Recognition of inter-transmembrane regions of acetylcholine receptor alpha subunit by antibodies, T cells and neurotoxins. Implications for membrane-subunit organization.

Three regions of the alpha chain of Torpedo californica acetylcholine receptor (AChR), corresponding to residues alpha 262-276, alpha 388, 408 and alpha 427-437 were synthesized, purified and characterized. The first two peptides have been proposed to occupy inter-transmembrane regions while the third represented the C-terminal segment, proposed by various models to be either extracellular or intracellular. Peptide alpha 388-408 stimulated a good response in the AChR-primed T cells of H-2s haplotype mice, a low response in the H-2q haplotype and no response in the H-2b haplotype. Peptide alpha 427-437 stimulated AChR-primed T cells of the H-2s haplotype, but caused no response in the q and b haplotypes. Peptide alpha 262-276 evoked no in vitro stimulation in any of the s, q or b haplotypes. In antibody binding studies, peptide alpha 388-408 bound antibodies raised against free AChR or against membrane-bound AChR. The other two peptides showed little or no binding activity. Further, peptide alpha 388-408 bound specifically both 125I-labelled bungarotoxin and cobratoxin, while the other two peptides had no binding activity. These results were consistent with only one of the models for subunit organization within the membrane.