Ca(II) and Zn(II) Cooperate To Modulate the Structure and Self-Assembly of S100A12.

ABSTRACT

S100A12 is a member of the Ca2+ binding S100 family of proteins that functions within the human innate immune system. Zinc sequestration by S100A12 confers antimicrobial activity when the protein is secreted by neutrophils. Here, we demonstrate that Ca2+ binding to S100A12's EF-hand motifs and Zn2+ binding to its dimeric interface cooperate to induce reversible self-assembly of the protein. Solution and magic angle spinning nuclear magnetic resonance spectroscopy on apo-, Ca2+-, Zn2+-, and Ca2+,Zn2+-S100A12 shows that significant metal binding-induced chemical shift perturbations, indicative of conformational changes, occur throughout the polypeptide chain. These perturbations do not originate from changes in the secondary structure of the protein, which remains largely preserved. While the overall structure of S100A12 is dominated by Ca2+ binding, Zn2+ binding to Ca2+-S100A12 introduces additional structural changes to helix II and the hinge domain (residues 38-53). The hinge domain of S100A12 is involved in the molecular interactions that promote chemotaxis for human monocyte, acute inflammatory responses and generates edema. In Ca2+-S100A12, helix II and the hinge domain participate in binding with the C-type immunoglobulin domain of the receptor for advanced glycation products (RAGE). We discuss how the additional conformational changes introduced to these domains upon Zn2+ binding may also impact the interaction of S100A12 and target proteins such as RAGE.