The cytochrome bc1 complex of yeast mitochondria. Isolation and partial characterization of the cytochrome bc1 complex and cytochrome b.

We have isolated the cytochrome bc1 complex and some of its constituent polypeptides from bakers yeast and have studied its spectroscopy, electrophoresis and amino acid analysis. The isolated complex contained 6 mumol of b heme and approximately 3 mumol of c1 heme per g of protein. The electron paramagnetic resonance spectrum was similar to that of the beef-heart preparation. The complex consisted of 7 polypeptides with mobilities on sodium dodecylsulphate polyacrylamide gel electrophoresis corresponding to Mr 44,000, 40,000, 32,000, 32,000, 17,000, 14,000 and 11,000. One of the polypeptides with Mr 32,000 was identified on sodium dodecylsulphate gels as cytochrome c1 by porphyrin fluorescence. Cytochrome b was isolated from the complex by treating it with guanidine hydrochloride; it had a purity of 20 mumol per g of protein and consisted of a polypeptide with Mr 32,000 plus two minor bands with Mr 14,000 and 11,000. We have isolated the polypeptide of Mr 32,000 from cytochrome b and the polypeptides of Mr 44,000 and 40,000 ("core proteins") from the complex, both by preparative sodium dodecylsulphate gel electrophoresis and determined their amino acid composition. Only the b polypeptide of Mr 32,000 shows the low proportion of polar amino acid residues that is considered typical of membrane proteins.