Fig. 1. ASIC subunits and activation of ASICs by extracellular acidification. (A) Each subunit has two hydrophobic transmembrane domains, a large cysteine-rich extracellular loop, and short intracellular N- and C- termini. (B) Three subunits assemble to form a functional homo- or hetero- trimeric channel. (C) ASIC currents evoked by extracellular pH fall in tsA cells. ASIC1a, ASIC2a, and ASIC3 were activated by application of pH 6.0, 4.5, and 5.0 solution, respectively. The membrane potential was clamped to −70 mV. The dashed line indicates zero current level.