GARP complex controls Golgi physiology by stabilizing COPI machinery and Golgi v-SNAREs
GARP is an evolutionary conserved heterotetrameric protein complex that is thought to tether endosome-derived vesicles and promotes their fusion in the trans-Golgi network. We have previously discovered the GARPs role in maintaining Golgi glycosylation machinery. To further investigate the importance of the GARP complex for Golgi physiology, we employed Airyscan superresolution and electron microscopy, as well as the unbiased quantitative proteomic analysis of Golgi in RPE1 cells. Both cis and trans-Golgi compartments were significantly enlarged in GARP deficient cells with pronounced alterations of TGN morphology. In GARP-KO cells, proteomic analysis revealed a depletion of a subset of Golgi resident proteins, including Ca2+ binding proteins, glycosylation enzymes, and v-SNAREs. We validated proteomics studies and discovered that two Golgi-resident proteins SDF4 and ATP2C1, related to Golgi calcium homeostasis, as well as intra-Golgi v-SNAREs GOSR1 and BET1L, are significantly depleted in GARP-KO cells. To test if SNARE depletion is responsible for the Golgi defects in GARP deficient cells, we created and analyzed GOSR1 and BET1L KO cell lines. Since GARP-KO was more deleterious to the Golgi physiology than SNARE-KOs, we have investigated other components of intra-Golgi vesicular trafficking, particularly COPI vesicular coat and its accessory proteins. We found that COPI is partially relocalized to the ERGIC compartment in GARP-KO cells. Moreover, COPI accessory proteins GOLPH3, ARFGAP1, GBF1 were displaced from the membrane and BIG1 was relocated to endolysosomal compartment in GARP-KO cells. We propose that the dysregulation of COPI machinery along with degradation of intra-Golgi v-SNAREs and alteration of Golgi Ca2+ homeostasis are the major driving factors for the instability of Golgi resident proteins and glycosylation defects in GARP deficient cells.