INFLUENCE OF STAPHYLOCOCCAL DELTA-TOXIN ON THE PHOSPHATIDYLCHOLINE HEADGROUP AS OBSERVED USING H-2-NMR

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 (H-2) and phosphorus (P-31) nuclear magnetic resonance (NMR) spectroscopy. At relatively low peptide/lipid ratios (P/L < 0.10), all H-2- and P-31-NMR spectral lineshapes at 25-degrees-C were indicative of a single population of liquid-crystalline lipids in a bilayer arrangement. At these P/L ratios, delta-toxin had only marginal effects on the size of the quadrupole splitting measured from POPC labelled at either the alpha-methylene (POPC-alpha-d2) or the beta-methylene segment (POPC-beta-d2) of the choline headgroup and, similarly small effects on the magnitude of the chemical shift anisotropy (CSA) of the P-31-NMR spectrum. With increasing amounts of delta-toxin (0.10 < P/L < 0.15) the size of the H-2 quadrupole splitting from POPC-alpha-d2, as well as the magnitude of the P-31-CSA, decreased progressively and rapidly. The quadrupole splitting from POPC-beta-d2, however, remained relatively unaffected. At yet higher levels of delta-toxin (P/L > 0.15), all H-2- and P-31-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 bilayer-type lineshapes upon lowering the measuring temperature to 5-degrees-C. At the utmost highest level of delta-toxin measured here (P/L = 0.20), all H-2- and P-31-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 delta-toxin on bilayer surface electrostatics.