Serine residues 110 and 114 are required for agonist binding but not antagonist binding to the melatonin MT1 receptor

Site-directed mutation of serine 110 (Ser(3.35)) and serine 114 (Ser(3.39)) in the human melatonin MT1, receptor to alanine residues reduced ligand bi nding affinities of seven known melatonin receptor agonists and partial ago nists by 3- to 15-fold. These mutants also displayed a relative reduction i n their affinities for melatonin-mediated functional responses of 30- and 1 4-fold, respectively. In contrast to the observed effects of the agonists a nd partial agonists, the melatonin receptor antagonist luzindole was found to bind to mutants Ser(3.35)Ala and Ser(3.39)Ala with affinities equivalent to that determined for the wild-type melatonin MT1 receptor. Luzindole was subsequently confirmed as an antagonist of melatonin-mediated functional r esponses for both mutant receptors. These studies have identified that in t he human melatonin MT1 receptor, Ser(3.35) and Ser(3.39), in transmembrane domain 3, are critical for the formation of the high-affinity ligand bindin g site for agonists and partial agonists but not for the antagonist luzindo le. (C) 2001 Academic Press.