MMGT1

MMGT1 (EMC5/EMC5) is a two-transmembrane subunit of the ER membrane complex (EMC) that supports the co-translational biogenesis of multi-pass membrane proteins (PMID:32494008, PMID:31263175). Within the EMC, the yeast ortholog Emc5 contributes to a five-transmembrane-helix fold centered on Emc3 that structurally resembles the prokaryotic YidC insertase and shapes a hydrophilic client-protein pocket required for transmembrane helix insertion (PMID:32494008). Consistent with an insertase role, loss of the Drosophila ortholog impairs rhodopsin biosynthesis and causes photoreceptor degeneration independent of the ERAD pathway (PMID:31263175). Independent of its EMC function, MMGT1 mediates saturable Mg2+ transport when heterologously expressed, with localization to the Golgi and post-Golgi/endosomal compartments that increases under low-magnesium conditions (PMID:18057121). In macrophages, MMGT1 restricts lipid droplet accumulation through the orphan GPCR GPR156 axis, and its loss promotes Mycobacterium tuberculosis persistence via upregulated triacylglycerol synthesis (PMID:37269834).

1. 2007 Medium

   Established a first molecular function for MMGT1 by showing it can move Mg2+ across membranes, framing it as a candidate magnesium transporter.

   Evidence Heterologous expression in Xenopus oocytes with two-electrode voltage-clamp electrophysiology and fluorescence Mg2+ measurements

   PMID:18057121

   Open questions at the time

   • Single study, not independently replicated
   • Transport mechanism (channel vs carrier) and physiological substrate range unresolved
   • Does not reconcile transport activity with later EMC subunit role
2. 2007 Medium

   Placed the protein in the secretory pathway by mapping its localization to Golgi and post-Golgi vesicles and linking abundance to magnesium availability.

   Evidence Immunohistochemistry of HA-tagged MMgT1 in transfected COS-7 cells with real-time RT-PCR and fractionation

   PMID:18057121

   Open questions at the time

   • Relies on overexpressed tagged protein in a single lab
   • Does not establish endogenous localization or ER-resident EMC pool
   • Mechanism linking low Mg2+ to increased expression unknown
3. 2019 High

   Demonstrated an in vivo requirement of the EMC5 ortholog for membrane protein biogenesis, showing rhodopsin synthesis depends on it independent of ERAD.

   Evidence Genome-wide genetic screen and loss-of-function alleles in Drosophila with phototransduction assays and ERAD epistasis

   PMID:31263175

   Open questions at the time

   • Client repertoire beyond rhodopsin not defined
   • Molecular step (insertion vs folding vs stability) not resolved in this system
4. 2020 High

   Resolved the structural basis of EMC5 function, showing it forms part of a YidC-like transmembrane insertase architecture that creates a client pocket for TMH insertion.

   Evidence Cryo-EM structure of S. cerevisiae EMC with validating mutagenesis

   PMID:32494008

   Open questions at the time

   • Structure is of the yeast complex; human MMGT1-specific contacts inferred
   • Does not address the reported Golgi Mg2+ transport activity
   • Dynamics of client engagement not captured
5. 2023 High

   Connected MMGT1 to host defense, showing it suppresses macrophage lipid droplet accumulation through GPR156 to limit M. tuberculosis persistence.

   Evidence Genome-wide CRISPR screen, MMGT1-knockout macrophages, transcriptomics, lipid droplet imaging, and pharmacological targeting of triacylglycerol synthesis

   PMID:37269834

   Open questions at the time

   • Mechanistic link between MMGT1 loss and GPR156 induction not defined
   • Whether the phenotype derives from EMC insertase function or Mg2+ transport is unresolved

• It remains unresolved how MMGT1's role as an EMC insertase subunit relates mechanistically to its reported Golgi/post-Golgi Mg2+ transport activity and its macrophage lipid-droplet phenotype.
• No reconciliation of ER EMC localization with Golgi Mg2+ transport
• Human structure and direct client list not established
• Whether Mg2+ transport is an intrinsic activity or indirect remains open