Investigating the Mechanism and Mode of Action of Golgi Toxins
Auxin-inducible forward genetics uncovered point mutations within Golgi Brefeldin A Resistant Guanine Nucleotide Exchange Factor 1 or GBF1 following lethal dose selection with a synthetic disubstituted pyrimidine toxin called Golgitox (GTOX). Resistant clones were also cross-resistant to the fungal toxin, Brefeldin A (BFA), and synthetic GBF1 inhibitor, Golgicide A (GCA). Like BFA and GCA, GTOX triggered Golgi disassembly via GBF1. Given that BFA is a reported molecular glue, we profiled Gbf1-Arf interactions in 293T cell lysates pre-treated with Golgi toxin. Both GTOX and GCA promoted GBF1-dependent interactions with Arfs 4 and 5, whereas BFA also interacted with Arf1. GBF1 domain mapping revealed that the HUS-SEC7-HDS1 domains were sufficient for promoting GTOX-dependent engagement with Arfs 4 and 5. Meanwhile, structural activity relationship studies showed that modifying the methyl group on the benzimidazole ring preserved GTOX activity and interactions between Gbf1 and Arfs 4 and 5. To assess which Arfs regulate BFA and GTOX cytotoxicities, genome-wide CRISPR/Cas9 compound enrichment screens were carried out, which identified ARF4 as being the most enriched hit. Next, we validated that ARF4 loss-of-function partially confers resistance to BFA and GTOX. Next, we asked whether GTOX preferentially interacts with ARF4-GDP versus ARF4-GTP. We found that exogenous ARF4 T31N (GDP-locked mutant) sensitized HCT116 scramble control cells 4-fold to GTOX, interacted with Gbf1 just as well as WT Arf4, but failed to rescue Arf4-mediated BiP retrieval. Collectively, these results suggest that the Gbf1-GTOX-Arf4-GDP complex is functionally inactive but deleterious to cell viability. Taken together, GTOX may act as a molecular glue to suppress GBF1 functions through downstream effector substrates like Arfs 4 and 5.