CANX

Calnexin is a type I transmembrane ER chaperone that transiently associates with diverse nascent glycoproteins and persistently retains unassembled or misfolded substrates—including MHC class I and II subunits and ΔF508 CFTR—in the ER, protecting them from premature degradation and preventing their forward transport until correct folding or assembly is achieved (PMID:8486646, PMID:8278813, PMID:7513695). The primary interaction with substrates such as MHC class I heavy chains is mediated through the transmembrane segment rather than the luminal domain (PMID:8349678). Terminally misfolded glycoproteins are extracted from the calnexin cycle by EDEM, which binds calnexin via its own transmembrane region and hands substrates to ER-associated degradation (ERAD) (PMID:12610305, PMID:12610306). Calnexin-dependent chaperoning of MHC class I molecules promotes their surface presentation, enabling CD8⁺ T-cell immune surveillance (PMID:34324740).

1. 1993 High

   Establishing calnexin as a chaperone: its transient association with diverse newly synthesized proteins and persistent retention of unassembled subunits revealed that calnexin functions as a general ER quality-control chaperone rather than a narrow-specificity receptor.

   Evidence Metabolic pulse-chase labeling and co-immunoprecipitation across multiple substrates in T cell lines

   PMID:8486646

   Open questions at the time

   • Mechanism by which calnexin distinguishes folded from unfolded substrates was unknown
   • Whether lectin-like carbohydrate recognition or direct polypeptide contact mediates binding was unresolved
2. 1993 High

   Identifying the transmembrane segment as the primary interaction interface with MHC class I heavy chains overturned the assumption that luminal domains mediate chaperone–substrate contact and pointed to a distinct membrane-embedded recognition mechanism.

   Evidence Systematic domain deletion and hybrid protein mutagenesis with co-immunoprecipitation in transfected cells

   PMID:8349678

   Open questions at the time

   • Whether transmembrane-mediated binding is a general mode or specific to MHC-I substrates was unclear
   • Structural basis of transmembrane recognition was not determined
3. 1994 High

   Reconstitution in Drosophila cells demonstrated that calnexin is both necessary and sufficient to retard ER-to-surface transport of incompletely assembled MHC-I and to protect free heavy chains from degradation, confirming a dual retention/stabilization function.

   Evidence Heterologous expression in Drosophila melanogaster cells with pulse-chase transport and degradation assays

   PMID:8278813

   Open questions at the time

   • Whether calnexin actively recruits degradation machinery or passively shields substrates was not resolved
   • Role in a native mammalian ER with competing chaperones (calreticulin, BiP) was not tested
4. 1994 High

   Extending the substrate repertoire: calnexin associates with MHC class II α, β, and invariant chains during stepwise nonameric assembly and dissociates only upon completion, generalizing its role as a quality-control checkpoint for multi-subunit complexes beyond MHC-I, and also binds both wild-type and ΔF508 CFTR, with only wild-type escaping the association.

   Evidence Co-immunoprecipitation with kinetic pulse-chase in human B cells (MHC-II) and human epithelial/CHO cells with glycerol gradient cosedimentation (CFTR)

   PMID:7513695 PMID:8313912

   Open questions at the time

   • Whether calnexin distinguishes misfolded substrates by lectin-cycle timing or conformational sensing was unknown
   • Direct role of calnexin in CFTR degradation versus simple retention was not separated
5. 2003 High

   Discovery that EDEM physically interacts with calnexin through its transmembrane region and extracts terminally misfolded glycoproteins from the calnexin cycle for ERAD connected the retention function to a defined disposal pathway and explained how persistent calnexin binding is resolved.

   Evidence Reciprocal co-immunoprecipitation plus gain- and loss-of-function (overexpression and siRNA/antisense knockdown of EDEM) with quantitative ERAD kinetic assays

   PMID:12610305 PMID:12610306

   Open questions at the time

   • Structural details of the calnexin–EDEM transmembrane interface were not determined
   • Whether additional factors cooperate in substrate handoff from calnexin to EDEM was unresolved
6. 2006 Medium

   Proteomic mapping of the CFTR interactome placed calnexin within a broader Hsp90-centered chaperone network, suggesting cooperative folding surveillance in the ER rather than isolated chaperone action.

   Evidence Affinity purification mass spectrometry of CFTR-associated proteins plus siRNA rescue of ΔF508 CFTR surface expression

   PMID:17110338

   Open questions at the time

   • Calnexin's specific functional contribution within the Hsp90 chaperome was inferred from complex membership, not from direct calnexin perturbation
   • Stoichiometry and temporal order of chaperone handoffs were not defined
7. 2021 Medium

   Demonstrating that calnexin promotes MHC class I surface presentation and CD8⁺ T-cell-mediated cytotoxicity linked its ER chaperoning function to adaptive immune surveillance, revealing a physiological consequence of calnexin-dependent MHC-I folding.

   Evidence Luciferase reporter validation of miR-148a-3p targeting CANX mRNA, flow cytometric MHC-I surface quantification, in vitro and in vivo CD8⁺ T-cell killing assays in colorectal cancer models

   PMID:34324740

   Open questions at the time

   • Whether direct CANX knockout/rescue (rather than miRNA-mediated modulation) yields the same MHC-I phenotype was not shown
   • Contribution of calreticulin or other chaperones to the same MHC-I presentation pathway was not controlled for

• The structural basis of calnexin's transmembrane-mediated substrate recognition, the rules governing whether a substrate is retained for further folding attempts versus handed to EDEM for degradation, and the in vivo immune consequences of complete calnexin loss in humans remain unresolved.
• No high-resolution structure of a calnexin–substrate transmembrane complex exists
• Decision mechanism (timer vs. conformation sensor) that commits substrates to ERAD via EDEM is not established
• In vivo loss-of-function phenotype in human or mammalian genetic models is incompletely characterized