The goal of this project is to develop an improved understanding of controls regulating the cell cycle. Elucidation of the major cell cycle control mechanisms is of fundamental importance in biology. Moreover, understanding cell cycle control will have a major impact on the rational investigation and treatment of human diseases that are related to human cell proliferation abnormalities. The project focuses on the positive regulation of the Cdc2 kinase that is directly responsible for bringing about the initiation of DNA replication and mitosis. Emphasis is placed on the investigation of Cdc25, a protein phosphatase that promotes the onset of mitosis by dephosphorylating tyrosine-15 of Cdc2. The project has three major specific aims. The first is to understand the regulation of Cdc25. Biochemical and genetic experiments will be performed to learn how Cdc25 is activated at the G2/M transition. Genetic approaches will be used to identify new proteins that are involved in the regulation of Cdc25. The second major aim is to develop a comprehensive understanding of the protein interactions that impact Cdc2. Proteins that directly interact with Cdc2 will be discovered and analyzed. The third aim is to understand the biological functions of the three Cdc25 subtypes that exist in human cells. Experiments will be carried out to measure the activity of these subtypes during the cell cycle. Cell lines will be established that ectopically express Cdc25 genes and anti-Cdc25 ribozymes. These studies will contribute to an improved understanding of the roles of Cdc25 proteins in regulating the cell cycle in humans.