Zinc homeostasis governed by Golgi-resident ZnT family members regulates ERp44-mediated proteostasis at the ER-Golgi interface

Many secretory enzymes acquire essential zinc ions (Zn(2+)) in the Golgi complex. ERp44, a chaperone operating in the early secretory pathway, also binds Zn(2+) to regulate its client binding and release for the control of protein traffic and homeostasis. Notably, three membrane transporter complexes, ZnT4, ZnT5/ZnT6 and ZnT7, import Zn(2+) into the Golgi lumen in exchange with protons. To identify their specific roles, we here perform quantitative Zn(2+) imaging using super-resolution microscopy and Zn(2+)-probes targeted in specific Golgi subregions. Systematic ZnT-knockdowns reveal that ZnT4, ZnT5/ZnT6 and ZnT7 regulate labile Zn(2+) concentration at the distal, medial, and proximal Golgi, respectively, consistent with their localization. Time-course imaging of cells undergoing synchronized secretory protein traffic and functional assays demonstrates that ZnT-mediated Zn(2+) fluxes tune the localization, trafficking, and client-retrieval activity of ERp44. Altogether, this study provides deep mechanistic insights into how ZnTs control Zn(2+) homeostasis and ERp44-mediated proteostasis along the early secretory pathway.