A molecular investigation of the antimicrobial functions of the human S100 host-defense proteins

Abstract

The human host is continually exposed to potentially harmful organisms and the innate immune response is the first line of defense against microbial invasion. One strategy employed by the human innate immune system includes the release of antimicrobial host-defense proteins (HDPs). The goal of this thesis is to understand the antimicrobial functions of four host-defense proteins of the S100 family of proteins: calprotectin (CP), S100A12, S100A7, and S100A15. In the first half of this thesis, we elucidate the Zn(lI)-binding and antimicrobial properties of S100A12 and S100A7 through the use of solution and microbiology studies. We evaluate the affinity of S100A12 for Zn(ll), the scope of its antimicrobial activity, and put forward a model whereby S100A12 uses Ca(ll) ions to tune its Zn(Il)-chelating properties and antimicrobial activity. Our work with S1 00A7 demonstrates that the protein may exist in more than one redox state under physiological conditions, and that unlike CP and S100A12, the antimicrobial properties of S100A7 are not directly modulated by Ca(ll) ions. We report a model whereby the local redox environment of S100A7 tunes its Zn(ll)-sequestration capacity through intramolecular disulfide-bond redox chemistry, and that Ca(II) ions exert an indirect modulatory effect on the Zn(Il)-binding properties of this protein. In the second half of this thesis, we examine the bactericidal properties of the four S100 proteins. Our results agree with prior work on the bactericidal properties of S100A7. Furthermore, we show that CP and S100A15, but not S100A12, possess bactericidal activity at pH 5, and that CP is a broad-spectrum Gram-negative bactericidal factor that functions through a mechanism of membrane permeabilization. Taken together, our studies provide new insights into the multifunctionality of the antimicrobial S100 HDPs.