Lipid-binding and antimicrobial properties of synthetic peptides of bovineapolipoprotein A-II

We previously showed that bovine apolipoprotein A-II (apoA-II) had antimicr obial activity against Escherichia coli and the yeast Saccharomyces cerevis iae in PBS. We have characterized here the active domain of apoA-II using s ynthetic peptides, A peptide corresponding to C-terminal residues Leu(49)-T hr(76) exhibited significant antimicrobial activity against E. coli in PBS, but not against S. cerevisiae, Experiments using amino-acid-substituted pe ptides indicated that the residues Phe(52)-Phe(53)-Lys(54)-Lys(55) are requ ired for the activity. Peptide Leu(49)-Thr(76) induced the release of calce in trapped inside the vesicles whose lipid composition resembles that of E. coli membrane, suggesting that peptide Leu(49)-Thr(76) can destabilize the E. coli membrane. CD measurements showed that the alpha-helicity of peptid e Leu(49)-Thr(76) increased from 3.5 to 36% by addition of the vesicles. Wh en E. coli cells were incubated with peptide Leu(49)-Thr(76), some proteins were released to the external medium, probably owing to membrane destabili zation caused by the peptide. In electron micrographs of E, coli cells trea ted with peptide Leu(49)-Thr(76), transparent nucleoids and granulated cyto plasm were observed. Amino acid substitutions, Phe(52)Phe(53) --> AlaAla (P he(52,53) --> Ala) in peptide Leu(49)-Thr(76) caused the loss of antimicrob ial activity against E. coli, although protein-releasing activity was retai ned. Electron micrographs of the cells treated with peptide Leu(49)-Thr(76) (Phe(52,53) --> Ala) revealed morphological change only at the nucleoids, t herefore peptide Leu(49)-Thr(76) appears to primarily target the cytoplasm rather than the membrane of E. coli cells.