HOPS-dependent lysosomal fusion controls Rab19 availability for ciliogenesis in polarized epithelial cells

Primary cilia are sensory cellular organelles crucial for organ development and homeostasis. Ciliogenesis in polarized epithelial cells requires Rab19-mediated clearing of apical cortical actin to allow the cilium to grow from the apically docked basal body into the extracellular space. Loss of the lysosomal membrane -tethering homotypic fusion and protein sorting (HOPS) complex disrupts this actin clearing and ciliogenesis, but it remains unclear how the ciliary function of HOPS relates to its canonical function in regulating late endosome-lysosome fusion. Here, we show that disruption of HOPSdependent lysosomal fusion indirectly impairs actin clearing and ciliogenesis by disrupting the targeting of Rab19 to the basal body, and that this effect is specific to polarized epithelial cells. We also find that Rab19 functions in endolysosomal cargo trafficking in addition to having its previously identified role in ciliogenesis. In summary, we show that inhibition of lysosomal fusion leads to the abnormal accumulation of Rab19 on late endosomes, thus depleting Rab19 from the basal body and thereby disrupting Rab19-mediated actin clearing and ciliogenesis in polarized epithelial cells.