The catalytic activity of the Src family of tyrosine kinases is suppressed by phosphorylation on a tyrosine residue located near the C terminus (Tyr 527 in c-Src), which is catalyzed by C-terminal Src Kinase (Csk). Given the promiscuity of most tyrosine kinases, it is remarkable that the C-terminal tails of the Src family kinases are the only known targets of Csk. We have determined the crystal structure of a complex between the kinase domains of Csk and c-Src at 2.9 Å resolution, revealing that interactions between these kinases position the C-terminal tail of c-Src at the edge of the active site of Csk. Csk cannot phosphorylate substrates that lack this docking mechanism because the conventional substrate binding site used by most tyrosine kinases to recognize substrates is destabilized in Csk by a deletion in the activation loop.
Bibliographical noteFunding Information:
We thank David King for the synthesis of the two substrate peptides, Caleb Cassidy-Amstutz for help with cloning and mutagenesis, and Luke Chao and Natalia Jura for providing CamKII and EGFR kinase domain used as controls in the surface plasmon resonance experiments. We are grateful to David Wemmer and Jeff Pelton for support and discussion regarding the NMR experiments. We also thank Xuewu Zhang, Tanya Freedman, Brian Kelch, and Meindert Lamers for helpful discussions and Tanya Freedman, Natalia Jura, and Jonathon Winger for critical reading of the manuscript. Supported in part by grants from the NIH (P.C.). M.A.S. is supported by NIH grant K99 GM080097.
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