Edinburgh Napier University

  While it has been completely established that the AChR** as isolated from the electroplaques of electric fish is composed of four different polypeptides, such that its subunit structure is α2βγδ (as reviewed in detail elsewhere in this Volume), the evidence for this structure has so far been less complete for the related AChRs from the skeletal muscle of various vertebrates (Dolly and Barnard, 1984). Of course, sequencing of the cDNAs encoding several of these has revealed strong amino acid homology between their subunits and those of the Torpedo AChR (summarised by Kubo et al., 1985). However, what appears to be a full complement of the subunits of a muscle AChR has been identified, via their DNAs, so far only in calf (Kubo et al., 1985) and even there a difference from the α2βγδ structure has been revealed. Thus, an additional cDNA of bovine muscle was recently identified (Takai et al., 1985), which encodes a fifth polypeptide, designated the e subunit. Electrophysiological measurements on Xenopus oocytes injected with combinations of subunit-specific mRNAs, in conjunction with Northern blot analysis, have shown that the s subunit replaces the γ subunit during muscle development (Mishina et al., 1986). These findings have led to the conclusion that the fetal and adult forms of the receptor are α2βγδ and α2βδε respectively in their subunit complements. Even in this case, although there is considerable evidence on the subunits present in an AChR species isolated from bovine muscle (Conti-Tronconi et al., 1982a), a correspondence of each of the putative cDNA products with an identified subunit and a proof that they form two different oligomers as just noted, has not yet been demonstrated.