On the mechanism of gating defects caused by the R117H mutation in cystic fibrosis transmembrane conductance regulator

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Abstract

Cystic fibrosis (CF) is caused by loss-of-function mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding a phosphorylation-activated, but ATP-gated chloride channel. In the current study, we investigated the mechanism responsible for the gating defects manifested in R117H-CFTR, an arginine-to-histidine substitution at position 117 of CFTR that is associated with mild forms of CF. We confirmed previous findings of a 25% decrease of the single-channel conductance (g) in R117H-CFTR, but found a ˜13-fold lower open probability (Po). This dramatic gating deficit is not due to dysfunctional nucleotide binding domains (NBDs) as the mutation does not alter the apparent affinity for ATP, and the mutant channels respond to ATP analogues in a similar manner as wild-type CFTR. Furthermore, once ATP hydrolysis is abolished, the R117H mutant can be trapped in a prolonged ‘burst opening’ conformation that is proposed to be equipped with a stable NBD dimer. On the other hand, our results support the conclusion that the R117H mutation decreases Po by perturbing the gating conformational changes in CFTR's transmembrane domains as even when NBDs are kept at a dimerized configuration, Po is reduced by ˜10-fold. Moreover, our data demonstrate that a synergistic improvement of R117H-CFTR function can be accomplished with a combined regiment of VX-770 (Ivacaftor), nitrate ion (NO3−) and N6-(2-phenylethyl)-2′-deoxy-ATP (d-PATP), which almost completely rectifies the gating defect of R117H-CFTR. Clinical implications of our results are discussed.

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