The Lysosomal Potassium Channel TMEM175 Adopts a Novel Tetrameric Architecture

TMEM175 is a lysosomal K(+) channel important for maintaining the lysosomal membrane potential and pH stability(1). It contains two homologous copies of a six-transmembrane (6-TM) domain, which has no sequence homology to the canonical tetrameric K(+) channels and lacks the TVGYG selectivity filter motif(2–4). Present in a subset of bacteria and archaea, the prokaryotic TMEM175 contains only a single 6-TM domain and functions as a tetramer. Here, we present the crystal structure of a prokaryotic TMEM175 from Chamaesiphon minutus, CmTMEM175, whose architecture represents a completely different fold from that of canonical K(+) channels. All six transmembrane helices of CmTMEM175 are tightly packed within each subunit without undergoing domain swap. The highly conserved TM1 acts as the pore lining inner helix, creating an hour-glass shaped ion permeation pathway in the channel tetramer. Three layers of hydrophobic residues on the C-terminal half of TM1s form a bottle neck along the ion conduction pathway and serve as the selectivity filter of the channel. Mutagenesis analysis suggests that the first layer of the highly conserved isoleucine residues in the filter plays the central role in channel selectivity. Thus, the structure of CmTMEM175 represents a novel architecture of a tetrameric cation channel whose ion selectivity mechanism appears distinct from that of the classical K(+) channel family.