Cloning and sequence analysis of human genomic DNA encoding γ subunit precursor of muscle acetylcholine receptor


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Human genomic DNA encoding the γ subunit precursor of the skeletal muscle acetylcholine receptor has been cloned by screening a gene library with a calf cDNA probe and has been subjected to nucleotide sequence analysis. Comparison of the nucleotide sequence of the cloned human genomic DNA with that of the calf cDNA has revealed that the protein-coding sequence of this gene is divided by 11 introns into 12 exons. Evidence is presented to suggest that the human muscle acetylcholine receptor γ and δ subunit genes are juxtaposed. The primary structure of the γ subunit precursor of the human muscle acetylcholine receptor has been deduced from the corresponding gene sequence. This polypeptide is composed of 517 amino acids including a hydrophobic prepeptide of 22 amino acids. The γ subunit of the human muscle acetylcholine receptor, like the α subunit of the same receptor as well as the α, β and γ subunits of its calf counterpart, shares structural features common to all four subunits of the Torpedo electroplax receptor, such as the putative disulphide bridge corresponding to that in the α subunit proposed as being in close proximity to the acetylcholine binding site and the four putative, hydrophobic transmembrane segments M1 – M4. Thus, the human γ subunit molecule apparently exhibits the same transmembrane topology as proposed for the fish receptor subunits. The 12 exons seem to correspond to different structural and functional domains of the γ subunit precursor molecule. Some exons and the protein regions encoded by them are more highly conserved between the mammalian and Torpedo sequences. The pattern of regional homology observed is consistent with the relatively high conservation of the region encompassing the putative disulphide bridge and of the region containing the putative transmembrane segments M1, M2 and M3.