Cloning and sequence analysis of calf cDNA and human genomic DNA encoding α-subunit precursor of muscle acetylcholine receptor

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Abstract
The nicotinic acetylcholine receptor (AChR) from fish electric organ is well characterized and is known to consist of five subunits present in a molar stoichiometry of α2βγδ (reviewed in refs 1–3). The mammalian skeletal muscle AChR is thought to have a similar subunit structure4–6. We have recently elucidated the primary structures of the α-, β-, γ- and δ-subunit precursors of the Torpedo californica AChR by cloning and sequencing cDNAs for these polypeptides7–9; cDNA sequences for the γ-subunit precursor of the T. californica AChR10 and the α-subunit precursor of the Torpedo marmorata AChR11,12 have also been reported by other groups. The four subunits exhibit conspicuous sequence homology and are similar in hydrophilicity profile and predicted secondary structure, thus being most probably oriented in a pseudosymmetric fashion across the membrane. The transmembrane topology of the subunit molecules and the locations of functionally important regions, such as the acetylcholine binding site and the transmembrane segments which may be involved in the ionic channel, have been proposed. We have now cloned cDNA for the α-subunit precursor of the calf skeletal muscle AChR and a human genomic DNA segment containing the corresponding gene. Nucleotide sequence analysis of the cloned DNAs has revealed the primary structures of the calf and human AChR α-subunit precursors, which exhibit marked sequence homology with their Torpedo counterpart. The protein-coding sequence of the human AChR α-subunit precursor gene is divided by eight introns into nine exons, which seem to correspond to different structural and functional domains of the subunit precursor molecule.