The influence of K+ binding to the selectivity filter on potassium channel conformation and function

Voltage-gated potassium (Kv) channels are multi-ion, single-file pores that regulate the transmembraneous flow of potassium (K+) in excitable cells. The pore has been modeled as a rigid structure allowing for a high degree of ionic selectivity (K+ relative to Na +) and flux in normal physiological solutions. This model was recently supported by interpretations based on the crystal structure obtained from a K+ channel from Streptomyces lividans, KcsA (Doyle et al., 1998). However, the pore of Kv channels undergoes conformational. changes during gating and the relative permeability of K+ and Na+ change during these processes. The activation and inactivation gates respond to changes in membrane voltage and the rates of these conformational changes are modulated by K+. In addition, some Kv channels do not exclude Na+ under all conditions. For example, Kv2.1 conducts Na+ in the absence of K+, displays an affinity-based selectivity mechanism in the presence of K +, and shows a cation-dependence for TEA block. We found that the competitive interaction between K+ and Na+ at the selectivity filter is determined not only by the S5-S6 loop but also by the scaffolding that holds the S5-S6 loop. This suggests that the scaffolding contributed to the three dimensional orientation of the selectivity filter. We also found that occupancy of the external K+ binding site associated with the selectivity filter alters the channel conformation both internal and external to the selectivity filter. These K+-dependent conformational changes correlate well with rate of slow inactivation in Kv2.1. In channels not occupied by K+, W interacted with the modulatory site with μM affinity. In channels occupied by a single K+, the modulatory site bound K+ with an affinity of ∼10 mM. These results indicate that ion-ion interactions within the selectivity filter provide the means for high selectivity with high throughput in potassium channels.