Molecular determinants for the differential coupling of Gα16 to the melatonin MT1, MT2 and Xenopus Mel1c receptors

The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G16. Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the α subunit of G16 (Gα16) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Gα16 even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Gα16. Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. All chimeras were fully capable of binding 2-[125I]iodomelatonin and inhibiting adenylyl cyclase. Chimeras containing the full-length Mel1c tail were incapable of activating Gα16, while those that contained the complete C-terminal region of either MT1 or MT2 stimulated PLC. Incorporation of the extra portion of the C-terminal tail of Mel1c to either MT1 or MT2 completely abolished the chimeras' ability to stimulate PLC via Gα16. In contrast, trunc- ation of the C-terminal tail of Mel1c allowed interaction with Gα16. Our results suggest that Gα16 can discern structural differences amid the three melatonin receptors and provide evidence for functional distinction of Mel1c from MT1 and MT2 receptors.