Since the pioneering work of Peter Mitchell 1, the central role of proton gradients in biological energy transduc- tion has been widely ackrLowledged. The enzymes directly involved in generating and harnessing the energy of proton gradients (H+-ATPases), are found in nearly all cells and presumably appeared very early in cell evolution 2. It has been suggested that they orig- inally functioned as a pH-stat mech- anism, disposing of the excess protons generated by fermentative metab- olism 3'4. The antiquity of the H +- ATPases makes them an attractive family of enzymes for evolutionary studies, particularly since functional constraints have preserved a high de- gree of sequence similarity across king- doms. Indeed, analyses of eubacterial H+-ATPase sequences l'lave pro- vided strong support for the endosym- biotic origin of mitochondria and chloroplasts 5. Recently, a new class of proton ATPases has been identified, the vacuolar ATPase (V-ATPase), which is specifically associated with the endo- membrane system of eukaryotes 6-1~. Comparisons of the structure and properties of the V-ATPases with those of prokaryotic plasma membrane H +- ATPases are now providing unexpec- ted insights into the possible origin of the eukaryotic host cell 1~ (Gogarten, J. P., Kibak, H., Taiz, L., Bowman, E. J., Bowman, B. J., Manolson, M. F., Poole, R. J., Denda, K., Oshima, T., Konishi, J., Date, T. and Yoshida, M., submitted). Classes of proton ATPases Proton ATPases have been divided into three major families: F', F and V (Ref. 12) (Table I). P-ATPases (or El-E2 type) operate via a phospho- enzyme intermediate, are irLhibited by vanadate, and have an H+/ATP stoi- chiometry of one 2'6. P-ATPases encompass a variety of cation-specific pumps, such as the Na+/K+-ATPase of animal plasma membranes, the Ca 2+- ATPase of the sarcoplasmic reticulum,