Analyzing import intermediates of mitochondrial proteins by blue native gel electrophoresis

Formation of the redox cofactor centers during Cox1 maturation in yeast cytochrome oxidase

The biogenesis of cytochrome c oxidase initiates with synthesis and maturation of the mitochondrion-encoded Cox1 subunit prior to the addition of other subunits. Cox1 contains redox cofactors, including the low-spin heme a center and the heterobimetallic heme a(3):Cu(B) center. We sought to identify the step in the maturation of Cox1 in which the redox cofactor centers are assembled. Newly synthesized Cox1 is incorporated within one early assembly intermediate containing Mss51 in Saccharomyces cerevisiae. Subsequent Cox1 maturation involves the progression to downstream assembly intermediates involving Coa1 and Shy1. We show that the two heme a cofactor sites in Cox1 form downstream of Mss51- and Coa1-containing Cox1 intermediates. These Cox1 intermediates form normally in cells defective in heme a biosynthesis or in cox1 mutant strains with heme a axial His mutations. In contrast, the Shy1-containing Cox1 assembly intermediate is perturbed in the absence of heme a. Heme a(3) center formation in Cox1 appears to be chaperoned by Shy1. Cu(B) site formation occurs near or at the Shy1-containing Cox1 assembly intermediate also. The Cu(B) metallochaperone Cox11 transiently interacts with Shy1 by coimmunoprecipitation. The Shy1-containing Cox1 complex is markedly attenuated in cells lacking Cox11 but is partially restored with a nonfunctional Cox11 mutant. Thus, formation of the heterobimetallic Cu(B):heme a(3) site likely occurs in the Shy1-containing Cox1 complex.