MOLECULAR MECHANISM OF CYCLIC-NUCLEOTIDE-GATED CHANNEL ACTIVATION

STUDIES on the activation of ligand- and voltage-gated ion channels ha ve identified regions involved in both ligand binding(1) and voltage s ensing(2), but relatively little is known about how such domains are c oupled to channel opening. Here we investigate the structural basis fo r the activation of cyclic-nucleotide-gated channels, which are direct ly opened by cytoplasmic cyclic nucleotides(3,4) but are structurally homologous to voltage-gated channels(5-7). By constructing chimaeras b etween cyclic-nucleotide-gated channels cloned from bovine retinal pho toreceptors(8) and catfish olfactory neurons(7), we identify two disti nct domains that are important for ligand binding and channel gating. A putative alpha-helix in the carboxy-terminal binding domain determin es the selectivity of the channel for activation by cGMP relative to c AMP. A domain in the amino-terminal region determines the ease with wh ich channels open and thus influences agonist efficacy. We propose tha t channel opening is coupled to an allosteric conformational change in the binding site which enhances agonist binding. Thus, cyclic nucleot ides activate the channel by binding tightly to the open state and sta bilizing it.