Acidic pH is crucial for the function of intracellular organelles in the secretory and endocytic pathways. Furthermore, it is one of the pathological hallmarks of many diseases, including cerebral and cardiac ischemia, cancer, infection and inflammation. However, the molecular mechanisms of acid sensing and regulation are not fully understood.
Exposure of cells to acidic conditions activates a ubiquitous proton-activated Clˉ channel, whose molecular identity has been a long-standing mystery in the field. Through an unbiased RNAi screen, the Qiu lab recently identified a novel and evolutionarily conserved membrane protein, PAC (encoded by PACC1 gene), as the proton-activated chloride channel. The discovery of such a new ion channel represents a major breakthrough, making it possible to reveal its fundamental structural and functional properties. I joined the Qiu lab when it opened its door at Hopkins five years ago, and decided to focus on this exciting new Clˉ gatekeeper.
Taking advantage of single-particle cryo-electron microscopy, we solved two distinct cryo-EM structures of human PAC at a high-pH resting closed state (pH 8.0) and a low-pH, proton-bound, nonconducting state (pH 4.0). I identified key residues critical for pH sensing mechanism, channel inactivation and anion selectivity. My work provides the first glimpse of the molecular assembly of PAC, and a basis for understanding the mechanism of proton-dependent activation.
We also showed that PAC, initially identified as a plasma membrane protein, traffics to the endosomes. It encodes a bona fide acid-sensitive Clˉ leak channel in endosomes and regulates endosomal pH, Clˉ level, and transferrin-receptor-mediated endocytosis. My research has uncovered a mechanism of endosomal Clˉ permeability and revealed a previously unappreciated complexity in endosomal biology.