Dynamic structure of disulfide-removed linear analogs of tachyplesin-I in the lipid bilayer from solid-state NMR

Tachyplesin-I (TP-I) is a 17-residue beta-hairpin antimicrobial peptide containing two disulfide bonds. Linear analogs of TP-I where the four Cys residues were replaced by aromatic and aliphatic residues, TPX4, were found to have varying degrees of activities, with the aromatic analogs similarly potent as TP-I. Understanding the different activities of the linear analogs should give insight into the mechanism of action of TP-I. To this end, we have investigated the dynamic structures of the active TPF4 and the inactive TPA4 in bacteria-mimetic anionic POPE/POPG bilayers and compared them with the wild-type TP-I using solid-state NMR spectroscopy. 13C isotropic chemical shifts and backbone (phi, psi) torsion angles indicate that both TPF4 and TPA4 adopt beta-strand conformations without a beta-turn at key residues. 1H spin diffusion from lipid chains to the peptide indicates that the inactive TPA4 binds to the membrane-water interface, similar to the active TP-I. Thus, neither the conformation nor the depth of insertion of the three peptides correlates with their antimicrobial activities. In contrast, the mobility of the three peptides correlates well with their activities: the active TP-I and TPF4 are both highly mobile in the liquid-crystalline phase of the membrane while the inactive TPA4 is completely immobilized. The different mobilities are manifested in the temperature-dependent 13C and 15N spectra, 13C-1H and 15N-1H dipolar couplings and 1H rotating-frame spin-lattice relaxation times. The dynamics of TP-I and TPF4 are both segmental and global. Combined, these data suggest that TP-I and TPF4 disrupt the membrane by large-amplitude motion in the plane of the membrane. The loss of this motion in TPA4 due to aggregation significantly weakens its activity because a higher peptide concentration is required to disturb lipid packing. Thus molecular motion, rather than structure, appears to be the key determinant for the membrane-disruptive activities of tachyplesins.