VIPAS39

VIPAS39 is a core subunit of the VPS33B–VIPAS39 complex that functions in endosomal maturation, cargo sorting, and delivery to lysosomes and lysosome-related organelles. VIPAS39 and VPS33B form a ~315 kDa bidirectional complex with 2:3 stoichiometry—the only known bidirectional Sec1/Munc18 assembly—in which a central VIPAS39 region (aa 143–316) is sufficient for binding, and the two proteins are mutually required for stability (PMID:37062417, PMID:31501156, PMID:35325493). The complex interacts with RAB11A and HOPS subunits to regulate apical–basolateral polarity in epithelia, platelet α-granule biogenesis, epidermal lamellar body formation, and late-endosomal sorting of cargoes including ACSL4 for exosomal release (PMID:20190753, PMID:19109425, PMID:23002115, PMID:29409756, PMID:40088627). Loss-of-function mutations in VIPAS39 cause arthrogryposis–renal dysfunction–cholestasis (ARC) syndrome, with disease variants such as L30P disrupting complex formation while others alter endosomal localization (PMID:23918659, PMID:37062417).

1. 2008 High

   Establishing VIPAS39 as a HOPS-associated endolysosomal regulator resolved its molecular identity: the human SPE-39 orthologue interacts with VPS33 and other HOPS subunits, and its depletion delays cathepsin D delivery and EGFR degradation, placing it in the lysosomal delivery pathway.

   Evidence Co-IP, co-sedimentation, in vitro binding, siRNA knockdown with endosome morphology and cargo trafficking readouts in human cells and C. elegans genetics

   PMID:19109425

   Open questions at the time

   • Whether VIPAS39 is a stoichiometric HOPS subunit or an accessory factor was unclear
   • Direct membrane fusion activity not tested
   • No structural information on the VIPAS39–VPS33B interaction
2. 2010 High

   Demonstrating that VIPAS39–VPS33B interacts with RAB11A and controls apical–basolateral polarity broadened its role beyond lysosomal delivery to include epithelial membrane protein sorting (Ceacam5, E-cadherin) and tissue morphogenesis in liver and kidney.

   Evidence Co-IP, zebrafish vipar knockdown, mouse mIMCD-3 Vipas39/Vps33b-deficient cells with membrane protein mis-sorting and tight junction defects

   PMID:20190753

   Open questions at the time

   • How RAB11A interaction is regulated was unknown
   • Whether the polarity defect is cell-autonomous in all tissues was untested
3. 2012 High

   Identifying VIPAS39 as essential for platelet α-granule biogenesis and showing it localizes to TGN, late endosomes, and α-granules linked the complex to lysosome-related organelle formation and explained bleeding phenotypes in ARC syndrome patients.

   Evidence Yeast two-hybrid, mass spectrometry, Co-IP, immunofluorescence, electron microscopy of patient platelets lacking α-granules

   PMID:23002115

   Open questions at the time

   • The trafficking step at which cargo is diverted remained unresolved
   • Whether VIPAS39 acts at the recycling endosome versus late endosome was unclear
4. 2012 Medium

   Revealing EGF-induced Tyr-11 phosphorylation and C-terminal ubiquitination of VIPAS39, counteracted by VPS33B binding, established post-translational regulation of the complex and a functional interplay with EGFR downregulation.

   Evidence IP, phosphorylation/ubiquitination assays, Tyr-11 mutagenesis, EGF stimulation in COS-7 cells

   PMID:22677173

   Open questions at the time

   • The kinase responsible for Tyr-11 phosphorylation was not identified
   • The E3 ligase mediating ubiquitination was not identified
   • Findings from a single cell type and single lab
5. 2013 Medium

   Mapping ARC-causing VIPAS39 mutations showed most preserve VPS33B binding but alter endosomal localization or fragment VIPAS39-positive endosomes, separating the interaction interface from the membrane-targeting mechanism.

   Evidence Y2H, Co-IP, quantitative fluorescent microscopy of WT and mutant VIPAS39/VPS33B

   PMID:23918659

   Open questions at the time

   • Whether endosome fragmentation reflects a fusion defect or maturation arrest was not distinguished
   • No in vivo validation of individual mutant phenotypes
6. 2018 Medium

   Extending the complex's role to epidermal lamellar body biogenesis in Vipas39-knockout mice showed it is a general regulator of lysosome-related organelle formation, not restricted to hematopoietic or epithelial polarity contexts.

   Evidence Mouse Vipas39 knockout with histology, immunofluorescence, and electron microscopy of skin

   PMID:29409756

   Open questions at the time

   • Molecular cargo sorted by the complex in keratinocytes was not fully defined
   • Whether VIPAS39 acts at the same endosomal compartment in skin as in megakaryocytes was not resolved
7. 2019 High

   CRISPR knockout of VPS33B in megakaryocytes demonstrated that VIPAS39 protein is destabilized in the absence of VPS33B and that the complex acts at recycling endosomes to prevent lysosomal degradation of α-granule cargo, pinpointing the trafficking step.

   Evidence CRISPR/Cas9 KO, SEC, immunoblotting, live-cell cargo trafficking, GFP-VPS33B reconstitution

   PMID:31501156

   Open questions at the time

   • Whether VIPAS39 loss reciprocally destabilizes VPS33B to the same extent was not shown in this system
   • The SNARE partners or membrane fusion mechanism at the recycling endosome were not identified
8. 2023 High

   Structural characterization revealed the VPS33B–VIPAS39 complex has a unique bidirectional Sec1/Munc18 architecture with 2:3 stoichiometry and a two-lobed shape, and that the ARC variant L30P disrupts complex formation while S243F and H344D do not, defining the structural basis for pathogenic versus benign variants.

   Evidence SEC-MALS, SAXS, negative-staining EM, circular dichroism, domain truncation, ARC variant expression

   PMID:37062417

   Open questions at the time

   • High-resolution atomic structure is lacking
   • How the bidirectional arrangement relates to SNARE engagement is unknown
   • No membrane fusion reconstitution with the purified complex
9. 2025 Medium

   Identification of ACSL4 as a VIPAS39-sorted cargo destined for exosomal release via late endosomes extended the complex's function to ferroptosis resistance, showing that VIPAS39 knockdown restores ACSL4 and sensitizes ovarian cancer cells to ferroptosis.

   Evidence IP-MS, live-cell CD63-pHuji imaging, protein binding assays, xenograft models

   PMID:40088627

   Open questions at the time

   • Whether ACSL4 sorting is direct or mediated by an adaptor is unresolved
   • Generalizability to other cancer types not tested
   • Single-lab finding

• Key open questions include the atomic structure of the VPS33B–VIPAS39 complex, the identity of its SNARE partners and the mechanism by which the bidirectional SM architecture promotes membrane fusion, and whether the complex directly participates in tethering or acts as an effector recruited by Rab GTPases.
• No high-resolution structure of the complex
• SNARE partners and membrane fusion mechanism unknown
• Direct tethering versus Rab-effector role not distinguished