Voltage-gated potassium channels (Kvs) regulate K + permeation to modulate membrane excitability and cellular repolarization. Delayed-rectifier channel Kv2.1 plays critical roles in many physiological processes including neuronal excitability, cardiac rhythm, and insulin secretion. A distinctive feature of Kv2.1 is its increased macroscopic K + current amplitude as extracellular K + (K + o) rises, even as the driving force for the ion decreases. K + o elevation occurs during periods of physiologically (i.e. exercise) and pathophysiologically (i.e. ischemia) heightened neuronal activity, and mice in Kv2.1-KO studies display epileptic and hyperactive phenotypes. A de novo disease-causing mutation (K391N) in the gene encoding Kv2.1 (KCNB1) causes epileptic encephalopathy and developmental delays. K391 resides in the outer pore of Kv2.1 near the selectivity filter. The Kv2.1 outer vestibule is uniquely basic compared to canonical channels like Shaker as it is dominated by a ring of lysines, including K391. Electrophysiological characterization of K391N mutant reveals exaggerated K + o-dependent changes in current size, which enlarges with increases in K + o. In addition, we observe drastic changes in macroscopic conductance at varying K + o compared to wild-type and, surprisingly, rapid onset of inactivation. Site-directed mutagenesis studies at position 391 show that negative (Glu) and smaller side chains (Thr and Gly) also magnify outward macroscopic current and conductance at heightened K + o without promoting rapid onset of inactivation. Our preliminary findings suggest K391 is important for modulating Kv2.1 ionic conductance in conditions of varying K + o. We hypothesize that this lysine contributes to the stability of the selectivity filter, and that the ring of lysines in the outer pore coordinates an energetically favorable pathway for K + permeation.
Heebner et al. (Sun,) studied this question.