Except in Every Detail: Comparing and Contrasting G-Protein Signaling inSaccharomyces cerevisiaeandSchizosaccharomyces pombe

When asked to explain the difference between the mechanisms controlling mating type switching in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe, Brandeis University's Jim Haber once replied, “It's exactly the same, except in every detail.” He may as well have been referring to how these two yeasts utilize guanine nucleotide-binding proteins (G proteins) to detect the two most essential objects in the extracellular milieu, sexual partners and food. Both pheromone signaling and glucose signaling in these yeasts occur through the action of G-protein-coupled receptors (GPCRs) and associated G proteins, though the composition of the G proteins and the relative roles of individual subunits vary among the four pathways. In this review, I will compare and contrast these S. cerevisiae and S. pombe G-protein-mediated signaling pathways. I will also discuss the genetic implications of Gα-mediated signaling compared to Gβγ-mediated signaling as well as the need to revisit some generally accepted ideas regarding receptor-mediated G-protein signaling. For more comprehensive reviews on fungal signaling pathways, see recent reviews by Lengeler et al. and Kays and Borkovich (48, 61). Note that recent studies of mitotic spindle positioning in Drosophila melanogaster and Caenorhabditis elegans (36) have uncovered both receptor-independent regulation of and novel biological roles for Gα subunits, providing further evidence that our understanding of G-protein signaling is far from complete.