Mechanisms underlying drug-mediated regulation of membrane protein function.

The hydrophobic coupling between membrane proteins and their host lipid bilayer provides a mechanism by which bilayer-modifying drugs may alter protein function. Drug regulation of membrane protein function thus may be mediated by both direct interactions with the protein and drug-induced alterations of bilayer properties, in which the latter will alter the energetics of protein conformational changes. To tease apart these mechanisms, we examine how the prototypical, proton-gated bacterial potassium channel KcsA is regulated by bilayer-modifying drugs using a fluorescence-based approach to quantify changes in both KcsA function and lipid bilayer properties (using gramicidin channels as probes). All tested drugs inhibited KcsA activity, and the changes in the different gating steps varied with bilayer thickness, suggesting a coupling to the bilayer. Examining the correlations between changes in KcsA gating steps and bilayer properties reveals that drug-induced regulation of membrane protein function indeed involves bilayer-mediated mechanisms. Both direct, either specific or nonspecific, binding and bilayer-mediated mechanisms therefore are likely to be important whenever there is overlap between the concentration ranges at which a drug alters membrane protein function and bilayer properties. Because changes in bilayer properties will impact many diverse membrane proteins, they may cause indiscriminate changes in protein function.