Transmembrane Domain Dominance Drives Emergent Signaling and Allosteric Inversion in mGlu1/5 Heterodimers

Class C GPCRs function as obligate dimers in which only one G protein can engage the complex at a time, but how each protomer contributes to heterodimer coupling has remained unresolved. Using CODA-RET, a BRET-based assay reporting direct Gq recruitment to defined, full-length receptor pairs, we show that signaling at the mGlu/ heterodimer flows predominantly through the mGlu protomer; domain-swapped chimeras localize this dominance to the transmembrane domain. The dominance generates emergent signaling: cis-acting mGlu PAMs and NAMs undergo allosteric inversion when coupling is restricted to mGlu. By contrast, the mGlu-selective NAM MTEP is silent at the heterodimer, mirroring mGlus minimal role in driving Gq. Because the mGlu PAM tested acts only in cis, a trans-acting mGlu PAM would theoretically be selective for mGlu/ homomers. These findings open a pharmacological design space in which protomer target and cis-versus-trans mode of action tune selectivity across mGlu/, mGlu/, and mGlu/ dimers.