Importance of the second extracellular loop for melatonin MT₁ receptor function and absence of melatonin binding in GPR50

Abstract

Our work demonstrated the crucial role of the E2 loop for MT receptor and GPR50 function by proposing a model in which the E2 loop is important in stabilizing active MT receptor conformations and by showing how evolutionary processes appear to have selected for modifications in the E2 loop in order to make GPR50 unresponsive to melatonin.

Melatonin receptor structures shed new light on melatonin research

“…The structures show that Q181 ECL2 (MT 1 )/ Q194 ECL2 (MT 2 ) forms hydrogen bonds with the alkylamide tail of melatonin ligands (Figure B), demonstrating for the first time the direct implication of ECL2 in ligand binding. Evolutionary conservation of Q ECL2 and mutagenesis studies further support the importance of this residue for melatonin binding . Interestingly, Q ECL2 is replaced by a tyrosine residue in GPR50 and replacement of this tyrosine by glutamine in a functionally inactive MT 1 chimera containing part of the ECL2 loop of GPR50 was sufficient to restore melatonin binding further highlighting the importance of this residue in the ECL2 …”

Detection of recombinant and endogenous mouse melatonin receptors by monoclonal antibodies targeting the C‐terminal domain

“…Melatonin receptors belong to the G protein‐coupled receptor superfamily, which preferentially couple to Gα i/o proteins . The melatonin receptor (MTR) subfamily is composed of three members in mammals: MT 1 and MT 2 , which are both binding to the neurohormone melatonin with high affinity, and GPR50, which has 70% sequence homology with MT 1 and MT 2 but lost the capacity to bind melatonin during evolution due to modifications in the second extracellular loop …”

“…1 The melatonin receptor (MTR) subfamily is composed of three members in mammals: MT 1 and MT 2 , which are both binding to the neurohormone melatonin with high affinity, 2 and GPR50, which has 70% sequence homology with MT 1 and MT 2 but lost the capacity to bind melatonin during evolution due to modifications in the second extracellular loop. 3,4 MT 1 and MT 2 are involved in various biological functions including the regulation of biological rhythms, sleep, pain, retinal, neuronal, and immune functions. 1,5 Melatonin receptor-knockout mice show deficits in many of these functions.…”

Recent developments in research of melatonin and its potential therapeutic applications

“…Their paper describes the results from MT 1 -GPR50 chimera studies, in combination with molecular modelling in silico, and leads to the conclusion that it is the primary and secondary structure of this loop that determines its ligand binding and option(s) for signal transduction. It turns out that GPR50 forms heterodimers with the MT 1 receptor, which blunts activation of this receptor; Clement et al (2018) propose that this could be important for research of melatonin and mental disorders.…”

“…In an accompanying research paper from this group, Clement et al (2018) provide evidence that the function of the MT 1 receptor depends on the structure of the second extracellular (E2) loop. This is gleaned from sequencing of the GPR50 (loss of function) orphan receptor, which binds melatonin in lower species but not mammals.…”