Identification of a molecular weight 43,000 protein kinase in acetylcholine receptor-enriched membranes.

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Abstract
A photoaffinity ATP ligand is used to identify the protein kinase present in acetylcholine receptor-enriched membranes from Torpedo californica. Incubation of these membranes with 8-azido-[.alpha.-32P]ATP and subsequent irradiation with UV light resulted in covalent labeling of a major band of MW 43,000. Alkali-stripped membranes that show a selective reduction in the MW 43,000 polypeptide also show a corresponding reduction in incorporation of photoaffinity label. The neutralized alkaline extract also showed 1 band at MW 43,000 when labeled with the photoaffinity ligand. After alkali extraction endogenous protein kinase activity decreased in the membranes in proportion to the loss of MW 43,000 peptide. The alkaline extract was able to phosphorylate casein in an exogenous assay system. Evidently, a MW 43,000 polypeptide in acetyecholine receptor-enriched membranes is the acetylcholine receptor.