McsA and the roles of metal-binding motif in Staphylococcus aureus

McsA is a key modulator of stress response in Staphylococcus aureus that contains four CXXC potential metal-binding motifs at the N-terminal. Staphylococcus aureus ctsR operon encodes ctsR, clpC, and putative mcsA and mcsB genes. The expression of the ctsR operon in S. aureus was shown to be induced in response to various types of heavy metals such as copper and cadmium. McsA was cloned and overexpressed, and purified product was tested for metal-binding activity. The protein bound to Cu(II), Zn(II), Co(II), and Cd(II). No binding with any heavy metal except copper was found when we performed site-directed mutagenesis of Cys residues of three CXXC motifs of McsA. These data suggest that two conserved cysteine ligands provided by one CXXC motif are required to bind copper ions. In addition, using a bacterial two-hybrid system, McsA was found to be able to bind to McsB and CtsR of S. aureus and the CXXC motif was needed for the binding. This indicates that the Cys residues in the CXXC motif are involved in metal binding and protein interaction.