the plasma membrane, suggesting that Mdg1p is an extrinsically bound membrane protein. Deletion of MDG1 causes sterility in cells in which the wild-type G beta has been replaced by partly defective G beta derivatives but does not cause any other obvious phenotypes.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 binds to Ste20p and is necessary for the development of cell polarity. Elevated levels of STE20 and BEM1 are capable of suppressing a temperature-sensitive mutation in CDC42. This complex network of genetic interactions points to a role for Bem1p and Mdg1p in G-protein mediated signal transduction and indicates a functional linkage between components of the pheromone signalling pathway and regulators of cell polarity during yeast mating.
Biotechnology Research Institute, National Research Council of Canada, Montreal, Quebec, Canada.
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E
Leberer
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J
Chenevert
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T
Leeuw
D
D
Harcus
I
I
Herskowitz
D Y
DY
Thomas
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D001483:Base Sequence; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D016764:Cell Polarity / Q000235:genetics; D002874:Chromosome Mapping; D004251:DNA Transposable Elements / Q000235:genetics*; D005656:Fungal Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism; D044387:GTP-Binding Protein beta Subunits; D019204:GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D017353:Gene Deletion; D018628:Gene Dosage; D015966:Gene Expression Regulation, Fungal; D016153:Genes, Suppressor; D020662:Guanine Nucleotide Exchange Factors; D020962:Heterotrimeric GTP-Binding Proteins; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D009154:Mutation; D010675:Pheromones / Q000235:genetics / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D017422:Sequence Analysis, DNA; D015398:Signal Transduction / Q000235:genetics*; D013347:Subcellular Fractions; D020846:cdc42 GTP-Binding Protein, Saccharomyces cerevisiae; D018909:src Homology Domains
D048868:Adaptor Proteins, Signal Transducing; C073594:CDC24 protein, S cerevisiae; D018797:Cell Cycle Proteins; D004251:DNA Transposable Elements; D005656:Fungal Proteins; D044387:GTP-Binding Protein beta Subunits; D020662:Guanine Nucleotide Exchange Factors; C102153:MDG1 protein, S cerevisiae; D008565:Membrane Proteins; D010675:Pheromones; D011518:Proto-Oncogene Proteins; D029701:Saccharomyces cerevisiae Proteins; C067576:Ste4 protein, S cerevisiae; C073129:BEM1 protein, S cerevisiae; D019204:GTP-Binding Proteins; D020962:Heterotrimeric GTP-Binding Proteins; D020846:cdc42 GTP-Binding Protein, Saccharomyces cerevisiae