Variation in virion phosphatidylserine content drives differential GAS6 binding among closely related flaviviruses
Many enveloped viruses engage phosphatidylserine (PS) receptors to enter cells, a phenomenon known as "apoptotic mimicry". We previously reported that Zika virus (ZIKV), but not closely-related West Nile virus (WNV) or dengue virus (DENV), utilized AXL to infect cells because only ZIKV could bind the AXL ligand GAS6, a PS-binding protein. In this study, we investigated the mechanisms underlying the differential ability of these viruses to bind GAS6. Although immature virions expose larger patches of the viral membrane than do mature ones, our data show that virion maturity levels did not contribute to GAS6 binding. Surprisingly, while ZIKV contains PS comparable to cellular membranes, PS on WNV and DENV is markedly reduced. These findings explain why only ZIKV can bind GAS6 and provide insights into a novel mechanism by which closely-related flaviviruses differentially utilize cellular entry factors. IMPORTANCEAmong flaviviruses, Zika virus uniquely causes microcephaly and congenital defects. While no flavivirus-specific entry receptors have been identified, they commonly take advantage of phosphatidylserine (PS) receptors to enter cells. Our previous studies revealed that Zika virus uniquely utilizes AXL, found in immune-privileged sites such as the brain and placenta, via binding to its ligand, GAS6. Our current study shows that despite being produced from the same cells, Zika virus has substantially higher PS content than closely related dengue and West Nile viruses, which likely explains Zika viruss unique ability to bind GAS6. These findings provide insight into how closely related flaviviruses can vary substantially in their use of cellular entry factors, potentially contributing to the distinct diseases they cause.