Characterization of protein phosphorylation in acetylcholine receptor-enriched membrane preparations from Torpedo fuscomaculata.

When the acetylcholine receptor (AChR) from Torpedo is phosphorylated, ATP is found to bind non-covalently to the preparation. After correction is made for this binding of ATP, the phosphorylation reaction has a Km of 0,16 mM, a pH optimum of 8,6 and reaches maximal activity within 3 min. The intrinsic kinase activity is very specific for the AChR and does not phosphorylate either histones or phosvitin. The only amino acid to be phosphorylated is serine and cyclic GMP causes stimulation of the reaction.