RABL2 Regulates Ciliation via Controlling IFT-B1 Basal Body Recruitment and ARL3-mediated BBSome Ciliary Retrieval

Highly conserved intraflagellar transport (IFT) trains and certain small GTPases coordinate to direct ciliation and to maintain the ciliary dynamics of signaling molecules via the IFT cargo adaptor BBSome. Unlike murine Rab-like 2 (RABL2) GTPase that enters cilia to drive outward transition zone (TZ) passage of the BBSome, human orthologue fails to enter cilia but resides at the ciliary base, indispensable for ciliation. However, mechanisms underlying how RABL2 regulates ciliation and BBSome barrier passage remain elusive. Here, we show that Chlamydomonas RABL2 regulates basal body targeting of the IFT-B1 subcomplex component of IFT trains as a RABL2-specific effector, mediating ciliation via controlling IFT-B1 basal body amount available for assembling anterograde IFT trains. RABL2GTP binds IFT-B1 to perform IFT; sheds from retrograde IFT trains at the proximal ciliary region right above the TZ; and converts to RABL2GDP rapidly. Next, RABL2GDP activates the ciliary membrane anchored Arf-like 3 (ARL3) GTPase (ARL3GDP) as a ARL3-specific guanine nucleotide exchange factor. Upon detaching from the ciliary membrane, the active ARL3GTP recruits its BBSome effector, autonomous of retrograde IFT train association, to move cross the TZ for ciliary retrieval. This ensures proper BBSome ciliary turnover for maintaining phototactic response of Chlamydomonas cells. For finishing RABL2 ciliary cycle, RABL2GDP passes the TZ for ciliary retrieval by loading onto the ARL3GTP/BBSome as a BBSome cargo. Our data thus propose that RABL2 mediates ciliation and BBSome ciliary retrieval simultaneously but via distinct molecular pathways. Significance statement Intraflagellar transport (IFT) and its cargo adaptor BBSome are indispensable for ciliation and ciliary singling. Rab-like 2 (RABL2) GTPase mediates ciliation and outward transition zone (TZ) passage of BBSomes with mechanisms yet to be determined. Here, we report that RABL2 decides ciliation by controlling the basal body amount of its effector IFT-B1 available for the assembly of anterograde IFT trains. RABL2GTP cycles through cilia as an IFT-B1 cargo; sheds from IFT at the ciliary base; and undergoes nucleotide exchange for activating ARL3 as an ARL3-specfic guanine nucleotide exchange factor. ARLGTP recruits IFT-shed BBSomes to pass the TZ for ciliary retrieval. RABL2GDP exists cilia via ARL3GTP/BBSome as a BBSome cargo. Therefore, RABL2 functions both outside and inside cilia for initiating IFT and BBSome ciliary retrieval, respectively. ### Competing Interest Statement The authors have declared no competing interest.