Unique biochemical properties of the protein tyrosine phosphatase activity of PTEN—Demonstration of different active site structural requirements for phosphopeptide and phospholipid phosphatase activities of PTEN

Missense PTEN mutations of the active site residues Asp-92, Cys-124 and Gly-129 contribute to Cowden syndrome. How their mutations affect phospholipid phosphatase activity and tumor suppressor function of PTEN has been defined. In this study, we investigated how their mutations affect the kinetics and catalytic mechanism of PTEN phosphoprotein phosphatase activity. Our data suggest that PTEN catalysis of phosphoprotein dephosphorylation follows a two-step mechanism with Cys-124 transiently phosphorylated to form the phosphoenzyme intermediate. In spite of this, we were unable to trap the genuine phosphoenzyme intermediate; instead, we unexpectedly discovered a novel phosphotransfer reaction in which the phosphate group is transferred from a tyrosyl phosphopeptide to PTEN to form a unique phosphorylated protein. Even though the finding is novel, the phosphotransfer reaction is likely an in vitro non-enzymatic reaction. Kinetic analysis revealed that mutation of Asp-92 has negligible impacts on phosphopeptide phosphatase activity of PTEN, suggesting that Asp-92 does not participate in the phosphopeptide dephosphorylation reaction. The results also imply that allosteric regulators facilitating the recruitment of Asp-92 to participate in catalysis will increase the activity of PTEN in dephosphorylating phosphoprotein and phosphopeptide substrates. Furthermore, kinetic analysis revealed that the G129E mutation has different effects on phospholipid and phosphoprotein phosphatase activities. Taken together, the data show that while the two phosphatase activities of PTEN follow a similar catalytic mechanism, they have notable differences in the requirements of the active site structure.