Fig. 7. Functional assay of vacuole acidification by V-ATPase. (A) Quinacrine staining of acidified vacuoles for vma2 strains carrying wild-type Drosophila vha55 constructs in low-copy plasmids. Upper panels show Quinacrine-stained cells under epifluorescence; lower panels are phase-contrast images. A bright vacuole indicates functional acidification; a dark vacuole indicates inactivation of the V-ATPase. The vacuolar acidification phenotype is thus rescued both by VMA2, and by all constructs encoding vha55. (B) As A, except that yeast were carrying vha55::GFP plasmids with mutations corresponding to the alleles shown. Although the yeast can survive under these permissive conditions, none of the constructs rescue the acidification phenotype. (Although the vacuole in e.g. vha55^7e1 can appear less dark, this is due to an out-of-focus contribution from the cytoplasm; the contrast with functional vacuoles in the top panel is clear.)