Proton-induced ionic currents were discovered in 1980 by Krishtal and Pidoplichko ^[1], who suggested that reduced pH in an extracellular medium would trigger a population of proton-gated ion channels ^[2]. Cloning of proton-gated ion channels in the mid-1990s classified them as a new family (ASICs – acid-sensing ion channels) belonging to the superfamily of degenerin/epithelial sodium channels (DEG/ENaC) ^[3]. The genes encoding ASICs have been identified in many vertebrate species, starting with cyclostomata. Four mammalian accn1-4 genes encoding at least six different subunits (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4) are currently known ^[2].

ASICs are trimers whose subunits are symmetrically arranged around the central channel pore. The extracellular domain (ECD) of each subunit resembles a clenched fist attached to the transmembrane segments by a movable "wrist" ^[4]. Given this similarity, Jasti et al. ^[4] described the ECD in terms of a human hand holding a ball. Subsequently, this terminology has become commonly used as it turns out to be quite convenient. ECD can be divided into five subdomains: the palm, the finger, the thumb, the knuckle and the β-ball domains.

The transmembrane segment of the ASIC domain is formed by six α-helices: two (TM1 and TM2) from each of the three subunits composing the functioning ASIC channel pore. The transmembrane segments of each subunit are involved in channel pore formation. TM2 directly lines the pore lumen, while TM1 plays a supporting role: it is in contact with the lipid bilayer and forms many bonds with TM2 of the same subunit, as well as with TM2 and TM1 of the neighboring subunit. Only a small C-terminal portion of TM1 directly lines the channel pore ^[5].

Ligand action consists in a shifting of the activation curve, which leads to either inhibition or potentiation of currents. Mambalgin-2 binds mainly in a region of ASIC1a involving the upper part of the thumb domain (residues Asp-349 and Phe-350), the palm domain of an adjacent subunit, and the β-ball domain (residues Arg-190, Asp-258, and Gln-259). This region overlaps with the acidic pocket (pH sensor) of the channel. The peptide exerts both stimulatory and inhibitory effects on ASIC1a. ^[6]

ASIC1a pore blockers are typical members of this group. Binding of these compounds in the pore leads to inhibition of ion transport independently of the degree of channel activation and desensitization. Three amiloride molecules reside in the upper portion of the channel pore, at the interfaces between subunits. Their charged groups are exposed to the pore lumen. On the other hand, mutagenesis data are indicative of deeper location of the amiloride binding sites in the pore. Kellenberger et al.^[8] put forward a hypothesis that the binding sites found in the X-ray crystallographic structure represent the intermediate position of amiloride; one molecule may go deeper, thus sterically blocking the channel. The second binding site of amiloride is located in the acidic pocket. Two molecules form a dimer, which is stabilized by stacking interactions between aromatic groups and the oppositely oriented guanidine groups. The functional role of this binding site is still unknown, but it seems likely that it is related to the ability of amiloride to activate ASICs ^[9].

Acidic pocket (ECD) can be divided into five subdomains: the palm, the finger, the thumb, the knuckle and the β-ball domains. (There are closely located three pairs of acidic amino-acid residues (Asp238-Asp350, Glu239-Asp346 and Glu220-Asp408) inside this pocket). The electrostatic repulsion between the negative charges of the side chains in these pairs of amino-acid residues retains the expanded conformation of the acidic pocket; so the channel is closed. Binding of the protons between carboxyl pairs takes place when the external medium is acidified, making the pocket take a more compact conformation. This causes conformational changes in the thumb domain, which in turn alters the wrist and the transmembrane domain. Hence, the acidic pocket is a site of the proton binding required for channel activation ^[7].

several asparagine, glutamine and histidine residues in the lower portion of the palm domain (their pKa value also lies within the pH range that activates ASIC1a channels) ^[10].