Structural heterogeneity of the .alpha. subunits of the nicotinic acetylcholine receptor in relation to agonist affinity alkylation and antagonist binding

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Abstract
The structural basis for the heterogeneity of the two agonist binding sites of the Torpedo californica acetylcholine receptor with respect to antagonist binding and reactivity toward affinity alkylating reagents was investigated. There is one agonist binding site on each of the two .alpha. subunits in a receptor monomer. One of these sites is easily affinity labeled with bromoacetylcholine, while more extreme conditions are required to label the other. Evidence is presented that the site which is easily labeled with bromoacetylcholine is the site with higher affinity for the antagonist d-tubocurarine. Digestion of purified .alpha. subunits with staphylococcal V8 protease gave two limit fragments with apparent molecular weights of 17K and 19K. Both of these fragments began at residue 46 of the .alpha. sequence, and both reacted with monoclonal antibodies specific for the sequence .alpha. 152-159 but not with antibodies specific for .alpha.235-242. Their tryptic peptide maps and reactivity with a number of monoclonal antibodies were virtually identical. Only the 17-kilodalton (17-kDa) fragments stained heavily for sugars with Schiff''s reagent. However, both fragments bound 125I-labeled concanavalin A. Complete removal of carbohydrate detectable with concanavalin A from V8 protease digests of .alpha. subunits resulted in two fragments of lower apparent molecular weights, indicating that these fragments differed not only in carbohydrate content but also in their C-termini or by another covalent modification. Covalent labeling of one of the two agonist sites of the intact receptor with bromo[3H]acetylcholine followed by digestion with V8 protease resulted in labeling of only the 19-kDa fragment. When both agonist sites were affinity labeled, the 17-kDa fragment was also labeled. This suggests that the 19-kDa fragment is derived from one .alpha. subunit and the 17-kDa fragment from the other. It appears that while both .alpha. subunits of the receptor are glycosylated, one of the subunits is more heavily glycosylated than the other. The less glycosylated subunit contains the site of high reactivity with affinity alkylating reagents such as bromoacetylcholine and also the site with high affinity for d-tubocurarine, while the more heavily glycosylated subunit contains the site of low reactivity with these affinity reagents, which is also the site with low affinity for antagonists. The two .alpha. subunits might also differ by other covalent modifications.