PROTEIN-TYROSINE PHOSPHATASES AS REGULATORS OF PROTEIN-KINASE ACTIVITY

The enzyme activity of many tyrosine-protein kinases and some serine/threonine protein kinases, which are critically involved in signal transduction pathways, is dependent on the level of phosphorylation at one or more tyrosines. Dephosphorylation of these phosphorylated tyrosines by protein-tyrosine phosphatases (PTPases) can activate or inactivate these enzymes. Activation occurs when PTPase acts on the negative regulatory site whereas inactivation of the enzyme occurs when PTPase acts on positive regulatory site. Both positive and negative regulatory sites are present on some protein kinases; these, therefore, may require two distinct PTPases for the regulation of their activity, although other mechanisms of regulation may also exist.