Alpha-lactalbumin binding to membranes: evidence for a partially buried protein.
The self-incorporation of apo-alpha-lactalbumin (alpha-LA) into single unilamellar vesicles (SUV) of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine was demonstrated by column chromatographic analyses on Sephadex G-200 (10 mM Tris-HCl, pH 7.4, 26-28 degrees C) and by intrinsic fluorescence emission of SUV-bound alpha-LA. It was shown that apo-alpha-LA slowly incorporated into the DMPC vesicle bilayer after equilibrating different mixtures of protein and SUV for several hours. The intrinsic fluorescence properties of the bound apo-alpha-LA were altered only slightly (lambda maxem = 333 nm vs. 337 nm in aqueous solution). The large blue shift in apo-alpha-LA fluorescence in solution induced by monovalent cations, such as Na(I), was almost completely prevented when apo-alpha-LA was membrane bound. Furthermore, the addition of calcium caused a slow conversion from apo-alpha-LA to Ca(II)-alpha-LA by a mechanism consistent with passive diffusion of Ca(II) into the bilayer interior to the (buried) calcium binding site. The release of Ca(II)-alpha-LA from the membrane is discussed with reference to alpha-LA release from the smooth endoplasmic reticulum in vivo.
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism*; D007768:Lactalbumin / Q000378:metabolism*; D008051:Lipid Bilayers; D008563:Membrane Lipids / Q000378:metabolism*; D008954:Models, Biological; D013050:Spectrometry, Fluorescence
D008051:Lipid Bilayers; D008563:Membrane Lipids; D015060:1,2-Dipalmitoylphosphatidylcholine; D007768:Lactalbumin; D002118:Calcium; D004134:Dimyristoylphosphatidylcholine