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Accessibility of Cations to the Selectivity Filter of KcsA in the Inactivated State: An Equilibrium Binding Study

1
Instituto de Investigación, Desarrollo e Innovación en Biotecnología Sanitaria de Elche (IDiBE), and Instituto de Biología Molecular y Celular (IBMC), Universidad Miguel Hernández, Elche, E-03202 Alicante, Spain
2
CQFM-IN and IBB-Institute for Bioengineering and Bioscience, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
3
Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, E-03080 Alicante, Spain
*
Authors to whom correspondence should be addressed.
Int. J. Mol. Sci. 2019, 20(3), 689; https://doi.org/10.3390/ijms20030689
Received: 22 December 2018 / Revised: 25 January 2019 / Accepted: 25 January 2019 / Published: 5 February 2019
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Abstract

Cation binding under equilibrium conditions has been used as a tool to explore the accessibility of permeant and nonpermeant cations to the selectivity filter in three different inactivated models of the potassium channel KcsA. The results show that the stack of ion binding sites (S1 to S4) in the inactivated filter models remain accessible to cations as they are in the resting channel state. The inactivated state of the selectivity filter is therefore “resting-like” under such equilibrium conditions. Nonetheless, quantitative differences in the apparent KD’s of the binding processes reveal that the affinity for the binding of permeant cations to the inactivated channel models, mainly K+, decreases considerably with respect to the resting channel. This is likely to cause a loss of K+ from the inactivated filter and consequently, to promote nonconductive conformations. The most affected site by the affinity loss seems to be S4, which is interesting because S4 is the first site to accommodate K+ coming from the channel vestibule when K+ exits the cell. Moreover, binding of the nonpermeant species, Na+, is not substantially affected by inactivation, meaning that the inactivated channels are also less selective for permeant versus nonpermeant cations under equilibrium conditions. View Full-Text
Keywords: potassium channels; C-type inactivation; selectivity filter conformation; protein thermal stability; fluorescence; ion-protein interactions potassium channels; C-type inactivation; selectivity filter conformation; protein thermal stability; fluorescence; ion-protein interactions
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Giudici, A.M.; Renart, M.L.; Díaz-García, C.; Morales, A.; Poveda, J.A.; González-Ros, J.M. Accessibility of Cations to the Selectivity Filter of KcsA in the Inactivated State: An Equilibrium Binding Study. Int. J. Mol. Sci. 2019, 20, 689.

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