RABL2B

RABL2B is a small GTPase of the RAB family that functions as a gatekeeper for intraflagellar transport (IFT) initiation at the ciliary base. Upon intrinsic nucleotide exchange to its GTP-bound state, RABL2B is captured by CEP19—itself recruited via the centriolar CEP350/FOP complex—and then binds and releases the IFT-B holocomplex from pre-docked pools to trigger ciliary entry (PMID:28625565). RABL2B also physically associates with ciliary GPCRs such as GPR161 and HTR6, controlling their trafficking to the ciliary membrane independently of TULP3, with its recruitment to the mother centriole depending on distal appendage proteins CEP164 and CEP83 (PMID:30578315). RABL2B is ubiquitously expressed and is the preferentially transcribed paralog over RABL2A in most human tissues (PMID:20138207).

1. 1999 Medium

   Molecular identity of RABL2B was established as a ubiquitously expressed small GTPase nearly identical to its paralog RABL2A, providing the foundational classification needed for functional studies.

   Evidence cDNA cloning, Northern blot, and chromosomal mapping in human tissues

   PMID:10444334

   Open questions at the time

   • No functional role assigned beyond GTPase family membership
   • Whether RABL2A and RABL2B are functionally redundant was unknown
2. 2010 Medium

   Demonstration that RABL2B is preferentially expressed over RABL2A across human tissues suggested incomplete functional redundancy and pointed to RABL2B as the dominant paralog.

   Evidence Pyrosequencing-based paralogous sequence quantification of genomic and transcriptional ratios across multiple human and chimpanzee tissues

   PMID:20138207

   Open questions at the time

   • Protein-level expression differences between paralogs were not measured
   • Functional consequences of differential expression were not tested
3. 2011 Medium

   Discovery of regulated alternative splicing of RABL2B—including isoforms with premature stop codons—revealed post-transcriptional complexity that could modulate functional protein output.

   Evidence RT-PCR isoform quantification across genetically diverse human and chimpanzee lymphoblastoid cell lines

   PMID:21220357

   Open questions at the time

   • Whether alternative splice isoforms encode functional proteins was not determined
   • No link to ciliary biology was yet apparent
4. 2017 High

   The core mechanism of RABL2B at the ciliary base was elucidated: GTP-bound RABL2B is captured by CEP19 (recruited via CEP350/FOP), then binds and releases the IFT-B holocomplex to initiate intraflagellar transport, resolving how IFT entry is triggered from pre-docked complexes.

   Evidence Affinity-purification mass spectrometry, co-immunoprecipitation, and biochemical reconstitution of GTP-binding and effector interactions

   PMID:28625565

   Open questions at the time

   • Structural basis of the RABL2B–CEP19 and RABL2B–IFT-B interfaces was not resolved
   • Whether RABL2A can substitute for RABL2B in this pathway was not directly tested
   • GAP identity or regulation of GTP hydrolysis cycle was not identified
5. 2019 Medium

   RABL2B's role was extended beyond IFT-B release to direct physical association with ciliary GPCRs (GPR161, HTR6) and TULP3-independent control of their ciliary trafficking, with recruitment depending on distal appendage proteins CEP164 and CEP83.

   Evidence siRNA knockdown, overexpression, co-immunoprecipitation, and immunofluorescence localization in mammalian cells

   PMID:30578315

   Open questions at the time

   • RABL2A and RABL2B were not fully distinguished in functional assays
   • Whether RABL2B directly binds GPCRs or acts through an intermediary was not resolved
   • Cargo selectivity of RABL2B beyond GPR161 and HTR6 was not defined

• Key unresolved questions include the structural basis of RABL2B interactions with CEP19 and IFT-B, the identity of a GAP for RABL2B, the full range of ciliary cargo controlled by RABL2B, and whether RABL2A is functionally redundant in vivo.
• No GAP for RABL2B has been identified
• No high-resolution structure of RABL2B in complex with its effectors exists
• In vivo genetic distinction between RABL2A and RABL2B function is lacking