Proton-activated chloride channel PAC regulates endosomal acidification and transferrin receptor- mediated endocytosis

During vesicular acidification, chloride (Cl ), as the counterion, provides the electrical shunt for proton pump- ing by the vacuolar H + ATPase. Intracellular CLC transporters mediate Cl influx to the endolysosomes through their 2Cl /H + exchange activity. However, whole-endolysosomal patch-clamp recording also re- vealed a mysterious conductance releasing Cl from the lumen. It remains unknown whether CLCs or other Cl channels are responsible for this activity. Here, we show that the newly identified proton-activated Cl (PAC) channel traffics from the plasma membrane to endosomes via the classical YxxL motif. PAC deletion abolishes the endosomal Cl conductance, raises luminal Cl level, lowers luminal pH, and increases trans- ferrin receptor-mediated endocytosis. PAC overexpression generates a large endosomal Cl current with properties similar to those of endogenous conductance, hypo-acidifies endosomal pH, and reduces trans- ferrin uptake. We propose that the endosomal Cl PAC channel functions as a low pH sensor and prevents hyper-acidification by releasing Cl from the lumen.