The Schizosaccharomyces pombe mam2 gene encodes a putative pheromone receptor which has a significant homology with the Saccharomyces cerevisiae Ste2 protein

Abstract

The fission yeast Schizosaccharomyces pombe has two mating-types, h+ (P) and h- (M). The mam2 mutant exhibits an h(-)-specific sterile phenotype. Nucleotide sequencing of the mam2 gene isolated from an S. pombe genomic library revealed an open reading frame composed of 348 amino acids. The deduced mam2 product is a hydrophobic protein of 39 kDa that has significant sequence similarity (26.3% for identical amino acids) with the transmembrane domains of the Saccharomyces cerevisiae STE2 product, the alpha-pheromone receptor. Hydropathicity analysis suggests that the Mam2 protein contains seven possible membrane-spanning domains and a carboxy-terminal hydrophilic region. The mam2 gene was disrupted and found to be non-essential for growth. An h- haploid strain harbouring this disrupted null allele failed to respond to the pheromone of h+ cells, P-factor. These observations imply that the mam2 gene encodes a receptor for P-factor. Transcription of mam2 was induced only when strains containing functional mat1-M allele were cultured under conditions of nitrogen starvation. The mam2 gene was also transcribed in h+/h- diploid strains. The fact that the map1/mam2 homozygous diploid cells are incapable of sporulation implies that the pheromone signalling system is necessary for sporulation in diploid cells.