UC San Diego

A long-held tenet of heterotrimeric G protein signal transduction is that it is triggered by G protein-coupled receptors (GPCRs) at the PM. Here, we demonstrate that Gi is activated in the Golgi by GIV/Girdin, a non-receptor guanine-nucleotide exchange factor (GEF). GIV-dependent activation of Gi at the Golgi maintains the finiteness of the cyclical activation of ADP-ribosylation factor 1 (Arf1), a fundamental step in vesicle traffic in all eukaryotes. Several interactions with other major components of Golgi trafficking-e.g., active Arf1, its regulator, ArfGAP2/3, and the adaptor protein β-COP-enable GIV to coordinately regulate Arf1 signaling. When the GIV-Gαi pathway is selectively inhibited, levels of GTP-bound Arf1 are elevated and protein transport along the secretory pathway is delayed. These findings define a paradigm in non-canonical G protein signaling at the Golgi, which places GIV-GEF at the crossroads between signals gated by the trimeric G proteins and the Arf family of monomeric GTPases.