The Tether Connecting Cytosolic (N Terminus) and Membrane (C Terminus) Domains of Yeast V-ATPase Subunit a (Vph1) Is Required for Assembly of V0 Subunit d

V-ATPases are molecular motors that reversibly disassemble in vivo. Anchored in the membrane is subunit a. Subunit a has a movable N terminus that switches positions during disassembly and reassembly. Deletions were made at residues securing the N terminus of subunit a (yeast isoform Vph1) to its membrane-bound C-terminal domain in order to understand the role of this conserved region for V-ATPase function. Shrinking of the tether made cells pH-sensitive (vma phenotype) because assembly of V-0 subunit d was harmed. Subunit d did not co-immunoprecipitate with subunit a and the c-ring. Cells contained pools of V-1 and V-0(-d) that failed to form V1V0, and very low levels of V-ATPase subunits were found at the membrane. Although subunit d expression was stable and at wild-type levels, growth defects were rescued by exogenous VMA6 (subunit d). Stable V1V0 assembled after yeast cells were co-transformed with VMA6 and mutant VPH1. Tether-less V1V0 was delivered to the vacuole and active. It retained 63-71% of the wild-type activity and was responsive to glucose. Tether-less V1V0 disassembled and reassembled after brief glucose depletion and readdition. The N terminus retained binding to V-1 subunits and the C terminus to phosphofructokinase. Thus, no major structural change was generated at the N and C termini of subunit a. We concluded that early steps of V-0 assembly and trafficking were likely impaired by shorter tethers and rescued by VMA6.