Vacuolar-type H+-ATPase (V-ATPase) is a multisubunit enzyme responsible for the acidification of eukaryotic intracellular organelles. V-ATPases pump protons against an electrochemical gradient, while F-ATPases reverse the process, thereby synthesizing ATP. A peripheral V1 domain, which is responsible for ATP hydrolysis, and an integral V0 domain, which is responsible for proton translocation, comprise the V-ATPase complex. Nine subunits (A–H) make up the V1 domain and five subunits (A, D, C, C' and C") make up the V0 domain. V-ATPase D1 (ATPase, H+ transporting, lysosomal, V0 subunit D1), also known as ATP6V0D1, P39, VATX, VMA6, ATP6D or VPATPD, is the D subunit of the V0 domain. Expressed ubiquitously, V-ATPase D1 acts in concert with other V0 subunits to catalytically acidify a variety of intracellular compartments, thereby synthesizing ATP to be used for vacuolar transport.