VPS26A

VPS26A is a structural subunit of the retromer complex, a heterotrimeric assembly that governs endosomal cargo retrieval and recycling (PMID:11102511). It binds directly to the N-terminal scaffold region of VPS35, and in the absence of VPS35 it fails to incorporate into the large retromer complex (PMID:11102511); structurally, VPS26A adopts an arrestin-like fold of two curved β-sandwich domains, and its VPS35-binding determinant maps to a mobile loop (residues 235-246) whose hydrophobic residues and conserved glycine are required for complex integration, endosomal localization, and cargo sorting (PMID:16732284). Within the assembled complex VPS26A contributes allosterically to Rab7-mediated endosomal recruitment of retromer, enhancing the affinity of the VPS35-containing subcomplex for activated Rab7 (PMID:25367362), and its phosphorylation state gates cargo selection, as engineered phospho-mimetic and phospho-null mutations alter cargo binding affinity and sorting (PMID:28362258). Acting together with the cargo adaptor SNX27, VPS26A-retromer recycles transmembrane receptors including CI-M6PR, GLUT1, and mGluR5 from endosomes to the trans-Golgi network and plasma membrane (PMID:21920005, PMID:26538661), and the paralogue-specific C-terminal region of VPS26 dictates which cargoes a given retromer engages (PMID:21920005). Beyond canonical recycling, the VPS26A-SNX27 retromer also mediates an unconventional Golgi-bypass secretion pathway, delivering trafficking-deficient ΔF508-CFTR and the SARS-CoV-2 spike protein to the cell surface under cellular stress (PMID:41942457). Through these trafficking functions VPS26A is required for diverse downstream processes, including Notch signaling in Drosophila oogenesis (PMID:29031909), neural differentiation via a Nox4/ROS/ERK redox cascade (PMID:30464227), and endosome-to-TGN delivery of APP and CI-M6PR in neurons, where its loss elevates Aβ and p-tau (PMID:35297035).

1. 2000 High

   Established that human VPS26 is not an autonomous factor but a subunit that requires VPS35 to assemble into the large retromer complex, defining the architectural dependency of the complex.

   Evidence Yeast two-hybrid, reciprocal co-IP, gel filtration, and deletion mapping in COS7 cells

   PMID:11102511

   Open questions at the time

   • Did not resolve the atomic interface or fold of VPS26
   • Cargo specificity of the complex not addressed
   • Membrane recruitment mechanism not defined
2. 2006 High

   Resolved the arrestin-like fold of VPS26A and pinpointed the VPS35-binding mobile loop (residues 235-246), explaining at residue resolution how the subunit integrates into retromer and supports cargo sorting.

   Evidence 2.1-Å X-ray crystallography with site-directed mutagenesis, mammalian endosomal localization assay, and yeast carboxypeptidase Y sorting assay

   PMID:16732284

   Open questions at the time

   • Structure of the full assembled trimer on membranes not determined
   • How the arrestin fold contributes to cargo binding not defined
3. 2011 High

   Showed that the two VPS26 paralogues define functionally distinct retromers, with VPS26A-retromer (not VPS26B) recycling CI-M6PR, and localized cargo discrimination to the VPS26B C-terminal variable region.

   Evidence Stable myc-tagged paralogue expression in HEK293, co-IP, Rab colocalization, and deletion analysis with receptor degradation and cathepsin D secretion readouts

   PMID:21920005

   Open questions at the time

   • Molecular basis of how the C-terminal region selects cargo not resolved
   • Whether paralogue choice is regulated in vivo unknown
4. 2012 Medium

   Identified Rabankyrin-5 as a partner needed to maintain VPS26 localization and M6PR retrieval, extending the set of factors organizing retromer-based transport.

   Evidence Co-IP, colocalization, and siRNA knockdown with functional rescue

   PMID:22284051

   Open questions at the time

   • Direct vs indirect interaction with VPS26 not distinguished
   • Single lab, no structural detail of the contact
5. 2014 Medium

   Defined a mechanistic role for VPS26 in membrane recruitment, showing it allosterically enhances binding of the VPS35 subcomplex to activated Rab7.

   Evidence FRET interaction assay in HeLa cells, biophysical binding measurements, and mutagenesis disrupting the VPS35-VPS26 interaction

   PMID:25367362

   Open questions at the time

   • Structural basis of the allosteric effect not resolved
   • Single lab; in vitro reconstitution of the allostery not performed
6. 2015 Medium

   Connected VPS26A to SNX27-dependent recycling of a specific GPCR cargo (mGluR5) in neurons and linked this trafficking to a behavioral phenotype.

   Evidence Co-IP, immunofluorescence, and siRNA knockdown in rat dorsal horn neurons with an allodynia behavioral assay

   PMID:26538661

   Open questions at the time

   • Whether VPS26A-SNX27-mGluR5 contact is direct not established
   • Single lab, single cargo in one neuronal context
7. 2017 Medium

   Demonstrated phosphorylation-based gating of cargo selection, showing that the phosphatase Mih1 sets the VPS26 phospho-state and that phospho-state mutations change cargo binding and sorting.

   Evidence Activating mutation screen, in vitro phosphatase assay, and phospho-mimetic/null mutagenesis with cargo sorting readout in yeast

   PMID:28362258

   Open questions at the time

   • Mammalian kinase/phosphatase regulating VPS26A not identified
   • Phosphosites and structural consequences in human VPS26A unmapped
8. 2017 Medium

   Placed VPS26-dependent retromer upstream of Notch signaling, establishing a developmental signaling output of the complex.

   Evidence Drosophila germline clonal loss-of-function with Notch pathway marker immunofluorescence and LysoTracker staining

   PMID:29031909

   Open questions at the time

   • Specific Notch-pathway cargo trafficked by retromer not identified
   • Single model organism
9. 2018 Medium

   Linked Vps26a to redox signaling and differentiation through a physical interaction with the NADPH oxidase Nox4 driving ERK1/2-dependent neurogenesis.

   Evidence Vps26a knockout ESCs, co-IP, ROS measurement, inhibitor treatments, and neurogenesis markers

   PMID:30464227

   Open questions at the time

   • Whether the Nox4 interaction is retromer-dependent unclear
   • Single lab; direct vs scaffolded contact not resolved
10. 2018 Medium

    Showed that the PLA2G6 ortholog iPLA2-VIA binds VPS35/VPS26 and enhances retromer function, tying retromer activity to lipid homeostasis and neurodegeneration.

    Evidence Fly protein-interaction assay, genetic epistasis with vps26/vps35 loss, lipid profiling, and neurodegeneration phenotype

    PMID:29909971

    Open questions at the time

    • Direct VPS26 contact vs VPS35-mediated association not separated
    • Mechanism by which iPLA2-VIA enhances retromer unknown
11. 2022 Medium

    Connected VPS26a loss to Alzheimer-relevant pathology, showing its down-regulation traps APP and CI-M6PR in endosomes to raise Aβ and p-tau, with recovery reversing the defect.

    Evidence Gain/loss-of-function in iPSC-derived neurons and SH-SY5Y cells, endosome/TGN trafficking assays, and a streptozotocin diabetic mouse model

    PMID:35297035

    Open questions at the time

    • Whether APP is a direct retromer cargo not established here
    • Single lab
12. 2023 Low

    Reported a link between VPS26 and Wnt/β-catenin signaling promoting osteogenesis, extending putative functions beyond endosomal trafficking.

    Evidence Co-localization, co-IP with β-catenin, TOP/FOP luciferase reporter, and lentiviral overexpression with osteogenic/adipogenic staining

    PMID:37005781

    Open questions at the time

    • Single Co-IP plus reporter assay with no mechanism for how VPS26 activates β-catenin
    • Direct vs indirect β-catenin interaction not tested
    • Not independently confirmed
13. 2024 Medium

    Implicated membrane atg8ylation, via ATG5, as a modulator of retromer-dependent GLUT1 sorting independent of canonical autophagy.

    Evidence Co-IP of ATG5 with VPS26/VPS29/VPS35 and atg8ylation-pathway knockouts with GLUT1 trafficking readout (preprint)

    Open questions at the time

    • Preprint, not peer-reviewed
    • Whether ATG5 contacts VPS26 directly not established
    • Generality across other retromer cargoes unknown
14. 2024 Medium

    Provided in vitro reconstitution showing SNX27 plus Retromer tubulates membranes and that VARP is required to assemble a SNX27-ESCPE-1-Retromer supercomplex, clarifying the higher-order organization of the recycling machinery.

    Evidence Membrane reconstitution with purified proteins, binding assays, AlphaFold2-Multimer modeling, and point mutagenesis (preprint)

    Open questions at the time

    • Preprint, not peer-reviewed
    • Specific role of VPS26A within the supercomplex not isolated
    • Cargo selectivity of the reconstituted supercomplex not fully defined
15. 2026 Medium

    Established a non-canonical role for VPS26A-retromer with SNX27 in unconventional Golgi-bypass secretion of ΔF508-CFTR and SARS-CoV-2 spike, expanding retromer function from recycling to stress-induced surface delivery.

    Evidence CRISPR knockout screen, co-IP of SNX27 with ΔF508-CFTR and VPS26A-retromer, and surface trafficking assays with VPS26A/SNX27 knockouts

    PMID:41942457

    Open questions at the time

    • Molecular trigger linking cellular stress to UPS routing not defined
    • Whether VPS26A directly contacts the secreted cargoes unknown

• How VPS26A phosphorylation, paralogue choice, and partner inputs (Rab7, SNX27, VARP, atg8ylation) are integrated to switch retromer between canonical recycling and unconventional secretion in human cells remains unresolved.
• No structure of the full assembled, membrane-bound supercomplex with cargo
• Human kinases/phosphatases controlling VPS26A phospho-gating unidentified
• Mechanism selecting canonical vs Golgi-bypass routing uncharacterized