Melatonin is a hormone mainly produced by the pineal gland and
MT1 is one of the two G protein-coupled receptors (GPCRs)
mediating its action. Despite an increasing number of available GPCR
crystal structures, the molecular mechanism of activation of a large
number of receptors, including MT1, remains poorly understood.
The purpose of this study is to elucidate the structural elements
involved in the process of MT1’s activation using
naturally occurring variants affecting its function. Thirty-six nonsynonymous
variants, including 34 rare ones, were identified in MTNR1A (encoding MT1) from a cohort of 8687 individuals and
their signaling profiles were characterized using Bioluminescence
Resonance Energy Transfer-based sensors probing 11 different signaling
pathways. Computational analysis of the experimental data allowed
us to group the variants in clusters according to their signaling
profiles and to analyze the position of each variant in the context
of the three-dimensional structure of MT1 to link functional
selectivity to structure. MT1 variant signaling profiles
revealed three clusters characterized by (1) wild-type-like variants,
(2) variants with selective defect of βarrestin-2 recruitment,
and (3) severely defective variants on all pathways. Our structural
analysis allows us to identify important regions for βarrestin-2
recruitment as well as for Gα12 and Gα15 activation. In
addition to identifying MT1 domains differentially controlling
the activation of the various signaling effectors, this study illustrates
how natural variants can be used as tools to study the molecular mechanisms
of receptor activation.