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Thus, in each individual cell type, the V-ATPase functions in a variety of distinct cellular processes. The V-ATPase is a complex enzyme that is composed of many subunits and is divided into two distinct sectors. The V0 sector is responsible for proton translocation and is composed of the transmembrane subunits previously designated by the lowercase letters a, c, c', c'', and d. The V1 sector forms a large cytosolic domain composed of eight subunits, designated by the uppercase letters A-H in a stoichiometry of A3B3CDEFG2H and is anchored to the plasma membrane via its interaction with the V0 domain. In view of the complexity of this enzyme, a new nomenclature for each of its subunits has been recently presented and approved by both the Human and the Mouse Gene Nomenclature Committees. This revised nomenclature is shown in Table 1. For simplicity, the abbreviated, single-letter subunit identification will be used throughout the text. The precise function of many of the V-ATPase subunits remains unknown, although subunit deletion studies, mainly in yeast, have described a variety of effects on assembly and function of the holoenzyme.