Vacuolar H+-ATPase (V-ATPase) is a ubiquitous multi-subunit proton pump that acidifies a wide variety of intracellular compartments, which in turn affects many biological processes, including membrane trafficking, protein degradation and coupled transport of small molecules and pH homeostasis. Subunit 'a' of V0 (the functional domain responsible for proton transport) sector is highly conserved across eukaryotic species and exists in multiple isoforms. It is the largest subunit of V-ATPases and partitioned almost equally into an N-terminal cytosolic domain and a C-terminal integral membrane. Structure analysis of the N-terminal cytosolic domain from the Meiothermus ruber subunit 'I' homolog of subunit a shows that it is composed of a curved long central alpha-helix bundle capped on both ends by two lobes with similar alpha/beta architecture [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