This domain is found in the cytoplasmic N-terminal domain of vacuolar ATP synthase subunit I present in Meiothermus ruber. Subunit I is a homolog of subunit A which associates with the membrane-bound complex of eukaryotic vacuolar H+-ATPase (V-ATPase) acidification machinery. The domain forms the proximal lobe that caps one end of the alpha helix bundle, with the distal lobe capping the other end. Although the two lobes exhibit a similar motif, the molecular nature of the coupling with the identical stalks is thought to be dissimilar [1].

1: Crystal structure of the cytoplasmic N-terminal domain of subunit I, a homolog of subunit a, of V-ATPase. Srinivasan S, Vyas NK, Baker ML, Quiocho FA; J Mol Biol. 2011;412:14-21. PMID:21787787