Solid-state 2H NMR and 31P NMR of ,H-enriched chains and polar head groups, respectively, of dipalmitoylglycerophosphatidylcholine/water dispersions were undertaken to investigate the action of 6-haemolysin from Staphylococcus aureus on biomembranes. When the lipid/toxin molar ratio, Ri, is 3 10, the gelphase ,H powder patterns and the temperature of the gel -fluid phase transition, t,, are unchanged by the presence of the toxin whereas the 31P powder spectra of polar head groups are perturbed. At t > t,, a detailed analysis of methylene ordering indicates that 6-haemolysin orders the lipid chains near t, and disorders them for t % t,. These findings are interpreted in terms of peptide location with regard to the membrane and suggest that the position of the toxin depends on the temperature relative to t,.Experiments carried out at Ri = 4 exhibit sharp, isotropic ,H-NMR lines, at t > t,, indicating that 6-haemolysin promotes the appearance of very small objects undergoing fast isotropic reorientation which average to zero the quadrupolar interaction. Below t,, one observes gel-phase powder patterns which indicate that the bacterial toxin is unable to form such small structures with ordered dipalmitoylglycerophosphocholine phospholipids. From comparison of the action of 6-haemolysin with that of melittin on same lipids Biochemistry 25, 6448 -64551 it results that both toxins perturb similarly fluid-phase lipids at elevated temperature, but they behave differently with gel-phase lipids, the former toxin being less efficient in membrane restructuring than the latter.Of all the toxins secreted by the Gram-positive bacterium Staphylococcus aureus, 6-haemolysin is the smallest and the best characterized chemically. This toxin is a 26-amino-acid peptide which does not possess cysteine, arginine, proline, histidine or tyrosine residues (Fig. 1). Polar and ionic amino acids alternate in the sequence with hydrophobic residues. This gives rise to a peptide with an amphipatic character distributed along the sequence [l]. 6-Haemolysin is often in oligomeric form around pH z 7 and tetrameric in very acidic (pH < 4) or basic conditions (pH > 9). According to Fitton[2], the secondary structure of 6-haemolysin would be essentially a-helical in a water/ethanol (1 : 1) medium. NMR data for the toxin in methanol solution demonstrate an a-helix for residues 2 -20 [3], while by the same technique residues 5 -23 form an extended helix when bound to lipid micelles [4]. This toxin is cytolytic, but weaker in magnitude when compared to other cytolytic agents secreted by S. aureus. In addition, it is not specific since it lyses a wide variety of cells or subcellular compartments [5]. Such properties led Bernheimer [6] to postulate that its mode of action was comparable to that of a detergent. 6-Haemolysin, in relatively high doses, is lethal for laboratory animals, and increases vascular permeability when injected into rabbits [7]. Like melittin, it activates phospholipases A,. However, on comparing the action of melittin and 6-haemolysi...
