Structures and pH sensing mechanism of proton-activated chloride channel

The activity of the proton-activated chloride channel (PAC) is widespread and is involved in acid-induced cell death and tissue injury(12,3). Its molecular identity has recently been identified as a novel and evolutionarily conserved protein family(4,5). We present two cryo-EM structures of human PAC in a high-pH resting closed state and a low-pH proton-bound non-conducting state. PAC is a trimer; each subunit consists of a transmembrane domain (TMD) formed by two helices, TM1–2, and an extracellular domain (ECD). We observed striking conformational changes in the ECD–TMD interface and the TMD when the pH drops from 8 to 4. The rearrangement of the ECD–TMD interface is characterized by the movement of histidine-98, which is, upon acidification, decoupled from the resting position and inserted into an acidic pocket that is about 5-Å away. Within the TMD, TM1 undergoes a rotational movement, switching its interaction partner from the cognate to the adjacent TM2. The anion selectivity of PAC is determined by the positively charged lysine-319 on TM2. Replacement of lysine-319 by a glutamate converts PAC to a cation-selective channel. Our data provide the first glimpse of the molecular assembly of PAC, and a basis for understanding the mechanism of proton-dependent activation.