ROLE OF DISULFIDE LINKAGES IN TACHYPLESIN LIPID INTERACTIONS

In order to elucidate the role of the two disulfide linkages of tachyp lesin I (T-SS), a membrane-acting cyclic antimicrobial peptide from Ta chypleus tridentatus, we synthesized the acyclic analog (T-Acm) with t he four SH groups protected by acetamidomethyl groups and also investi gated the interactions of these peptides with lipid bilayers. T-SS ind uced leakage of calcein from egg yolk L-alpha-phosphatidylglycerol lar ge unilamellar vesicles (PG LUVs) at peptide concentrations 1 order of magnitude smaller than those at which leakage was induced by T-Acm, w hich coincides with the stronger antimicrobial activities of T-SS. The micellization of PG LUVs was also more efficient for the cyclic pepti de. Fluorescence titration studies revealed that binding affinities of both peptides to the PG membranes were similar. Fourier transform inf rared polarized attenuated total reflection spectroscopy and fluoresce nce quenching experiments demonstrated that T-SS and T-Acm both form a mphiphilic antiparallel beta-sheet structures in the membranes. They a re formed in such a way that the sheet planes lie parallel to the memb rane surface with the sheet hydrophobic surfaces penetrating slightly into the hydrophobic region of the bilayers. Furthermore, the observat ion that the linear T-Acm, the weaker membrane permeabilizer, caused a far more serious membrane disruption suggests the possibility that th e mechanisms of membrane permeabilization by the cyclic peptide are di fferent from those by the linear peptide, the latter being the disrupt ion of the lipid organization.