The Na+-Activated Potassium Channel Slack Shares a Similar Na+ Coordination Site with Kir3 Channels

Abstract

Characteristics of Na+ activated potassium channel (Slack or Slo2.2) currents, including high conductance, rundown, regulation by Na+, Cl- and phosphorylation have long been reported but underlying mechanisms remain unknown. Here we report identification of a sodium regulatory site in the RCK2 domain of Slack channels by screening the C-terminus with the conserved sodium coordination motif of Kir channels. While the charge preserving D818E mutation exhibited similar Na+ sensitivity as the wild-type Slack channel, both a neutralization mutant D818N and a charge reversal D818R mutant within this site dramatically decreased Na+ sensitivity. Thus, D818 is engaged in an important electrostatic interaction with Na+. Similarly, the H823N mutant within this site also greatly decreased Na+ sensitivity of Slack channels. Simulations of the Slack RCK2 domain based on the crystallized structure of a prokaryotic RCK domain structure (Jiang et al., 2001, Neuron 29:593) provided a model of the Na+ coordination site in Slack channels. Moreover, simulations of the Na+ coordination site in Slo2.2 channels predicted a 5∼7 fold selectivity for Na+ over Li+ that were confirmed by electrophysiological data. Our results suggest that the Slack channel shares a similar Na+ regulatory mechanism with Kir channels but with important differences, such as an intricate coupling mechanism to Cl- co-regulation and possibly additional Na+ sensitive sites.