Primary structure of delta subunit precursor of calf muscle acetylcholine receptor deduced from cDNA sequence

Abstract

Clones carrying cDNA sequences for the δ subunit precursor of the acetylcholine receptor from calf skeletal muscle have been isolated. Nucleotide sequence analysis of the cloned cDNA has indicated that this polypeptide consists of 516 amino acids including a hydrophobic prepeptide of 21 amino acids. The δ subunit of the calf muscle acetylcholine receptor, like the α, β and γ subunits of the same receptor as well as the α and γ subunits of its human counterpart, exhibits structural features common to all four subunits of the Torpedo electroplax receptor, apparently being oriented across the membrane in the same manner as proposed for the fish receptor subunits. The degree of amino acid sequence homology between the calf and Torpedoδ subunits (60%) is comparable to that between the β subunits (59%) and to that between the γ subunits (56%), but is lower than that between the α subunits of the two species (81%). This suggests that the α subunit evolved more slowly than the three other subunits. A dendrogram representing the sequence relatedness among the four subunit precursors of the mammalian and fish acetylcholine receptors has been constructed. Some regions of the δ subunit molecule, including the region containing the putative disulphide bridge and that encompassing the clustered putative transmembrane segments M1, M2 and M3, are relatively well conserved between calf and Torpedo. The relative pattern of regional homology is similar for all four subunit precursors.