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Selective exclusion and selective binding both contribute to ion selectivity in KcsA, a model potassium channel.

ABSTRACT: The selectivity filter in potassium channels, a main component of the ion permeation pathway, configures a stack of binding sites (sites S1-S4) to which K⁺ and other cations may bind. Specific ion binding to such sites induces changes in the filter conformation, which play a key role in defining both selectivity and permeation. Here, using the potassium channel KcsA as a model, we contribute new evidence to reinforce this assertion. First, ion binding to KcsA blocked by tetrabutylammonium at the most cytoplasmic site in the selectivity filter (S4) suggests that such a site, when in the nonconductive filter conformation, has a higher affinity for cation binding than the most extracellular S1 site. This filter asymmetry, along with differences in intracellular and extracellular concentrations of K⁺versus Na⁺ under physiological conditions, should strengthen selection of the permeant K⁺ by the channel. Second, we used different K⁺ concentrations to shift the equilibrium between nonconductive and conductive states of the selectivity filter in which to test competitive binding of Na⁺ These experiments disclosed a marked decrease in the affinity of Na⁺ to bind the channel when the conformational equilibrium shifts toward the conductive state. This finding suggested that in addition to the selective binding of K⁺ and other permeant species over Na⁺, there is a selective exclusion of nonpermeant species from binding the channel filter, once it reaches a fully conductive conformation. We conclude that selective binding and selective exclusion of permeant and nonpermeant cations, respectively, are important determinants of ion channel selectivity.

SUBMITTER: Renart ML

PROVIDER: S-EPMC5602411 | biostudies-literature | 2017 Sep

REPOSITORIES: biostudies-literature

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Selective exclusion and selective binding both contribute to ion selectivity in KcsA, a model potassium channel.

Renart M Lourdes ML Montoya Estefanía E Giudici A Marcela AM Poveda José A JA Fernández Asia M AM Morales Andrés A González-Ros José M JM

The Journal of biological chemistry 20170804 37

The selectivity filter in potassium channels, a main component of the ion permeation pathway, configures a stack of binding sites (sites S1-S4) to which K<sup>+</sup> and other cations may bind. Specific ion binding to such sites induces changes in the filter conformation, which play a key role in defining both selectivity and permeation. Here, using the potassium channel KcsA as a model, we contribute new evidence to reinforce this assertion. First, ion binding to KcsA blocked by tetrabutylammo ...[more]

PMID: 28778926

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Project description:Potassium (K(+)) channels are specialized membrane proteins that are able to facilitate and regulate the conduction of K(+) through cell membranes. Comprising five specific cation binding sites (S(0)-S(4)) formed by the backbone carbonyl groups of conserved residues common to all K(+) channels, the narrow selectivity filter allows fast conduction of K(+) while being highly selective for K(+) over Na(+). To extend our knowledge of the microscopic mechanism underlying selectivity in K(+) channels, we characterize the free energy landscapes governing the entry and translocation of a Na(+) or a K(+) from the extracellular side into the selectivity filter of KcsA. The entry process of an extracellular ion is examined in the presence of two additional K(+) in the pore, and the three-ion potential of mean force is computed using extensive all-atom umbrella sampling molecular dynamics simulations. A comparison of the potentials of mean force yields a number of important results. First, the free energy minima corresponding to configurations with extracellular K(+) or Na(+) in binding site S(0) or S(1) are similar in depth, suggesting that the thermodynamic selectivity governed by the free energy minima for those two binding sites is insignificant. Second, the free energy barriers between stable multi-ion configurations are generally higher for Na(+) than for K(+), implying that the kinetics of ion conduction is slower when a Na(+) enters the pore. Third, the region corresponding to binding site S(2) near the center of the narrow pore emerges as the most selective for K(+) over Na(+). In particular, while there is a stable minimum for K(+) in site S(2), Na(+) faces a steep free energy increase with no local free energy well in this region. Lastly, analysis shows that selectivity is not correlated with the overall coordination number of the ion entering the pore, but is predominantly affected by changes in the type of coordinating ligands (carbonyls versus water molecules). These results further highlight the importance of the central region near binding site S(2) in the selectivity filter of K(+) channels.

Dataset Information

Selective exclusion and selective binding both contribute to ion selectivity in KcsA, a model potassium channel.

Publications

Selective exclusion and selective binding both contribute to ion selectivity in KcsA, a model potassium channel.

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