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 pumping 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 revealed 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 transferrin 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 transferrin 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.