Structure-function study of cathelicidin-derived bovine antimicrobial peptide BMAP-28: design of its cell-selective analogs by amino acid substitutions in the heptad repeat sequences

Although BMAP-28 is a potent cathelicidin-derived bovine antimicrobial peptide, its cytotoxic activity against the human and other mammalian cells is of concern for converting it into a novel antimicrobial drug. We have identified a short leucine and isoleucine zipper sequences at the N- and C-terminals of BMAP-28, respectively. To understand the possible role of these structural elements in BMAP-28, a number of alanine-substituted analogs were designed, synthesized and characterized along with the wild-type peptide. The substitution of amino acids at single or multiple 'a' position(s) of these structural motifs by alanine showed significant effects on the cytotoxic activity of the molecule on the human red blood cells (hRBCs) and 3T3 cells without showing much effects on their MIC values against the selected bacteria. BMAP-28 and all its analogs depolarized the Escherichia coli cells with almost equal efficacy. In contrast, the alanine-substituted analogs of BMAP-28 depolarized hRBCs much less efficiently than the parent molecule. Results further showed that BMAP-28 assembled appreciably onto the live E. coli and hRBC. However, the selected less toxic analogs of BMAP-28 although assembled as good as the parent molecule onto the live E. coli cells, their assembly onto the live mammalian hRBCs was much weaker as compared to that of the wild-type molecule. Looking at the remarkable similarity with the data presented in our previous work on melittin, it appears that probably the heptad repeat sequence possesses a general role in maintaining the cytotoxicity of the antimicrobial peptides against the mammalian cells and assembly therein.