Determination of the Substrate-Docking Site of Protein Tyrosine Kinase C-terminal Src Kinase

Sungsoo Lee
Xiaofeng Lin
Nguyen Hai Nam
Keykavous Parang, Chapman University
Gongqin Sun

This article was originally published in Proceedings of the National Academy of Sciences, volume 100, issue 25, in 2003. doi: 10.1073/pnas.2534493100

Abstract

Protein tyrosine kinases (PTK) are key enzymes of mammalian signal transduction. For the fidelity of signal transduction, each PTK phosphorylates only one or a few proteins on specific Tyr residues. Substrate specificity is thought to be mediated by PTK–substrate docking interactions and recognition of the phosphorylation site sequence by the kinase active site. However, a substrate-docking site has not been determined on any PTK. C-terminal Src kinase (Csk) is a PTK that specifically phosphorylates Src family kinases on a C-terminal Tyr. In this study, by sequence alignment and site-specific mutagenesis, we located a substrate-docking site on Csk. Mutations in the docking site disabled Csk to phosphorylate, regulate, and complex with Src but only moderately affected its general kinase activity. A peptide mimicking the docking site potently inhibited (IC50 = 21 μM) Csk phosphorylation of Src but only moderately inhibited (IC50 = 422 μM) its general kinase activity. Determination of the substrate-docking site provides the structural basis of substrate specificity in Csk and a model for understanding substrate specificity in other PTKs.