ABSTRACT
From the earlier days of ligand-receptor binding studies, opioid receptors belonging to the family of membrane receptors which transduce their signals via another family of membrane proteins, G-proteins, has been postulated. Until recently, extensive efforts by many laboratories either using conventional protein purification techniques or heterologous expression methods did not result in isolating cDNA clones which might have structures related to the G-protein receptors, or cDNA clones which when expressed in eukaryotic cells resulted in opioid receptor activities. Using heterologous expression cloning strategy and ligand binding activity to monitor expressed receptors, Evans et al. (1992) and Kieffer et al. (1992) independently reported cloning of ¿-opioid receptor from neuroblastoma x glioma NG108-15 hybrid cells. The product of cloned receptor cDNA when expressed in COS7 cells, stereoselectively binds ¿-opioid ligands with high affinities and could inhibit intracellular production of cAMP when activated by agonist (Evans et al., 1992) and could be demonstrated to be ¿2-opioid receptor subtype in CHO cells stably expressing the cloned receptor (Law et al., 1994). As discussed in Chapter 3 on cloning of opioid receptors, the most striking feature is that the 372 amino acids protein coded by cloned opioid receptor cDNA has putative seven transmembrane domains, extracellular N-terminus with two putative glycosylation sites, third cytosolic loop with amphiphilic helical structures which could interact with G-proteins. Similar structures were also deduced from /¿-opioid receptor (Chen et al., 1993) and K-opioid receptor (Yasuda et al., 1993) cDNAs which were later cloned with the reported ¿-opioid receptor sequence. The putative structures of these opioid receptors suggest that opioid receptors do belong to the family of receptors which transduce their signal via heterotrimeric G-proteins. In one sense, the cloning of opioid receptors validated all biochemical studies in which involvement of G-proteins in opioid receptors' function are documented and that opioid receptors belong to the family of G-protein coupled receptors.
