1. It has been suggested that the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel may utilize a novel gating mechanism in which open and closed states are not in thermodynamic equilibrium. This suggestion is based on the assumption that energy of ATP hydrolysis drives the gating cycle. 2. We demonstrate that CFTR channel gating occurs in the absence of ATP hydrolysis and hence does not depend on an input of free energy from this source. The binding of ATP or structurally related analogues that are poorly or non-hydrolysable is sufficient to induce opening. Closing occurs on dissociation of these ligands or the hydrolysis products of those that can be cleaved. 3. Not only can channel opening occur without ATP hydrolysis but the temperature dependence of the open probability (P(o)) is reversed, i.e. P(o) increases as temperature is lowered whereas under hydrolytic conditions, P(o) increases as temperature is elevated. This indicates that there are different rate-limiting steps in the alternate gating pathways (hydrolytic and non-hydrolytic). 4. These observations demonstrate that phosphorylated CFTR behaves as a conventional ligand-gated channel employing cytoplasmic ATP as a readily available cytoplasmic ligand; under physiological conditions ligand hydrolysis provides efficient reversibility of channel opening.
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