The objective of this research is to understand in molecular detail the regulation of enzymes that modulate cellular growth and metabolism. Reversible phosphorylation of proteins on tyrosine residues (Tyr) is an essential step in transduction of extracellular signals. Various growth factor receptors and a class of retroviral oncogene products exhibit protein Tyr kinase activity. Protein Tyr(P) phosphatases (PTPases) oppose the action of these receptors and oncogenes to terminate the intracellular signals, thereby acting as growth and tumor suppressors. Elevation of intracellular cAMP inhibits growth, can reverse cellular transformation, and provokes differentiation of leukemia cells. Evidence is presented that PTPase is activated selectively in a membrane fraction in vivo by increased cAMP. Anti-PTPase-peptide antibodies have been prepared and used to demonstrate that a single catalytic protein of Mr=40 kDa undergoes translocation from the cytoplasm into the nucleus during the cell division cycle. Proposed studies will identify and purify proteins that bind the PTPase catalytic subunit to regulate its activity and localize it within the cell by using selective immunoprecipitation, PTPase inhibition, PTPase affinity chromatography and blot overlays with radiolabeled PTPase. The mechanism of cAMP activation of membrane PTPase changes and by identifying which PTPase-binding proteins are substrates for cAMP-dependent protein kinase (PK-A), and what effect phosphorylation has on their PTPase binding or inhibition. Physiological and hormonal regulation of PTPase activity via cAMP will be correlated to in vivo phosphorylation of PTPase proteins, monitored by 2D-gel electrophoresis and peptide mapping of immunoprecipitated proteins from metabolically labeled cells. Using agents that arrest growth at specific times, changes in PTPase activity and subunit phosphorylation during the cell cycle will be examined. Establishing the links of PTPase cAMP will yield new insights into how different signal transduction pathways modulate one another to control cell growth.
