Cystinosis is a genetic disease caused by mutations in the CTNS gene, which in the most severe cases results in end-stage renal failure and death. The CTNS protein has been studied extensively as a lysosomal cystine transporter, but restoration of cystine export at the lysosome is insufficient to treat disease symptoms, suggesting that CTNS has additional cellular functions. CTNS has previously been reported to have an alternative splicing isoform, CTNS-LKG, that localizes not only to the lysosome but also to other subcellular localizations such as the Golgi and plasma membrane, suggesting that it may function outside of lysosomal cystine export. Ers1 is the yeast homolog of human CTNS and has been identified in genetic screens to be involved in the maintenance of ER and Golgi proteins. However, the mechanism by which Ers1 functions in this process is unknown. My lab recently observed that Ers1 is localized at the early Golgi in yeast, is a multicopy suppressor of mutants defective in Golgi protein recycling, and exhibits genetic interaction with mutants defective in ER/Golgi redox homeostasis, further supporting its functions at the ER/Golgi. Overall, I hypothesize that Ers1 contributes to maintaining ER/Golgi homeostasis through a mechanism outside of its known lysosomal cystine transport function. In this project, I aim to use a proteomics and live-cell fluorescence microscopy-based approach to identify ER and Golgi proteins whose localization is dependent on Ers1 function (Aim 1). Based on a Golgi isolation method that I have already optimized, I will also use untargeted metabolomics to compare the metabolome of the Golgi compartment of WT and ers1Δ yeast cells to identify the solute transported by Ers1 across the Golgi membrane (Aim 2). I also plan to understand the role of Ers1 in regulating redox homeostasis at the Golgi by functionally characterizing its genetic interaction with the ER/Golgi glutaredoxins Grx6 and Grx7 (Aim 3). Understanding how Ers1 contributes to ER/Golgi proteostasis is of biomedical significance because the function of Ers1 at the ER/Golgi may be conserved in the LKG isoform of human CTNS and be relevant to disease pathology.