FIXATION OF ALLOSTERIC STATES OF THE NICOTINIC ACETYLCHOLINE-RECEPTORBY CHEMICAL CROSS-LINKING

Receptor activity can be described in terms of ligand-induced transiti ons between functional states, The nicotinic acetylcholine receptor (n AChR), a prototypic ligand-gated ion channel, is an ''unconventional a llosteric protein'' which exists in at least three interconvertible co nformations, referred to as resting (low agonist affinity, closed chan nel), activated (open channel), and desensitized (high agonist affinit y, closed channel), Here we show that 3,3'-dimethyl suberimidate (DMS) is an agonistic bifunctional cross-linking reagent, which irreversibl y ''freezes'' the nAChR in a high agonist affinity/closed-channel stat e, The monofunctional homologue methyl acetoimidate, which is also a w eak cholinergic agonist, has no such irreversible effect. Glutardialde hyde, a cross-linker that is not a cholinergic effector, fixes the rec eptor in a low-affinity state in the absence of carbamoylcholine, but, like DMS, in a high-affinity state in its presence, Covalent cross-li nking thus allows us to arrest the nAChR in defined conformational sta tes.