scholarly journals The M1 and pre-M1 segments contribute differently to ion selectivity in ASICs and ENaCs

2021 ◽  
Vol 153 (10) ◽  
Author(s):  
Zeshan P. Sheikh ◽  
Matthias Wulf ◽  
Søren Friis ◽  
Mike Althaus ◽  
Timothy Lynagh ◽  
...  
Keyword(s):  
Ion Channels ◽  
Ion Selectivity ◽  
Ionic Species ◽  
Variable Degree ◽  
Transmembrane Segment ◽  
Ion Conductance ◽  
Acid Sensing Ion Channels ◽  
High Preference ◽  
High Degree ◽  
Pore Lining

The ability to discriminate between different ionic species, termed ion selectivity, is a key feature of ion channels and forms the basis for their physiological function. Members of the degenerin/epithelial sodium channel (DEG/ENaC) superfamily of trimeric ion channels are typically sodium selective, but to a surprisingly variable degree. While acid-sensing ion channels (ASICs) are weakly sodium selective (sodium:potassium ratio ∼10:1), ENaCs show a remarkably high preference for sodium over potassium (>500:1). This discrepancy may be expected to originate from differences in the pore-lining second transmembrane segment (M2). However, these show a relatively high degree of sequence conservation between ASICs and ENaCs, and previous functional and structural studies could not unequivocally establish that differences in M2 alone can account for the disparate degrees of ion selectivity. By contrast, surprisingly little is known about the contributions of the first transmembrane segment (M1) and the preceding pre-M1 region. In this study, we used conventional and noncanonical amino acid–based mutagenesis in combination with a variety of electrophysiological approaches to show that the pre-M1 and M1 regions of mASIC1a channels are major determinants of ion selectivity. Mutational investigations of the corresponding regions in hENaC show that these regions contribute less to ion selectivity, despite affecting ion conductance. In conclusion, our work suggests that the remarkably different degrees of sodium selectivity in ASICs and ENaCs are achieved through different mechanisms. These results further highlight how M1 and pre-M1 are likely to differentially affect pore structure in these related channels.

scholarly journals Differential contributions of M1 and pre-M1 to ion selectivity in ASICs and ENaCs

2021 ◽  
Author(s):  
Z.P. Sheikh ◽  
M. Wulf ◽  
S. Friis ◽  
M. Althaus ◽  
T. Lynagh ◽  
...  
Keyword(s):  
Ion Channels ◽  
Ion Selectivity ◽  
Ionic Species ◽  
Variable Degree ◽  
Transmembrane Segment ◽  
Ion Conductance ◽  
Acid Sensing Ion Channels ◽  
High Preference ◽  
Canonical Amino Acid ◽  
High Degree

AbstractThe ability to discriminate between different ionic species, termed ion selectivity, is a key feature of ion channels and forms the basis for their physiological function. Members of the degenerin/epithelial sodium channel (DEG/ENaC) superfamily of trimeric ion channels are typically sodium selective, but to a surprisingly variable degree. While acid-sensing ion channels (ASICs) are weakly sodium selective (sodium:potassium around 10:1), ENaCs show a remarkably high preference for sodium over potassium (>500:1). The most obvious explanation for this discrepancy may be expected to originate from differences in the pore-lining second transmembrane segment (M2). However, these show a relatively high degree of sequence conservation between ASICs and ENaCs and previous functional and structural studies could not unequivocally establish that differences in M2 alone can account for the disparate degrees of ion selectivity. By contrast, surprisingly little is known about the contributions of the first transmembrane segment (M1) and the preceding pre-M1 region. In this study, we use conventional and non-canonical amino acid-based mutagenesis in combination with a variety of electrophysiological approaches to show that the pre-M1 and M1 regions of mASIC1a channels are major determinants of ion selectivity. Mutational investigations of the corresponding regions in hENaC show that they contribute less to ion selectivity, despite affecting ion conductance. In conclusion, our work supports the notion that the remarkably different degrees of sodium selectivity in ASICs and ENaCs are achieved through different mechanisms. The results further highlight how M1 and pre-M1 are likely to differentially affect pore structure in these related channels.

scholarly journals Conserved His-Gly motif of acid-sensing ion channels resides in a reentrant ‘loop’ implicated in gating and ion selectivity

2020 ◽  
Author(s):  
Nate Yoder ◽  
Eric Gouaux
Keyword(s):  
Ion Channels ◽  
Ion Selectivity ◽  
Fear Memory ◽  
Amino Terminus ◽  
Ion Permeation ◽  
Structural Explanation ◽  
Cryo Electron Microscopy ◽  
Acid Sensing Ion Channels ◽  
And Function

ABSTRACTAcid-sensing ion channels (ASICs) are proton-gated members of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily of ion channels and are expressed throughout central and peripheral nervous systems. The homotrimeric splice variant ASIC1a has been implicated in nociception, fear memory, mood disorders and ischemia. Here we extract full-length chicken ASIC1a (cASIC1a) from cell membranes using styrene maleic acid (SMA) copolymer, yielding structures of ASIC1a channels in both high pH resting and low pH desensitized conformations by single-particle cryo-electron microscopy (cryo-EM). The structures of resting and desensitized channels reveal a reentrant loop at the amino terminus of ASIC1a that includes the highly conserved ‘His-Gly’ (HG) motif. The reentrant loop lines the lower ion permeation pathway and buttresses the ‘Gly-Ala-Ser’ (GAS) constriction, thus providing a structural explanation for the role of the His-Gly dipeptide in the structure and function of ASICs.

scholarly journals Molecular Basis for Ion Selectivity in Heteromeric Acid-Sensing Ion Channels

2019 ◽  
Vol 116 (3) ◽  
pp. 110a
Author(s):  
Zeshan P. Sheikh ◽  
Timothy Lynagh ◽  
Emelie Flood ◽  
Celine Boiteux ◽  
Toby W. Allen ◽  
...  
Keyword(s):  
Ion Channels ◽  
Molecular Basis ◽  
Ion Selectivity ◽  
Acid Sensing Ion Channels

scholarly journals The His-Gly motif of acid-sensing ion channels resides in a reentrant ‘loop’ implicated in gating and ion selectivity

eLife ◽  
2020 ◽  
Vol 9 ◽  
Author(s):  
Nate Yoder ◽  
Eric Gouaux
Keyword(s):  
Ion Channels ◽  
Ion Selectivity ◽  
Fear Memory ◽  
Amino Terminus ◽  
Ion Permeation ◽  
Structural Explanation ◽  
Cryo Electron Microscopy ◽  
Acid Sensing Ion Channels ◽  
And Function

Acid-sensing ion channels (ASICs) are proton-gated members of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily of ion channels and are expressed throughout the central and peripheral nervous systems. The homotrimeric splice variant ASIC1a has been implicated in nociception, fear memory, mood disorders and ischemia. Here, we extract full-length chicken ASIC1 (cASIC1) from cell membranes using styrene maleic acid (SMA) copolymer, elucidating structures of ASIC1 channels in both high pH resting and low pH desensitized conformations by single-particle cryo-electron microscopy (cryo-EM). The structures of resting and desensitized channels reveal a reentrant loop at the amino terminus of ASIC1 that includes the highly conserved ‘His-Gly’ (HG) motif. The reentrant loop lines the lower ion permeation pathway and buttresses the ‘Gly-Ala-Ser’ (GAS) constriction, thus providing a structural explanation for the role of the His-Gly dipeptide in the structure and function of ASICs.

scholarly journals Ion Selectivity in Acid-Sensing Ion Channels and Epithelial Sodium Channels

2018 ◽  
Vol 114 (3) ◽  
pp. 24a
Author(s):  
Zeshan P. Sheikh ◽  
Timothy P. Lynagh ◽  
Stephan A. Pless
Keyword(s):  
Ion Channels ◽  
Sodium Channels ◽  
Ion Selectivity ◽  
Epithelial Sodium Channels ◽  
Acid Sensing Ion Channels

scholarly journals Indirect Determinants of Ion Selectivity in Acid-Sensing Ion Channels and Epithelial Sodium Channels

2019 ◽  
Vol 116 (3) ◽  
pp. 110a
Author(s):  
Zeshan P. Sheikh ◽  
Timothy Lynagh ◽  
Anders S. Kristensen ◽  
Stephan A. Pless
Keyword(s):  
Ion Channels ◽  
Sodium Channels ◽  
Ion Selectivity ◽  
Epithelial Sodium Channels ◽  
Acid Sensing Ion Channels

scholarly journals Determinants of ion selectivity in ASIC1a- and ASIC2a-containing acid-sensing ion channels

2020 ◽  
Vol 152 (2) ◽  
Author(s):  
Timothy Lynagh ◽  
Emelie Flood ◽  
Céline Boiteux ◽  
Zeshan Pervez Sheikh ◽  
Toby W. Allen ◽  
...  
Keyword(s):  
Ion Channels ◽  
Essential Role ◽  
Ion Selectivity ◽  
Site Directed Mutagenesis ◽  
Ion Permeation ◽  
Extracellular Acidification ◽  
Acid Sensing Ion Channels ◽  
Molecular Determinants ◽  
Energy Simulations

Trimeric acid-sensing ion channels (ASICs) contribute to neuronal signaling by converting extracellular acidification into excitatory sodium currents. Previous work with homomeric ASIC1a implicates conserved leucine (L7′) and consecutive glycine-alanine-serine (GAS belt) residues near the middle, and conserved negatively charged (E18′) residues at the bottom of the pore in ion permeation and/or selectivity. However, a conserved mechanism of ion selectivity throughout the ASIC family has not been established. We therefore explored the molecular determinants of ion selectivity in heteromeric ASIC1a/ASIC2a and homomeric ASIC2a channels using site-directed mutagenesis, electrophysiology, and molecular dynamics free energy simulations. Similar to ASIC1a, E18′ residues create an energetic preference for sodium ions at the lower end of the pore in ASIC2a-containing channels. However, and in contrast to ASIC1a homomers, ion permeation through ASIC2a-containing channels is not determined by L7′ side chains in the upper part of the channel. This may be, in part, due to ASIC2a-specific negatively charged residues (E59 and E62) that lower the energy of ions in the upper pore, thus making the GAS belt more important for selectivity. This is confirmed by experiments showing that the L7′A mutation has no effect in ASIC2a, in contrast to ASIC1a, where it eliminated selectivity. ASIC2a triple mutants eliminating both L7′ and upper charges did not lead to large changes in selectivity, suggesting a different role for L7′ in ASIC2a compared with ASIC1a channels. In contrast, we observed measurable changes in ion selectivity in ASIC2a-containing channels with GAS belt mutations. Our results suggest that ion conduction and selectivity in the upper part of the ASIC pore may differ between subtypes, whereas the essential role of E18′ in ion selectivity is conserved. Furthermore, we demonstrate that heteromeric channels containing mutations in only one of two ASIC subtypes provide a means of functionally testing mutations that render homomeric channels nonfunctional.

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