Influence of staphylococcal δ-toxin on the phosphatidylcholine headgroup as observed using 2H-NMR

Abstract

The interaction of the 8-toxin peptide isolated from Staphylococcus aureus with the headgroup region of lipid bilayer membranes composed of 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC) was investigated using deuterium ( 2H) and phosphorus ( 31P) nuclear magnetic resonance (NMR) spectroscopy. At relatively low peptide/lipid ratios (P/L < 0.10), all 2H- and 31P-NMR spectral lineshapes at 25°C were indicative of a single population of liquid-crystalline lipids in a bilayer arrangement. At these P/L ratios, δ-toxin had only marginal effects on the size of the quadrupole splitting measured from POPC labelled at either the α-methylene (POPC- α- d 2) or the ß-methylene segment (POPC- ß- d 2) of the choline headgroup and, similarly small effects on the magnitude of the chemical shift anisotropy (CSA) of the 31P-NMR spectrum. With increasing amounts of δ-toxin (0.10 < P/L < 0.15) the size of the 2H quadrupole splitting from POPC- α- d 2, as well as the magnitude of the 31P-CSA, decreased progressively and rapidly. The quadrupole splitting from POPC- ß- d 2, however, remained relatively unaffected. At yet higher levels of δ-toxin (P/L > 0.15), all 2H- and 31P-NMR spectra indicated the presence of multiple lipid populations experiencing varying degrees of increased conformational disordering. The spectral lineshapes of these apparently nonbilayer spectral components reverted to bilaler-type lineshapes upon lowering the measuring temperature to 5°C. At the utmost highest level of δ-toxin measured here (P/L = 0.20), all 2H- and 31P-NMR spectra consisted of a single, broad, apparently nonbilayer-type component, indicative of hindered but virtual isotropic motional averaging of the POPC headgroups. In this case no reversion to bilayer-type spectra could be obtained by decreasing the temperature. We could obtain no evidence that the conformation of the choline headgroup of POPC was responding to any specific influence of δ-toxin on bilayer surface electrostatics.