Coupling factor B is a component of the Fo proton channel of mitochondrial H+-ATPase.

Repeated extraction of bovine heart submitochondrial particles with ammonia and EDTA (AE) yields a preparation that is highly deficient in coupling factor B (FB). The activity of the thrice extracted particle (AE-P3) in ATP-driven NAD+ reduction by succinate and the 32Pi-ATP exchange activity were substantially stimulated, 8-fold and 5-fold, respectively, by purified FB. To decrease the basal activity of the particle further, the residual FB in AE-P3 was inactivated by treatment with the -SH reagent, 4-vinylpyridine. The resulting particle was depleted of F1 by treatment with 3.5 M NaBr. This particle was incorporated into asolectin liposomes alone and in the presence of added FB. Passive proton conduction in the FB-deficient proteoliposomes was negligible and restored in the presence of FB. The H+ conductance was inhibited extensively by oligomycin and partially by F1-ATPase. The results show absolute dependence on FB for functioning of the FO proton channel.