Voltage-dependent electrogenic chloride/proton exchange by endosomal CLC proteins

Eukaryotic members of the CLC gene family function as plasma membrane chloride channels, or may provide neutralizing anion currents for V-type H+-ATPases that acidify compartments of the endosomal/lysosomal pathway1. Loss-of-function mutations in the endosomal protein ClC-5 impair renal endocytosis2 and lead to kidney stones3, whereas loss of function of the endosomal/lysosomal protein ClC-7 entails osteopetrosis4 and lysosomal storage disease5. Vesicular CLCs have been thought to be Cl- channels, in particular because ClC-4 and ClC-5 mediate plasma membrane Cl- currents upon heterologous expression6,7. Here we show that these two mainly endosomal CLC proteins instead function as electrogenic Cl-/H+ exchangers (also called antiporters), resembling the transport activity of the bacterial protein ClC-e1 (ref. 8), the crystal structure of which has already been determined9. Neutralization of a critical glutamate residue not only abolished the steep voltage-dependence of transport7, but also eliminated the coupling of anion flux to proton counter-transport. ClC-4 and ClC-5 may still compensate the charge accumulation by endosomal proton pumps, but are expected to couple directly vesicular pH gradients to Cl- gradients.