Interaction of Spin-Labeled Apocytochrome c and Spin-Labeled Cytochrome c with Negatively Charged Lipids Studied by Electron Spin Resonance

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Abstract
Apocytochrome c has been spin-labeled with a nitroxide derivative of maleimide on a cysteine residue at either position 14 or position 17 in the N-terminus. Yeast cytochrome c was spin-labeled with the same maleimide derivative on its single free cysteine residue at position 102 in the C-terminus. The ESR spectra of spin-labeled apocytochrome c have been characterized in different environments with respect both to the conformation of the protein and to its association with lipid. In buffer, the spectrum of spin-labeled apocytochrome c indicates high mobility, characteristic of the unfolded structure of the apoprotein, and that of spin-labeled cytochrome c is only slightly less mobile, suggesting that the site labeled is situated at the surface of the folded holoprotein. Upon binding the spin-labeled protein to negatively charged lipid membranes composed of dioleoylphosphatidylglycerol (DOPG), the ESR spectra of apocytochrome c evidence a large reduction in the mobility of the spin-label group, as also do those of yeast cytochrome c. In the case of apocytochrome c, this immobilization most likely arises from both an increase in secondary structure and a partial penetration of the protein into the lipid bilayer, in addition to the electrostatic interaction with the lipid headgroups, whereas for cytochrome c the immobilization observed arises primarily from an intimate association with the membrane surface. When the spin-labeled holocytochrome c is denatured by heating and is bound to DOPG bilayer membranes, a rather mobile ESR spectrum is observed, which demonstrates that the spin-label is located at the surface of the membrane in this case. The ESR spectra of spin-labeled apocytochrome c bound to mixed bilayers of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine (DMPC) consist of both an immobile and a mobile component. The proportion of the mobile component is increased by increasing the mole fraction of the zwitterionic DMPC in the mixed bilayers. The mobile component represents a localization of apocytochrome c at the membrane surface, whereas the immobile component most probably represents the penetration of the precursor protein into the membrane interior. The immobile component assigned to membrane penetration of the precursor protein is still present at negatively charged lipid contents comparable to those in the native mitochondrial system. The results are discussed in relation to the conformation of apocytochrome c, its interaction with lipid, and the import of the apoprotein into mitochondria.

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