# Proton-selective conductance and gating of the lysosomal cation channel TMEM175

## TL;DR

This study shows that the lysosomal channel TMEM175 conducts protons and how it responds to pH changes, which is important for understanding its role in diseases like Parkinson’s.

## Contribution

The study identifies H57 as a key residue regulating proton flux in TMEM175 and clarifies its proton selectivity and pH-dependent gating.

## Key findings

- Luminal acidification activates TMEM175-mediated H+ conductance, collapsing the lysosomal pH gradient.
- H57 forms salt bridges with D279 and E282, playing a critical role in TMEM175 function.
- The H57Y mutant shows reduced H+ and K+ conductance and lower H+/K+ selectivity.

## Abstract

Malfunction of the lysosomal ion channel TMEM175 disrupts luminal pH homeostasis and has been linked to neurodegenerative disorders, such as Parkinson’s disease. The channel’s principal ion selectivity remains a subject of ongoing debate, with conflicting evidence supporting K+ or H+ as the dominant permeant species. To investigate the channel’s selectivity and pH dependence, we analyzed TMEM175-mediated currents in response to changes in luminal pH. Electrophysiological recordings revealed that luminal acidification activates a H+-conductance, leading to rapid collapse of the pH-gradient. Integrating experimental and computational approaches, we identified H57 as a key residue regulating TMEM175-mediated H+ flux. Presented findings deepen our understanding of human TMEM175 structure–function and broaden the possibilities for developing therapeutic approaches for the treatment of TMEM175-associated neurodegenerative diseases.

The lysosomal cation channel TMEM175 plays a key role in luminal pH homeostasis and lysosome function, with aberrant activity linked to Parkinson’s disease. Although initially described as a K+-selective channel, TMEM175 exhibits substantial H+ permeability. Here, we dissect complex changes affecting human TMEM175 conductance and ionic properties of TMEM175-mediated current in response to pH shifts on the luminal side of the protein. A drop in pH from 7.4 to 4.7 on the side equivalent to the lysosomal lumen triggers a sustained increase in TMEM175-mediated inward and outward currents, which is accompanied by a transient shift in the reversal potential (Erev) toward the theoretical equilibrium voltage for H+, yet remaining ~100 mV below the expected value even in the absence of K+. This discrepancy, along with low sensitivity of Erev to the concentration gradient for K+, supports a model in which TMEM175-mediated H+ flux rapidly collapses the lysosomal pH-gradient. Molecular dynamics simulations identify H57 as a key residue on the luminal side of the open channel, which forms intra- and intersubunit salt bridges with D279 and E282. Supporting the functional importance of these interactions, the TMEM175 mutant H57Y displayed reduced H+- and K+-conductance and a reduced H+/K+ selectivity in whole-cell and lysosomal electrophysiological analyses. Our findings contribute to a better understanding of TMEM175’s complex electrophysiological properties, thereby expanding the possibilities of understanding the channel’s function in lysosomal physiology and pathophysiology.