The lysosomal potassium channel TMEM175 adopts a novel tetrameric architecture

Abstract

TMEM175 is a lysosomal K+ channel important for maintaining the lysosomal membrane potential and pH stability1. 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 motif2–4. 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.

Molecular dysregulation of ciliary polycystin-2 channels caused by variants in the TOP domain

“…Second, the TOP domain also forms homotypic contacts between each subunit, which likely stabilize the oligomeric channel structure. Since there is strong precedence for external domains' being essential for tetrameric ion channel formation and native membrane localization, we hypothesize that TOP domain variants may disrupt the channels' structure, assembly, and trafficking to the primary cilia (27)(28)(29). Thus, the effect of ADPKD mutations found in the TOP domain forms the basis of the present study.…”

Genetic, Structural, and Functional Evidence Link TMEM175 to Synucleinopathies

“…To determine the potential structural impact of the p.M393T and p.Q65P mutations, we modeled the human TMEM175 using the crystal structure of an ortholog from the bacterium Chamaesiphon minutus as a template . HsTMEM175 is characterized by 2 TM domains in tandem, each showing sequence homology to the single TM domain of CmTMEM175 (see Fig A, B).…”

“…To determine the potential structural impact of the p.M393T and p.Q65P mutations, we modeled the human TMEM175 using the crystal structure of an ortholog from the bacterium Chamaesiphon minutus as a template. 26 HsTMEM175 is characterized by 2 TM domains in tandem, each showing sequence homology to the single TM domain of CmTMEM175 (see Fig 1A, B). Because CmTMEM175 forms a K + channel as a tetramer, 26 HsTMEM175 can adopt 2 possible pseudotetrameric configurations, where the 2 N-terminal domains are on either adjacent or opposite sides (Fig 3).…”

Lysosomes as a therapeutic target

“…Deficiency in TMEM175 may play a critical role in PD pathogenicity 322 . Importantly, the structure of TMEM175 has been recently refined 323 .…”