Original Manufacturer High Purity MT2

MT1 and MT2 both belong to the G protein-coupled receptor (GPCR) superfamily. Their transmembrane structure consists of seven α-helices, forming a typical "serpentine" topology. When melatonin binds to the orthoform site of its receptor, it triggers a conformational change, activating different G protein isoforms: MT1 primarily couples to the Gi/o protein, inhibiting adenylate cyclase (AC) activity, reducing intracellular cAMP levels, and thus inhibiting the phosphorylation cascade of protein kinase A (PKA); MT2, on the other hand, couples to both Gi/o and Gq proteins, inhibiting cAMP and activating phospholipase C (PLC), promoting IP3 and DAG production, and inducing calcium ion influx. This "dual-track signaling" makes MT1 and MT2 functionally complementary-MT1 dominates the "inhibitory signaling," while MT2 possesses both "inhibition + excitation" regulatory capabilities.

Although MT1 and MT2 share up to 60% amino acid sequence homology, key differences in their transmembrane regions determine their functional specificity. MT1's third intracellular loop (ICL3) is rich in basic amino acids, making it more readily bound to Gi/o proteins; while MT2's ICL3 contains unique cysteine ​​residues, which can form disulfide bonds to stabilize Gq protein interactions. Furthermore, MT2's C-terminus contains a cluster of phosphorylation sites, which can be phosphorylated by G protein-coupled receptor kinase (GRK), thereby recruiting β-arrestin protein and mediating receptor endocytosis and signal termination. These structural "molecular fingerprints" result in significant differences between MT1 and MT2 in signal duration and intensity.

MT2's core function lies in "phase shift," that is, adjusting the circadian rhythm phase. At night, melatonin activates MT2 receptors, inhibiting the firing activity of SCN neurons and delaying the "start time" of the biological clock, allowing the body to adapt to a late-to-bed, late-to-wake rhythm; while before dawn, MT2 activation advances the phase, helping the body adapt to an early-to-bed, early-to-wake pattern. This "phase shift" mechanism relies on MT2's inhibition of cAMP and regulation of calcium ions: decreased cAMP levels reduce CREB protein phosphorylation, thereby inhibiting the transcription of Per1/Per2 genes; while calcium ion influx activates calmodulin kinase (CaMK), promoting Cry gene expression, ultimately achieving a "time reset" of the biological clock.

From an evolutionary perspective, the functional differentiation between MT1 and MT2 is driven by positive selection. The MT1 gene is highly conserved across species, mainly undergoing purifying selection, retaining its basal response to melatonin; while the MT2 gene exhibits significant positive selection characteristics, with higher amino acid substitution frequencies in its transmembrane region and intracellular loop, suggesting a more flexible regulatory role in adapting to changes in environmental photoperiod. This "conservatism + innovation" evolutionary strategy allows MT1 and MT2 to maintain their basal rhythms while coping with light and dark variations at different latitudes, providing molecular-level adaptive advantages for the survival and reproduction of organisms.

In summary, MT1 and MT2 act like "dual engines" in the circadian rhythm system. Through G protein-coupled "dual-track signals," structural differences forming "molecular fingerprints," phase shifts providing "time calibration," and evolutionary "positive selection," they collectively construct the life's mechanism for perceiving and responding to time. They are not only key targets for sleep regulation but also "molecular bridges" connecting environmental signals and cellular metabolism, offering novel intervention strategies for the treatment of insomnia, depression, and metabolic syndrome.