Primary structure of gamma subunit precursor of calf-muscle acetylcholine receptor deduced from the cDNA sequence

Clones carrying cDNA sequences for the gamma subunit precursor of the acetylcholine receptor from calf skeletal muscle have been isolated. Nucleotide sequence analysis of the cloned cDNA has revealed the primary structure of this polypeptide, which consists of 519 amino acids including a hydrophobic prepeptide of 22 amino acids. The gamma subunit of the calf muscle acetylcholine receptor, like the alpha subunit of the calf as well as the human muscle receptor, shares features characteristic of all four subunits of the Torpedo electroplax receptor, such as the putative disulphide bridge corresponding to that in the alpha subunit proposed as being in close proximity to the acetylcholine binding site and the four putative, hydrophobic transmembrane segments M1-M4. Thus, the calf gamma subunit molecule apparently exhibits the same transmembrane topology as proposed for the fish receptor subunits. The degree of sequence homology between the calf and Torpedo gamma subunits (56%) is lower than that between the alpha subunits of the two species (81%). Some regions of the gamma subunit molecule, including the region encompassing the putative disulphide bridge and the region containing the putative transmembrane segments M1, M2 and M3, are relatively well conserved between the two species.