NMR Solution Structure of Subunit F of the Methanogenic A<sub>1</sub>A<sub>O</sub> Adenosine Triphosphate Synthase and Its Interaction with the Nucleotide-Binding Subunit B<sup>†</sup><sup>,</sup><sup>‡</sup>

The A<sub>1</sub>A<sub>O</sub> adenosine triphosphate (ATP) synthase from archaea uses the ion gradients generated across the membrane sector (A<sub>O</sub>) to synthesize ATP in the A<sub>3</sub>B<sub>3</sub> domain of the A<sub>1</sub> sector. The energy coupling between the two active domains occurs via the so-called stalk part(s), to which the 12 kDa subunit F does belong. Here, we present the solution structure of the F subunit of the A<sub>1</sub>A<sub>O</sub> ATP synthase from <i>Methanosarcina mazei</i> Gö1. Subunit F exhibits a distinct two-domain structure, with the N-terminal having 78 residues and residues 79−101 forming the flexible C-terminal part. The well-ordered N-terminal domain is composed of a four-stranded parallel β-sheet structure and three α-helices placed alternately. The two domains are loosely associated with more flexibility relative to each other. The flexibility of the C-terminal domain is further confirmed by dynamics studies. In addition, the affinity of binding of mutant subunit F, with a substitution of Trp100 against Tyr and Ile at the very C-terminal end, to the nucleotide-binding subunit B was determined quantitatively using the fluorescence signals of natural subunit B (Trp430). Finally, the arrangement of subunit F within the complex is presented.