Cystinosin, the protein defective in cystinosis, is a H+-driven lysosomal cystine transporter

Cystinosis is an inherited lysosomal storage disease characterized by defec tive transport of cystine out of lysosomes. However, the causative gene, CT NS, encodes a seven transmembrane domain lysosomal protein, cystinosin, unr elated to known transporters. To investigate the molecular function of cyst inosin, the protein was redirected from lysosomes to the plasma membrane by deletion of its C-terminal GYDQL sorting motif (cystinosin-Delta GYDQL), t hereby exposing the intralysosomal side of cystinosin to the extracellular medium. COS cells expressing cystinosin-Delta GYDQL selectively take up l-c ystine from the extracellular medium at acidic pH. Disruption of the transm embrane pH gradient or incubation of the cells at neutral pH strongly inhib its the uptake. Cystinosin-Delta GYDQL is directly involved in the observed cystine transport, since this activity is highly reduced when the GYDQL mo tif is restored and is abolished upon introduction of a point mutation indu cing early-onset cystinosis. We conclude that cystinosin represents a novel H+-driven transporter that is responsible for cystine export from lysosome s, and propose that cystinosin homologues, such as mammalian SL15/Lec35 and Saccharomyces cerevisiae ERS1, may perform similar transport processes at other cellular membranes.