The carboxy-terminal half of the c-src protein fused to the protein A moiety was expressed in bacteria. The protein A/truncated c-src fusion protein, which does not have SH2 and SH3 domains, is found in the periplasmic space allowing for a simple one-step purification and demonstrated high efficiency in autophosphorylation and exogeneous substrate phosphorylation. The missense mutation at codon 294 (Ile → Thr), which is located in the ATP-binding domain of the c-src, resulted in dramatic reduction of tyrosine kinase activity of the fusion protein. Using the fusion protein. we also revealed that staurosporin, a well-known kinase inhibitor, directly affects autophosphorylation of the C-terminal half of the c-src protein. This truncated c-src expression system provides a good source of enzyme for diverse experiments and is an ideal model for understanding the implication of structural alterations in the catalytic activity of the c-src kinase by site-directed mutagenesis experiments.
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