STV1 gene encodes functional homologue of 95-kDa yeast vacuolar H⁺-ATPase subunit Vph1p

The Saccharomyces cerevisiae gene, VPH1 (Vacuolar pH 1), encodes a 95-kDa integral membrane subunit of the vacuolar-type H⁺-ATPase (V-ATPase) that is required for enzyme assembly; disruption of the VPH1 gene impairs vacuolar acidification (Manolson, M. F., Proteau, D., Preston, R. A., Stenbit, A., Roberts, B. T., Hoyt, M. A., Preuss, D., Mulholland, J., Botstein, D., and Jones, E. W. (1992) J. Biol. Chem. 267, 14294-14303). Here we show that STV1 (Similar To VPH1) encodes an integral membrane polypeptide of 102 kDa with 54% identity with the peptide sequence of Vph1p. High copy expression of STV1 partially restores vacuolar acidification in a \vphl mutant strain; solubilization and fractionation of membrane proteins from these vacuoles show that Stv1p co-purifies with bafilomycin A₁-sensitive ATPase activity and with the 60- and 69-kDa V-ATPase subunits. Immunofluorescence microscopy of strains bearing a single copy of epitope-tagged STV1 reveals punctate staining of the cytoplasm; overexpression of epitope-tagged Stv1p reveals both punctate cytoplasmic staining and vacuolar membrane staining. Northern analysis shows that disruption of STV1 does not affect the level of transcription of VPH1 and that disruption of VPH1 does not affect the level of transcription of STV1. Strains bearing disruption of genes encoding other V-ATPase subunits (VMA1, VMA2, VMA3, and VMA4) fail to grow on media supplemented with 100 mM CaCl₂ or 4 mM ZnCl₂, media buffered to pH 7.5, or media with a glycerol carbon source. On the same types of media only a Δvph1 Δstv1 double disruption mutant has growth phenotypes equivalent to strains bearing a single disruption of the VMA1, VMA2, VMA3, and VMA4 genes; a Δvph1 strain has only moderate growth inhibition while a Δstv1 strain has wild type growth on the conditions listed above. We conclude that Stv1p is a functional homologue of Vphlp and suggest that Stv1p and Vphlp may be equivalent subunits for V-ATPases located on different organelles. The function of these 100-kDa homologues may be to target or regulate other common V-ATPase subunits for two distinct cellular locations.