Structure and mechanism of action of an indolicidin peptide derivative with improved activity against gram-positive bacteria

Indolicidin, an antimicrobial peptide with a unique amino acid sequence (IL PWKWPWWPWRR-NH2) is found in bovine neutrophils, A derivative of indolicidi n, CP10A, has alanine residues substituted for proline residues and has imp roved activity against Gram-positive organisms. Transmission electron micro scopy of Staphylococcus aureus and Staphylococcus epidermidis treated with CP10A showed mesosome-like structures in the cytoplasm, The peptide at a-fo ld the minimal inhibitory concentration did not show significant killing of S, aureus ISP67 (a histidine, uridine, and thymidine auxotroph) but did sh ow an early effect on histidine and uridine incorporation and, later, an ef fect on thymidine incorporation. Upon interaction with liposomes, detergent s, and lipoteichoic acid, CP10A was shown by circular dichroism spectroscop y to undergo a change in secondary structure. Fluorescence spectroscopy ind icated that the tryptophan residues were located at the hydrophobic/hydroph ilic interface of liposomes and detergent micelles and were inaccessible to the aqueous quencher KI. The three-dimensional structure of CP10A in the L ipid mimetic dodecylphosphocholine was determined using two-dimensional NMR methods and was characterized as a short, amphipathic helical structure, w hereas indolicidin was previously shown to have an extended structure. Thes e studies have introduced a cationic peptide with a unique structure and an ability to interact with membranes and to affect intracellular synthesis o f proteins, RNA, and DNA.