Folding of the mitochondrial proton adenosinetriphosphatase proteolipid channel in phospholipid vesicles

The mitochondrial H+-ATPase proteolipid from Neurospora crassa was incorporated into small unilamellar dimyristoylphosphatidylcholine vesicles and its conformation determined by circular dichroism spectroscopy (CD). While the largely alpha-helical conformation is relatively independent of the method of incorporation into vesicles, i.e., rehydration, detergent dialysis, or detergent dilution, the proteolipid conformation was significantly different in detergent micelles and in organic solvents. Only very slight changes in the CD spectrum were observed upon binding of the H+-ATPase inhibitor dicyclohexylcarbodiimide to the proteolipid in vesicles, thus suggesting that the inhibitor acts either by blocking the channel or by masking an essential charge group, rather by than causing an overall conformational change in the channel. Additionally, very similar CD spectra were obtained for vesicles with different lipid/protein mole ratios, indicating either that no substantial conformational differences exist between monomer and multimers or that monomers self-associate to form stable complexes during incorporation into vesicles. This study has provided a physical basis for model-building studies of the proteolipid channel structure.