(757f) Kir-Cholesterol Interactions: Molecular Simulations Reveal Cholesterol-Mediated De-Coupling between Functionally Important Domains Essential for Gating

Cholesterol is a major component of cell membranes and an important regulator of multiple types of ion channels, but the specific mechanisms through which it mediates ion channel function are not well understood. The cholesterol sensitivity of one family of ion channels, inwardly rectifying potassium (Kir) channels, has been shown to depend on direct interactions between the sterol and “cholesterol-sensitive” regions on the channel protein. Our previous simulation work has indicated that these cholesterol-sensitive regions represent multiple distinct, but contiguous, binding sites that are state-specific and dependent on membrane cholesterol concentration.

While putative cholesterol binding sites have been identified, the molecular mechanism through which cholesterol binding alters channel function (i.e., gating) remains unknown. Using a combination of Martini coarse-grained molecular dynamics simulations and a network theory-based analysis, we explored the impact of cholesterol binding on structure-function relationships of the Kir2.2 ion channel. We found that the presence of increased levels of cholesterol in a model POPC/cholesterol bilayer reduced the likelihood of contact between specific regions of the cytoplasmic and transmembrane domains. This resulted in an overall decoupling of structures known to be important for gating. Although the binding residues were not part of the zone of separation, the decoupling phenomenon could be correlated to the stoichiometry of cholesterol binding events. Finally, we identified key residues implicated in the change of tertiary structure and demonstrated distinct changes in their time-resolved shifts in radius of gyration. These again correlated with binding stoichiometry. Thus, we demonstrate a causal link between cholesterol binding and shifts in the structural dynamics of the Kir2.2 channel.