Abstract
Acid-sensing ion channels (ASICs) are proton-gated cation channels that are involved in diverse neuronal processes including pain sensing. The peptide toxin Mambalgin1 (Mamba1) from black mamba snake venom can reversibly inhibit the conductance of ASICs, causing an analgesic effect. However, the detailed mechanism by which Mamba1 inhibits ASIC1s, especially how Mamba1 binding to the extracellular domain affects the conformational changes of the transmembrane domain of ASICs remains elusive. Here, we present single-particle cryo-EM structures of human ASIC1a (hASIC1a) and the hASIC1a-Mamba1 complex at resolutions of 3.56 and 3.90 Å, respectively. The structures revealed the inhibited conformation of hASIC1a upon Mamba1 binding. The combination of the structural and physiological data indicates that Mamba1 preferentially binds hASIC1a in a closed state and reduces the proton sensitivity of the channel, representing a closed-state trapping mechanism.
Highlights
Acid-sensing ion channels (ASICs) are a group of voltage-independent proton-gated cation channels belonging to the degenerin/epithelial sodium channel (DEG/ENaC) superfamily (Kellenberger and Schild, 2002; Krishtal and Pidoplichko, 1981; Waldmann et al, 1997)
At saturation (10 mM Mamba1), hASIC1a and cASIC1 showed decreases in the measured sodium currents of 78.9 ± 6.2% and 31.9 ± 4.7% (Figure 1b). These data indicated that Mamba1 acts as an inhibitor targeting both hASIC1a and cASIC1, with comparable affinities but different efficacies. These observations suggested that functional and pharmacological differences exist between chicken ASIC1 and human ASIC1a, leading to the question of whether the structures of cASIC1 can fully recapitulate the functional states of hASIC1a
Truncated hASIC1a with the 60 carboxyl terminal residues removed was determined to be functional by whole-cell patch-clamp electrophysiology
Summary
Acid-sensing ion channels (ASICs) are a group of voltage-independent proton-gated cation channels belonging to the degenerin/epithelial sodium channel (DEG/ENaC) superfamily (Kellenberger and Schild, 2002; Krishtal and Pidoplichko, 1981; Waldmann et al, 1997). At saturation (10 mM Mamba1), hASIC1a and cASIC1 showed decreases in the measured sodium currents of 78.9 ± 6.2% and 31.9 ± 4.7% (Figure 1b) These data indicated that Mamba acts as an inhibitor targeting both hASIC1a and cASIC1, with comparable affinities but different efficacies. These observations suggested that functional and pharmacological differences exist between chicken ASIC1 and human ASIC1a, leading to the question of whether the structures of cASIC1 can fully recapitulate the functional states of hASIC1a. Direct interactions between residues in Mamba and acidsensing residues in the ‘acidic pocket’ of hASIC1a were observed to reduce the apparent proton sensitivity of the hASIC1a channel, leading to channel inhibition These data indicate that the mechanism by which Mamba inhibits hASIC1a channel is closed-state trapping
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