3.A.2.2.3
H⁺-translocating V-type ATPase. The d-subunit couples ATP hydrolysis to H⁺ transport (Owegi et al., 2006).  The a subunit (Vph1p) most likely has an 8 TMS topology (Kartner et al. 2013; Knight and Behm 2012).  Structures for three rotational states show ten proteolipid subunits in the c-ring, setting the ATP:H⁺ ratio for proton pumping. Long tilted TMSs in the a-subunit interact with the c-ring.  V-ATPase's membrane sector, Vo, has been implicated in functions including membrane fusion and neurotransmitter release. Couoh-Cardel et al. 2016 reported that the purified V-ATPase c subunit ring (c-ring) forms dimers mediated by the c subunits' cytoplasmic loops. Electrophysiological measurements of the c-ring reconstituted into a planar lipid bilayer revealed a large unitary conductance of ~8.3 nS.  A role of the c-ring in membrane fusion and neuronal communication was suggested (Couoh-Cardel et al. 2016).
   The three different maps reveal the conformational changes that occur to couple rotation in the symmetry-mismatched soluble catalytic region to the membrane-bound proton-translocating region. Almost all of the subunits undergo conformational changes during the transitions between these three rotational states (Zhao et al. 2015). The structures of these states provide direct evidence that deformation during rotation enables the smooth transmission of power through rotary ATPases. Increases in glucose stimulate V-ATPase assembly and activity while glucose deprivation triggers rapid V-ATPase disassembly and inactivation in yeast. However, the opposite phenomenon is observed in mammalian cells, specifically that V-ATPase assembly and activation increases when glucose is lost (Parra and Hayek 2018).
   The 3.5 Å resolution cryoEM structure of the lipid nanodisc-reconstituted Vo proton channel, captured in a physiologically relevant autoinhibited state, revealing the residues that constitute the proton pathway at the interface of the proteolipid ring and subunit a has been solved (Roh et al. 2018). The chemical basis of transmembrane proton transport was proposed. Roh et al. 2018 discovered that the C terminus of the assembly factor Voa1 (TC# 9.B.206.2.1)) is an integral component of the mature Vo. Voa1's C-terminal transmembrane alpha helix is bound inside the proteolipid ring, where it contributes to the stability of the complex. The structure rationalizes possible mechanisms by which mutations in human Vo can result in disease phenotypes (Roh et al. 2018). The cytosolic N-terminal domain of the V-ATPase a-subunits is a regulatory hub targeted by multiple signals (Tuli and Kane 2023).     An arginine residue (Arg-735) in transmembrane helix 7 (TMS7) of subunit a of the yeast ATPase is known to be essential for proton translocation. Arginine residues are usually assumed to 'snorkel' toward the protein surface when exposed to a hydrophobic environment. However, Hohlweg et al. 2018 obtained evidence for the formation of a transient, membrane-embedded cation-π interaction in TM7 between Arg-735 and two highly conserved nearby aromatic residues, Tyr-733 and Trp-737. They proposed a mechanism by which the transient, membrane-embedded cation-π complex provides the necessary energy to keep the charged side chain of Arg-735 within the hydrophobic membrane. Such cation-π interactions may define a general mechanism to retain charged amino acids in a hydrophobic membrane environment (Hohlweg et al. 2018). A  2.7-A cryo-EM structure of the yeast Vo proton channel revealed the location of ordered water molecules along the proton path, details of specific protein-lipid interactions, and the architecture of the membrane scaffold protein (Roh et al. 2020) as well as a state of Vo showing the c-ring rotated by ~14 degrees. Two rotary states are in thermal equilibrium and depict how the protonation state of essential glutamic acid residues couples water-mediated proton transfer with c-ring rotation. Resuts suggest a mechanism for inhibition of passive proton transport as observed for free Vo that is generated as a result of V-ATPase regulation by reversible disassembly in vivo (Roh et al. 2020). Vacuolar H⁺-ATPase dysfunction rescues intralumenal vesicle cargo sorting in yeast lacking PI(3,5)P2 or Doa4 (Wilson et al. 2021). The V-ATPase a-subunit is a two-domain protein containing a C-terminal transmembrane domain responsible for proton transport and an N-terminal cytosolic domain that is a regulatory hub, integrating environmental inputs to regulate assembly, localization, and V-ATPase activity. S. cerevisiae encodes two organelle-specific a-isoforms, Stv1 in the Golgi and Vph1 in the vacuole (Tuli and Kane 2023). The significance of the plasma membrane H⁺-ATPase and V-ATPase for growth and pathogenicity in pathogenic fungi has been discussed (Yang and Peng 2023).