| Abstract: | Combined phosphorus-31 nuclear magnetic resonance (31P NMR) and electron microscopic studies were performed on the ADP/ATP carrier protein from beef heart mitochondria. The protein was incorporated into phospholipids by addition of Triton-protein micelles to a lipid suspension or to the dry lipid. All of the phospholipid (egg phosphatidylcholine or mixtures of egg phosphatidylcholine and egg phosphatidylethanolamine) that contributed to the observed 31P NMR signal under these conditions appeared to be in a bilayer configuration. Freeze-fracturing and negative-staining electron microscopy showed unilamellar vesicles and multilayers. An isotropic signal could be attributed to vesicle rotation, judging from its sensitivity to increasing viscosity. The presence of small vesicles was also noticeable in the 31P NMR spectra of planar oriented membranes. In the presence of phosphatidylethanolamine, aggregation of protein particles was observed. Gel chromatography of the protein-Triton-phospholipid mixture revealed that, before Triton removal, large amounts of protein are associated with multibilayers. Separation of loaded and unloaded membranes by centrifugation in D2O showed that, upon stepwise addition, protein incorporates preferentially into unloaded liposomes. From these findings a mechanism of protein reincorporation was deduced. |
