EMB

EMBIGIN (EMB/GP70) is a transmembrane glycoprotein that acts as the dedicated ancillary protein for monocarboxylate transporter 2 (MCT2), forming a complex distinct from the CD147/basigin partnership that supports MCT1 and MCT4 (PMID:15917240). The identity of this ancillary protein determines the catalytic properties of the transporter it serves: the MCT2/EMB complex is insensitive to organomercurial inhibition by pCMBS, and rat erythrocytes in which MCT1 associates with EMB rather than CD147 are likewise pCMBS-insensitive, establishing that EMB dictates MCT functional behavior (PMID:15917240). EMB further supports proton-coupled monocarboxylate transport by directly binding carbonic anhydrase IV (CAIV) through Arg-130 in its extracellular Ig1 domain, which hydrogen-bonds to His-88 of CAIV—the residue central to CAIV's intramolecular proton shuttle—and this interaction is required for CAIV-mediated facilitation of MCT lactate/pyruvate transport (PMID:30446621). Beyond its role in chaperoning MCT2 and bridging CAIV to enable proton-coupled transport, no further mechanistic detail has been characterized in the available corpus.

1. 2005 High

   It was unknown which ancillary protein supports MCT2; this work established EMB/GP70 as the dedicated partner distinct from CD147 and showed that ancillary-protein identity, not the transporter alone, sets MCT catalytic sensitivity.

   Evidence Co-immunoprecipitation and FRET between fluorescent-tagged MCT2 and GP70, with organomercurial (pCMBS) inhibition assays across cell lines and rat erythrocytes

   PMID:15917240

   Open questions at the time

   • Structural basis of the MCT2/EMB interface not resolved
   • Stoichiometry and trafficking mechanism of complex assembly not defined
   • Whether EMB performs functions independent of MCT chaperoning not addressed
2. 2018 High

   How EMB contributes to proton-coupled transport was unclear; this work showed EMB directly recruits carbonic anhydrase IV via a specific Ig1-domain residue, linking the proton-handling enzyme to the transporter.

   Evidence Pulldown assays, site-directed mutagenesis of GP70 (Arg-130) and CAIV (His-88), and transport measurements in HEK-293 cells and Xenopus oocytes

   PMID:30446621

   Open questions at the time

   • Interaction mapped in rat GP70; conservation of Arg-130 contact in human EMB not established
   • No high-resolution structure of the EMB–CAIV interface
   • Quantitative contribution of CAIV recruitment versus chaperone function to net transport not separated

• Whether EMB has roles beyond MCT chaperoning and CAIV recruitment, including any developmental or signaling function, remains unaddressed in the available corpus.
• No data on EMB function outside the MCT/CAIV context
• Tissue-specific roles not characterized
• No structural model of full-length EMB