Structural model of the transmembrane F o rotary sector of H +-transporting ATP synthase derived by solution NMR and intersubunit cross-linking in situ

Abstract

H +-transporting, F 1F o-type ATP synthases utilize a transmembrane H + potential to drive ATP formation by a rotary catalytic mechanism. ATP is formed in alternating β subunits of the extramembranous F 1 sector of the enzyme, synthesis being driven by rotation of the γ subunit in the center of the F 1 molecule between the alternating catalytic sites . The H + electrochemical potential is thought to drive γ subunit rotation by first coupling H + transport to rotation of an oligomeric rotor of c subunits within the transmembrane F o sector. The γ subunit is forced to turn with the c-oligomeric rotor due to connections between subunit c and the γ and ε subunits of F 1. In this essay we will review recent studies on the Escherichia coli F o sector. The monomeric structure of subunit c, determined by NMR, shows that subunit c folds in a helical hairpin with the proton carrying Asp 61 centered in the second transmembrane helix (TMH). A model for the structural organization of the c 10 oligomer in F o was deduced from extensive cross-linking studies and by molecular modeling. The model indicates that the H +-carrying carboxyl of subunit c is occluded between neighboring subunits of the c 10 oligomer and that two c subunits pack in a “front-to-back” manner to form the H + (cation) binding site. In order for protons to gain access to Asp 61 during the protonation/deprotonation cycle, we propose that the outer, Asp 61-bearing TMH-2s of the c-ring and TMHs from subunits composing the inlet and outlet channels must turn relative to each other, and that the swiveling motion associated with Asp 61 protonation/deprotonation drives the rotation of the c-ring. The NMR structures of wild-type subunit c differs according to the protonation state of Asp 61. The idea that the conformational state of subunit c changes during the catalytic cycle is supported by the cross-linking evidence in situ, and two recent NMR structures of functional mutant proteins in which critical residues have been switched between TMH-1 and TMH-2. The structural information is considered in the context of the possible mechanism of rotary movement of the c 10 oligomer during coupled synthesis of ATP.