Structural Plasticity of the Selectivity Filter in Cation Channels

Ion channels allow for the passage of ions across biological membranes, which is essential for the functioning of a cell. In pore loop channels the selectivity filter (SF) is a conserved sequence that forms a constriction with multiple ion binding sites. It is becoming increasingly clear that there are several conformations and dynamic states of the SF in cation channels. Here we outline specific modes of structural plasticity observed in the SFs of various pore loop channels: disorder, asymmetry, and collapse. We summarize the multiple atomic structures with varying SF conformations as well as asymmetric and more dynamic states that were discovered recently using structural biology, spectroscopic, and computational methods. Overall, we discuss here that structural plasticity within the SF is a key molecular determinant of ion channel gating behavior.

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K2P channel C-type gating involves asymmetric selectivity filter order-disorder transitions

10.1101/2020.03.20.000893 ◽

2020 ◽

Cited By ~ 3

Author(s):

Marco Lolicato ◽

Andrew M. Natale ◽

Fayal Abderemane-Ali ◽

David Crottès ◽

Sara Capponi ◽

...

Keyword(s):

Conformational Changes ◽

Ion Selectivity ◽

Transmembrane Helix ◽

Selectivity Filter ◽

Ion Binding ◽

Anomalous Scattering ◽

X Ray Crystallography ◽

Functional Studies ◽

Ion Binding Sites ◽

Physical And Chemical

K2P channels regulate nervous, cardiovascular, and immune system functions1,2 through the action of their selectivity filter (C-type) gate3-6. Although structural studies show K2P conformations that impact activity7-13, no selectivity filter conformational changes have been observed. Here, combining K2P2.1 (TREK-1) X-ray crystallography in different potassium concentrations, potassium anomalous scattering, molecular dynamics, and functional studies, we uncover the unprecedented, asymmetric, potassium-dependent conformational changes underlying K2P C-type gating. Low potassium concentrations evoke conformational changes in selectivity filter strand 1 (SF1), selectivity filter strand 2 (SF2), and the SF2-transmembrane helix 4 loop (SF2-M4 loop) that destroy the S1 and S2 ion binding sites and are suppressed by C-type gate activator ML335. Shortening the uniquely long SF2-M4 loop to match the canonical length found in other potassium channels or disrupting the conserved Glu234 hydrogen bond network supporting this loop blunts C-type gate response to various physical and chemical stimuli. Glu234 network destabilization also compromises ion selectivity, but can be reversed by channel activation, indicating that the ion binding site loss reduces selectivity similar to other channels14. Together, our data establish that C-type gating occurs through potassium-dependent order-disorder transitions in the selectivity filter and adjacent loops that respond to gating cues relayed through the SF2-M4 loop. These findings underscore the potential for targeting the SF2-M4 loop for the development of new, selective K2P channel modulators.

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Structural basis for C-type inactivation in a Shaker family voltage gated K+ channel

10.1101/2021.10.01.462615 ◽

2021 ◽

Author(s):

Ravikumar Reddi ◽

Kimberly Matulef ◽

Erika A. Riederer ◽

Matthew R. Whorton ◽

Francis I. Valiyaveetil

Keyword(s):

Crystal Structure ◽

Binding Sites ◽

Structural Changes ◽

Selectivity Filter ◽

Structural Basis ◽

K Channel ◽

Ion Binding ◽

Channel Pore ◽

Voltage Gated ◽

Ion Binding Sites

AbstractC-type inactivation is a process by which ion flux through a voltage-gated K+ (Kv) channel is regulated at the selectivity filter. While prior studies have indicated that C-type inactivation involves structural changes at the selectivity filter, the nature of the changes have not been resolved. Here we report the crystal structure of the Kv1.2 channel in a C-type inactivated state. The structure shows that C-type inactivation involves changes in the selectivity filter that disrupt the outer two ion binding sites in the filter. The changes at the selectivity filter propagate to the extracellular mouth and the turret regions of the channel pore. The structural changes observed are consistent with the functional hallmarks of C-type inactivation. This study highlights the intricate interplay between K+ occupancy at the ion binding sites and the interactions of the selectivity filter in determining the balance between the conductive and the inactivated conformations of the filter.

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

International Journal of Molecular Sciences ◽

10.3390/ijms20030689 ◽

2019 ◽

Vol 20 (3) ◽

pp. 689 ◽

Cited By ~ 2

Author(s):

Ana Giudici ◽

Maria Renart ◽

Clara Díaz-García ◽

Andrés Morales ◽

José Poveda ◽

...

Keyword(s):

Potassium Channel ◽

Binding Sites ◽

Cation Binding ◽

Selectivity Filter ◽

Ion Binding ◽

Channel State ◽

Equilibrium Conditions ◽

Equilibrium Binding ◽

Potassium Channel Kcsa ◽

Ion Binding Sites

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.

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