nu 1, a Mr 43,000 component of postsynaptic membranes, is a protein kinase.

Acetylcholine receptor-enriched membranes from the electric organ of Torpedo californica show a major band at Mr 43,000 on NaDodSO4/polyacrylamide gels. This band is composed of three polypeptides: nu 1, nu 2, and nu 3. Polypeptide nu 1 has been found to be localized exclusively at the innervated face of the electrocyte and at the neuromuscular junction in rat muscle. We show here that monoclonal antibody to nu 1 precipitates a radioactive Mr 43,000 polypeptide from detergent-solubilized extracts of Torpedo membranes covalently labeled with periodate-oxidized [alpha-32P]ATP. The monoclonal antibody also precipitates protein kinase activity from neutralized pH 11 extracts of the acetylcholine receptor-rich membranes. These data suggest that nu 1 is a postsynaptic membrane protein kinase.