State-dependent blocker interactions with the CFTR chloride channel: implications for gating the pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel is subject to voltage-dependent open-channel block by a diverse range of cytoplasmic anions. However, in most cases the ability of these blocking substances to influence the pore opening and closing process has not been reported. In the present work, patch clamp recording was used to investigate the state-dependent block of CFTR by cytoplasmic Pt(NO₂)₄²⁻ ions. Two major effects of Pt(NO₂)₄²⁻ were identified. First, this anion caused fast, voltage-dependent block of open channels, leading to an apparent decrease in single-channel current amplitude. Secondly, Pt(NO₂)₄²⁻ also decreased channel open probability due to an increase in interburst closed times. Interestingly, mutations in the pore that weakened (K95Q) or strengthened (I344K, V345K) interactions with Pt(NO₂)₄²⁻ altered blocker effects both on Cl⁻ permeation and on channel gating, suggesting that both these effects are a consequence of Pt(NO₂)₄²⁻ interaction with a single site within the pore. Experiments at reduced extracellular Cl⁻ concentration hinted that Pt(NO₂)₄²⁻ may have a third effect, possibly increasing channel activity by interfering with channel closure. These results suggest that Pt(NO₂)₄²⁻ can enter from the cytoplasm into the pore inner vestibule of both open and closed CFTR channels, and that Pt(NO₂)₄²⁻ bound in the inner vestibule blocks Cl⁻ permeation as well as interfering with channel opening and, perhaps, channel closure. Implications for the location of the channel gate in the pore, and the operation of this gate, are discussed.