Functional Diversity of Csk, Chk, and Src SH2 Domains due to a SingleResidueVariation

Abstract

The C-terminal Src kinase (Csk) family of protein tyrosine kinases containstwo members: Csk and Csk homologous kinase (Chk). Both phosphorylate andinactivate Src family kinases. Recent reports suggest that the Src homology(SH) 2 domains of Csk and Chk may bind to different phosphoproteins, whichprovides a basis for different cellular functions for Csk and Chk. To verifyand characterize such a functional divergence, we compared the bindingproperties of the Csk, Chk, and Src SH2 domains and investigated thestructural basis for the functional divergence. First, the study demonstratedstriking functional differences between the Csk and Chk SH2 domains andrevealed functional similarities between the Chk and Src SH2 domains. Second,structural analysis and mutagenic studies revealed that the functionaldifferences among the three SH2 domains were largely controlled by oneresidue, Glu127 in Csk, Ile167 in Chk, andLys200 in Src. Mutating these residues in the Csk or Chk SH2 domainto the Src counterpart resulted in dramatic gain of function similar to SrcSH2 domain, whereas mutating Lys200 in Src SH2 domain to Glu (theCsk counterpart) resulted in loss of Src SH2 function. Third, a single pointmutation of E127K rendered Csk responsive to activation by a Src SH2 domainligand. Finally, the optimal phosphopeptide sequence for the Chk SH2 domainwas determined. These results provide a compelling explanation for thefunctional differences between two homologous protein tyrosine kinases andreveal a new structure-function relationship for the SH2 domains.