A 1,350-base-pair-long cDNA clone, named pa-2, was isolated by hybridization to the previously characterized clone pa-i and found to be specific for the a-subunit of the Torpedo marmorata acetylcholine receptor. The nucleotide sequences of both cDNA inserts were analyzed and the sequence of the complete coding region and part of the 5' and 3' untranslated regions of the a-chain mRNA was determined. The complete amino acid sequence of the a-chain precursor is presented and used to develop a model for the transmembrane organization of the polypeptide.

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Consistent with the model of an H+ cotransport, amino acid uptake can be driven by a proton gradient generated by an efflux of sugar when the normal energy sources are suppressed. Heterologous countertransport is completely inhibited by uncouplers unlike homologous countertransport. Positive coupling was obtained with methyl thiogalactoside/proline, methyl thiogalactoside/phenylalanine, gluconate/proline; however, the poor coupling efficiency suggests a more complex sequence of reactions.

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