Lysosomal Targeting of Cystinosin Requires AP-3

Cystinosin is a lysosomal cystine transporter defective in cystinosis, an autosomal recessive lysosomal storage disorder. It is composed of seven transmembrane (TM) domains and contains two lysosomal targeting motifs: a tyrosine-based signal (GYDQL) in its C-terminal tail and a non-classical motif in its fifth inter-TM loop. Using the yeast two-hybrid system, we showed that the GYDQL motif specifically interacted with the μ subunit of the adaptor protein complex 3 (AP-3). Moreover, cell surface biotinylation and total internal reflection fluorescence microscopy revealed that cystinosin was partially mislocalized to the plasma membrane (PM) in AP-3-depleted cells. We generated a chimeric CD63 protein to specifically analyze the function of the GYDQL motif. This chimeric protein was targeted to lysosomes in a manner similar to cystinosin and was partially mislocalized to the PM in AP-3 knockdown cells where it also accumulated in the trans-Golgi network and early endosomes. Together with the fact that the surface levels of cystinosin and of the CD63-GYDQL chimeric protein were not increased when clathrin-mediated endocytosis was impaired, our data show that the tyrosine-based motif of cystinosin is a 'strong' AP-3 interacting motif responsible for lysosomal targeting of cystinosin by a direct intracellular pathway.