Fluorescent probes for the acetylcholine receptor surface environments.

Abstract

The acetylcholine receptor (AchR) was the first neurotransmitter receptor to be identified and purified in an active form. It is a complex transmembrane glycoprotein present in the postsynaptic side of the neuromuscular junctions. When an action potential reaches the motor nerve terminals, acetylcholine is released into the synaptic cleft, where its local concentration can rise transiently to 10-4 to 10-3 M. Binding of Ach to specific sites located on the extracellular domains of the AchR molecules triggers the opening of short-lived cation channels, thus increasing the permeability of the postsynaptic membrane and causing the muscle fiber membrane to be depolarized beyond a critical threshold. The final result of this chain of events is muscle contraction. The AchR is present in high amounts in the electric organ of certain fishes. Using Torpedo (electric ray) electroplax as the starting material, one can purify milligram quantities of active protein, as well as substantial amounts of its constituent subunits. Moreover, a group of closely related protein toxins (α-neurotoxins) have been isolated from the venom of several Elapid snakes, which bind to the AchR with dissociation constants in the nanomolar to subnanomolar range [for review see Karlsson (1979) and Low (1979)]. The high affinity of α-neurotoxins for AchR, combined with their extreme specificity, has greatly facilitated the purification and characterization of AchR from different sources.