Fig. 6: Schematic representation of maturation differences during the infectious cycle of TBFVs and MBFVs.

Schematic representation of flavivirus maturation process upon assembly of new particles in an infected cell, reflecting current understanding based on structural studies, experimental observations, and model-based predictions. a–I Immature particles, which bud at neutral pH into the ER lumen, contain 60 spikes formed of (prM/E)₃ trimers depicted with prM in yellow and E in blue. PrM has a globular pr head masking the fusion loop in E, and a long linker that connects to its transmembrane domains. The linker contains a zippering element (green), which inserts at neutral pH into a non-polar groove in E, thereby protecting the adjacent FCS (in red) from exposure and cleavage. a–II During transport through the GA, the particle is exposed to increasingly acidic pH, which triggers (E/prM)₃ dissociation and ejection of the zipper from the E groove, effectively exposing the FCS. a–III The E/prM protomers reorganize to make 90 (prM/E)₂ dimers in a smooth herringbone lattice, with the pr moiety of prM still masking the fusion loop but with the linker unzipped and the FCS exposed. a–IV The presence of furin (green pacman) results in cleavage and maturation of the particle. MBFVs and TBFVs released from furin-deficient cells (into the extracellular environment with neutral pH – panel B) show different behavior: while the former reverts to the spike form adopted in the ER, resulting in re-zippering and re-formation of (prM/E)₃ trimers (b–V) this is not the case for TBFVs. b–VI In this case, the more stable interface between E protomers of the (prM/E)₂ dimer prevents the zipper from re-entering the E groove, which is located beneath the dimer, facing the viral membrane. The FCS thus remains exposed at the surface of smooth particles and furin cleaves it at neutral pH, in striking contrast to MBFVs. In TBFVs, this cleavage can occur at the host cell membrane upon infection, leading to the release of the cleaved pr fragment and subsequent endocytosis (b–VI and c). During endocytosis, the decreasing pH activates the fusion loop, facilitating fusion with the endosomal membrane and initiating a new infectious cycle (c). Selected graphics were created in BioRender. Ruzek, D. (https://BioRender.com/4n4b27g.