External surface charge neutralization induces outward rectification on the Calcium- and Voltage- activated potassium channel BK

Abstract

There are two acidic amino acid residues in an extracellular loop (D326 and E329) of the large-conductance Calcium- and voltage- activated K+ channel (BK).Those are not present in lower conductance K+ channels. In order to determine the effect of these charges on channel conductance, we expressed BK mutants D326N, E329Q and D326N-E329Q on Xenopus oocytes and measured the expressed currents under path clamp. The double mutation D326N-E329Q induces outward rectification evidenced by a small decrease on a lower channel conductance at negative voltages (12 % at -100 mV, 110 mM K+ symmetrical). The outward rectification induced by charge neutralization increases to 22% at 30 mM symmetrical K+ concentration and disappears at 1 M symmetrical K+ suggesting an electrostatic mechanism. Surface charge effect of D326 on outward rectification accounts for the total effect of the double mutant. On the other hand, E329 charge neutralization promotes no rectification. To understand the differential effect of D326 and E329, we built a molecular model for the open pore of the BK channel using the MthK channel as template. Electrostatic potential calculations (Poisson-Boltzmann) were carried out on the molecular model embedded in a lipid bilayer. Our calculations indicate that the differential effect of the charges on the electrostatic potential at the selectivity filter is due to a differential dielectric shielding. The electrostatic potential induced E329 is more strongly shielded that D326 by the high dielectric constant of the external solution. (Supported by FONDECYT grants 103-0830 (RL), 104-0254 (FG) and D-200518 DID U.Austral (IC)).CECS hosts a Millennium Institute.