Regulation of Ras signaling in fission yeast

How specificity is maintained in signaling systems where a single regulatory molecule such as Ras often controls distinct, and sometimes opposing, pathways remains poorly understood. Schizosaccharomyces pombe represents a genetically tractable, simple system that contains a single Ras protein, Ras1, that is regulated by two exchange factors, Ste6 and Efc25, to control two distinct signaling pathways. Ras1 is regulated by Ste6 to control mating through the Byr2-MAPK pathway, while Efc25 activates Ras1 to maintain an elongated cellular morphology through Cdc42 and its exchange factor Scd1.; Recent studies in metazoan systems have suggested that subcellular compartmentalization of Ras may explain, in part, differential signaling. My data demonstrate that, like mammalian Ras proteins, Ras1 is palmitoylated, and that palmitoylation is essential for proper Ras1 signaling. Palmitoylation-deficient Ras1 fails to traffic to the plasma membrane, accumulates on endomembranes and regulates the Scd1-Cdc42 pathway but not the Byr2-MAPK pathway. Conversely, Ras1 targeted exclusively to the plasma membrane signals to the Byr2 mating pathway, but not to the Scd1 morphology pathway. These observations provide an unambiguous demonstration of compartment-specific Ras signaling and indicate that spatial specificity in the Ras pathway is evolutionarily conserved. Related studies demonstrate that mammalian K-Ras signaling is also compartmentalized in fission yeast, and that the mechanism by which mammalian K-Ras localizes to different cellular membranes is conserved.; In order to examine the specificity of Ras1 signaling at the level of its GEFs, a structure-function analysis of Efc25 was performed in order to identify residues that influence its ability to exclusively activate the Ras1-Scd1-Cdc42 pathway. Two conserved residues, S849 and N915, were found to be critical for the catalytic function of the protein and which cause Efc25 to interfere with Ras1 signaling when altered. Collectively, these data provide novel insights into Ras signaling and regulation, and suggest new approaches to understanding Ras and its oncogenic properties.