TERTIARY STRUCTURAL-CHANGES OF THE ALPHA-HEMOLYSIN FROM STAPHYLOCOCCUS-AUREUS ON ASSOCIATION WITH LIPOSOME MEMBRANES

The interaction of alpha-hemolysin (also called alpha-toxin) from Stap hylococcus aureus with mixed egg-yolk phosphatidylcholine/cholesterol liposomes has been investigated using the intrinsic tryptophan fluores cence emission (ITFE) signal. The ITFE intensity of alpha-hemolysin, w hich was obtained using a novel purification protocol, showed a tripha sic increase on incubation with liposomes at low protein/lipid ratios. The first, rapid phase results in an increase in ITFE of 10%, which r eflects rapid conformation changes in the alpha-hemolysin on associati on with the liposome membrane, The second phase of the ITFE increase i s associated with a red shift from 334 to 339 nm in the maximum emissi on wavelength, suggesting the transition to a partially unfolded inter mediate in the oligomerization process. The third phase of the ITFE in tensity change demonstrates a temporal correlation with the appearance of SDS-stable oligomers. The results demonstrate the feasibility of i dentification of intermediate protein conformations in complex membran e-associated processes by manipulation of the liposomal membrane compo sition. (C) 1998 Academic Press.