Transition Metal Sequestration by the Host-Defense Protein Calprotectin

Abstract

In response to microbial infection, the human host deploys metal-sequestering host-defense proteins, which reduce nutrient availability and thereby inhibit microbial growth and virulence. Calprotectin (CP) is an abundant antimicrobial protein released from neutrophils and epithelial cells at sites of infection. CP sequesters divalent first-row transition metal ions to limit the availability of essential metal nutrients in the extracellular space. While functional and clinical studies of CP have been pursued for decades, advances in our understanding of its biological coordination chemistry, which is central to its role in the host-microbe interaction, have been made in more recent years. In this review, we focus on the coordination chemistry of CP and highlight studies of its metal-binding properties and contributions to the metal-withholding innate immune response. Taken together, these recent studies inform our current model of how CP participates in metal homeostasis and immunity, and they provide a foundation for further investigations of a remarkable metal-chelating protein at the host-microbe interface and beyond.

Keywords: S100 protein; antimicrobial activity; host–microbe interaction; metal homeostasis; nutritional immunity.

Figure 1

Timeline of notable events during the discovery and evaluation of transition metal binding of calprotectin (CP). A number of investigations of the role of CP in infectious disease dating back to 1980 built upon each other to elucidate the structure and function of CP. While the involvement of metal ions in the activity of CP was noted as early as 1991, the metal-binding characteristics of CP were not thoroughly evaluated until approximately 20 years later.

Figure 2

Structure of human calprotectin (CP). S100A8 subunits are green, and S100A9 subunits are blue. (a) Amino acid sequence alignment of human (h) and murine (m) CP. The secondary structural elements are presented above the alignment for the human form. The transition metal–binding residues are presented in orange. For the murine S100A9 subunit, His105 and His107 are predicted to contribute to the His₆ site. (b) The heterodimer bound to Ni(II) (teal ), Ca(II) ( yellow), and Na(I) ( purple) [Protein Data Bank (PDB) 5W1F]. Site 1 is expanded to show the Ni(II)-bound His₃Asp motif. (c) The dimer bound to Mn(II) (magenta), Ca(II) ( yellow), and Na(I) ( purple) (PDB 4XJK). Site 2 is expanded to show the Mn(II)-bound His₆ motif. (d ) S100A9 canonical and noncanonical EF-hands and proximity to the Ni(II)-bound His₃Asp site (site 1) (PDB 5W1F). Both EF-hands are Ca(II) bound. (e) S100A8 canonical and noncanonical EF-hands and proximity to the Mn(II)-bound His₆ site (site 2) (PDB 4XJK). The canonical EF-hand is Ca(II) bound, and the noncanonical EF-hand is Na(I) bound. ( f ) The Ca(II)-, Na(I)-, and Mn(II)-bound (S100A8/S100A9)₂ tetramer (PDB 4XJK). Panel a modified from Reference .

Figure 3

Model for the extracellular role of calprotectin (CP) in metal sequestration. CP is released from neutrophils or epithelial cells and encounters high concentrations of Ca(II) ( 2 mM) in the extracellular space, causing the protein to form the (S100A8/S100A9)₂ heterotetramer. Ca(II)-induced tetramerization affords protease resistance, enhanced transition metal affinities, and enhanced antimicrobial activity. In the extracellular space, CP competes with microbes for bioavailable metals in the 2 oxidation state to impart its growth inhibitory activity.

Figure 4

The S100A9 C-terminal tail and encapsulation of a Mn(II) ion at site 2 [Protein Data Bank (PDB) 4XJK]. S100A8 is shown in green, S100A9 is shown in blue, and the Mn(II) ion is shown in magenta. Primary and secondary coordination sphere residues are shown in orange. Water molecules are shown as red spheres. Hydrogen-bonding interactions are shown as black dashed lines and labeled with the internuclear distance. An asterisk (^*) indicates residues from the S100A9 subunit in the other heterodimer of the heterotetramer.