Froehner_1977_Proc.Natl.Acad.Sci.U.S.A_74_4685

The acetylcholine receptor from denervated rat skeletal muscle was purified by affinity chromatography and, after reduction, was treated with the affinity alkylating agent 4-(N-maleimido)benzyltri[3H]methylammonium iodide. The receptor specifically incorporated approximately 1 mol of alkylating agent per mol of 125I-labeled alpha-bungarotoxin bound. Analysis of the labeled receptor by polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that two subunits were labeled; their apparent molecular weights were 45,000 and 49,000. These results suggest that the affinity reagent labels a second site for acetylcholine binding in the muscle receptor that is not labeled in receptors from Electrophorus or Torpedo.