K+-Dependent Selectivity and External Calcium Block of Shab Potassium Channels

Abstract

Potassium channels allow the selective flux of K+ excluding the smaller, and more abundant in the extracellular solution, Na+ ions. Here we show that Shab is a typical K+ channel that excludes Na+ under bi-ionic, Nao/Ki or Nao/Rbi, conditions. However, when internal K+ is replaced by Cs+ (Nao/Csi), stable inward Na+ and outward Cs+ currents are observed. These currents show that Shab selectivity is not accounted for by protein structural elements alone, as stated in the snug-fit model of selectivity. Additionally, here we report the block of Shab channels by external Ca2+ ions, and compare the effect that internal K+ replacement exerts on both Ca2+ and TEA blockage. Our observations indicate that Ca2+ blocks the channels in a site located near to the external TEA binding site, and that this pore region changes conformation under conditions that allow Na+ permeation. Based on our observations and the structural information derived from the NaK bacterial channel, we hypothesize that Ca2+ is probably coordinated by main chain carbonyls of the first K+-binding site of the pore.