A major obstacle in the study of membrane proteins is their solubilization in a stable and active conformation when using detergents. Here, we explored a detergent-free approach to isolating the tetrameric potassium channel KcsA directly from the membrane of Escherichia coli, using a styrene-maleic acid copolymer. This polymer self-inserts into membranes and is capable of extracting membrane patches in the form of nanosize discoidal proteolipid particles or "native nanodiscs." Using circular dichroism and tryptophan fluorescence spectroscopy, we show that the conformation of KcsA in native nanodiscs is very similar to that in detergent micelles, but that the thermal stability of the protein is higher in the nanodiscs. Furthermore, as a promising new application, we show that quantitative analysis of the co-isolated lipids in purified KcsA-containing nanodiscs allows determination of preferential lipid-protein interactions. Thin-layer chromatography experiments revealed an enrichment of the anionic lipids cardiolipin and phosphatidylglycerol, indicating their close proximity to the channel in biological membranes and supporting their functional relevance. Finally, we demonstrate that KcsA can be reconstituted into planar lipid bilayers directly from native nanodiscs, which enables functional characterization of the channel by electrophysiology without first depriving the protein of its native environment. Together, these findings highlight the potential of the use of native nanodiscs as a tool in the study of ion channels, and of membrane proteins in general.membrane-protein solubilization | styrene-maleic acid copolymer | lipid-protein interactions | nanodisc | ion channels

Dynamic nuclear polarization (DNP) has become a powerful method to enhance spectroscopic sensitivity in the context of magnetic resonance imaging and nuclear magnetic resonance spectroscopy. We show that, compared to DNP at lower field (400 MHz/263 GHz), high field DNP (800 MHz/527 GHz) can significantly enhance spectral resolution and allows exploitation of the paramagnetic relaxation properties of DNP polarizing agents as direct structural probes under magic angle spinning conditions. Applied to a membrane-embedded K(+) channel, this approach allowed us to refine the membrane-embedded channel structure and revealed conformational substates that are present during two different stages of the channel gating cycle. High-field DNP thus offers atomic insight into the role of molecular plasticity during the course of biomolecular function in a complex cellular environment.

We have investigated specific lipid binding to the pore domain of potassium channels KcsA and chimeric KcsA-Kv1.3 on the structural and functional level using extensive coarse-grained and atomistic molecular dynamics simulations, solid-state NMR, and single channel measurements. We show that, while KcsA activity is critically modulated by the specific and cooperative binding of anionic nonannular lipids close to the channel's selectivity filter, the influence of nonannular lipid binding on KcsA-Kv1.3 is much reduced. The diminished impact of specific lipid binding on KcsA-Kv1.3 results from a point-mutation at the corresponding nonannular lipid binding site leading to a salt-bridge between adjacent KcsA-Kv1.3 subunits, which is conserved in many voltage-gated potassium channels and prevents strong nonannular lipid binding to the pore domain. Our findings elucidate how protein-lipid and protein-protein interactions modulate K(+) channel activity. The combination of MD, NMR, and functional studies as shown here may help to dissect the structural and dynamical processes that are critical for the functioning of larger membrane proteins, including Kv channels in a membrane setting.

Potassium (i.e., K + ) channels allow for the controlled and selective passage of potassium ions across the plasma membrane via a conserved pore domain. In voltage-gated K + channels, gating is the result of the coordinated action of two coupled gates: an activation gate at the intracellular entrance of the pore and an inactivation gate at the selectivity filter. By using solid-state NMR structural studies, in combination with electrophysiological experiments and molecular dynamics simulations, we show that the turret region connecting the outer transmembrane helix (transmembrane helix 1) and the pore helix behind the selectivity filter contributes to K + channel inactivation and exhibits a remarkable structural plasticity that correlates to K + channel inactivation. The transmembrane helix 1 unwinds when the K + channel enters the inactivated state and rewinds during the transition to the closed state. In addition to wellcharacterized changes at the K + ion coordination sites, this process is accompanied by conformational changes within the turret region and the pore helix. Further spectroscopic and computational results show that the same channel domain is critically involved in establishing functional contacts between pore domain and the cellular membrane. Taken together, our results suggest that the interaction between the K + channel turret region and the lipid bilayer exerts an important influence on the selective passage of potassium ions via the K + channel pore. membrane protein | ion channel | solid-state NMR spectroscopy P otassium (i.e., K + ) channels are embedded in the plasma membrane to control the selective passage of potassium ions across the lipid bilayer. The channels open and close their conduction pathway by sensing changes in physicochemical parameters such as pH, ligand concentration, and membrane voltage (1). Structure-function studies on voltage-gated K + (Kv) channels suggested that lipid molecules are an integral part of the voltage-sensing domains, which transfer during the gating process electrical charges across the cell membrane (2-4). In some of the available Kv channel crystal structures, lipid molecules appear most densely packed against the pore domain, presumably providing an appropriate environment for the stability and the operation of the gating machinery to open and close the conduction pathway. In general, the activity of Kv pore domains is thought to be determined by the activity of two gates in series, one for activation and one for inactivation. These gates jointly control the conduction of ions through the pore (5-11). The activation gate is located at the intracellular entrance of the pore and the inactivation gate is situated toward the extracellular entrance at the selectivity filter (i.e., C-type inactivation). In addition, some potassium channels possess close to the activation gate a receptor for an N-terminal inactivating domain (i.e., N-type inactivation).The K + channel pore domain is conserved across all K + channels. It comprises a tetrameric assembly of t...