The transforming growth factor-b (TGFb) superfamily of cytokines, including TGFbetas and bone morphogenic proteins (BMPs), play important roles in the regulation of cell growth and vertebrate development. Smad proteins are critical mediators of TGFbeta and BMP signaling. Upon phosphorylation and activation by the TGFbeta or BMP receptor kinases, the Smad proteins translocate into the nucleus and regulate transcription of TGFbeta or BMP responsive genes. In the nucleus, the activities of the Smads are tightly regulated by both positive and negative cellular co-factors. The P.I. has previously identified a nuclear proto-oncoprotein Ski as an important negative regulator of TGFb and BMP signaling. The long-term goal of this proposal is to understand the function of Ski oncoprotein in development and transformation, in particular related to its ability to interact with the Smad proteins. Ski is a unique oncoprotein in that it can induce both oncogenic transformation as well as terminal differentiation of muscle precursors. However, the mechanism by which Ski regulates these two processes has not been defined. The P.I. has shown that in the nucleus, Ski interacts with the TGFbeta- as well as BMP- specific Smad complexes and represses their abilities to activate transcription of TGFbeta and BMP target genes. Since TGFbeta-Smad pathway has been shown to be an important tumor suppressor pathway and BMPs are critical regulators or embryonic development, we hypothesize that the ability to interact with the Smad proteins is required for both the transforming activity of Ski and its ability to modulate muscle and neuronal differentiation. This proposal is designed to test this hypothesis and carry out detailed functional analyses of the Ski-Smad interaction during mammalian epithelial carcinogenesis and embryonic development as well as to investigate the molecular mechanisms of Ski function. The specific aims are: 1) Analysis of the role of Ski in oncogenic transformation; 2) Microarray analysis of Ski inducible genes; 3) Analysis of the role of the Ski-Smad interaction in embryonic development. These studies will allow a mechanistic understanding of functions of the Ski oncoprotein as well as the role of Ski and the Ski-Smad interaction in mammalian epithelial carcinogenesis and embryonic development.