Bioinorganic Explorations of Host-defense Proteins

Transition metal ions are essential nutrients for all organisms. The availability of these nutrients plays a criticalrole in the host-microbe interaction and microbial pathogenesis. The primary objective of this researchproposal is to elucidate how the host-defense protein calprotectin (CP) sequesters transition metals frommicrobes and thereby contributes to the innate immune response. CP provides a remarkable example ofunique biological coordination chemistry that is relevant to infectious disease and microbial pathogenesis.Each CP heterodimer (S100A8/S100A9) exhibits six different sites for chelating divalent cations, includingcalcium (Ca) and transition metals. Our central hypothesis is that CP responds to physiological Ca(II)gradients to tune its coordination chemistry for transition metals and to modulate its biological function as anantimicrobial protein that deprives invading pathogens of essential nutrient metals (e.g. manganese, iron, zinc).The proposed investigations are based on preliminary data that Ca(II) binding by human CP (hCP) at the EF-hand domains triggers high-affinity chelation of transition metals at sites formed at the interface of the S100A8and S100A9 subunits. In Aim 1, we will investigate how Ca(II) ions modulate hCP structure and tune itsaffinities for transition metals. In Aim 2, we will evaluate how the murine orthologue (mCP) sequesterstransition metals and thereby provide needed molecular and biophysical insights into literature results of CPfrom animal models of infection. In Aim 3, we will investigate the competition between CP and bacterial metal-transport machinery for manganese(II). These fundamental bioinorganic and biophysical initiatives constitutean innovative departure from biological and medical studies of CP, and highlight the importance of applyingquantitative analytical and spectroscopic methods to a problem central to human health and disease. Takentogether, the results will provide new molecular insights into how CP contributes to innate immunity and metalhomeostasis. Moreover, the ability to acquire metal ions is an important facet of microbial pathogenesis, andboth intercepting microbial metal acquisition and boosting the metal-withholding response of the host presentopportunities for antibiotic development. We anticipate that the results from our work will, in the long term, helpto guide the development of new antimicrobial therapeutics that target these processes central to the host-pathogen interaction.