Drosophila Ar18 is a general positive regulator of lysosomal fusion events

The small GTPase Ar18 is known to be involved in the periphery-directed motility of lysosomes. However, the overall importance of moving these vesicles is still poorly understood. Here we show that Drosophila Ar18 is required not only for the proper distribution of lysosomes, but also for autophagosome-lysosome fusion in starved fat cells, endosome-lysosome fusion in garland nephrocytes, and developmentally programmed secretory granule degradation (crinophagy) in salivary gland cells. Moreover, proper Ar18 localization to lysosomes depends on the shared subunits of the BLOC-1 and BORC complexes, which also promote autophagy and crinophagy. In conclusion, we demonstrate that Ar18 is responsible not only for positioning lysosomes but also acts as a general lysosomal fusion factor.