RAB40B

RAB40B is an atypical Rab GTPase that couples vesicular trafficking to substrate ubiquitylation, functioning as the substrate-recognition module of a CRL5-type E3 ubiquitin ligase to drive actin remodeling, invadopodia function, and breast cancer cell invasion (PMID:23902685, PMID:33999101). Its C-terminal SOCS box mediates binding to Cullin5, and loss of this interaction increases stress fibers and stable focal adhesions while suppressing invadopodia formation, actin dynamics at the leading edge, and 3D migration (PMID:33999101, PMID:34453716). Through this complex RAB40B ubiquitylates the actin regulator EPLIN, targeting it for degradation, and ubiquitylates the Ras-like GTPase Rap2 to control its activation and recycling from the endolysosomal compartment to lamellipodia (PMID:33999101, PMID:35293963). In parallel, RAB40B sorts MMP2 and MMP9 into VAMP4-positive secretory vesicles and, via its effector Tks5, tethers and targets these vesicles to extending invadopodia for extracellular matrix degradation; RAB40B and Tks5 are jointly regulated by the tumor suppressor miR-204 (PMID:23902685, PMID:27789576). A nonsense mutation (Y83X) that deletes the SOCS box causes axonal Charcot-Marie-Tooth disease type 2, and model-organism epistasis with Cul5 ties the neuropathy to loss of SOCS box–Cullin5 function (PMID:38196136).

1. 2013 High

   Established RAB40B's cellular role by showing it is required to sort matrix metalloproteinases into secretory vesicles and deliver them to invadopodia, defining a trafficking step needed for tumor cell invasion.

   Evidence Rab siRNA library screen with knockdown, co-localization, and in vitro ECM degradation/invasion assays in breast cancer cells

   PMID:23902685

   Open questions at the time

   • Did not identify the effector tethering vesicles to invadopodia
   • GTPase biochemistry and nucleotide cycling not characterized
2. 2016 High

   Identified Tks5 as the RAB40B effector that physically tethers MMP-containing vesicles to invadopodia, and placed both proteins under miR-204 control, linking the trafficking machinery to a tumor-suppressive regulatory input.

   Evidence Reciprocal co-immunoprecipitation, vesicle-targeting assays, siRNA knockdown, and miR-204 gain/loss of function with ECM degradation readout

   PMID:27789576

   Open questions at the time

   • Did not address whether RAB40B has enzymatic/ligase activity beyond trafficking
   • Direct vs indirect nature of the Tks5 interaction surface unresolved
3. 2021 High

   Reframed RAB40B as a CRL5 E3 ligase component by showing its SOCS box binds Cullin5 and that the complex ubiquitylates and degrades EPLIN, mechanistically connecting RAB40B to actin, focal adhesion, and invadopodia dynamics.

   Evidence SOCS-domain mutant constructs, Co-IP with Cullin5, in-cell ubiquitylation assay, mass spectrometry substrate ID, and actin/focal adhesion imaging plus a methods paper formalizing substrate discovery and 3D migration

   PMID:33999101 PMID:34453716

   Open questions at the time

   • Full substrate repertoire beyond EPLIN not defined
   • How GTPase state regulates ligase assembly unresolved
4. 2022 High

   Extended the ligase model to a second substrate by showing RAB40B/Cullin5 ubiquitylates Rap2 to control its activation and endolysosome-to-lamellipodia recycling, linking ubiquitylation directly to leading-edge actin and migration.

   Evidence Co-IP, in-cell ubiquitylation assay, dominant-negative/constitutively-active constructs, Rap2 activation pulldown, and live-cell imaging with migration/invasion assays

   PMID:35293963

   Open questions at the time

   • Whether EPLIN and Rap2 ubiquitylation are coordinated or independent unknown
   • Lysine sites and ubiquitin chain topology not defined
5. 2023 Medium

   Connected RAB40B to human disease by showing a SOCS box-truncating Y83X mutation causes axonal CMT2, with model-organism epistasis attributing the defect to lost Cullin5 interaction—extending its biology beyond cancer to neuromuscular function.

   Evidence Human genetic analysis, zebrafish and Drosophila transgenic expression with NMJ immunostaining, locomotion assays, and Cul5 genetic interaction

   PMID:38196136

   Open questions at the time

   • Disease-relevant substrate(s) in neurons/glia not identified
   • Glial vs neuronal site-of-action mechanism unresolved
6. 2025 Medium

   Broadened the trafficking machinery by placing RAB40B in a complex with DCLK1 and the kinesin KIF16B for motor-driven MMP9 cargo transport along invadopodia, and showed viral suppression of RAB40B disrupts invadopodia, indicating the pathway is a target for invasion control.

   Evidence Proximity ligation assay, Co-IP, super-resolution microscopy, TMT proteomics, and gelatin/Boyden invasion assays (DCLK1/KIF16B study); qRT-PCR, Western, Co-IP, and rescue co-transfection (ARV p17 study)

   PMID:39994636 PMID:40575485

   Open questions at the time

   • Whether DCLK1/KIF16B coupling requires RAB40B ligase activity unknown
   • Mechanistic basis of the proposed RAB40B-PI3K-Akt axis limited to expression-level evidence

• How RAB40B integrates its GTPase nucleotide cycle with CRL5 ligase assembly and substrate selection, and the full substrate repertoire across cancer and neuronal contexts, remain open.
• No structural model of the RAB40B-Cullin5 complex or substrate engagement
• GAP/GEF regulators and nucleotide state effects on ligase function uncharacterized
• Substrates relevant to the neuropathy phenotype unidentified