INTERACTION OF INTEGRAL AND PERIPHERAL MEMBRANE PROTEINS - AFFINITY LABELING OF YEAST CYTOCHROME-OXIDASE BY MODIFIED YEAST CYTOCHROME-C

To identify possible substrate-binding subunit(s) of yeast [Saccharomyces cerevisiae] cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1), the purified enzyme was reacted with yeast iso-1-cytochrome c whose single free sulfhydryl group at position 107 was activated with 5,5''-dithiobis(2-nitrobenzoate). The resulting cytochrome c derivative appeared to function as an affinity-label of cytochrome oxidase, since it rapidly inactivated the enzyme. Inactivation was competitively prevented by underivatized cytochrome c. When the affinity-labeled oxidase was analyzed by 2-dimensional polyacrylamide electrophoresis in dodecyl sulfate (separation in the second dimension being carried out in the presence of excess sulfhydryl compound), the derivatized cytochrome c had specifically formed a mixed disulfide with the mitochondrially made subunit III (apparent MW 24,000) of the oxidase. Similar results were obtained when underivatized iso-1-cytochrome c was crosslinked to the oxidase by oxidative disulfide bridge formation in the presence of o-phenanthroline and Cu++. The hydrophobic mitochondrially made subunit III of yeast cytochrome c oxidase is in close proximity to a cytochrome c binding site on the enzyme. Since cytochrome c and the mitochondrially made cytochrome oxidase subunit III are typical peripheral and integral membrane proteins, respectively, the present study suggests a useful approach for analyzing specific interactions between these different classes of membrane proteins.