Molecular investigations on the nicotinic acetylcholine receptor - Conformational mapping and dynamic exploration using photoaffinity labeling

The nicotinic acetylcholine receptor (nAChR) is a well-understood member of the ligand-gated ion channels superfamily. The members of this signaling p roteins group, including 5HT(3), GABA(A), glycine, and ionotropic glutamate receptors, are thought to share common secondary, tertiary, and quaternary structures on the basis of a very high degree of sequence similarity. Desp ite the absence of X-ray crystallographic data, considerable progress on st ructural analysis of nAChR was achieved from biochemical, mutational, and e lectron microscopy data allowing the emergence of a three-dimensional image . Photoaffinity labeling and site-directed mutagenesis gave information on the tertiary structure with respect to the agonist/antagonist binding sites , the ion channel, and its selectivity filter. nAChR is an allosterical protein that undergoes interconversion among sever al conformational states. Time-resolved photolabeling Mras used in an attem pt to elucidate the structural changes that occur in nAChR on neurotransmit ter activation. Tertiary and quaternary rearrangements were found in the ch olinergic binding pocket and in the channel lumen, but the structural deter minant and the functional link between the binding of agonist and the chann el gating remain unknown. Time-resolved photolabeling of the functional act ivated A state using photosensitive agonists might help in understanding th e dynamic process leading to the interconversion of the different states.