Molecular weight and structural nonequivalence of the mature ɑ subunits of Torpedo californica acetylcholine receptor

A discrepancy of about 20% exists between the molecular weight of the ɑ subunit of Torpedo californica electroplax acetylcholine receptor as determined by gel electrophoresis of the mature protein (M_r 40,000 ± 2000) and by nucleotide sequence analysis of cDNA (M_r ≈z50,000). We demonstrate by amino acid sequence analysis that post-translational processing does not occur and that the mature subunit has a M_r of ≈50,000. The functional acetylcholine receptor contains two copies of this ɑ subunit in addition to one each of related β, y, and δ subunits. The binding sites for cholinergic ligands that are located on the a subunits have been shown to be nonequivalent. Amino acid sequence analysis of peptides obtained by proteolytic cleavage of the a subunit reveals that N-asparagine glycosylation at a single site (residue 141) occurs to a different extent in the two copies of this polypeptide in the mature protein and provides an explanation for nonequivalence of their binding sites