Structure and transmembrane nature of the acetylcholine receptor in amphibian skeletal muscle as revealed by cross-reacting monoclonal antibodies.

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Abstract
A collection of 126 monoclonal antibodies (mAb) made against acetylcholine receptors (AChR) from the electric organs of Torpedo californica or Electrophorus electricus was tested for cross-reactivity with AChR in cryostat sections of skeletal muscle from Rana pipiens and Xenopus laevis by indirect immunofluorescene. mAb (49, 39%) cross-reacted with AChR from Rana, and 25 mAb (20%) cross-reacted with AChR from Xenopus. mAb specific for each of the 4 subunits of electric organ AChR (.alpha., .beta., .gamma., .delta.) cross-reacted with AChR from each amphibian species. mAbs cross-reacting with Xenopus AChR were, with 1 exception, a subset of the mAb cross-reacting with Rana AChR. The major difference detected between the 2 spp. was in binding by mAb specific for the main immunogenic region (MIR) of the .alpha.-subunit. Whereas 22 of 33 anti-MIR mAb tested cross-reacted with Rana AChR, only one of these mAb cross-reacted with Xenopus AChR. Some (32) of the cross-reacting mAb were tested for binding to AChR in intact muscle. Of these mAb 21, bound to AChR only when membranes were made permeable with saponin. EM using immunoperoxidase or colloidal Au techniques revealed that these mAb recognize cytoplasmic determinants and that mAb that do not require saponin to bind AChR in intact muscle recognize extracellular determinants. Evidently, AChR in skeletal muscle of Rana and Xenopus are composed of subunits corresponding to the .alpha.-, .beta.-, .gamma.- and .delta.-subunits of AChR from fisk electric organs. The subunit specificity of mAb whose binding was examined by EM suggests that parts of each subunit (.alpha., .beta., .gamma., .delta.) are exposed on the cytoplasmic surface and that, as in AChR from fish electric organs and mammalian muscle, the MIR on .alpha.-subunits of Rana AChR is exposed on the extracellular surface.