Engineering disulfide bridges to dissect antimicrobial and chemotactic activities of human β-defensin 3

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Abstract
Human defensins form a family of small, cationic, and Cys-rich antimicrobial proteins that play important roles in innate immunity against invading microbes. They also function as effective immune modulators in adaptive immunity by selectively chemoattracting T lymphocytes and immature dendritic cells. On the basis of sequence homology and the connectivity of six conserved Cys residues, human defensins are classified into α and β families. Structures of several β-defensins have recently been characterized, confirming the disulfide connectivity conserved within the family, i.e., Cys1–Cys5, Cys2–Cys4, and Cys3–Cys6. We found that human β-defensin 3 (hBD3), a recently described member of the growing β family, did not fold preferentially into a native conformation in vitro under various oxidative conditions. Using the orthogonal protection of Cys1–Cys5 and of Cys1–Cys6, we chemically synthesized six topological analogs of hBD3 with predefined disulfide connectivities, including the (presumably) native β pairing. Unexpectedly, all differently folded hBD3 species exhibited similar antimicrobial activity against Escherichia coli, whereas a wide range of chemotactic activities was observed with these analogs for monocytes and cells transfected by the chemokine receptor CCR6. Furthermore, whereas substitution of all Cys residues by α-aminobutyric acid completely abolished the chemotactic activity of hBD3, the bactericidal activity remained unaffected in the absence of any disulfide bridge. Our findings demonstrate that disulfide bonding in hBD3, although required for binding and activation of receptors for chemotaxis, is fully dispensable for its antimicrobial function, thus shedding light on the mechanisms of action for human β-defensins and the design of novel peptide antibiotics.