Binding of water-soluble, globular proteins to anionic model membranes

The role of electrostatics is studied in the adsorption of proteins to negatively charged (phosphatidylcholine/phosphatidylglycerol, PC/PG) and neutral (PC) small unilamellar vesicles (SUVs). For model proteins the interaction is monitored vs. pH at low ionic strength. The adsorption behaviour of lysozyme, myoglobin and albumin (isoelectronic point, pI 5–11) is investigated in SUVs, along with changes of the fluorescence emission spectra of the charged proteins, via their adsorption on SUVs. Significant adsorption of the proteins to negatively charged SUVs is found only at pH values, where the number of positive charge moieties exceeds the number of negative charge moieties on the protein, by at least 3 e.u. The fluorescence emission of positively charged proteins increases, after adsorption on negatively charged SUVs. With increasing protein to phospholipid ratio, the increase in the fluorescence emission levels off and reaches a plateau. Neutralization of the SUV charge is the controlling factor in their adsorption. The plateau level depends on the type of protein and pH of the incubation medium. The pH dependency can be ascribed to the mean positive charge of the protein. The effective charge of all proteins is calculated from the charge differences between empty and protein-coated SUVs.