Outlines of the pore in open and closed conformations describe the gating mechanism of ASIC1

Abstract

The proton-activated sodium channel AsIC1 belongs to the EnaC/Degenerins family of ion channels. Little is known about gating of the pore in any member of this class. Here we outline the shape of the ion pathway of AsIC1 in the open and closed conformations by measuring apparent rates of cysteine modification by thiol-specific reagents in the two transmembrane helices that form the pore (Tm1 and Tm2). Closed channels have a narrowing in the external end of the pore, whereas open channels have a narrowing midway, the length of Tm2 that serves as selectivity filter. Thus, gating of the pore entails straightening the tilt of Tm2 without significant rotation. The findings imply that the external narrowing serves as opening, closing and desensitization gate, and that the selectivity filter of AsIC1 is a transient structure that assembles in the open state and is pulled apart in the closed state.

Contribution of Residues in Second Transmembrane Domain of ASIC1a Protein to Ion Selectivity

“…Gly-442 and Ser-444 are located at sites analogous to the first and third residues in the conserved (G/S)XS tract of ENaC that contribute to its Na ϩ selectivity (15,(17)(18)(19). ENaCs and ASICs bearing substitutions at these sites have reduced activity (15,18,19,28 (Fig. 5).…”

“…We hypothesize that substitutions in all three subunits at position 443 may compromise the functionality of the (G/S)XS tract as a molecular sieve (18). Interestingly, concatemeric lASIC1 bearing a subunit with a Cys substitution at the first position of the (G/S)XS tract has a K ϩ /Na ϩ selectivity similar to wild type channels, but a reduced Li ϩ /Na ϩ permeability (28). A443C shows enhanced tachyphylaxis when compared with wild type channels, indicating that in addition to changing the selectivity toward alkali metals, this mutation also alters gating.…”

Structure of the Acid-sensing ion channel 1 in complex with the gating modifier Psalmotoxin 1

“…Th is highlights the importance of the architecturally conserved central vestibule for ion conductance in trimeric sodium channels. In ASIC1, Leu78 separates the central vestibule that possibly functions as a cation reservoir from the extracellular vestibule that features the ion inlet windows 10,31,32 , whereas in P2X receptors, both vestibules are connected and accessible for ions that can enter the lateral fenestrations of the extracellular vestibule 31 . Strikingly, the only positively charged residue of the central vestibule, Arg370, is located right above the putative ion.…”

Acid-sensing ion channel (ASIC) structure and function: Insights from spider, snake and sea anemone venoms

“…Interestingly, the TM domain swap was also observed in the 2009 desensitised apo structure upon reanalyses of the electron densities (Gonzales et al, 2009), but not in either of the PcTx1 complex structures (Baconguis and Gouaux, 2012). This swapping of TM domains results in a continuous structure that contains a GAS motif that is important for creating the Na + -selectivity filter of ASICs (Carattino and Della Vecchia, 2012;Kellenberger et al, 2001;Kellenberger et al, 1999;Li et al, 2011). Cs + -soaked MitTx-cASIC1 crystals revealed coordination of Cs + ions by carbonyl atoms of Gly443 of the GAS motif.…”