Résumé
In order to investigate the effect of primary amphipathic peptides on mollicutes (wall-less bacteria), we have synthesised five molecules (P1, P2, P3, JM123, and JM133) comprising a 16 to 18-residue hydrophobic sequence and the nuclear localization sequence (NLS) PKKKRKV of simian virus 40 large-T antigen,
C-terminated by a cysteamide group. The hydrophobic cluster was in P1 the signal sequence of the heavy chain of
Caiman crocodilus immunoglobulin G and in JM123 the fusion peptide of human immunodeficiency virus 1 glycoprotein gp41 in which phenylalanine
7 was replaced by a tryptophan residue. The homologues P2, P3, and JM133 were obtained by slight alterations of these sequences. Circular dichroism spectroscopy revealed that, in liposomes, P-series peptides were mainly under the form of β-sheets whereas JM-series peptides displayed a high proportion of turns. These peptides proved to be bactericidal for some mollicutes, notably
Acholeplasma laidlawii, but were much less potent than melittin. Furthermore, their antibiotic activity was independent of the average thickness of the plasma membrane hydrophobic core whilst that of melittin was inversely related to the thickness. Melittin and the synthetic peptides abolished spiroplasma cell motility and helicity, but only melittin and P-series peptides split the cells into globular forms displaying an average diameter of ca. 1 μm. In contrast to melittin, the synthetic peptides agglutinated spiroplasmas, suggesting that their polycationic NLS was exposed on the cell surface. P-series peptides decreased, though less efficiently than melittin,
A.
laidlawii and
Spiroplasma melliferum membrane potential (Δ
Ψ) and transmembrane pH gradient (ΔpH), at concentrations much lower than their minimal inhibitory concentrations whilst JM-series peptides had no effect on Δ
Ψ and ΔpH in the same conditions. Actually, the bactericidal activity of these peptides towards mollicutes was proportional to their ability to collapse the electrochemical transmembrane potential.