Membrane-protein interaction and the molten globule state: interaction of alpha-lactalbumin with membranes.

The insertion of soluble proteins into membranes has been a topic of considerable interest. We have studied the insertion of bovine alpha-lactalbumin into single-bilayer vesicles prepared from egg phosphatidylcholine (PC). Fluorescence studies indicated rapid and tight binding of apo-alpha-lactalbumin (apo-alpha-LA) to PC vesicles as a function of pH. The binding was maximal at pH values which favor the formation of the molten globule state. As an increase of hydrophobic surface is observed in the molten globule state, this conformational state can provide a molecular basis for insertion of soluble proteins into membranes. The membrane-bound complex formed at low pH (3.0) could be isolated and was found to be stable at neutral pH. The structural characterization of the apo-alpha-LA-PC complex was studied by fluorescence quenching using iodide, acrylamide, and 9,10-dibromostearic acid. The results obtained indicated that some of the tryptophans of apo-alpha-LA were buried in the membrane interior and some were exposed on the outer side. Fluorescence quenching and CD studies indicated the membrane-bound conformation of apo-alpha-LA was some conformational state that is between the soluble, fully folded conformation and the molten globule state.