Integral polypeptide composition of Complex III of the mitochondrial electron-transfer chain

Phospholipids in isolated Complex III of the mitochondrial electron-transfer chain were depleted by hydrophobic chromatography. The complex was further purified by affinity chromatography. The polypeptide composition of the complex was examined using SDS-polyacrylamide gel electrophoresis. Ten polypeptides were demonstrated in the gel pattern of the complex containing more than 10% (w/w) phospholipids; and 9 polypeptides, in the pattern of the complex containing 5% phospholipids. Although the enzymic activity of the complex composed of the 9 polypeptides was about a half of that of the original enzyme, it was fully restored when soybean phospholipid mixture was added. Further depletion of phospholipids to 0.6% makes the iron-sulfur protein dissociable from the complex, resulting in a loss of the enzymic activity (Shimomura, Y. and Ozawa, T. (1982) Biochem. Int. 5, 1-6). These results suggest that Complex III consists of 9 polypeptides, and the smallest polypeptide is a contaminant embedded in phospholipids with respect to the electron-transfer capability of the complex.