University of Virginia Health System

Mammalian cells express receptors for various peptide growth factors. Growth factor receptors are proteins in the plasma membrane that bind specifically a particular peptide, generating signals for changes in metabolism, gene expression, and/or cell division. Expression of these receptors (as well as receptors for other hormonal substances) provides for external control of individual cell activity. My laboratory is studying the protein kinases in the pathways used for signal transduction by peptide growth factors. MAP (mitogen-activated protein) kinases are serine/threonine kinases that are regulated positively by tyrosine and threonine phosphorylation. Two closely related 42 kDa and 44 kDa MAP kinases are expressed ubiquitously in mammalian cells. MAP kinases function in cascades important for mediating some of the actions of many peptide growth factors. The sites recognized by MAP kinases in target proteins all conform minimally to the amino acid sequence serine (or threonine) followed by proline. We are interested in several aspects of MAP kinase regulation and function. Several proteins have been identified as candidates for regulation by MAP kinase, these include important regulatory proteins, e.g. protein kinases, cytoplasmic phospholipase A2, and several transactivating factors. We are attempting to identify additional proteins regulated by MAP kinase. This protein is translocated into the nucleus but lacks an apparent nuclear localization domain, thus the mechanism of its transport to the nucleus is an interesting problem. MAP kinases are activated by a new family of protein kinases with dual specificity for the regulatory tyrosine and threonine amino acid residues in MAP kinases, the MAP kinase kinases (MKKs). These enzymes are remarkably specific for their MAP kinase substrates. MKKs are activated also by phosphorylation. Present evidence suggests that several protein kinases including Raf-1 and the MEK kinase may activate MKKs. The mechanism of activation of Raf-1 and MEK kinase is unknown and is a major interest at present. The mechanisms for communication with other pathways remains to be explored. Recently, cAMP has been shown to regulate negatively the MAP kinase pathways. Finally, and perhaps most exciting of all, there must exist interactions of the MAP kinase pathway with the cell cycle machinery.