Translation-independent association of mRNAs that encode protomers of the 5-HT-mGlu2 receptor complex.

G protein-coupled receptors (GPCRs) constitute the largest family of plasma membrane proteins and regulate cell signaling by activating heterotrimeric G proteins. The serotonin 5-HT receptor (5-HTR) and the metabotropic glutamate 2 receptor (mGluR2) are GPCRs that play a pivotal role in processes related to perception, memory, and mood regulation. These receptors can interact to form heteromeric GPCR complexes through direct physical interactions, which modulate the signaling and trafficking properties of both protomers. Co-translational association of mRNAs encoding subunits of heteromeric ion channels has been reported, but whether complex assembly of GPCRs occurs during translation remains unknown. Here, our in vitro data reveals evidence of co-translational modulation in 5-HTR and mGluR2 mRNAs following siRNA-mediated knockdown. Interestingly, immunoprecipitation of either 5-HTR or mGluR2, using an antibody targeting epitope tags at their N-terminus, results in detection of both transcripts associated with ribonucleoprotein complexes containing RPS24. Additionally, we demonstrate that the mRNA transcripts of 5-HTR and mGluR2 associate autonomously of their respective encoded proteins. Validation of this translation-independent association is extended ex vivo using mouse frontal cortex samples. Together, these findings provide mechanistic insights into the co-translational assembly of GPCR heteromeric complexes in mammalian cells, unraveling regulatory processes governing protein-protein interactions and complex formation.