An internalization signal in ClC-5, an endosomal Cl- channel mutated in Dent's disease

The C1C-5 chloride channel resides mainly in vesicles of the endocytotic pathway and contributes to their acidification, Its disruption in mice entails a broad defect in renal endocytosis and causes secondary changes in calciotropic hormone levels. Inactivating mutations in Dent's disease lead to proteinuria and kidney stones. Possibly by recycling, a small fraction of C1C-5 also reaches the plasma membrane. Here we identify a carboxyl-terminal internalization motif in C1C-5, It resembles the PY motif, which is crucial for the endocytosis and degradation of epithelial Na+ channels. Mutating this motif increases surface expression and currents about a-fold. This is probably because of interactions with WW domains, because dominant negative mutants of the ubiquitin-protein ligase WWP2 increased surface expression and currents of C1C-5 only when its PY motif was intact. Stimulating endocytosis by expressing rab5 or its GTPase-deficient Q79L mutant decreased WT C1C-5 currents but did not affect channels with mutated motifs, Similarly, decreasing endocytosis by expressing the inactive S34N mutant of rab5 increased C1C-5 currents only if its PY-like motif was intact. Thus, the endocytosis of C1C-5, which itself is crucial for the endocytosis of other proteins, depends on the interaction of a carboxyl-terminal internalization signal with ubiquitin-protein ligases containing WW domains.