In vitro phosphorylation of the acetylcholine receptor

Abstract

IN a study of the properties of the acetylcholine receptor protein (AChR) solubilised from extrasynaptic and sub-synaptic areas of rat diaphragm, Brockes and Hall¹ reported differences in the behaviour of these receptors on isoelectric focusing and concluded that extrasynaptic and subsynaptic receptors “have similar properties but are still distinct molecules”. Applying the same technique to crude detergent extracts of membrane fragments from Electrophorus electricus electric organ², we also found two forms of AChR with isoelectric points similar to those reported by Brockes and Hall. Furthermore a catalytic interconversion between these two forms was achieved by incubating in vitro the membrane extract in the presence of 100 mM NaCl or 100 mM NaF. This finding was interpreted as resulting from a phosphorylation-dephosphorylation of the AChR. In agreement with this interpretation, it has been shown³ that endogenous protein kinase and phosphoprotein phosphatase activities are indeed present in membrane fragments isolated from the electric organ of E. electricus. We report here that in detergent extracts of these membrane fragments, a phosphorylation of AChR takes place when incubated in the presence of γ-³²P-ATP, Mn²⁺ and the endogenous protein kinases.