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The ATP-dependent proton pump V-ATPase ensures low intralysosomal pH, which is essential for lysosomal hydrolase activity. Based on studies with the V-ATPase inhibitor BafilomycinA1, lysosomal acidification is also thought to be required for fusion with incoming vesicles from the autophagic and endocytic pathways. Here we show that loss of V-ATPase subunits in the Drosophila fat body causes an accumulation of non-functional lysosomes, leading to a block in autophagic flux. However, V-ATPase-deficient lysosomes remain competent to fuse with autophagosomes and endosomes, resulting in a time-dependent formation of giant autolysosomes. In contrast, BafilomycinA1 prevents autophagosome-lysosome fusion in these cells, and this defect is phenocopied by depletion of the Ca2+ pump SERCA, a secondary target of this drug. Moreover, activation of SERCA promotes fusion in a BafilomycinA1-sensitive manner. Collectively, our results indicate that lysosomal acidification is not a prerequisite for fusion, and that BafilomycinA1 inhibits fusion independent of its effect on lysosomal pH.
Bibliographical noteFunding Information:
We thank Drs X. Huang and M. Boutros for the generous gifts of fly stocks. We are grateful to Drs Michael O’Connor and Aidan Peterson for access and support for confocal microscopy, and to the members of the Neufeld laboratory for helpful discussions. Stocks obtained from the Bloomington Drosophila Stock Center (NIH P40OD018537), Drosophila Genetic Resource Center (Kyoto, Japan) and Vienna Drosophila Resource Center were used in this study. This work was supported by funding from the Wellcome Trust (087518/Z/08/Z) to G.J. and from the National Institute of General Medical Sciences (RO1 GM62509) and a seed grant from the Norwegian Centennial Chair Program (UMF1175) to T.P.N.