CANX

Calnexin (CANX/IP90/p88) is an ER transmembrane chaperone that mediates quality control of newly synthesized membrane proteins by retaining and stabilizing incompletely assembled or misfolded intermediates in the ER until proper assembly is achieved (PMID:8278813, PMID:8486646). It associates transiently with a broad range of ER-transiting proteins and holds onto unassembled subunits—peptide-deficient MHC class I heavy chain–β2-microglobulin heterodimers and free heavy chains, unassembled TCR β-chains, and partially assembled MHC class II α/β/invariant-chain complexes—releasing them only upon completion of assembly and ER egress (PMID:8278813, PMID:8486646, PMID:8313912). This recognition is directed primarily at the transmembrane segment and flanking residues of client proteins rather than their luminal domains, as GPI-anchored class I variants fail to bind calnexin unless engineered to carry a transmembrane region (PMID:8349678). Through this activity calnexin supports biogenesis and surface delivery of clients including ΔF508-sensitive CFTR (PMID:7513695), ERG/hERG-type K+ channels where it facilitates tetrameric assembly (PMID:29941431), and cell-surface MHC-I, with calnexin levels setting MHC-I display and consequent CD8+ T-cell-mediated cytotoxicity (PMID:34324740, PMID:35356876). Beyond classical ER quality control, calnexin acts in protein-degradation and autophagy pathways: it partners with the ER-phagy receptor FAM134B and LC3 at ER–endolysosome membrane contact sites to drive ERLAD of misfolded alpha-1-antitrypsin (ATZ) polymers (PMID:41179805), serves as an interaction partner required for ULK1 recruitment to the ER and for autophagy initiation (PMID:39813987), and—following KAT7-mediated K525 crotonylation—translocates to lysosomes where it binds LAMP2 and inhibits Ragulator GEF activity toward RRAG GTPases to suppress leucine-stimulated mTORC1 (PMID:35266843). Calnexin abundance is controlled by ubiquitin-mediated degradation via the E3 ligases CBL and TRIM27 (PMID:40514420, PMID:41557161), and it can repress IRF7 transcription by recruiting HDAC3 to the IRF7 promoter (PMID:40514420).

1. 1993 Medium

   Established calnexin as a general ER retention chaperone by showing it transiently binds many nascent ER-transiting proteins and stably holds unassembled subunits when assembly partners are absent.

   Evidence Pulse-chase metabolic labeling and co-immunoprecipitation of unassembled TCR β-chains with IP90 in T cell lines

   PMID:8486646

   Open questions at the time

   • Did not define the structural basis of client recognition
   • Single-lab co-IP without reconstitution
2. 1993 High

   Resolved how calnexin recognizes clients, showing the interaction is mediated by the transmembrane segment and flanking residues rather than the luminal domain.

   Evidence Domain-deletion mutagenesis, GPI-anchor substitution hybrids, and co-IP with class I heavy chains and TCR α-chain

   PMID:8349678

   Open questions at the time

   • Does not address calnexin's lectin/glycan-dependent recognition mode
   • Mapped on a limited set of clients
3. 1994 High

   Demonstrated a functional consequence of calnexin binding—retention and protection of MHC class I assembly intermediates and partially assembled class II complexes, slowing transport and impeding degradation until assembly completes.

   Evidence Heterologous reconstitution in Drosophila cells with transport assays, and pulse-chase co-IP tracking class II assembly intermediates

   PMID:8278813 PMID:8313912

   Open questions at the time

   • Mechanism of release upon completed assembly not defined
   • Quantitative contribution to degradation kinetics not isolated
4. 1994 High

   Extended calnexin's quality-control role to a disease-relevant client, showing it discriminates wild-type from misfolded CFTR by retaining the misfolded form in the ER.

   Evidence Co-immunoprecipitation and glycerol density gradient cosedimentation of nascent CFTR in epithelial and CHO cells

   PMID:7513695

   Open questions at the time

   • Did not establish whether retention promotes folding or commits CFTR to degradation
5. 2018 High

   Showed calnexin actively assists multimeric channel assembly, not merely retention, by facilitating tetramerization of ERG-type K+ channels and being required for hERG surface expression.

   Evidence C. elegans forward genetic screen, liposome-assisted cell-free assembly assay, calnexin KO with electrophysiology, and co-IP across species

   PMID:29941431

   Open questions at the time

   • Structural basis of assembly assistance not resolved
   • Whether assembly chaperoning generalizes to other tetrameric channels unknown
6. 2021 Medium

   Identified upstream regulation of calnexin by miR-148a-3p, linking calnexin abundance to MHC-I surface levels and antitumor immunity.

   Evidence Luciferase reporter validation of miRNA targeting, CANX knockdown/rescue, and CD8+ T cell cytotoxicity assays with in vivo tumor models

   PMID:34324740

   Open questions at the time

   • Does not establish the molecular step at which calnexin controls MHC-I trafficking
   • Single-lab
7. 2022 High

   Revealed a non-canonical calnexin function as a negative regulator of nutrient signaling, showing crotonylation-driven lysosomal translocation inhibits Ragulator GEF activity to suppress leucine-stimulated mTORC1.

   Evidence Cell-free mTORC1 reconstitution, lysosomal fractionation, CANX–LAMP2 and CANX–Ragulator co-IPs, mass spectrometry of K525 crotonylation, and KAT7 KO with functional rescue

   PMID:35266843

   Open questions at the time

   • How an ER membrane protein relocates to lysosomes mechanistically unresolved
   • Crosstalk with calnexin's ER chaperone pool unclear
8. 2022 Medium

   Provided gain- and loss-of-function evidence that calnexin sets MHC-I surface levels and tunes CD8+ T-cell killing in colorectal cancer.

   Evidence Lentiviral overexpression and siRNA knockdown with MHC-I Western blot, CD8+ T cell co-culture cytotoxicity, and cytokine ELISA

   PMID:35356876

   Open questions at the time

   • Mechanistic step controlling surface MHC-I not pinpointed
   • Single-lab
9. 2025 High

   Placed calnexin upstream of autophagy initiation by identifying it as a ULK1 partner required for ULK1 recruitment to the ER, with in vivo cognitive consequences in an AD model.

   Evidence Co-IP, proximity ligation, BiFC, CANX KO in hippocampal neurons with LC3/flux assays, domain-deletion rescue, and Morris water maze testing

   PMID:39813987

   Open questions at the time

   • Structural ULK1-binding determinant within calnexin not mapped
   • Relationship to calnexin's chaperone function unknown
10. 2025 Medium

    Connected calnexin to ER-to-lysosome degradation by showing it forms a segregation complex with FAM134B and LC3 at ER–endolysosome contact sites to deliver misfolded ATZ polymers for degradation.

    Evidence Fluorescence microscopy of membrane contact sites, proximity assays, and ATZ fate tracking with manipulation of CANX, FAM134B, VAPA, ORP1L, RAB7

    PMID:41179805

    Open questions at the time

    • Direct binding interfaces within the complex not defined
    • Single study
11. 2026 Medium

    Characterized post-translational control of calnexin stability, identifying CBL and TRIM27 as E3 ligases that ubiquitinate and degrade calnexin, and a transcriptional repressor role via HDAC3 recruitment to IRF7.

    Evidence Co-IP, ubiquitination assays, chromatin-associated HDAC3 assays at the IRF7 promoter, and CANX knockdown/overexpression in tumor cells with PI3K/AKT readouts

    PMID:40514420 PMID:41557161

    Open questions at the time

    • Ubiquitination sites not mapped
    • TRIM27 link rests on co-IP/ubiquitination without mutagenesis or reconstitution
    • How an ER chaperone reaches gene promoters mechanistically unresolved

• How calnexin partitions between its canonical ER chaperone pool and its non-canonical roles at lysosomes, ER-phagy contact sites, and gene promoters—and how post-translational marks like K525 crotonylation switch between these functions—remains unresolved.
• No structural model linking client recognition to relocalization
• Quantitative flux of calnexin between compartments unknown