CONFORMATIONAL-CHANGES DUE TO MEMBRANE-BINDING AND CHANNEL FORMATION BY STAPHYLOCOCCAL ALPHA-TOXIN

Conformational changes occurring upon membrane binding and subsequent insertion of staphylococcal ol-toxin were studied using complementary spectroscopic techniques. Experimental conditions were established whe re binding could be uncoupled from membrane insertion but insertion an d channel formation seemed to be concomitant. Binding led to changes i n tertiary structure as witnessed by an increase in tryptophan fluores cence, a red shift of the tryptophan maximum emission wavelength, and a change in the near UV CD spectrum, In contrast to what was observed for the soluble form of the toxin, 78% of the tryptophan residues in t he membrane-bound form were accessible to the hydrophilic quencher KI, At this stage, the tryptophan residues were not in the immediate vici nity of the lipid bilayer, Upon membrane insertion, a second conformat ional change occurred resulting in a dramatic drop of the near UV CD s ignal but an increase of the far UV signal, Tryptophan residues were n o longer accessible to KI but could be quenched by brominated lipids. In the light of the available data on channel formation by cytoxin, ou r results suggest that the tryptophan residues might be dipping into t he membrane in order to anchor the extramembranous part of the channel to the lipid bilayer.