Reactive Cysteine in the Structural Zn2+ Site of the C1B Domain from PKCα

Structural cysteine-rich Zn2+ sites that stabilize protein folds are considered to be unreactive. In this article, we identified a reactive cysteine residue, Cys151, in a treble-clef zinc finger with a Cys3His coordination sphere. The protein in question is the C1B domain of Protein Kinase Cα (PKCα). Mass-tagging cysteine assays of several C1B variants were employed to ascertain the site specificity of the covalent modification. The reactivity of Cys151 in C1B also manifests itself in the structural dynamics of the Zn2+ coordination sphere where the Sγ of Cys151 alternates between the Zn2+-bound thiolate and free thiol states. We used NMR-detected pH titrations, ZZ-exchange spectroscopy, and residual dipolar coupling (RDC)-driven structure refinement to characterize the two exchanging conformations of C1B that differ in zinc coordination. Our data suggest that Cys151 serves as an entry point for the reactive oxygen species that activate PKCα in a process involving Zn2+ release.