A proton pump ATPase with testis-specific E1-subunit isoform required for acrosome acidification

The vacuolar-type H+-ATPases (V-ATPases) are a family of multimeric proton pumps involved in a wide variety of physiological processes. We have identified two novel mouse genes, Atp6e1 and Atp6e2, encoding testis-specific (E1) and ubiquitous (E2) V-ATPase subunit E isoforms, respectively. The E1 transcript appears about 3 weeks after birth, corresponding to the start of meiosis, and is expressed specifically in round spermatids in seminiferous tubules. Immunohistochemistry with isoform-specific antibodies revealed that the V-ATPase with E1 and a2 isoforms is located specifically in developing acrosomes of spermatids and acrosomes in mature sperm. In contrast, the E2 isoform was expressed in all tissues examined and present in the perinuclear compartments of spermatocytes. The E1 isoform exhibits 70% identity with the E2, and both isoforms functionally complemented a null mutation of the yeast counterpart VMA4, indicating that they are bona fide V-ATPase subunits. The chimeric enzymes showed slightly lower K-m(ATP) than yeast V-ATPase. Consistent with the temperature-sensitive growth of Deltavma4-expressing E1 isoform, vacuolar membrane vesicles exhibited temperature-sensitive coupling between ATP hydrolysis and proton transport. These results suggest that E1 isoform is essential for energy coupling involved in acidification of acrosome.