Surface properties of membrane systems. Transport of staphylococcal δ-toxin from aqueous to membrane phase

Abstract

Hemolytic δ-toxin from Staphylococcus aureus was soluble in either water, methanol or chloroform/methanol (2 : 1, v/v). The toxin spread readily from distilled water into films with pressures (π) of 10 dynes/cm on water and 30 dynes/cm on 6 M urea; from chloroform/methanol it produced 40 dynes/cm pressure on distilled water. The toxin adsorbed barely from water (π= 1 dyne/cm) but it did rapidly from 6 M urea (π= 35 dynes/cm). The protein films had unusually high surface potentials, which increased with the film pressure and decreased with increasing both pH and urea concentration in the aqueous phase. The fluorescence of 1-aniline 8-naphthalene sulfonate with δ-toxin was much greater than that with RNAase and dipalmitoyl phosphatidylcholine itself, indicating probably a marked lipid-binding character of the toxin. By circular dichroism the α-helix content of δ-toxin was 42% in water, 45% in methanol, 24% in 6 M urea. Infrared spectroscopy showed predominant a-helix in both ²H₂O and deuterated chloroform/methanol as well as in films spread from either solvent on ²H₂O. In spreading from 6 M [²H]urea, in which the major infrared absorption was that of [²H]urea with peaks at 1600 and 1480 cm^-1, and δ-toxin film showed prevalently non-a-helix structures with major peak intensities at 1633 cm^-1 > 1680 cm^-1, indicating the appearance of new β-aggregated and β-antiparallel pleated sheet structures in the film. The data prove that (1) high pressure protein films can consist of α-helix as well as non-α-helix structures and, differently from another cytolytic protein, melittin, δ-toxin does not resume the α-helix conformation in going into the film phase from the extended chain in 6 M urea; (2) conformational changes are important in the transport of proteins from aqueous to lipid or membrane phase; (3) δ-toxin is by far more versatile in structural dynamics and more surface active than α-toxin.