Structural and Functional Studies of Potassium Channels by Solid State NMR

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Potassium channels control membrane potential and signaling processes for humans and pathogens. Essentially all characterized K+ channels inactivate after opening due to a transmembrane allosteric process that appears to be a slow result of activation and involves residues near the selectivity filter in the pore domain (c-type inactivation). Since the activated state is the only conductive species, and is metastable, inactivation controls mean open time and thereby modulates function for many important channels and drug targets. For example, neurons use K+ channel inactivation kinetics to modulate their firing frequency, and inactivation kinetics in the channels of the human heart have strong effects on heart timing. Our research provided evidence in the K+ channel KcsA that the molecular basis of c-type inactivation is transmembrane allosteric coupling between the activation gate (H+ binding to the intracellular pH sensor) and the inactivation gate (K+ release from the extracellular selectivity filter). In recent efforts, we have delineated the mechanism for transmembrane allosteric control of channel activity by identifying residues that serve important roles in the allosteric response using NMR chemical shifts and mutation. We showed that activation and opening directly lead to K+ loss in the selectivity filter for wild type, but not for inactivation-reduced mutants, for which thermodynamic coupling between opening and K+ affinity is reduced. Our studies use solid state NMR measurements on full-length wild-type channels in hydrated proteoliposomes and offer atomistic access to structure, as well as the dynamics and thermodynamics of ligand binding. Thus, studies in the last period offer support for the hypothesis that allosteric coupling between activation and inactivation is the basis for inactivation and channel timing. The studies also give support for the specific identities of the “hotspots”. (Aim 1) In the upcoming period, a definitive test will be based on mutants that are accelerated in inactivation, in the sense that these mutants increase the “timing” function rather than abolish it. Many of these faster-inactivating mutants are also of interest because of similarities or analogies to eukaryotic channels that are fast-inactivating. (Aim 2) We plan to probe the conformational dynamics of the activated open state of the channel with recently developed NMR methods, to identify spontaneous conversion to early intermediates of inactivation. Specific methods developed in the last period allow us to carry out studies of the dynamics of key carbonyl and aromatic groups. Recent breakthroughs in the sensitivity of solid state NMR methods will be harnessed so that controls can be performed to test hypothesized relationships between dynamics and function, for example from molecular dynamics simulations. (Aim 3) Finally, in Ktr, a related channel that has been identified as a drug target for many pathogenic bacteria, we plan to clarify ligand-channel interactions determining the binding location of promising lead compounds. We will also characterize the inactivation mechanism and determine the consequence to allostery, inactivation and function when ligands bind. A clearer understanding of the thermodynamics and dynamics in these exemplars of transmembrane allostery is likely to lead to clarification of broad biophysical principles, as well as specific insights into small molecule modulators of disease-related channel function.