This proposal is focused on the functional role of a newly- discovered cytosolic protein kinase that is involved in the regulation of cell proliferation. The kinase phosphorylates its substrate proteins at serine or threonine residues that reside in positions adjacent to a proline residue. Because of this specificity, the kinase is designated as a proline-directed protein kinase (PDPK) by the PI. The active enzyme consists of a complex of two subunits, namely the catalytic p34cdc2 kinase and a regulatory A-type cyclin. The kinase is thought to function at the G1/S transition of the cell cycle in somatic mammalian cells and to phosphorylate a number of potentially-important substrates, including growth factor receptors and key regulatory enzymes. The activation of these substrates by phosphorylation may be an important facet of mitotic induction by growth factors and mitogens. The link between the activity of this kinase and the induction of proliferation by the treatment of cells with mitogenic growth factors will be explored in a series of biochemical, pharmacological and immunological studies that focus on the role played by the kinase in this process. The regulation of progression through the cell cycle and the induction of cell proliferation are areas of intense research that have merged because of the discovery of several key factors that are essential for both processes. Most prominent among these regulatory factors is a protein kinase known as p34cdc2 that is present in all organisms from yeast to humans, and, as part of a complex with certain regulatory subunits known as the cyclins, functions at particular times during the cell cycle to promote progression through mitosis. Studies on the cyclin proteins, performed largely in invertebrate embryonic cells, have demonstrated that the cyclins confer specific activity and substrate specificity to the kinase, but in these cells, the role of the A-type cyclins has remained less clear than that for B-type cyclins. Dr. Hall has shown that the binding of cyclin-A to p34cdc2 confers a particular type of activity for the kinase in somatic mammalian cells. This activity appears to be essential for progression of another part of the cell cycle, when the cell begins its DNA replication stage, long before mitosis. Dr. Hall has demonstrated that cyclin-A binding confers unique substrate specificity to p34cdc2 that could prove to be a major breakthrough in our understanding of how somatic cells enter the mitotic pathway prior to DNA synthesis.
