Membrane transporters are essential for cellular homeostasis and human health, and defects in transporter function underlie numerous human diseases. While transporters are traditionally studied for their roles in moving specific substrates across biological membranes, increasing evidence indicates that many also perform non-canonical functions that contribute to cell signaling, organelle homeostasis, and disease pathogenesis. Understanding these additional functions is therefore critical for elucidating disease mechanisms and identifying new therapeutic opportunities.

Cystinosis is an autosomal recessive disorder caused by mutations in the CTNS gene which encodes cystinosin, a highly conserved lysosomal proton-coupled cystine transporter. Although lysosomal cystine accumulation is a hallmark of the disease, therapies that reduce lysosomal cystine load fail to fully prevent disease progression, suggesting that cystinosin possesses additional disease-relevant functions beyond lysosomal cystine transport. In this thesis, I investigate the function of Ers1, the yeast homolog of cystinosin, to gain insight into these poorly understood roles.

I show that Ers1 localizes to and functions within the early secretory pathway rather than the lysosome membrane. Genetic analysis reveal functional interactions between Ers1 and ER and Golgi-localized redox-active Fe–S cluster-binding proteins, linking Ers1 to the regulation of redox homeostasis in the ER and Golgi. Furthermore, the extra-lysosomal splicing variant of human cystinosin, cystinosin-LKG, functionally complements Ers1 in yeast, demonstrating conservation of this pathway.

Together, these findings uncover a novel conserved role for cystinosin/Ers1 in the early secretory pathway and provide new insight into the molecular basis of cystinosis pathology. More broadly, this work establishes cystinosin as an example of a disease-associated transporter whose biological functions extend beyond substrate transport, highlighting the importance of considering non-canonical transporter activities in health and disease.