Subunit structure of the acetylcholine receptor from Electrophorus electricus.

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Abstract
The amino-terminal amino acid sequences of the 4 major peptides (MW 41,000, 50,000, 55,000 and 62,000) present in purified preparations of E. electricus nicotinic acetylcholine receptor (AcChoR) were determined for 24 cycles by automated sequence analysis procedures yielding 4 unique polypeptide sequences. The sequences showed a high degree of similarity, having identical residues in a number of positions ranging between 37-50% for specific pairs of subunits. Comparison of the sequences obtained with those of the subunits of similar MW from Torpedo californica AcChoR revealed an even higher degree of homology (from 46 to 71%) for these 2 highly diverged species. Simultaneous sequence analysis of the amino termini present in native, purified Electrophorus AcChoR showed that these 4 related sequences were the only ones present and that they occur in a ratio of 2:1:1:1, with the smallest subunit (.alpha.1) being present in 2 copies. Genealogical analysis suggests that the subunits of both Torpedo and Electrophorus AcChoRs derive from a common ancestral gene, the divergence having occurred early in the evolution of the receptor. This shared ancestry and the very early divergence of the 4 subunits, as well as the highly conserved structure of the AcChoR complex along animal evolution, suggest that each of the subunits evolved to perform discrete crucial roles in the physiological function of the AcChoR.