G protein-coupled receptors are major targets for the treatment of cardiovascular and psychiatric disorders. Receptors of this class transmit signals to a variety of effector enzymes including mitogen activated protein kinases (MAPKs). This proposal is focused on the regulation of two MAPKs in yeast, Fus3 and Kss1. Both of these MAPKs are activated by the same pheromone stimulus. However while activation of Fus3 requires a kinase scaffold (Ste5) Kss1 does not. Our investigations have revealed four additional proteins required for full activation of Fus3 but not Kss1; these include two phosphatidylinositol 4-kinases (Pik1 & Stt4) and two subunits of the phosphatidylinositol 3-kinase complex (Vps15 & Vps34). We postulate that these enzymes promote the selective activation of scaffolded MAP kinases. There are three aims that address the hypothesis: Aim 1: Determine the signaling function of Pik1 and Stt4. Pik1 and Stt4 are phosphatidylinositol 4- kinases, which produce phosphatidylinositol 4-P (PI4P). PI4P is known to bind to the MAPK scaffold Ste5, but the consequences for MAPK activation are not known. We will test the hypothesis that Ste5 binding to PI4P promotes the activation of Fus3 catalytic activity. Aim 2: Determine the signaling function of Vps34 and PI3P. Vps34 is a phosphatidylinositol 3-kinase and produces phosphatidylinositol 3-P (PI3P). PI3P binds to a defined set of proteins having PX or FYVE domains. We will test the hypothesis that one of these PI3P-binding proteins is specifically required for Fus3 catalytic activity. Aim 3: Determine the signaling function of Vps15. Vps15 is a protein kinase required for Vps34 function, as well as for activation of Fus3. We will test the hypothesis that autophosphorylation of Vps15 is required for PI3P production. In addition, we will identify other pathway components phosphorylated by Vps15, and determine how phosphorylation affects MAPK activity and substrate specificity. This proposal investigates new components of the G protein and MAPK signaling apparatus in yeast. Mechanisms discovered in yeast are typically recapitulated in more complex organisms, so the activities elucidated here will likely apply to hormone and neurotransmitter function in humans.