Identification of the Ca²⁺ Blocking Site of Acid-sensing Ion Channel (ASIC) 1 : Implications for Channel Gating

Acid-sensing ion channels ASIC1a and ASIC1b are ligand-gated ion channels that are activated by H⁺ in the physiological range of pH. The apparent affinity for H⁺ of ASIC1a and 1b is modulated by extracellular Ca²⁺ through a competition between Ca²⁺ and H⁺. Here we show that, in addition to modulating the apparent H⁺ affinity, Ca²⁺ blocks ASIC1a in the open state (IC₅₀ ∼ 3.9 mM at pH 5.5), whereas ASIC1b is blocked with reduced affinity (IC₅₀ > 10 mM at pH 4.7). Moreover, we report the identification of the site that mediates this open channel block by Ca²⁺. ASICs have two transmembrane domains. The second transmembrane domain M2 has been shown to form the ion pore of the related epithelial Na⁺ channel. Conserved topology and high homology in M2 suggests that M2 forms the ion pore also of ASICs. Combined substitution of an aspartate and a glutamate residue at the beginning of M2 completely abolished block by Ca²⁺ of ASIC1a, showing that these two amino acids (E425 and D432) are crucial for Ca²⁺ block. It has previously been suggested that relief of Ca²⁺ block opens ASIC3 channels. However, substitutions of E425 or D432 individually or in combination did not open channels constitutively and did not abolish gating by H⁺ and modulation of H⁺ affinity by Ca²⁺. These results show that channel block by Ca²⁺ and H⁺ gating are not intrinsically linked.