Ba 2+, a doubly charged analogue of K +, specifically blocks K + channels by virtue of electrostatic stabilization in the permeation pathway. Ba 2+ block is used here as a tool to determine the equilibrium binding affinity for various monovalent cations at specific sites in the selectivity filter of a noninactivating mutant of KcsA. At high concentrations of external K +, the block-time distribution is double exponential, marking at least two Ba 2+ sites in the selectivity filter, in accord with a Ba 2+-containing crystal structure of KcsA. By analyzing block as a function of extracellular K +, we determined the equilibrium dissociation constant of K + and of other monovalent cations at an extracellular site, presumably S1, to arrive at a selectivity sequence for binding at this site: Rb + (3 μM) > Cs + (23 μM) > K + (29 μM) > NH4 + (440 μM) >> Na + and Li + (>1 M). This represents an unusually high selectivity for K + over Na +, with |ΔΔG 0| of at least 7 kcal mol -1. These results fit well with other kinetic measurements of selectivity as well as with the many crystal structures of KcsA in various ionic conditions. © 2011 Piasta et al.
CITATION STYLE
Piasta, K. N., Theobald, D. L., & Miller, C. (2011). Potassium-selective block of barium permeation through single KcsA channels. Journal of General Physiology, 138(4), 421–436. https://doi.org/10.1085/jgp.201110684
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