Protein (serine and threonine) phosphate phosphatases

Abstract

Publisher Summary The protein phosphatases can be grouped into two classes: type 1 protein phosphatase and type 2 protein phosphatases. Type 1 protein phosphatase selectively dephosphorylates the β subunit of phorphorylase kinase and is inhibited by nanomolar concentrations of two thermostable regulatory proteins, termed as “inhibitor-1” and “inhibitor-2.” Type 2 protein phosphatases selectively dephosphorylate the α subunit of phosphorylase kinase and are insensitive to inhibitor-1 and inhibitor-2. Two mechanisms for regulating protein phosphatase 1 have been identified. One involves the phosphorylation of inhibitor-1 on a specific threonine residue by cyclic AMP-dependent protein kinase. A second major mechanism for regulating protein phosphatase 1 involves MgATP. Subsequently, this MgATP-dependent phosphorylase phosphatase activity has been found to be present in a number of other tissues and the activity has been resolved into two protein components, termed as factor “Fa” and “Fc enzyme.” This chapter discusses the procedures for the identification of protein phosphatases. Phosphotyrosyl protein phosphatases have been demonstrated to be distinct and separable protein phosphatase species. Hence, the use of uncharacterized phosphoprotein substrates for the isolation of protein phosphatases necessitates the identification of the 32P-labeled phosphoamino acids present in the substrates and the analysis of their specific dephosphorylation.