The structure and properties of the purified muscarinic acetylcholine receptor from rat forebrain.

The muscarinic acetylcholine receptor (mAChR) of both brain and myocardium is an integral membrane sialoglycoprotein of molecular mass approx. kDa (Birdsall r t al., 1979; Berrie ef al., 1985h; Haga & Haga, 1985; Peterson et al., 1986). The carbohydrate moiety has been estimated to contribute between 6 and 20 kDa to the overall molecular mass (Rauh CI al., 1986; Peterson et al., 1986). Variations in the extent or character of glycosylation may account for part of the molecular mass heterogeneity which has been reported for the mAChR in exocrine glands (Hootman ef al., 1985). The similarity of the gross molecular properties of mAChR's in several different tissues and species has been emphasized (Venter, 1983). Yet. the receptor is pleiotropic, regulating several different tissue responses through coupling to apparently distinct GTP-binding proteins; these directly G-coupled responses include inhibition of adenylate cyclase (Jakobs c't a/., 1979; Hulme et al., 1981), hydrolysis of polyphosphoinositides (Sasaguri et al., 1985) and the opening of K + channels (Pfaffinger et al., 1985, Breitweiser & Szabo, 1985). I t is possible that several different, but similar, mAChR gene products with specialized effector recognition sites are needed to interact selectively with these distinct mechanisms (Michell & Houslay, 1986), although it appears that the purified porcine brain receptor can activate both Gi and Go (Haga et al.. 1985; Kurose et ul.. 1986). A further complicating factor is the occurrence of subtypes of mAChR's, which are differentiated by selective antagonists such as pirenzepine (Pz) (Hammer P I al., 1980; Birdsall & Hulme, 1983) but not by 'classical' antagonists such as N-methylscopolamine (NMS) (Hulme et al., 1978). For Pz, the rank order of potency for mAChR in different tissues is forebrain/autonomic ganglia > exocrine glands (e.g. lacrimal) > myocardium. After solubilization of the receptors in digitonin, the selectivity of Pz, although still present, is altered, the rank order now being forebrain > myocardium > lacrimal (Berrie et al., 1985a, 1986; Birdsall (11 ul., 1986). These tissues express different balances between the mAChR-response coupling mechanisms, yet no clear linkage can be identified between the different subclasses of Pz-binding sites and individual coupling mechanisms (Brown P I al., 1985; Harden P I al., 1986). The origin of the subclasses of Pz-binding sites is therefore unknown. The resolution of these questions evidently requires the purification of the mAChR's from different tissues, elucidation of their primary structures, and pharmacological repertoire, and determination of the number of gene products expressed.