G protein-coupled receptors (GPCRs) are transmembrane proteins that play a pivotal role in cellular signaling and are involved in a wide range of physiological processes. As the largest family of membrane receptors in the human genome, GPCRs are invaluable targets for drug development and an important subject of research. While GPCRs can function as monomers, they can also form oligomeric complexes. Human melatonin receptors MT₁ and MT₂ are known to work as homo-and heterodimers. However, the fact that the two subtypes produce dissimilar signals in their monomeric states raises questions about whether G proteins preferentially bind to one of the protomers in MT₁/MT₂ heterodimers. To explore this issue, signaling-defective mutant receptors and G protein pre-fused receptors were used to l...[ Read more ]

G protein-coupled receptors (GPCRs) are transmembrane proteins that play a pivotal role in cellular signaling and are involved in a wide range of physiological processes. As the largest family of membrane receptors in the human genome, GPCRs are invaluable targets for drug development and an important subject of research. While GPCRs can function as monomers, they can also form oligomeric complexes. Human melatonin receptors MT₁ and MT₂ are known to work as homo-and heterodimers. However, the fact that the two subtypes produce dissimilar signals in their monomeric states raises questions about whether G proteins preferentially bind to one of the protomers in MT₁/MT₂ heterodimers. To explore this issue, signaling-defective mutant receptors and G protein pre-fused receptors were used to limit the downstream signaling pathway to one protomer. In Ca²⁺ mobilization assays, MT₁ exhibited a stronger response than MT₂, whether it was fused with G protein or not, suggesting that MT₁ plays a primary role in the heterodimer of melatonin receptors. The signal from MT2 was further diminished when replaced with mutants and became desensitized to antagonists. Additionally, while the C-terminal region is known to be directly involved in G protein recruitment, the crucial amino acid residues remain unclear. Therefore, progressive truncations(5-6 residues each) were performed at the C-terminal to test their signal transduction efficiency. The effects of C-terminal tail deletion were observed through ERK 1/2 activation and cAMP assays. These findings provide insight into MT₁ being the predominant protomer in MT₁/MT₂ heterodimers, and a clearer understanding of the melatonin receptor signaling mechanism can be attained through further investigation of the C-termini region of melatonin receptors.