Cell number is regulated, in part, by a balance between mitogenic and apoptotic responses. Heterotrimeric GTP-binding proteins (G proteins) can transmit growth signals leading to mitogenesis. We have shown that alpha subunits of heterotrimeric G12 and G13 proteins induce mitogenesis and neoplastic transformation in mouse fibroblasts. In contrast, Galpha12 and Galpha13 induce apoptosis in human endothelial cells. Our data suggest that Galpha12 and Galpha13 can stimulate multiple signaling pathways, including Ras and Rho family of monomeric G proteins and the mitogen-activated protein kinase (MAP) pathways. Ras and Rho families of G proteins play an important role in mitogenesis and neoplastic transformation of the cell. However, there is considerably evidence that Ras and Rho families of G proteins can promote apoptosis. The variable effects of Galpha12 and Galpha13 on mitogenic and apoptotic responses could reflect the utilization of the multiple signaling pathways under different circumstances and in different cell types. We will test the hypothesis that Galpha12 and Galpha13 proteins regulate multiple signaling pathways which produce bifurcating signals leading to mitogenic or apoptotic response. To test this hypothesis, we will use the combination of biochemical, cellular, and genetic approaches to investigate the specific signaling pathways leading to G protein-induced mitogenesis or apoptosis. In particular, we will determine the role of Ras and Rho family of G proteins and the MAP kinase pathways in mitogenic and apoptotic responses induced by Galpha12 and Galpha13 proteins. To understand the molecular basis of variable effects of Galpha12 and Galpha13, we will determine the role of caspases and Bcl-2 in the G protein- induced apoptotic response. Finally, we will identify the effector proteins interacting with Galpha12 and Galpha13 using yeast two-hybrid screening of cDNA libraries and will characterize the role of interacting proteins in mediating mitogenic or apoptotic responses. These studies are critical for understanding of the control of tissue proliferation and apoptosis and for design of novel strategies to control proliferation of cancerous cells.