Role of Coupling Factor B in the Fo Proton Channel

Abstract

The electrochemical proton gradient generated during the oxidations in the respiratory chain is known to be utilized for the synthesis of ATP from ADP and inorganic phosphate by the ATP synthase (H+- ATPase of Fo — F1. In this process, H+ reenters the mitochondria through a specific proton channel located in the membrane-embedded Fo segment. The proton channel of E. coli Fo contains three proteins, all of which are necessary for H+ conduction, but the mitochondrial Fo is more complex and contains at least six subunits: subunit 6, subunit 9, URFAGL gene product, OSCP (oligomycin sensitivity conferring protein), F6 (coupling factor 6), and FB (coupling factor B). F6 and OSCP are believed to be involved in the binding of F1 to Fo in the correct manner to render the ATPase activity sensitive to oligomycin (1), and indirect evidence has implicated FB in the H+ channel (2). Three properties of FB, a 14 kdalton dithiol protein, have been useful in identifying its role in the ATP synthase reaction. They are a) resolution of FB from submitochondrial particles concomitant with loss of activity in energy-linked reactions and reactivation by reconstitution of FB into the particle (3, 4), b) specific inhibition of FB, activity by Cd2+ in the presence of excess monothiol which restricts binding of Cd2+ to a dithiol site (5, 6) and c) retention of Cd2+ on Fb during SDS-PAGE (6, 7).