Our goal is to understand the molecular basis of cell growth control and cell cycle regulation. Many growth factor receptors are protein-tyrosine kinases (PTK), which are activated by ligand-induced dimerization. Mitogenic signalling by PTKs involves tyrosine phosphorylation of critical target proteins. We are currently investigating what signal pathways are activated by the mammalian PDGF and stem call factor receptor PTKs, and the EphA2 receptor PTK, and investigating the roles of the Nck and c-Cbl adaptor proteins that are phosphorylated by activated receptor PTKs. We are also studying what signalling pathways are stimulated when integrin receptors bind to extracellular matrix proteins, such as fibronectin, in mammalian cells. We are focusing on the roles of the focal adhesion kinase (FAK) nonreceptor PTK and its relative Pyk2, and the c-Src family of nonreceptor PTKs, which are activated upon adhesion. We have also identified a Drosophila FAK/Pyk2 homologue, DmFAK, and we are carrying out a genetic analysis of its function. To complement our work on PTKs, we are investigating the functions of protein-tyrosine phosphatases (PTPs) in cell signalling. We are studying the receptor-like PTP, RPTPa, which like most RPTPs has a twin catalytic domain structure. The crystal structure of the membrane proximal catalytic domain of RPTPa shows that it exists as an inactive dimer, suggesting that dimerization of RPTPa, and other RPTPs, may negatively regulate their activity. We are have shown that this is true for the CD45 RPTP, and have evidence that RPTPa activity is also inhibited by dimerization. This principle of regulation would be the exact opposite of that observed for receptor PTKs. Our studies on the role of phosphorylation in mammalian cell cycle regulation are focused on the cyclin-dependent kinases (Cdk), and their substrates and inhibitors, and on the Cdc7/Dbf4 kinase. Using a new protein kinase substrate screen, we have identified Prc1, a spindle-associated protein, as a Cdk substrate, and we have also shown that Cdc6, an essential component of the prereplication complex that is bound to chromosomal origins, is phosphorylated by cyclin E/Cdk2, resulting in its export from the nucleus thus preventing rereplication. We have identified a novel peptidyl-prolyl isomerase, Pin1, that contains a WW domain and a prolyl isomerase domain. Pin1 is the functional homologue of yeast Ess1p, a protein essential for progression through mitosis. We have solved the structure of Pin1, and shown that it binds to phosphorylated Ser.Pro sequences, which are generated by Cdk phosphorylation. We are studying the role of Pin1 in cell cycle progression.