SummaryThe SH2 domain of the C-terminal Src kinase [Csk] contains a unique disulfide bond not present in other known SH2 domains. To investigate whether this unusual disulfide bond could serve a novel function, the effects of disulfide bond formation on catalytic activity of the full-length protein and on the structure of the SH2 domain were investigated. The kinase activity of full-length Csk decreases by an order of magnitude upon formation of the disulfide bond in the distal SH2 domain. NMR spectra of the fully oxidized and fully reduced SH2 domains exhibit similar chemical shift patterns and are indicative of similar, well-defined tertiary structures. The solvent-accessible disulfide bond in the isolated SH2 domain is highly stable and far from the small lobe of the kinase domain. However, reduction of this bond results in chemical shift changes of resonances that map to a cluster of residues that extend from the disulfide bond across the molecule to a surface that is in direct contact with the small-lobe of the kinase domain in the intact molecule. Normal mode analyses and molecular dynamics calculations suggest that disulfide bond formation has large effects on residues within the kinase domain, most notably within the active-site cleft. Overall, the data indicate that reversible cross-linking of two cysteines in the SH2 domain greatly impacts catalytic function and domaindomain communication in Csk. KeywordsCsk; cysteine; disulfide; kinase; NMR; phosphorylation The Src family of tyrosine kinases [SFKs] 1 are modular enzymes responsible for controlling various aspects of cellular growth and differentiation.[1] SH3 and SH2 domains in the SFKs flank the kinase domain and repress catalytic activity through intrasteric constraints. Most notably, the SH2 domain interacts tightly with a phosphotyrosine in the C-terminus of the kinase domain, inducing a closed conformation that poorly phosphorylates protein substrates.