ELECTRON-MICROSCOPY OF CYTOCHROME-C-OXIDASE CRYSTALS - LABELING OF SUBUNIT-III WITH A MONOMALEIMIDE UNDECAGOLD CLUSTER COMPOUND

Two-dimensional crystals of beef heart mitochondrial cytochrome c oxid ase dimers were labeled at Cys-115 of subunit III with a monomaleimide derivative of an undecagold cluster compound. The binding site of the gold cluster compound and hence the site of subunit III were identifi ed by image processing of cryoelectron micrographs of the crystals pre served in a mixture of glucose and uranyl acetate. The shape of the cy tochrome oxidase dimer can be approximated as a parallelogram which is 44 by 82 Angstrom with an included angle of 80 degrees oriented with its long dimension along the a axis of the crystal. Labeling of subuni t III was confirmed by a shift in the mobility of approximately 50% of subunit III molecules upon electrophoresis in polyacrylamide gels in the presence of sodium dodecyl sulfate. Averaged images of undecagold cluster labeled crystals and of unlabeled crystals were calculated; ea ch image represents an average of approximately 17 000 molecules of ei ther labeled or unlabeled cytochrome oxidase. On the basis of a statis tical analysis of the differences between the two images, the gold clu ster binds along a line 30 degrees from the a axis and 29 Angstrom fro m the center of the dimer. This result is Interpreted in the context o f other structural studies including the site of cytochrome c binding which Frey and Murray found to be near the a axis and 18 Angstrom from the center of the dimer [Frey, T. G., and Murray, J. M. (1994) J. Mel . Biol. 237, 275-297]. A structural model is proposed with subunit III polyheptides near the acrystal axis approximately 30 Angstrom from th e center of the aimer and the subunit I and II polypeptides (which bin d both hemes, a and a(3), and copper centers, CUA and CUB) near the ce nter of the dimer.