Balance of electrostatic and hydrophobic interactions in the lysis of model membranes by E. coli alpha-haemolysin.
The relative weight of electrostatic interactions and hydrophobic forces in the process of membrane disruption caused by E. coli alpha-haemolysin (HlyA) has been studied with a purified protein preparation and a model system consisting of large unilamellar vesicles loaded with water-soluble fluorescent probes. Vesicles were prepared in buffers of different ionic strengths, or pHs, and the net surface charge of the bilayers was also modified by addition of negatively (e.g., phosphatidylinositol) or positively (e.g., stearylamine) charged lipids. The results can be interpreted in terms of a multiple equilibrium in which alpha-haemolysin may exist: aggregated HlyA <==> monomeric HlyA <==> membrane-bound HlyA. In these equilibria both electrostatic and hydrophobic forces are significant. Electrostatic forces become substantial under certain circumstances, e.g., membrane binding when bilayer and protein have opposite electric charges. Protein adsorption to the bilayer is more sensitive to electrostatic forces than membrane disruption itself. In the latter case, the irreversible nature of protein insertion may overcome electrostatic repulsions. Also of interest is the complex effect of pH on the degree of aggregation of an amphipathic toxin like alpha-haemolysin, since pH changes are not only influencing the net protein charge but may also be inducing protein conformational transitions shown by changes in the protein intrinsic fluorescence and in its susceptibility to protease digestion, that appear to regulate the presence of hydrophobic patches at the surface of the molecule, thus modifying the ability of the toxin to either aggregate or become inserted in membranes.
Ostolaza
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H
Grupo Biomembranas (Unidad Asociada al C.S.I.C.), Departamento de Bioquímica, Universidad del País Vasco, Aptdo. 644, 48080 Bilbao, Spain.
Bakás
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L
Goñi
F M
FM
D001426:Bacterial Proteins / Q000633:toxicity*; D001665:Binding Sites; D029968:Escherichia coli Proteins; D006460:Hemolysin Proteins / Q000633:toxicity*; D006863:Hydrogen-Ion Concentration; D008051:Lipid Bilayers / Q000737:chemistry; D008567:Membranes, Artificial; D009994:Osmolar Concentration; D055672:Static Electricity; D013499:Surface Properties; D014867:Water
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D006460:Hemolysin Proteins; C036507:Hlya protein, E coli; D008051:Lipid Bilayers; D008567:Membranes, Artificial; D014867:Water