Complementary DNA encoding core protein II of human mitochondrial cytochrome bc1 complex. Substantial diversity in...

Core protein II of mitochondrial cytochrome bc1 complex is the second largest nuclear-encoded subunit that is essential for the assembly of the functional complex. We have isolated, for the first time, a cDNA clone for a mammalian core protein II. Immunoscreening of a lambda gt11 rat liver cDNA library resulted in isolation of a cDNA encoding a partial sequence of rat mitochondrial core protein II, and this cloned cDNA was used as a probe for colony hybridization to screen a human fibroblast cDNA library. By comparison with the known NH2-terminal amino acid sequence of bovine core protein II, the amino acid sequence deduced from a cloned human cDNA was predicted to be composed of the part for the NH2-terminal extension of 14 amino acid residues and that for the mature polypeptide of 439 residues. The presequence is typical of precursors to mitochondrial proteins in being composed of a combination neutral and basic residues. The deduced sequence shows rather low homology with that of the corresponding subunit of yeast cytochrome bc1 complex; this finding is in contrast with the relatively high homology observed for the sequences of the redox center-carrying subunits of the same complex. On Northern blot analysis of human fibroblast poly(A)+ RNA, the cDNA hybridized to two mRNAs, a major species of 1700 nucleotides and a minor one of 2000 nucleotides. Southern blot hybridization of genomic DNA suggested that the human genome contains one or two genes for core protein II.