RECOMBINANT CSK EXPRESSED IN ESCHERICHIA-COLI IS AUTOPHOSPHORYLATED ON TYROSINE RESIDUE(S)

The C-terminal src kinase (Csk) is responsible for the phosphorylation of the carboxy-terminal tyrosine of several tyrosine kinases of the S rc family. This phosphorylation site has a negative regulatory functio n. Csk is unique among the members of the protein tyrosine kinase fami ly because it lacks the conserved tyrosine autophosphorylation site an d has been thought to be devoid of autophosphorylation activity. Using the glutathione S-transferase (GST) bacterial expression system, we h ave produced large amounts of a chimeric rat Csk protein. We have dete rmined that the GST-Csk fusion protein isolated from bacteria is autop hosphorylated on tyrosine residue(s). GST-Csk and purified Csk are cap able of undergoing autophosphorylation on tyrosine residue(s) in vitro . The GST-Csk fusion protein also phosphorylates exogenous substrates, including the heteropolymer poly-Glu/Tyr and enolase. This is the fir st indication that Csk is autophosphorylated on tyrosine residue(s) bo th in vivo in bacteria expressing Csk cDNA and in vitro. These finding s suggest that the autophosphorylation of Csk might play a role in the regulation of its kinase activity as well as its binding to other cel lular proteins.