RABL2B

RABL2B is an atypical RAB-family small GTPase that initiates intraflagellar transport and controls protein delivery into the primary cilium (PMID:28625565, PMID:30578315). When loaded with GTP through its intrinsic nucleotide exchange, RABL2B is specifically captured at the ciliary base by the centrosomal scaffold CEP19, which is itself recruited to the centriole by the CEP350/FOP module; activated RABL2B then engages and releases the IFT-B holocomplex from a pool of pre-docked complexes to license ciliary entry of intraflagellar transport (PMID:28625565). Beyond launching IFT, RABL2 physically associates with the ciliary GPCRs GPR161 and HTR6 and promotes their trafficking into primary cilia in a manner that depends on its recruitment to the mother centriole by the distal appendage proteins CEP164 and CEP83 and on CEP19 and IFT-B, but is independent of TULP3 (PMID:30578315). The gene is expressed ubiquitously with a muscle-specific isoform arising from differential 3' UTR splicing (PMID:10444334), and splicing variants of the orthologous gene that disrupt the transcript are associated with defects in sperm morphology and motility (PMID:28553998).

1. 1999 Medium

   Established RABL2B as a RAB-like small GTPase gene with defined chromosomal location and expression pattern, providing the molecular starting point before any function was known.

   Evidence Northern analysis, genomic mapping, and sequence analysis of the 22q13.3 subtelomeric locus

   PMID:10444334

   Open questions at the time

   • No biochemical activity or interaction partner identified
   • Functional role of the muscle-specific 3' UTR isoform undefined
2. 2011 Low

   Showed that RABL2B alternative splicing is tightly constrained and constant across genetic backgrounds, but did not link the splicing regulation to any functional output.

   Evidence Pyrosequencing of splice isoforms and quantitative splice-ratio analysis across lymphoblastoid cell lines

   PMID:21220357

   Open questions at the time

   • No functional consequence demonstrated for the splicing regulation
   • Single method/single lab
   • No protein-level readout
3. 2017 High

   Defined the core mechanism: how RABL2B is activated and how it triggers ciliary entry of IFT, answering what this GTPase actually does at the ciliary base.

   Evidence AP-MS, reciprocal Co-IP, GTPase nucleotide-state mutants, and ciliary entry assays establishing the CEP350/FOP→CEP19→GTP-RABL2B→IFT-B pathway

   PMID:28625565

   Open questions at the time

   • GAP/GEF regulating the RABL2B nucleotide cycle in cells not identified
   • Structural basis of IFT-B capture and release not resolved
   • Quantitative dynamics of IFT initiation not measured
4. 2017 Medium

   Linked RABL2B sequence variation to a physiological phenotype, suggesting splicing disruption impairs reproductive function.

   Evidence pSPL3 exon-trapping reporter assay in MLTC-1 cells with genotype-specific constructs plus bull sperm-quality association (bovine ortholog); and a human cohort splice-prediction association in oligoasthenoteratozoospermia (bioinformatics only)

   PMID:28138870 PMID:28553998

   Open questions at the time

   • Human variant splicing effect inferred bioinformatically without a functional assay
   • Small association cohorts
   • Mechanistic link between aberrant splice product and sperm defect not established
5. 2019 Medium

   Extended RABL2 function beyond IFT initiation to selective cargo delivery, defining how specific ciliary GPCRs reach the cilium and which upstream factors recruit RABL2.

   Evidence siRNA silencing with mislocalization readout, overexpression rescue, reciprocal Co-IP of RABL2 with GPR161/HTR6, and epistasis with CEP19/IFT-B and distal appendage proteins

   PMID:30578315

   Open questions at the time

   • Direct vs indirect nature of the RABL2–GPCR association not fully resolved
   • Single lab
   • Generality across other ciliary GPCRs untested

• How the RABL2B GTPase cycle is regulated in vivo (its GEF beyond intrinsic exchange and any GAP) and the structural basis of IFT-B engagement remain unresolved.
• No GAP identified
• No high-resolution structure of RABL2B–IFT-B or RABL2B–CEP19 complexes
• Direct GPCR binding interface uncharacterized