Zn2+ ions selectively induce antimicrobial salivary peptide histatin-5 to fuse negatively charged vesicles. Identification and characterization of a zinc-binding motif present in the functional domain

The salivary antimicrobial peptide histatin-5 is able to aggregate and fuse negatively charged small unilamellar vesicles, and this fusogenic activity is selectively induced by the presence of zinc ions. Circular dichroism sp ectroscopy shows that histatin-5, in the presence of negatively charged ves icles and zinc ions, undergoes a conformational change leading to the stabi lization of an ct-helical secondary structure. We attribute the specific ac tion of the zinc ions to the presence of a consensus sequence, HEXXH, locat ed in the C-terminal functional domain of histatin-5, a recognized zinc-bin ding motif in many proteins. Two-dimensional proton NMR spectroscopy of his tatin-5 in a trifluoroethanol/water mixture (a membrane mimetic environment ) has been performed and the results analyzed by means of distance geometry and restrained molecular dynamics simulations. Our results reveal that the peptide chain, including the Zn-binding consensus sequence corresponding t o residues 15-19, is in a helicoidal conformation. Comparison of the chemic al shifts of the individual amino acids in histatin-5 with those recently r eported in other solvents indicates that trifluoroethanol/water has a struc turing capability somewhere between water and dimethyl sulfoxide. The mecha nism of action of this antimicrobial peptide is discussed on the basis of i ts structural characteristics with particular attention to the Zn-binding m otif.