Bovine factor B: Cloning, expression, and characterization

Bovine factor B, a polypeptide required for the coupled activity of the mitochondrial ATP synthase complex, was cloned. A novel expression system for overproducing the recombinant bovine factor B was developed, which yielded the recombinant polypeptide at a level of 12–15 mg of protein per liter of bacterial culture. Reconstitution of the recombinant polypeptide with factor B-depleted ammonia, EDTA-treated submitochondrial particles (AE-SMP) restored the formation of substrate-driven ΔpH gradient across vesicular membranes, presumably by blocking a proton leak. The proton leak in the AE-SMP could also be blocked by the F0 inhibitors oligomycin and dicyclohexylcarbodiimide, but not the F1-ATPase inhibitors efrapeptin and aurovertin B. The six factor B thiols titrated rapidly with Ellman’s reagent, and two of these, presumably Cys92 and Cys94, gained protection following treatment of factor B with a vicinal dithiol-specific reagent phenylarsine oxide (PAO). Similarly, Cd2+, whose binding to factor B is believed to also involve a vicinal dithiol, and PAO, protected ∼2 Cys residues against labeling with sulfhydryl-specific fluorescent reagent fluorescein-5′-maleimide. The circular dichroism spectra showed that binding of Cd2+ and Zn2+, but not Ca2+ to bovine factor B caused small but reproducible changes in the secondary structure elements of the polypeptide.