SIGNALING IN THE YEASTS - AN INFORMATIONAL CASCADE WITH LINKS TO THE FILAMENTOUS FUNGI

All cells, from bacteria and yeasts to mammalian cells, respond to cue s from their environment. A variety of mechanisms exist for the transd uction of these external signals to the interior of the cell, resultin g in altered patterns of protein activity. Eukaryotic cells commonly t ransduce external cues via a conserved module composed of three protei n kinases, the mitogen-activated protein kinase (MAPK) cascade. This m odule can then activate substrates, some of which include transcriptio nal activators. Multiple MAPK signalling pathways coexist in a cell. T his review considers different MAPK cascade signalling pathways that g overn several aspects of the life cycle of budding and fission yeasts: conjugation and meiosis by the pheromone response pathway, stress res ponse by the high-osmolarity and heat-sensing pathway, and pseudohypha l growth in response to activation of a subset of the components of th e pheromone response pathway. Because the MAPK cascade components are highly conserved, a key question in studies of these pathways is the m echanism by which specificity of response is achieved. Several other i ssues to be addressed in this review concern the nature of the recepto rs used to sense the external signals and the mechanism by which the r eceptors communicate with other components leading to activation of th e MAPK cascade. Recently, it has become apparent the MAPK cascades are important in governing the pathogenicity of filamentous fungi.