EMC4

EMC4 (TMEM85) is a transmembrane subunit of the ER membrane complex (EMC) insertase, where its transmembrane domain tilts away from the main transmembrane region and remains partially mobile; this structural flexibility is functionally required for the EMC to insert transmembrane helices into the ER membrane (PMID:32494008). Beyond its core insertase role, EMC4 functions at ER–endosome interfaces: it engages late-endosome-associated Rab7 and binds ER-resident syntaxin18 to support late-endosome-to-ER targeting of SV40 polyomavirus (PMID:32111841), and it mediates ER-to-endosome transfer of phosphatidylserine that promotes fusion of dengue virus with endosomal membranes during viral entry (PMID:35834589). Heterologous expression studies establish a conserved cytoprotective function, with EMC4 promoting growth and viability under oxidative stress (PMID:18586032). EMC4 physically interacts with the GLUT9 urate transporter isoforms but does not regulate urate uptake (PMID:31695625), and its ablation reduces α-synuclein aggregation by enhancing ER-driven autophagic flux and lysosomal clearance (PMID:41911287).

1. 2008 Medium

   Established the first functional readout for EMC4 by showing it confers cytoprotection, revealing a conserved role in the oxidative stress response.

   Evidence Heterologous expression of human TMEM85 and its yeast ortholog in S. cerevisiae with viability assays under H2O2 stress

   PMID:18586032

   Open questions at the time

   • No molecular mechanism linking EMC4 to oxidative-stress protection identified
   • Did not connect protective effect to EMC insertase activity
2. 2019 Medium

   Identified a direct physical partner of EMC4 (GLUT9) while delineating the boundary of its function, showing binding does not equate to transport regulation.

   Evidence Dual-membrane yeast two-hybrid screen, co-IP in HEK293T and Xenopus oocytes, and urate transport assays

   PMID:31695625

   Open questions at the time

   • Functional consequence of the EMC4–GLUT9 interaction unknown
   • No structural basis for the interaction defined
3. 2020 High

   Defined the structural mechanism of EMC4 within the insertase, establishing that its transmembrane flexibility is essential for transmembrane helix insertion.

   Evidence Cryo-EM structure of the yeast EMC combined with mutagenesis functional assays

   PMID:32494008

   Open questions at the time

   • Substrate specificity contributed by EMC4 not resolved
   • How EMC4 mobility couples to the catalytic cycle not detailed
4. 2020 Medium

   Extended EMC4 function beyond insertase activity to membrane tethering, showing it bridges late endosomes and ER via Rab7 and syntaxin18 to support viral trafficking.

   Evidence Co-IP of EMC4 with Rab7 and syntaxin18, plus siRNA knockdown with SV40 infection and trafficking readouts

   PMID:32111841

   Open questions at the time

   • Whether tethering is independent of the full EMC complex unclear
   • Single lab; physiological (non-viral) role of the contact site untested
5. 2020 Low

   Placed EMC4 as a genetic suppressor of eIF2B-mediated translational control defects, linking it to stress-response phenotypes.

   Evidence High-copy suppressor screen and overexpression in eIF2B mutant yeast with growth/phenotype assays

   PMID:32476094

   Open questions at the time

   • Genetic epistasis only; no direct biochemical mechanism established
   • Connection to eIF2B translational control is indirect
   • Not validated in mammalian cells
6. 2022 Medium

   Defined a lipid-transfer mechanism for EMC4, showing it moves phosphatidylserine from ER to endosomes to enable viral membrane fusion.

   Evidence siRNA knockdown of EMC4 with DENV infection/fusion assays and phosphatidylserine transfer assays

   PMID:35834589

   Open questions at the time

   • Direct lipid-transfer activity of EMC4 versus indirect facilitation not distinguished
   • Single lab; structural basis of PS transfer unknown
7. 2025 Medium

   Connected EMC4 to proteostasis, showing its loss reduces α-synuclein aggregation by boosting ER-driven autophagic and lysosomal clearance.

   Evidence Arrayed CRISPR knockout screen in HEK293 and iPSC-derived neurons with pSyn129 microscopy, autophagic flux and lysosomal clearance assays

   PMID:41911287

   Open questions at the time

   • Molecular link between EMC4 ablation and enhanced autophagic flux not defined
   • Whether the effect requires EMC insertase activity unknown

• How EMC4's core insertase function mechanistically connects to its tethering, lipid-transfer, cytoprotective, and proteostasis roles remains unresolved.
• No single mechanism unifies the distinct EMC4 activities
• Unclear which functions require the intact EMC versus EMC4 alone