Saccharomyces cerevisiae expression of exogenous vacuolar ATPase subunits B

The precise function of subunit B of the vacuolar H(+)-ATPase class is unknown, but it is essential for proton pumping. We have previously reported the DNA sequence and predicted protein sequence of the vacuolar ATPase subunit B for Candida tropicalis (Gu, H.H., Gallagher, M.J., Rupkey, S. and Dean, G.E. (1990) Nucleic Acids Res. 18, 7446). When the Candida gene was expressed in a Saccharomyce cerevisiae delta vat2 mutant from which the homologous gene had been deleted, viability and vacuolar acidification was restored to apparently wild-type levels. The predicted identity between these two proteins is 90%. We have searched for vacuolar ATPase subunits B from other species that might show a difference in function, when expressed in yeast, relative to the endogenous gene. We have cloned an apparently full-length 1.8-kb bovine subunit B cDNA from adrenal medulla that is about 1 kb shorter than the previously reported bovine brain cDNA (Puopolo, K., Kumamoto, C., Adachi, I., Magner, R. and Forgac, M. (1992) J. Biol. Chem. 267, 3696-3706; Nelson, R.D., Guo, X.L., Masood, K., Brown, D., Kalkbrenner, M. and Gluck, S. (1992) Proc. Natl. Acad. Sci. USA 89, 3541-3545), but nearly identical throughout the coding nucleotide and protein sequences; it is only 74% identical to the Saccharomyces subunit B protein sequence. Upon expression of this cDNA in two different delta vat2 deletion strains, the bovine cDNA restored function only partially, as judged by both viability at high pH and vacuolar acidification. Current work is aimed at determining which regions of the bovine protein require alteration in order to fully restore the delta vat2 strain to wild-type acidification, with the eventual goal of identifying interactive residues between subunit B and other proteins required for pump function.