Acid Sensing Ion Channels (ASICs) are sodium channels gated by extracellular protons. ASICs are widely distributed throughout the peripheral and central nervous system. They are involved in pain perception, learning and memory, and ischemic neuronal death. 4 asic genes are present in mammalian species, but at least 6 asic genes in zebrafish. Among them, rat ASIC4 and zebrafish ASIC4.2 cannot be activated by protons. Their ligand is unknown.

zASIC4.1 is 83% identical to zASIC4.2 in the extracellular loop, yet it is responsive to protons. This distinct proton sensitivity in spite of a high sequence identity prompted us to further investigate their structure-function relationship. We first examined their ability to form heteromeric channels using the two- electrode voltage clamp technique in the oocyte expression system. Our data indicated that zASIC4.1 can form heteromeric channels with zASIC1.2 and with ?1.3. There is also evidence in support of the formation of heteromeric channels by zASIC4.2 and ?1.3, but not by zASIC4.2 and ?1.2. Next we analyzed chimeric channels made between zASIC4.1 and zASIC4.2 and identified a crucial region, which determined the distinct functional response to protons. This region is only 16 amino acids long and located at the proximal part of the ectodomain. These results from zebrafish ASIC4 provide information on regions important for proton activation of ASICs.