1.A.114. The Proton-activated Chloride Channel (PACC) Family
Severe local acidosis causes tissue damage and pain, and is one of the hallmarks of many diseases including ischemia, cancer, and inflammation. Yang et al. 2019 performed an unbiased RNA interference screen and identified PAC (TMEM206) as being essential for the widely observed proton-activated Cl- (PAC) currents (I Cl,H). Overexpression of human PAC in PAC knockout cells generated I Cl,H with the same characteristics as the endogenous ones. Zebrafish PAC encodes a PAC channel with distinct properties. Knockout of mouse Pac abolished I Cl,H in neurons and attenuated brain damage after ischemic stroke. The wide expression of PAC suggests a broad role for this conserved Cl- channel family in physiological and pathological processes associated with acidic pH (Yang et al. 2019). It is involved in lysosome function, hypoxia adaption, stroke, and carcinogenesis (Cai et al. 2021). Ion permeation is controlled by hydrophobic residues and proton binding (Cai et al. 2021). Downregulation of TMEM206 inhibits malignant properties of human osteosarcoma cells (Peng et al. 2021).
Ullrich et al. 2019 used a genome-wide siRNA screen to molecularly identify the widely expressed acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR. ASOR is formed by TMEM206 proteins which display two TMSs and are expressed at the plasma membrane. Ion permeation-changing mutations along the length of TMS2 and at the end of TMS1 suggest that these segments line ASOR's pore. TMEM206 has orthologs in probably all vertebrates, but possibly not in other orgamism. Currents from evolutionarily distant orthologs share activation by protons, a feature essential for ASOR's role in acid-induced cell death (Ullrich et al. 2019). Molecular determinants of pH sensing have been determined, revealing that distinct pH-sensing and gating mechanisms are operative (Osei-Owusu et al. 2022).
PAC is active across a wide range of mammalian cells and is involved in acid-induced cell death and tissue injury. Ruan et al. 2020 presented 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 in which each subunit consists of a transmembrane domain (TMD), which is formed of two helices (TMS1 and TMS2), and an extracellular domain (ECD). Upon a decrease of pH from 8 to 4, a conformational change in the ECD-TMD interface and the TMD is observed. The rearrangement of the ECD-TMD interface is characterized by the movement of the histidine 98 residue, which is, after acidification, decoupled from the resting position and inserted into an acidic pocket that is about 5 Å away. Within the TMD, TMS1 undergoes a rotational movement, switching its interaction partner from its cognate TMS2 to the adjacent TMS2. The anion selectivity of PAC is determined by the positively charged lysine 319 residue in TMS2, and replacing lysine 319 with a glutamate residue converts PAC to a cation-selective channel (Ruan et al. 2020).
ASOR (TMEM206/PAC) permeates anions across cellular membranes in response to acidification, thereby enhancing acid-induced cell death and regulating endocytosis. Wang et al. 2022 reconstituted function from purified protein and presented a 3.1-Å-resolution cryoEM structure of human ASOR at acidic pH in an activated conformation. The work contextualizes a previous acidic pH structure as a desensitized conformation. Combined with electrophysiological studies and high-resolution structures of resting and desensitized states, the work reveals mechanisms of proton sensing and ion pore gating. Clusters of extracellular acidic residues function as pH sensors and coalesce when protonated. Ensuing conformational changes induce metamorphosis of transmembrane helices to fashion an ion conduction pathway unique to the activated conformation. The studies identify a new paradigm of channel gating in this ubiquitous ion channel (Wang et al. 2022).
Plasma membrane proton-activated chloride channel, PACC1, or acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR or TMEM206, of 350 aas and 2 TMSs. Ion permeation-changing mutations along the length of TMS2 and at the end of TMS1 suggest that these segments line the pore. TMEM206 probably has orthologs in all vertebrates (Ullrich et al. 2019). Knockout of mouse Pac abolished I Cl,H in neurons and attenuated brain damage after ischemic stroke (Yang et al. 2019). The cryoEM structure (3.1 Å resolution) in active and desensitized states has been determined (Wang et al. 2022).
PACC1 of Homo sapiens
Proton-activated chloride channel of 298 aas and 2 TMSs. It mediates import of chloride ion in response to extracellular acidic pH (Yang et al. 2019) and displays channel activity with kinetic properties distinct from that of the human ortholog.
TMEM206 of Danio rerio (Zebrafish) (Brachydanio rerio)
TMEM206 of 469 aas and 2 TMSs
TMEM206 of Amphimedon queenslandica
TMEM206-like protein of 382 aas and 2 TMSs.
TMEM206 of Saccoglossus kowalevskii
TMEM206 of 254 aas and 2 TMSs.
TMEM206 of Callorhinchus milii
Proton-activated chloride channel of 351 aas and 2 TMSs near the N- and C-termini. TMEM206 is an evolutionarily conserved chloride channel that underlies ubiquitously expressed, proton-activated, outwardly rectifying, anion currents. Deng et al. 2021 reported the cryo-EM structure of pufferfish TMEM206, which forms a trimeric channel, with 6 TMSs, 2 per subunit, each with a large extracellular domain. An ample vestibule in the extracellular region is accessible laterally from the three side portals. The central pore contains multiple constrictions; a conserved lysine residue near the cytoplasmic end of the inner helix forms the presumed chloride ion selectivity filter. The core structure and assembly closely resemble those of the epithelial sodium channel/degenerin family of sodium channels that seem to be unrelated in amino acid sequence and conduct cations instead of anions. Together with electrophysiology, this structure provides insights into ion conduction and gating for these chloride channels (Deng et al. 2021).
TMEM206 of Tetraodon nigroviridis (Spotted green pufferfish) (Chelonodon nigroviridis)