Rab34 is necessary for early stages of intracellular ciliogenesis

Primary cilia are sensory organelles present on most vertebrate cells and are critical for development and health. Ciliary dysfunction is associated with a large class of human pathologies collectively known as ciliopathies. These include cystic kidneys, blindness, obesity, skeletal malformations, and other organ anomalies. Using a proximity biotinylation with Ift27 as bait, we identified the small guanosine triphosphatase (GTPase) Rab34 as a ciliary protein. Rab34 localizes to the centrosomes near the mother centriole, the axoneme of developed cilia, and highly dynamic tubule structures in the centrosomal region. Rab34 is required for cilia formation in fibroblasts, where we find that Rab34 loss blocks ciliogenesis at an early step of ciliary vesicle formation. In inner medullary collecting duct (IMCD3) epithelial cells, the requirement is more complex, with Rab34 needed in cells grown at low density but becoming less important as cell density increases. Ciliogenesis can proceed by an internal pathway where cilia form in the cytoplasm before being displayed on the ciliary surface or cilia can assemble by an external pathway where the centriole docks on the plasma membrane before ciliary assembly. Fibroblasts are thought to use the internal pathway, although IMCD3 cells are thought to use the external pathway. However, we find that IMCD3 cells can use the internal assembly pathway and significant numbers of internally assembling cilia are observed in low-density cells. Together, our work indicates that Rab34 is required for internal assembly of cilia, but not for cilia built on the cell surface.