2,3-butanedione monoxime affects cystic fibrosis transmembrane conductance regulator channel function through phosphorylation-dependent and phosphorylation-independent mechanisms: The role of bilayer material properties
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2,3-Butanedione monoxime (BDM) is widely believed to act as a chemical phosphatase. We therefore examined the effects of BDM on the cystic fibrosis transmembrane regulator ( CFTR) Cl- channel, which is regulated by phosphorylation in a complex manner. In guinea pig ventricular myocytes, forskolin-activated whole-cell CFTR currents responded biphasically to external 20 mM BDM: a rapid similar to 2-fold current activation was followed by a slower (tau similar to 20 s) inhibition (to similar to 20% of control). The inhibitory response was abolished by intracellular dialysis with the phosphatase inhibitor microcystin, suggesting involvement of endogenous phosphatases. The BDM-induced activation was studied further in Xenopus laevis oocytes expressing human epithelial CFTR. The concentration for half-maximal BDM activation (K-0.5) was state-dependent, similar to 2 mM for highly and similar to 20 mM for partially phosphorylated channels, suggesting a modulated receptor mechanism. Because BDM modulates many different membrane proteins with similar K-0.5 values, we tested whether BDM could alter protein function by altering lipid bilayer properties rather than by direct BDM-protein interactions. Using gramicidin channels of different lengths ( different channel-bilayer hydrophobic mismatch) as reporters of bilayer stiffness, we found that BDM increases channel appearance rates and lifetimes (reduces bilayer stiffness). At 20 mM BDM, the appearance rates increase similar to 4-fold (for the longer, 15 residues/monomer, channels) to similar to 10-fold (for the shorter, 13 residues/monomer channels); the lifetimes increase similar to 50% independently of channel length. BDM thus reduces the energetic cost of bilayer deformation, an effect that may underlie the effects of BDM on CFTR and other membrane proteins; the state- dependent changes in K-0.5 are consistent with such a bilayer-mediated mechanism.