Structural similarities between acetylcholine receptors from fish electric organs and mammalian muscle

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Abstract
Acetylcholine [Ach] receptors from fish electric organ tissue and mammalian muscle were compared by peptide mapping. The .alpha. subunits from receptors of Torpedo california and Electrophorus electricus electric organ tissue were digested with V8 protease and the resulting fragments separated on polyacrylamide gels and stained for protein or for carbohydrate. 125I-Labeled .alpha. subunits of ACh receptors from Electrophorus electric organ tissue and bovine muscle were digested with V8 protease, and the resulting fragments were also separated on polyacrylamide gels. Intact receptors from both the fish electric organs and mammalian muscle were labeled with [4-(N-maleimido)benzyl]tri[3H]-methylammonium iodide which binds specifically to the ACh binding site on .alpha. subunits, and the isolated .alpha. subunits were subjected to the same peptide mapping procedure. The fragment patterns produced were stained for protein and fluorographed to identify active site containing polypeptides. None of these peptide mapping approaches revealed extensive homologies between .alpha. subunits. Intact and V8 proteolyzed sodium dodecyl sulfate denatured receptors from Torpedo and Electrophorus electric organs and bovine muscle were electrophoretically transferred to diazophenyl thioether paper and probed with antisera to Torpedo receptor subunits and 2 monoclonal antibodies. Unique fragment patterns were obtained with each antisubunit serum. A fragment of the same size was derived from the .beta. subunit of each Ach receptor and specifically bound the same monoclonal antibody in each case. Only in the .beta. subunits from all of the species examined is a large sequence nearly identical. It is likely that corresponding receptor subunits from receptors of all of these species have extensive structural homologies.