Molecular basis for manganese sequestration by calprotectin and roles in the innate immune response to invading bacterial pathogens

The S100A8/S100A9 heterodimer calprotectin (CP) functions in the host response to pathogens through a mechanism termed nutritional immunity. CP binds Mn2+ and Zn2+ with high affinity and starves bacteria of these essential nutrients. Combining biophysical, structural, and microbiological analysis, we identified the molecular basis of Mn2+ sequestration. The asymmetry of the CP heterodimer creates a single Mn2+-binding site from six histidine residues, which distinguishes CP from all other Mn2+-binding proteins. Analysis of CP mutants with altered metal-binding properties revealed that, despite both Mn2+ and Zn2+ being essential metals, maximal growth inhibition of multiple bacterial pathogens requires Mn2+ sequestration. These data establish the importance of Mn2+ sequestration in defense against infection, explain the broad-spectrum antimicrobial activity of CP relative to other S100 proteins, and clarify the impact of metal depletion on the innate immune response to infection.