Department of Veterinary Medicine Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2762
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Item Characterization of the GBF1-Arf1 axis in enterovirus RNA replication(2024) Gabaglio Velazquez, Samuel Maria; Belov, George; Veterinary Medical Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Enterovirus genus includes many known and emerging pathogens, such as poliovirus, enteroviruses A71 and D68, rhinoviruses, and others. Enterovirus infection induces the massive remodeling of intracellular membranes and the development of specialized domains harboring viral replication complexes, called replication organelles. The cellular protein Golgi-specific brefeldin A-resistance guanine nucleotide exchange factor 1 (GBF1) is essential for the replication of enteroviruses, but its molecular role in the replication process is unclear. In uninfected cells, GBF1 activates small GTPases of the Arf family and coordinates multiple steps of membrane metabolism, including the functioning of the cellular secretory pathway. The nonstructural protein 3A of poliovirus and other enteroviruses directly interact with and recruits GBF1 to the replication organelles. Moreover, enterovirus infection induces the massive recruitment of all isoforms of the small cellular Arf GTPases to the replication organelles, but the mechanistic role of these proteins in the replication process is not understood either. Here, we sought to characterize the role of the GBF1-Arf1 axis in enterovirus replication. First, we systematically investigated the conserved elements of GBF1 to understand which determinants are important to support poliovirus replication. We demonstrated that multiple GBF1 mutants inactive in cellular metabolism could still be fully functional in the replication complexes. Our results showed that the Arf-activating property, but not the primary structure of the Sec7 catalytic domain is essential for viral replication. They also suggest a redundant mechanism for recruiting GBF1 to the replication sites. This mechanism depends not only on the direct interaction of the protein with the viral protein 3A but also on elements located in the noncatalytic C-terminal domains of GBF1. Next, we investigated the distribution of viral proteins and Arf1 on the replication organelles and their biochemical environment. Pulse-labeling of viral RNA with 5-ethynyl uridine showed that active RNA replication is associated with Arf1-enriched membranes. We observed that Arf1 forms isolated microdomains in the replication organelles and that viral antigens are localized in both Arf1-depleted and Arf1-enriched microdomains. We investigated the viral protein composition of the Arf1-enriched membranes using peroxidase-based proximity biotinylation. Viral protein biotinylation was detected as early as 3 h.p.i., and the non-cleaved fragments of the viral polyprotein were overrepresented in the Arf1-enriched domains. Furthermore, we show that after 4 h.p.i. viral proteins could be efficiently biotinylated only upon digitonin permeabilization of the replication organelle membranes, while such permeabilization inhibited the Arf1 biotinylation signal at the Golgi in non-infected cells. Together, these data support a model that recruitment of GBF1 to the replication organelles generates foci of activated Arfs on the membranes, which further differentiate into specific microdomains through the recruitment of a specific complex of viral proteins and cellular Arf effectors likely needed to establish the lipid and protein composition required for viral replication.