The flavivirus protein NS4B recruits the cis-Golgi protein ACBD3 to modify ER-Golgi trafficking for virion release

Flavivirus infection involves extensive remodeling of the endoplasmic reticulum (ER), which is key to both the replication of the viral RNA genome as well as the assembly and release of new virions. Yet, little is known about how viral proteins and host factors cooperatively facilitate such a vast transformation of the ER, and how this influences the different steps of the viral life cycle. In this study, we screened for host proteins that interact with the tick-borne encephalitis virus (TBEV) protein NS4B and found that the top candidates were coupled to trafficking between ER exit sites (ERES) and the Golgi. We characterized the role of ACBD3, one of the identified proteins, in flavivirus infection and show that it interacts with NS4B to promote infection across multiple flavivirus species. Using ACBD3 knockout cells, we found that the depletion of ACBD3 inhibited TBEV replication by preventing the trafficking of virions from the cell. We found that ACBD3 promotes flavivirus infection via a different mechanism than its previously described role in picornavirus infection. ACBD3 was enriched at modified ERES-Golgi contact sites to support virus replication. Therefore, we propose that ACBD3 promotes flavivirus replication by modifying the trafficking between the ERES and the Golgi to enable the release of new virions. Author summaryFlaviviruses including dengue virus and tick-borne encephalitis virus are group of viruses that widely affecting the health of human. During infection, flavivirus particles enter host cells and transform the endoplasmic reticulum (ER), which is the main structure for protein synthesis in cells. New flaviviral particles are produced in the transformed ER and then released to the Golgi apparatus, which is the main structure for protein transport in cells. It is unclear how the particles are transported from the ER to Golgi. Here we screened the factors that interact with viral proteins and identified a Golgi protein called ACBD3 as an important factor supporting flavivirus particles release from ER to Golgi. We showed that ACBD3 is recruited by flavivirus to a modified connection between ER and Golgi for viral particles release from the ER. Our work provides new insights into the fine coordination of virus replication and viral particles transport between organelles inside host cells.