Defining the metal specificity of a multifunctional biofilm adhesion protein.

The accumulation associated protein (Aap) of Staphylococcus epidermidis mediates intercellular adhesion events necessary for biofilm growth. This process depends upon Zn2+-induced self-assembly of G5 domains within the B-repeat region of the protein, forming anti-parallel, intertwined protein ropes between cells. Pleomorphism in the Zn2+-coordinating residues was observed in previously solved crystal structures, suggesting that the metal binding site might accommodate other transition metals and thereby support dimerization. By use of carefully selected buffer systems and a specialized approach to analyze sedimentation velocity analytical ultracentrifugation data, we were able to analyze low-affinity metal binding events in solution. Our data show that both Zn2+ and Cu2+ support B-repeat assembly, whereas Mn2+, Co2+, and Ni2+ bind to Aap but do not support self-association. As the number of G5 domains are increased in longer B-repeat constructs, the total concentration of metal required for dimerization decreases and the transition between monomer and dimer becomes more abrupt. These characteristics allow Aap to function as an environmental sensor that regulates biofilm formation in response to local concentrations of Zn2+ and Cu2+, both of which are implicated in immune cell activity.