A hidden Intermediate State for Priming ATP-Hydrolysis During Open Conformation in CFTR Channels

Cystic Fibrosis Transmembrane conductance Regulator (CFTR) anion channel, a member of ABC transporter superfamily, gates following ATP-dependent conformational changes of the nucleotide binding domains (NBD).Reflecting the hundreds of milliseconds duration of the channel open state corresponding to the dimerization of two NBDs, macroscopic WT-CFTR currents usually showed a fast, single exponential relaxation upon removal of cytoplasmic ATP. Mutations of tyrosine1219, a residue critical for ATP binding in second NBD (NBD2), induced a significant slow phase in the current relaxation, suggesting that weakening ATP binding affinity at NBD2 increases the probability of the stable open state. The slow phase was effectively diminished by a higher affinity ATP analogue.These data suggest that a stable binding of ATP to NBD2 is required for normal CFTR gating cycle and the instability of ATP binding frequently halts the gating cycle in the open state presumably through a failure of ATP hydrolysis at NBD2. In addition, two distinct components of the current relaxation suggests the existence of a hidden intermediate state in which success or failure of ATP-hydrolysis is destined during open conformation.