Conformation-dependent interaction of alpha-lactalbumin with model and biological membranes: a spin-label ESR study.

Alpha-lactalbumin (alpha-LA) is biosynthesized and stored at the smooth endoplasmic reticulum (ER), then transferred to the Golgi lumen when prolactin stimulation of lactose biosynthesis and secretion takes place. Because both environments are composed of membranes, it was of interest to examine the interactions of alpha-LA with relevant model and biological membranes. Using the ESR spin-labeled fatty acid analog 5-doxyl stearic acid, we found evidence reflecting the insertion of "acid-shocked" molten globule (MG) alpha-LA into lecithin or phosphatidylserine (PS) multi-lamellar vesicles. An additional approximately 3 G immobilization was observed in the alpha-LA-lecithin sample versus the lipid alone. With PS, the increased immobilization was almost 6 G, reflecting an enhanced effect caused by strong electrostatic interactions between the positively charged protein with the negatively charged headgroup at pH 2.4. This was also reflected in the broadening of the PS:alpha-LA phase transition. Additionally, we have demonstrated that alpha-LA in its apo-form also shows similar insertion characteristics with both model and natural lipid membranes. Upon addition of calcium, the apo-form is released from the membrane as the Ca(2+)-bound protein.