SOLUTION STRUCTURE OF THE TRANSMEMBRANE H-TRANSPORTING SUBUNIT-C OF THE F1F0 ATP SYNTHASE()

Subunit c is the H+-translocating component of the F1F0 ATP synthase c omplex. H+ transport is coupled to conformational changes that ultimat ely lead to ATP synthesis by the enzyme. The properties of the monomer ic subunit in a single phase solution of chloroform-methanol - water ( 4:4:1) have been shown to mimic those of the protein in the native com plex. Triple resonance NMR experiments were used to determine the comp lete structure of monomeric subunit c in this solvent mixture. The str ucture of the protein was defined by >2000 interproton distances, 64 ( 3)J(N alpha), and 43 hydrogen-bonding NMR-derived restraints. The root mean squared deviation for the backbone atoms of the two transmembran e helices was 0.63 Angstrom. The protein folds as a hairpin of two ant iparallel helical segments, connected by a short structured loop. The conserved Arg41-Gln42-Pro43 form the top of this loop. The essential H +-transporting Asp61 residue is located at a slight break in the middl e of the C-terminal helix, just prior to Pro64. The C-terminal helix c hanges direction by 30 +/- 5 degrees at the conserved Pro64. In its pr otonated form, the Asp61 lies in a cavity created by the absence of si de chains at Gly23 and Gly27 in the N-terminal helix. The shape and ch arge distribution of the molecular surface of the monomeric protein su ggest a packing arrangement for the oligomeric protein in the F-o comp lex, with the front face of one monomer packing favorably against the back face of a second monomer. The packing suggests that the proton (c ation) binding site lies between packed pairs of adjacent subunit c.