Aggregates of yeast mitochondrial cytochrome b observed after electrophoresis

Mitochondrial translation produces obtained from yeast cells labeled in vivo in the presence of cycloheximide were separated by dodecylsulfate polyacrylamide gel electrophoresis. The labeled band, with a molecular weight of 30,000 corresponding to cytochrome b, was excised and subsequently transferred to a second gel. After electrophoretic separation, two labeled polypetides with apparent molecular weights of 67,000 and 27,000 became visible in addition to the cytochrome b band of 30,000 molecular weight. Heating of the cytochrome b band prior to transfer resulted in an increase in the amount of the labeled polypeptides migrating with a molecular weight of 67,000. Longer exposure during autoradiography of the gels of mitochondrial translation products resulted in the appearance of a double band with an apparent molecular weight of 67,000. Limited proteolysis of this 67,000 dalton protein with Staphylococcus aureus V8 protease revealed a peptide map similar to that obtained after proteolysis of cytochrome b. These results suggest that the polypeptide with an apparent molecular weight of 67,000 represents an aggregate of cytochrome b that is either present as such in the membrane or is formed in vitro during the experimental manipulation to prepare mitochondria for gel electrophoresis.