EMC8

EMC8 is a subunit of the ER membrane protein complex (EMC) that functions in the biogenesis of multi-pass transmembrane proteins (PMID:25715730, PMID:37196677). Within the EMC, EMC8 contributes to a holdase chaperone activity for the pore-forming CaV1.2 subunit, engaging the channel through two defined contacts—a transmembrane dock and a cytoplasmic dock—that partially extract the pore subunit and splay open the CaVα2δ-interaction site; because EMC and CaVα2δ binding are mutually exclusive, the EMC stages channel assembly via a divalent ion-dependent hand-off to CaVα2δ, and disrupting the EMC–CaV interaction compromises channel function (PMID:37196677). Consistent with this role in transmembrane protein topogenesis, EMC (including EMC8/9) is required for the stable expression and folding of multi-pass membrane proteins such as rhodopsins, TRP, and Na+/K+-ATPase but not secreted or single-pass type I proteins, with loss causing rhabdomere degeneration in Drosophila photoreceptors (PMID:25715730). EMC8 (interchangeably with its paralog EMC9) is also required for neural crest development and craniofacial cartilage formation (PMID:37318954).

1. 2015 Medium

   Established that the EMC, including the EMC8/9 module, is selectively required for biogenesis of multi-pass transmembrane proteins rather than secreted or single-pass proteins, defining its client specificity in vivo.

   Evidence Genetic loss-of-function screen in Drosophila photoreceptors with ER localization and Co-IP of EMC subunits with calnexin

   PMID:25715730

   Open questions at the time

   • Does not separate the specific contribution of EMC8 from other EMC subunits
   • No structural mechanism for client recognition
   • Rhodopsin/TRP clients defined in Drosophila, not human EMC8
2. 2023 High

   Resolved the molecular mechanism by which the EMC acts as a holdase, showing it binds CaV1.2 through transmembrane and cytoplasmic docks and stages assembly via a mutually exclusive hand-off to CaVα2δ.

   Evidence Cryo-EM structures of human CaV1.2–CaVβ3–EMC and assembled CaV1.2–CaVβ3–CaVα2δ-1 channel plus functional disruption assays

   PMID:37196677

   Open questions at the time

   • Specific catalytic or structural role of the EMC8 subunit within the complex not isolated
   • Molecular identity of the divalent ion-dependent step not defined
   • Whether the same hand-off logic applies to other EMC clients untested
3. 2023 Low

   Linked EMC8 function to a developmental program by showing it (or paralog EMC9) is required for neural crest and craniofacial cartilage formation, connecting transmembrane protein topogenesis to organismal patterning.

   Evidence Morpholino-based depletion in Xenopus tropicalis with craniofacial and neural crest phenotypic assays

   PMID:37318954

   Open questions at the time

   • Cannot resolve EMC8-specific requirement versus EMC9 substitution
   • No identification of the client proteins responsible for the developmental phenotype
   • Morpholino depletion lacks genetic confirmation

• The specific structural and functional contribution of the EMC8 subunit itself—as distinct from the whole EMC and from its paralog EMC9—remains undefined.
• No EMC8-specific client repertoire established
• Functional non-redundancy between EMC8 and EMC9 not delineated
• No structural assignment of EMC8 contacts to client docking