Signal transduction pathways function in interconnected networks comprising common or shared proteins. How specific signals are directed through a common protein module to induce a highly specific response is not clear. Some diseases like cancer can arise because a signal that is meant to follow one path is misdirected into another. One MAPK pathway in yeast regulates a microbial differentiation response called filamentous growth. In fungal pathogens, filamentous growth is required for virulence. The MAPK pathway is composed of G proteins and protein kinases that function in multiple pathways. A major regulator of the pathway is the Rho GTPase, Cdc42, which is widely considered a master regulator of cell polarity and signaling. In this proposal is reported the discovery that Cdc42 is degraded by a mechanism that involves ubiquitin modification of the protein. This discovery is important because it provides a new way to regulate signaling pathways that require Cdc42 for function. Thus, one objective of the proposal is to determine the functional consequences of Cdc42 turnover on MAP kinase signaling (Aim 1). In addition to Cdc42, several protein kinases are shared among MAPK pathways. In Aim 2, a newly identified scaffold of the Ral GDS type will be examined that binds to kinases to direct them to the MAPK pathway. The filamentous growth pathway is regulated by a sensor protein of the mucin family. Mucins are large glycoproteins that regulate signaling pathways through mechanisms that remain uclear. In Aim 3 of the proposal, how the mucin connects to and regulates Cdc42 will be determined. One candidate is the cell's main polairty scaffold, which has recently been found to also regulate the MAPK pathway. Experiments are proposed to determine how the mucin directs the polarity scaffold to the Cdc42 module. Collectively, this proposal will define new mechanisms of G protein regulation, as well as how commom proteins are directed to specific pathways. Given that the proteins in this pathway are highly conserved from yeast to humans, insights gained from this proposal will likely extend to signaling pathways in general, including those that are relevant to human health.