RAB36

RAB36 is a Golgi-associated small GTPase that drives microtubule-dependent retrograde transport of late endosomal/lysosomal and melanosomal compartments toward the perinuclear region (PMID:19961360, PMID:22740695). In its active state it engages RILP-family effectors (RILP, RILP-L1, RILP-L2) and the JIP3/JIP4 scaffolds through a conserved coiled-coil RILP homology domain (RHD), and this interaction is required for RILP-induced perinuclear aggregation of melanosomes and late endosomes/lysosomes (PMID:19961360, PMID:22740695). In melanocytes RAB36 functions as a cargo receptor that, together with melanoregulin and Rab44, mediates retrograde melanosome transport, and it independently activates the myosin-5a motor by promoting RILPL2 binding to the myosin-5a globular tail domain, thereby exposing the melanophilin-binding site and stimulating motor ATPase and motility (PMID:22740695, PMID:31175157, PMID:36126775). RAB36 is also recruited to Arf6-positive recycling endosomes via MICAL-L1 acting downstream of RAB35, where it recruits its effector JIP4 to support neurite outgrowth (PMID:25086062). Its membrane association is terminated by the Rab9A effector RUTBC2, which acts as a RAB36-specific GTPase-activating protein (PMID:22637480).

1. 1999 Medium

   Established the basic subcellular address of RAB36 by showing it localizes to the Golgi, providing the first spatial framework for its function.

   Evidence Transfection and subcellular localization in the original cloning study

   PMID:9920784

   Open questions at the time

   • No functional activity linked to Golgi localization
   • No effector or pathway identified
2. 2010 Medium

   Connected RAB36's Golgi localization to organelle positioning by showing overexpression clusters late endosomes/lysosomes perinuclearly and identifying RILP as a binding partner, defining a candidate retrograde transport role.

   Evidence EGFP-tagged WT/mutant expression, co-localization with organelle markers, GST pull-down

   PMID:19961360

   Open questions at the time

   • Overexpression phenotype not validated by loss-of-function
   • Binding region on RAB36 not mapped
   • Motor machinery not identified
3. 2010 Low

   A Rab-wide interaction screen flagged GAPCenA/TBC1D11 as a RAB36 binding protein through a non-TBC domain, hinting at additional regulatory partners.

   Evidence GST pull-down of 60 mammalian Rabs with mass spectrometry

   PMID:20070612

   Open questions at the time

   • Single pull-down with no functional follow-up
   • Physiological relevance of the interaction unknown
4. 2012 High

   Defined the effector logic of RAB36 by mapping its binding to the RHD of RILP-family proteins and JIP3/JIP4 and demonstrating, via knockdown, that RAB36 is required for retrograde melanosome transport independent of RAB7.

   Evidence Binding screens, RHD mutagenesis, siRNA in melanocytes including Rab27A-KO cells, live-cell imaging

   PMID:22740695

   Open questions at the time

   • Direct motor adaptor at the melanosome not yet defined
   • GTP-dependence of effector engagement not biochemically dissected here
5. 2012 High

   Identified the off-switch for RAB36 by showing the Rab9A effector RUTBC2 acts as its GAP and reduces RAB36 membrane association, placing RAB36 in a Rab9A-coupled regulatory circuit.

   Evidence In vitro GAP assay against a Rab panel, co-localization, catalytically inactive R829A mutant in cells

   PMID:22637480

   Open questions at the time

   • GEF for RAB36 not identified
   • In vivo significance of RUTBC2-mediated inactivation untested
6. 2014 Medium

   Placed RAB36 in a RAB35→MICAL-L1 recruitment cascade at Arf6-positive recycling endosomes, showing it recruits JIP4 there and is required for neurite outgrowth, broadening its role beyond melanosomes.

   Evidence siRNA, live-cell imaging, epistasis in NGF-stimulated PC12 cells

   PMID:25086062

   Open questions at the time

   • Cargo transported in this pathway not defined
   • Mechanism linking JIP4 recruitment to outgrowth unresolved
7. 2019 High

   Resolved a direct biochemical mechanism by which RAB36 activates the myosin-5a motor, showing it drives RILPL2 binding to the myosin-5a GTD to expose the melanophilin-binding site and stimulate motor activity.

   Evidence ATPase and single-molecule motility assays, GST pull-down, analytical ultracentrifugation

   PMID:31175157

   Open questions at the time

   • Cellular reconstitution of this activation on melanosomes not shown
   • Integration with the retrograde (dynein) transport role unclear
8. 2021 Medium

   Distinguished RAB36 from its paralog Rab34 in ciliogenesis, showing RAB36 is not required for the extracellular ciliogenesis pathway in MDCK cells.

   Evidence MDCK knockout cell lines and ciliogenesis assays

   PMID:33989527

   Open questions at the time

   • Negative result; role in intracellular ciliogenesis not directly tested for RAB36
   • Other cell types not examined
9. 2022 Medium

   Defined RAB36 as one of at least three redundant melanosomal cargo receptors for retrograde transport, with full inhibition only on combined depletion with melanoregulin and Rab44.

   Evidence Triple siRNA knockdown in Rab27A-deficient melan-ash cells, melanosome distribution assays

   PMID:36126775

   Open questions at the time

   • Molecular basis of redundancy among the three receptors unresolved
   • Single-cell-type evidence

• The activating GEF for RAB36 and how its dynein-driven retrograde role is coordinated with myosin-5a activation on the same melanosome remain undefined.
• No GEF identified in the corpus
• Coordination of opposing/cooperating motors mechanistically unresolved
• In vivo physiological phenotype not established