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Mel 1a melatonin receptors belong to the superfamily of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. So far, interest in Mel 1a receptor signaling has focused mainly on the modulation of the adenylyl cyclase pathway via pertussis toxin (PTX)-sensitive G proteins. To further investigate signaling of the human Mel 1a receptor, we have developed an antibody directed against the C terminus of this receptor. This antibody detected the Mel 1a receptor as a protein with an apparent molecular mass of approximately 60 kDa in immunoblots after separation by SDS-PAGE. It also specifically precipitated the 2-[¹²⁵I]iodomelatonin (¹²⁵I-Mel)-labeled receptor from Mel 1a-transfected HEK 293 cells. Coprecipitation experiments showed that G_i2, G_i3, and G_q/11 proteins couple to the Mel 1a receptor in an agonist-dependent and guanine nucleotide-sensitive manner. Coupling was selective …