Similarity of Acetylcholine Receptors of Denervated, Innervated and Embryonic Chicken Muscles

The native (9‐S) form of the acetylcholine receptor, purified from chick embryonic or adult innervated or denervated chicken muscles, was shown in each case to contain three subunits of M r about 40000 (predominant), 50000 and 54000. The 40000‐ M r subunit (termed α) appears to exist in two forms; the minor variant form of it, with M r about 41000, may have a different carbohydrate content. Bromo[ 3 H]acetylcholine, an affinity alkylating reagent for this receptor, labels the 40000‐ M r subunit and the accompanying 41000‐ M r variant. The apparent ratio of the subunits varies with the several methods of detection used (protein staining, radio‐iodination or tritiation) but the α subunit always predominates. The α subunit was separated in urea gel isoelectric focussing; its isoelectric point was identical there for the three receptors from innervated, denervated and embryonic muscles. Their respective peptide maps for the α subunit were identical, and likewise for the 54000‐ M r subunit. All of the evidence obtained shows that the subunit structures of the junctional, extra‐junctional and embryonic chick muscle receptors are identical. Although the α subunit in the receptor from Torpedo electric organ, cat muscle and chicken muscles is distinguishable in all three cases in dodecylsulphate gel electrophoresis, it is affinity‐labelled in each case. The peptide map of this subunit after limited proteolysis is also very similar in all three cases. It is concluded that the α subunits from these three diverse sources are closely related structurally.