Conformational Dynamics in the Selectivity Filter of KcsA in Response to Potassium Ion Concentration

Conformational change in the selectivity filter of KcsA as a function of ambient potassium concentration is studied with solid-state NMR. This highly conserved region of the protein is known to chelate potassium ions selectively. We report solid-state NMR chemical shift fingerprints of two distinct conformations of the selectivity filter; significant changes are observed in the chemical shifts of key residues in the filter as the potassium ion concentration is changed from 50 mM to 1 μM. Potassium ion titration studies reveal that the site-specific Kd for K+ binding at the key pore residue Val76 is on the order of ∼7 μM and that a relatively high sample hydration is necessary to observe the low-K+ conformer. Simultaneous detection of both conformers at low ambient potassium concentration suggests that the high-K+ and low-K+ states are in slow exchange on the NMR timescale (kex