The atypical small GTPase RABL3 interacts with RAB11 to regulate early ciliogenesis in human cells.

Primary cilia are near-ubiquitously assembled on cells in the human body, and are broadly associated with genetic diseases and cancers. In the early stage of ciliogenesis, the ciliary vesicle (CV) is formed on the mother centriole, which nucleates the primary cilium. However, the regulatory mechanisms underlying CV formation have not yet been fully elucidated. Here, we found that the atypical small GTPase RAB-like 3 (RABL3) is necessary to assemble primary cilia in human cells. RABL3 directly interacts with RAB11 (herein referring to both RAB11A and RAB11B), which is involved in CV formation. RABL3 localizes around the centrosome during early ciliogenesis, reminiscent of RAB11 dynamics. Furthermore, RABL3 positively controls the CV formation like RAB11. These findings suggest that RABL3 plays an important role, in cooperation with RAB11, in CV formation during early ciliogenesis.

RABL2 positively controls localization of GPCRs in mammalian primary cilia.

The primary cilium, a solitary protrusion from most mammalian cells, functions as a cell sensor by receiving extracellular signals through receptors and channels accumulated in the organelle. Certain G-protein-coupled receptors (GPCRs) specifically localize to the membrane compartment of primary cilia. To gain insight into the mechanisms that regulate ciliary GPCR sorting, we investigated the atypical small GTPase RAB-like 2 (RABL2; herein referring to the near-identical human paralogs RABL2A and RABL2B). RABL2 recruitment to the mother centriole is dependent on the distal appendage proteins CEP164 and CEP83. We found that silencing of RABL2 causes mis-targeting of ciliary GPCRs, GPR161 and HTR6, whereas overexpression of RABL2 resulted in accumulation of these receptors in the organelle. Ablation of CEP19 and the intraflagellar transport B (IFT-B) complex, which interact with RABL2, also leads to mis-localization of GPR161. RABL2 controls localization of GPR161 independently of TULP3, which promotes entry of ciliary GPCRs. We further demonstrate that RABL2 physically associates with ciliary GPCRs. Taken together, these studies suggest that RABL2 plays an important role in trafficking of ciliary GPCRs at the ciliary base in mammalian cells.