Subunit structure and peptide mapping of junctional and extrajunctional acetylcholine receptors from rat muscle

Abstract

The junctional acetylcholine receptor was purified from normal rat skeletal muscle and compared its structure with that of the extrajunctional receptor from denervated muscle. The 2 receptors from leg muscle were distinguished by isoelectric focusing and by reaction with sera from patients with myasthenia gravis. The junctional form of the acetylcholine receptor was purified from normal leg muscle by affinity chromatography on concanavalin A/Sepharose and cobrotoxin/Sepharose followed by sucrose gradient centrifugation. Analysis of radioiodinated receptor by polyacrylamide gel electrophoresis in sodium dodecyl sulfate indicated that the subunit structure of the junctional receptor was similar to that previously determined for the extrajunctional form with 2 major polypeptides, whose apparent MW in 9% polyacrylamide gels were 45,000 and 51,000. Several minor polypeptides also were found. When the 2 receptors were labeled with different isotopes of iodine and run together on a sodium dodecyl sulfate gel, the subunits of 1 receptor could not be resolved from those of the other. As seen earlier with the extrajunctional form, the affinity alkylating reagent [3H]MBTA [4-(N-maleimido) benzyltri [3H] methylammonium iodide] labeled the 45,000- and 49,000-dalton polypeptides of the junctional receptor. Peptide mapping showed that the 2 MBTA binding subunits are structurally related, although they are unrelated to the other polypeptides, and that the 45,000- and 51,000-dalton polypeptides of the junctional receptor were indistinguishable from those of the extrajunctional receptor. Peptide mapping of the 4 subunits of acetylcholine receptor isolated from Torpedo californica electric organ showed that these 4 polypeptides appear to be structurally unrelated.