However, we could not exclude the possibility that CEP19/RABL2B could reside within a compartment in the most proximal portion of the distal appendage (Fig. as proteins organizing the first known mechanism directing ciliary access of IFT complexes. We discover that CEP19 is usually recruited to the ciliary base by the centriolar CEP350/FOP complex, and then specifically captures GTP-bound RABL2B, which is usually activated via its intrinsic nucleotide exchange. Activated RABL2B then captures and releases its single effector, the intraflagellar transport B holocomplex, from your large pool of pre-docked IFT-B complexes and thus initiates ciliary access of IFT. eTOC Blurb Intraflagellar transport protein (IFT) complexes accumulate at the ciliary base and periodically enter the cilium via an unknown mechanism. Kanie et al. show that CEP19 recruits the RABL2B GTPase complex to the ciliary base where it mediates release of IFT-B from pre-docked complexes to initiate IFT ciliary access. Introduction Cilia and flagella are evolutionarily conserved organelles with important motile and sensory functions. The primary, or immotile, cilium is found on almost all cell types in the human body, and senses extracellular signals including hormones and morphogens (Ishikawa and Marshall, 2011). Defects in the formation or function of cilia cause a variety of disorders, called ciliopathies, which result in FCRL5 clinical phenotypes including retinal degeneration and obesity (examined in (Waters and Beales, 2011)). Structurally, cilia consist of nine microtubule doublets that form the axoneme, surrounded by a C-178 ciliary membrane. Since cilia lack protein synthesis machinery, their generation (ciliogenesis) largely relies on the delivery of ciliary proteins from your cytoplasm into the cilium (Pedersen and Rosenbaum, 2008). This active protein transport process is called intraflagellar transport (IFT). IFT is usually mediated by two large, highly-conserved protein complexes, IFT-A and IFT-B, and motor proteins. Ciliary cargo proteins are transported from the base to the tip by anterograde trains, powered by kinesin II. Cargos are then transported from the tip to the base by retrograde trains, directed by cytoplasmic dynein II (Pedersen and Rosenbaum, 2008; Taschner and Lorentzen, 2016). Defects in IFT-B typically cause severe ciliation defects, often leading to the absence of the cilium in various organisms including humans and (examined in (Pedersen and Rosenbaum, 2008), observe Section 9) and embryonic lethality in mice (Bangs and Anderson, 2016), emphasizing the evolutionarily conserved role and essentiality of IFT-B in ciliogenesis. Despite its initial purification from isolated flagella, immunofluorescence indicates the predominant localization of IFT components and motor proteins is at the ciliary base (Cole et al., 1998; Pazour et al., 1999), or more precisely at the transition fiber/distal appendage at the apical end of the centriole (Deane et al., 2001). IFT trains periodically enter into the cilium from this large pool of IFT particles located at the base (Dentler, 2005), and it has been long hypothesized that regulatory components accumulate at the ciliary base to initiate anterograde IFT (Pedersen and Rosenbaum, 2008; Rosenbaum and Witman, 2002). To date, no crucial regulator for the initiation of IFT has been identified. Here, by tandem affinity purification and mass spectrometry of CEP19, we discover two interlinked protein complexes that trigger the access of IFT into the cilium: the FOP-CEP350 and the CEP19 complex with the highly conserved RABL2B C-178 GTPase. CEP19 was first discovered by centrosomal proteomics as a novel protein (called C3orf34) localized specifically to the distal end of the C-178 mother centriole (Jakobsen et al., 2011). Recently, human morbid obesity patients were recognized transporting a homozygous nonsense mutation (R82X) in.