Signal transduction pathways typically function in interconnected web-like networks comprising common or shared signaling modules. Yet, these shared molecules through a common module induce specific responses. Some diseases like cancer can arise because a signal that is meant to follow one path is misdirected to another. The Rho GTPase Cdc42 is one such molecule. In the model eukaryote, Saccharomyces cerevisiae. It is a major regulator of multiple signaling pathways, but it is not clear how Cdc42 induces a specific response in one setting (e.g mating) and a completely different response in another (e.g. filamentous growth). Importantly, Cdc42 also functions in determining the axis of cell polarity, and this provides a clue to its specificity. A recent discovery from our lab showed that proteins required for establishing polarity also direct Cdc42 function to a specific MAPK pathway. This discovery is important because it links positional information with the cell's decision-making process. Thus, the objective is to determine the molecular mechanisms for how positional information is used to direct pathway specificity. Aim 1 will be to define specificity determinants between mucins that regulate different MAPK pathways. Mucins are important but poorly understood proteins whose function is mechanistically distinct from the well-studied GPCR class of MAPK regulators. We will use a proteomics approach to identify mucin-interacting proteins, and a synthetic biology approach to construct and test chimeric mucis. Aim 2 will use molecular and biochemical approaches to elucidate the role of Bem4, a Cdc42- interacting protein and pathway-specific scaffold for the MAPK pathway. Aim 3 focuses on the new connection between spatial landmarks and the MAPK pathway. We will use genetic, biochemical and cell biological approaches to determine the spatial-temporal regulation of filamentous growth pathway regulators. Altogether, this effort will identify the mechanisms by which common modules switch from one set of outputs to another. Understanding this problem is necessary to combat genetic diseases that result from loss of pathway specificity.