Dengue virus (DENV) infection is the most common arboviral disease globally, with up to 400 million infections and 25,000 deaths annually. The related flavivirus Zika virus (ZIKV) has also spread rapidly across the tropics and subtropics, and outbreaks of both DENV and ZIKV infection have now reached the continental United States. There are currently no effective antiviral agents against either virus, no DENV vaccine approved for use in the United States, and no ZIKV vaccine. Our long-term goal is to comprehensively identify and characterize the cellular pathways required for flavivirus infection, as these may represent novel targets for antiviral treatment. We and others have identified the host Endoplasmic Reticulum Membrane Protein Complex (EMC) as required for infection by multiple flaviviruses, including DENV and ZIKV. The EMC appears to function as a molecular chaperone, promoting the biogenesis of multipass membrane proteins in the endoplasmic reticulum. However, how the EMC supports flavivirus infection is unknown. Strikingly, our published findings reveal that during infection, the EMC is required by flavivirus replication by promoting the biogenesis of flavivirus NS4A and NS4B at an early post-translational step. Both proteins are non-structural multipass transmembrane proteins essential for viral replication. The objective of this proposal is to define how the EMC supports DENV and ZIKV replication. Our central hypothesis, based on strong preliminary data, is that the EMC functions as a molecular chaperone for the proper biogenesis of select flavivirus-encoded multipass transmembrane proteins. This proposal leverages the complementary strengths of an investigator with extensive experience in flavivirus-host biology (Tai), and another in ER-quality control mechanisms hijacked during viral infection (Tsai). The specific aims of the project are to (1) Define and validate the viral determinants of EMC dependence; (2) Determine the specific role of the EMC in flaviviral replication; and (3) Characterize DENV and ZIKV mutants that bypass EMC dependency. Successful completion of this project will illuminate the mechanism by which the EMC supports flavivirus infection, thereby providing insights into a novel vulnerability shared by these medically important viruses. It will also increase our understanding of other viruses that require the EMC.