Abstract: Four proteases have been used to assess the topology of the H+-ATPase from Saccharomyces cerevisiae reconstituted into phosphatidylserine vesicles. Limited proteolysis by trypsin and alpha-chymotrypsin inactivates the enzyme and produces stable, membrane-bound fragments. Sequence analyses of these peptides have located the peptide bonds hydrolyzed. The labile bonds are on opposite sides of a central hydrophilic domain containing consensus sequences for the site of phosphorylation and fluorescein isothiocyanate binding of several related ATPases. Limited proteolysis of the ATPase by elastase cuts approximately 50 amino acids from the C terminus, leaving the remaining membrane-bound fragments active. Proteolysis by carboxypeptidase Y in the presence and absence of detergent suggests that the C terminus is on the inside of the vesicle in this reconstitution. A model for the transmembrane arrangement of the polypeptide is proposed. In this model, the C terminus is on the inside of the vesicle, the N terminus is on the outside, the ATP binding region is on the outside, and the polypeptide passes through the membrane a minimum of five times.