RABGGTB

RABGGTB is the catalytic β-subunit of geranylgeranyltransferase complexes that prenylate substrate proteins to direct their membrane localization and downstream activity (PMID:31209342, PMID:38042502). Beyond its canonical role in the Rab geranylgeranyltransferase (GGTase2/RabGGTase), RABGGTB partners with the orphan α-subunit PTAR1 to form a distinct prenyltransferase, GGTase3, that geranylgeranylates FBXL2 at its CaaX motif; crystallography of the full-length GGTase3–FBXL2–SKP1 assembly shows that substrate specificity is conferred by an extensive multivalent interface between the FBXL2 leucine-rich repeat domain and PTAR1, while RABGGTB supplies the catalytic prenyl-transfer activity, and this modification is required for FBXL2 membrane localization and polyubiquitylation of membrane-anchored proteins (PMID:31209342). In autophagy, RABGGTB-driven geranylgeranylation of Rab7 promotes autophagosome–lysosome fusion and clearance of misfolded protein aggregates: RABGGTB overexpression reduced abnormal SOD1 aggregation in ALS cell models and, delivered by AAV9 to SOD1G93A mouse motoneurons, delayed disease onset, extended survival, and lowered misfolded SOD1 accumulation and glial overactivation (PMID:38042502, PMID:36967828). The GGTase3 complex is also expressed in pancreatic islets and β-cells, where it supports glucose-stimulated insulin secretion, potentially via geranylgeranylation of the synaptobrevin homolog Ykt6 (PMID:40598917).

1. 1995 Low

   Established the basic expression behavior of the Rab GGTase β-subunit, showing it is ubiquitously expressed and transcriptionally responsive to retinoid signaling, the first hint that its abundance is regulated.

   Evidence Northern blot, in situ hybridization, and retinoic acid treatment of P19 embryonal carcinoma cells

   PMID:7544156

   Open questions at the time

   • No functional or catalytic mechanism dissected
   • Retinoic acid induction mechanism (direct vs indirect) not defined
2. 1997 Low

   Addressed how RABGGTB levels are controlled post-transcriptionally, showing the mRNA is stabilized upon translation inhibition and implicating a labile repressor of transcript stability.

   Evidence Northern blot with actinomycin D chase and cycloheximide treatment in P19 cells

   PMID:9031634

   Open questions at the time

   • The proposed labile repressor protein was not identified
   • Indirect pharmacological inference only
3. 2008 Medium

   Identified a candidate physical partner, showing the chondrolectin cytoplasmic domain binds RABGGTB, raising the possibility of substrate or regulatory connections beyond Rab proteins.

   Evidence Sos recruitment system yeast two-hybrid screen confirmed by in vitro pulldown and co-immunoprecipitation

   PMID:18161010

   Open questions at the time

   • No functional consequence of the interaction established
   • Whether chondrolectin is a prenylation substrate untested
   • Single lab
4. 2019 High

   Defined a new enzymatic identity for RABGGTB, showing it pairs with PTAR1 to form GGTase3, which geranylgeranylates FBXL2 to drive its membrane localization and membrane-protein polyubiquitylation; the structure explained how substrate specificity arises from a PTAR1–FBXL2 interface rather than from the catalytic subunit.

   Evidence Co-IP, in vitro prenylation, crystal structure of GGTase3–FBXL2–SKP1, interface/active-site mutagenesis, membrane fractionation, and ubiquitylation assays

   PMID:31209342

   Open questions at the time

   • Full substrate repertoire of GGTase3 beyond FBXL2 not enumerated
   • Physiological contexts requiring GGTase3 vs GGTase2 not mapped
   • Regulation of α-subunit partner choice unknown
5. 2023 Medium

   Connected RABGGTB catalytic activity to neurodegenerative proteostasis, showing that boosting RABGGTB enhances Rab7 geranylgeranylation, autophagosome–lysosome fusion, and clearance of misfolded SOD1 in both cell and mouse ALS models.

   Evidence Lentiviral overexpression in NSC34-hSOD1G93A and TDP-43 cell models with autophagy flux analysis, and AAV9 intrathecal overexpression in SOD1G93A mice with behavioral and histological readouts

   PMID:36967828 PMID:38042502

   Open questions at the time

   • Rab7 prenylation step not confirmed by in vitro reconstitution or epistasis
   • Single lab without independent replication of the in vivo result
   • Whether effect generalizes beyond SOD1 ALS unclear
6. 2025 Medium

   Extended GGTase3 function to endocrine physiology, implicating the RABGGTB-containing complex in glucose-stimulated insulin secretion, potentially through geranylgeranylation of Ykt6.

   Evidence Western blotting in human/mouse islets and INS-1 832/13 β-cells, siRNA knockdown of PTAR1, and insulin ELISA secretion assays

   PMID:40598917

   Open questions at the time

   • Only the α-subunit PTAR1 was knocked down; RABGGTB role inferred from complex membership
   • Ykt6 prenylation in β-cells not directly demonstrated
   • Single lab, single study

• How RABGGTB partitions between its GGTase2 (RABGGTA) and GGTase3 (PTAR1) complexes, and what governs subunit selection across cell types and physiological demands, remains unresolved.
• No quantitative measure of RABGGTB distribution among complexes
• Regulatory inputs controlling α-subunit choice unknown
• Complete substrate maps of each complex incomplete