The amphiphilic alpha-helix concept. Consequences on the structure of staphylococcal delta-toxin in solution and bound to lipids.
Staphylococcal delta-toxin, a synthetic analogue and a fragment were studied in order to determine their structure in solution and bound in lipids. In solution, a self-association process is observed. Analytical ultracentrifuge and quasi-elastic light-scattering experiments suggest an isodesmic aggregation in the high concentration domain above 2 microM up to very large asymmetrical species. Decreasing concentrations below 2 microM of delta-toxin and the analogue allows dissociation, probably into monomers. The self-associated species are essentially alpha-helical (70%) with buried and highly immobilized Trp either at position 15 for natural delta-toxin or 16 for the analogue. At the lowest concentration studied, the alpha-helix content severely decreases down to 35% while Trp fluorescence shows that these residues are exposed to buffer. The fragment 11-26 is always monomeric and structureless. From all the data, a structural model of aggregated species is proposed with stacked antiparallel amphipathic rods. When bound to lipids, whatever their initial structure in solution, 26-residue long peptides mainly adopt an alpha-helix conformation (80%) while fragment 11-26 exhibits about 50% alpha-helix. The lipid-peptide interactions were quantitatively analysed. For fragment 11-26, a single-step mechanism fits the spectroscopic changes and defines a single monomeric bound structure. On the other hand, for the 26-residue-long analogue, multiple-step processes must occur. The data were analysed with a partition of tetramers into lipids followed by a partial dissociation. Finally, the affinity of fragment 11-26 severely decreases from micelles to fluid and gel-state bilayers. The partition coefficient of the delta-toxin analogue is higher than those of other more apolar peptides, such as melittin and alamethicin, correlating with Eisenberg's hydrophobic moments. It is therefore proposed that delta-toxin probably lies parallel to the surface, only penetrating weakly in lipids, depending on their packing.
Thiaudière
E
E
Centre de Recherche Paul Pascal, Centre National de la Recherche Scientifique, Pessac, France.
Siffert
O
O
Talbot
J C
JC
Bolard
J
J
Alouf
J E
JE
Dufourcq
J
J
D000595:Amino Acid Sequence; D001427:Bacterial Toxins / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D002942:Circular Dichroism; D007700:Kinetics; D008027:Light; D008081:Liposomes; D008958:Models, Molecular; D008962:Models, Theoretical; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry; D010718:Phosphatidylserines / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D012542:Scattering, Radiation; D012996:Solutions; D013050:Spectrometry, Fluorescence / Q000379:methods; D013816:Thermodynamics
D001427:Bacterial Toxins; D008081:Liposomes; D010455:Peptides; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D012996:Solutions; C025971:staphylococcal delta toxin