Characterization of a Phosphotyrosyl Protein Phosphatase Activity Associated with a Phosphoseryl

A cytosolic phosphoprotein phosphatase of M, = 95,000 purified from bovine cardiac muscle, which contains a catalytic subunit of M, = 35,000, is known to be associated with a Mg2+-activated p-nitrophenyl phosphatase activity. We have found that the enzyme preparation is also active toward phosphotyrosyl-IgG and -casein phosphorylated by pp60"", the transforming gene product of Rous sarcoma virus. The properties of this phosphotyrosyl protein phosphatase activity closely resemble those of the p-nitrophenyl phosphatase activity but sharply differ from those of the phosphorylase phosphatase activity. Comparative studies of the activities of the M, = 95,000 phosphatase, bovine kidney alkaline phosphatase, and ATP. Mg-dependent phosphatase toward phosphoseryl, phosphothreonyl, and phosphotyrosyl proteins and p-nitrophenyl phosphate under various conditions have been carried out. The results indicate that the M, = 95,000 enzyme exhibits higher activity toward phosphoseryl and phosphothreonyl proteins than toward phosphotyrosyl proteins, while the kidney alkaline phosphatase preferentially dephosphorylates phosphotyrosyl proteins. ATP. Mg-dependent phosphatase is inactive toward phosphotyrosyl proteins.