A fundamental but poorly understood question in developmental biology is how cells stop proliferating when the density of cells of that cell type, n a tissue or the entire body, reaches the correct point. Theoretically, one way this can be accomplished is if the cells secrete a specific diffusible factor that inhibits proliferation of tht cell type. The extracellular concentration of such a factor, called a chalone, would increase as the density of the secreting cells increase, and when the density of that cell type reaches the correct point, the corresponding high levels of the chalone would stop their proliferation. Despite evidence for the existence of chalones, little is known about the identity of chalones and their signal transduction pathways. In the simple eukaryotic model system Dictyostelium discoideum, cells stop proliferating when they reach a high cell density even if adequate nutrients are presen+t. This is due to the extracellular accumulation of a chalone. We have partially purified the chalone and found that it is a heat- and protease-resistant anionic molecule smaller than 2 kDa. The chalone signal transduction pathway involves the PTEN and CnrN phosphatases and the BzpN transcription factor, and blocks proliferation by preventing the transition from the G2 to the M phase of the cell cycle. I propose three specific aims that will use the power of the Dictyostelium model system to elucidate this chalone and its signal transduction pathway. First, we will finish the purification and identify the chalone, and test theoretical models of chalone function. Second, we will test the hypothesis that the chalone regulates the PI3 kinase/ Akt pathway to regulate proliferation, and use genetic screens to identify additional components of the pathway. Third, we will test the hypothesis that the chalone blocks the cell cycle by inhibitin the activity of the G2/ M regulators Cyclin B and Cdc2. Together, this work combining natural products chemistry, physics and mathematical modeling, genetics, cell biology, and biochemistry in a versatile model system wil elucidate the molecular mechanism of a chalone.