| Abstract | Apocytochrome c, the heme-free precursor of cytochrome c, has been used extensively as a model to study molecular aspects of posttranslational translocation of proteins across membranes. In this report, we have used Fourier-transform infrared spectroscopy to gain further insight into the mechanism of apocytochrome c interation with membrane phospholipids. Association of apocytochrome c with model membranes containing acidic lipid dimyristoylphosphatidylglycerol (DMPG) as a single component results in a drastic perturbation of phospholipid structure, at the level of both the acyl chains and the interfacial carbonyl groups. However, in a binary mixture of DMPG with acyl chain perdeuterated dimyristoyl-phosphatidylcholine (DMPC-d₅₄), the perturbing effect of the protein on the acidic phospholipid is greatly attenuated. In such a membraine with mixed lipids, the physical properties of the DMPG and DMPC components are affected in a similar fashion, indicating that apocytochrome c does not induce any significant segregation or lateral-phase separation of acidic and zwitterionic lipids. Analysis of the apocytochrome c spectrum in the amide I region revealed that binding to phospholipids causes considerable changes in the secondary structure of the protein, the final conformation of which depends on the lipid to protein ration. In the presence of a large excess of DMPG apocytochrome c undergoes a transition from an essentially unordered conformation in solution to an α-helical structure. Howeer, in complexes of lower lipid to protein ratios (≤~40:1), infrared spectra ae indicative of an extended, intermolecularly hydrogen-bonded β-sheet structure. The latter is suggestive of an extensive aggregation of the membrane-associated protein. |
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