MgATP hydrolysis destabilizes the interaction between subunit H and yeast V 1 -ATPase, highlighting H's role in V-ATPase regulation by reversible disassembly.

Vacuolar H+ -ATPases (V-ATPases; V1 Vo -ATPases) are rotary-motor proton pumps that acidify intracellular compartments and, in some tissues, the extracellular space. V-ATPase is regulated by reversible disassembly into autoinhibited V1 -ATPase and Vo proton channel sectors. An important player in V-ATPase regulation is subunit H, which binds at the interface of V1 and Vo H is required for MgATPase activity in holo-V-ATPase but also for stabilizing the MgADP-inhibited state in membrane-detached V1 However, how H fulfills these two functions is poorly understood. To characterize the H-V1 interaction and its role in reversible disassembly, we determined binding affinities of full-length H and its N-terminal domain (HNT ) for an isolated heterodimer of subunits E and G (EG), the N-terminal domain of subunit a ( a NT ), and V1 lacking subunit H (V1 ΔH). Using isothermal titration calorimetry (ITC) and biolayer interferometry (BLI), we show that HNT binds EG with moderate affinity, that full-length H binds a NT weakly, and that both H and HNT bind V1 ΔH with high affinity. We also found that only one molecule of HNT binds V1 ΔH with high affinity, suggesting conformational asymmetry of the three EG heterodimers in V1 ΔH. Moreover, MgATP hydrolysis-driven conformational changes in V1 destabilized the interaction of H or HNT with V1 ΔH, suggesting an interplay between MgADP inhibition and subunit H. Our observation that H binding is affected by MgATP hydrolysis in V1 points to H's role in the mechanism of reversible disassembly.