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 c onsiderable interest. We have studied the insertion of bovine cy-lacta lbumin into single-bilayer vesicles prepared from egg phosphatidylchol ine (PC), Fluoresence 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 observe d 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 w as found to be stable at neutral pH. The structural characterization o f the apo-alpha-LA-PC complex was studied by fluorescence quenching us ing iodide, acrylamide, and 9,10-dibromostearic acid, The results obta ined indicated that some of the tryptophans of apo-alpha-LA were burie d in the membrane interior and some were exposed on the outer side. Fl uorescence quenching and CD studies indicated the membrane-bound confo rmation of apo-alpha-LA was some conformational state that is between the soluble, fully folded conformation and the molten globule state.