Phosphorylation of acetylcholine receptor by endogenous membrane protein kinase in receptor-enriched membranes of Torpedo californica

Abstract

THE molecular basis of synaptic transmission is a fundamental problem in neurobiology. Although neurotransmitters produce striking changes in post-synaptic ion permeability, little is known about the molecular events following binding of neurotransmitter to its receptor. It has been postulated that the effects of neurotransmitters on post-synaptic membranes are mediated by changes in protein phosphorylation^1,2. There has been no direct evidence for neurotransmitter-dependent changes in protein phosphorylation in membranes prepared from innervated tissues, however. We investigated this problem using an acetylcholine receptor-enriched membrane fraction prepared from the electric organ of Torpedo californica and found that several membrane polypeptides are phosphorylated when this fraction is incubated with γ-³²P-ATP and 100 mM K⁺ (ref. 3). In particular, we demonstrated that a polypeptide of 65,000 molecular weight (MW) is phosphorylated and that phosphorylation of this and other polypeptides is inhibited by cholinergic ligands. A 65,000 MW polypeptide in the same membrane preparation also reacted with antibody prepared against the purified acetylcholine receptor. This suggested that a component of the acetylcholine receptor itself could be phosphorylated by an endogenous membrane protein kinase. Here we present proof that the 65,000 MW component of the acetylcholine receptor is a substrate for an endogenous membrane protein kinase.