Effect of Autophosphorylation on the Catalytic and Regulatory Properties of Protein Tyrosine Kinase Src

The function of autophosphorylation in Src family protein tyrosine kinases is not fully understood. In this paper we compared the catalytic and ligand-binding properties of autophosphorylated and nonautophosphorylated (control) Src. The following are the main differences we found. First, while both forms had the same Km for ATP-Mg, autophosphorylated Src had significantly higher Km values for the phosphate-accepting substrates, polyE4Y, and RCM-lysozyme. The autophosphorylated form also had significantly higher Vmax values than the control. The substrate specificity, as measured by Vmax/Km ratio, was altered by autophosphorylation and was dependent on the phosphate-accepting substrate. Second, while autophosphorylation did not affect Src activation by free Mg2+, Zn2+, which inhibited Src by competing against an essential Mg2+ activator, inhibited the control threefold more potently than the autophosphorylated form. Third, autophosphorylation significantly reduced the ability of its SH2 domain to bind phosphotyrosine. Fourth, a Pro-rich Src SH3 domain binding peptide activated the control, but not the autophosphorylated Src even though the apparent binding affinity was not significantly affected by autophosphorylation. These differences indicated that autophosphorylation induced significant and widespread changes in the catalytic and regulatory properties of Src. The implications of these findings relative to Src biological regulation are discussed.