Platelet-derived growth factor (PDGF) is a potent mitogen for connective tissue cells, such as human skin fibroblasts. The plasma membranes of these cells contain specific receptor proteins for PDGF. When PDGF binds to the receptor, a protein kinase activity is activated which phosphorylates the receptor at tyrosine and serine amino acid resides. The first aim of this proposal is to solubilize the PDGF receptor from human fibroblasts and characterize its binding and kinase activities in solution. The second aim is to use the technique of peptide mapping to determine which subdomains of the molecule contain the functional sites. These studies should lead to a clearer picture of the receptor and provide insight into the mechanism by which PDGF and it receptor stimulate cell proliferation. (J)