ARCN1

ARCN1 encodes the delta subunit of the COPI coatomer complex, which mediates intracellular protein transport between the endoplasmic reticulum and the Golgi apparatus (PMID:27476655). Loss-of-function heterozygous mutations in ARCN1 impair type I collagen transport and reduce collagen secretion, triggering ER stress and underlying a craniofacial/skeletal syndrome characterized by micrognathia, rhizomelic shortening, and microcephalic dwarfism (PMID:27476655). Independently, ARCN1 acts as a pro-viral host factor required for efficient influenza A virus internalization; its expression is suppressed by miR-33a through targeting of the ARCN1 3' UTR, and this suppression restricts viral replication (PMID:26498766). Beyond these two functions, no further mechanistic detail has been characterized in the available corpus.

1. 2015 Medium

   Whether ARCN1 contributes to influenza A virus infection was unknown; this work established it as a pro-viral host factor whose suppression by a host miRNA limits viral entry.

   Evidence Luciferase 3' UTR reporter assay, siRNA knockdown, and ectopic ARCN1 overexpression rescue in 293T and HeLa cells

   PMID:26498766

   Open questions at the time

   • The molecular step in internalization that requires ARCN1 is not defined
   • Whether ARCN1's COPI function is mechanistically involved in viral entry was not tested
   • No direct interaction between ARCN1 and viral components was demonstrated
2. 2016 Medium

   The cellular consequence of ARCN1 loss-of-function in human disease was unresolved; patient-derived cells linked heterozygous mutations to impaired collagen secretion and ER stress, providing the basis for the skeletal/craniofacial syndrome.

   Evidence ER stress assays and collagen secretion assays in ARCN1 mutant patient-derived cell lines; loss-of-function mutation analysis in a human genetics cohort

   PMID:27476655

   Open questions at the time

   • No in vitro reconstitution or structural validation of the delta-COP role was performed
   • Whether all secretory cargo or specifically collagen is affected was not delineated
   • The molecular basis for the tissue-specific (skeletal/craniofacial) phenotype is unexplained

• How ARCN1's role in COPI-mediated ER-Golgi transport mechanistically connects to its requirement during influenza A internalization remains unresolved.
• No structural model of ARCN1 within the COPI coatomer in the available corpus
• No defined direct binding partners or substrates beyond inferred collagen cargo
• Unclear whether the viral and secretory functions share a common molecular mechanism