Point mutations in the pore region directly or indirectly affect glibenclamide block of the CFTR chloride channel

The sulfonylurea glibenclamide is a relatively potent inhibitor of the CFTR Cl^- channel. This inhibition is thought to be via an open channel block mechanism. However, nothing is known about the physical nature of the glibenclamide-binding site on CFTR. Here we show that mutations in the pore-forming 6th and 12th transmembrane regions of CFTR affect block by intracellular glibenclamide, confirming previous suggestions that glibenclamide enters the pore in order to block the channel. Two mutations in the 6th transmembrane region, F337A and T338A, significantly weakened glibenclamide block, consistent with a direct interaction between glibenclamide and this region of the pore. Interestingly, two mutations in the 12th transmembrane region (N1138A and T1142A) significantly strengthened block. These two mutations also abolished the dependence of block on the extracellular Cl^- concentration, which in wild-type CFTR suggests an interaction between Cl^- and glibenclamide within the channel pore that limits block We suggest that mutations in the 12th transmembrane region strengthen glibenclamide block not by directly altering interactions between glibenclamide and the pore walls, but indirectly by reducing interactions between Cl^- ions and glibenclamide within the pore. This work demonstrates that glibenclamide binds within the CFTR channel pore and begins to define its intrapore binding site.