Immunofluorescence localization at the mammalian neuromuscular junction of the Mr 43,000 protein of Torpedo postsynaptic membranes.

Highly purified cholinergic postsynaptic membranes from Torpedo electric tissue contain, in addition to the acetylcholine receptor (AcChoR), major proteins of Mr 43,000 and Mr approximately 90,000 and minor proteins that can be removed from the membranes by alkaline treatment. We have prepared an antiserum to these alkaline-extractable proteins that reacts with the Mr 43,000 protein but not with any of the other major membrane proteins, including the AcChoR subunits. Immunofluorescent staining of sections of Torpedo electric tissue shows that this antiserum binds to the innervated but not the uninnervated surface of the electrocytes. In rat diaphragm muscle, the antigens recognized by this antiserum are highly concentrated at the synapse. Synaptic staining of muscle is eliminated by prior incubation of the antiserum with the Mr 43,000 protein but not by incubation with affinity-purified AcChoR. This antiserum stains end plates of muscles denervated for 7 days. Antiserum to AcChoR binds to the subsynaptic membranes of electrocytes and muscle but does not react with the Mr 43,000 protein. Purified AcChoR blocks staining of synapses by anti-AcChoR but the Mr 43,000 protein does not. These results indicate that the Mr 43,000 protein is located in the innervated membrane of Torpedo electrocytes and that an immunologically similar component is highly concentrated in the postsynaptic membrane of mammalian muscle.