Forty-Five Years of Marburg Virus Research

Brauburger, Kristina; Hume, Adam J.; Mühlberger, Elke; Olejnik, Judith

Following assembly, newly synthesized nucleocapsids are recruited to the sites of virus budding (Figure 7) . Release of viral particles is mainly mediated by VP40 via recruitment of nucleocapsids from the inclusions to the plasma membrane, recruiting GP to the sites of budding, and inducing the formation and release of filamentous VLPs. VP40-induced budding is enhanced by NP, GP, and VP24 [98, 111, 132] . As is the case with many other viruses, MARV exploits the vesicular transport machinery of the infected cell for viral egress, including the COPII vesicular transport system and the ESCRT machinery. The COPII vesicular transport system is used by VP40 for its intracellular trafficking to the multivesicular bodies, where MARV budding takes place [118, 187] . Cellular proteins that promote particle release and are linked to the ESCRT machinery include Tsg101, Vps4A/B, and Nedd4.1 [111, 119, 121, 122] . MARV budding not only takes place at internal membranes but also at the plasma membrane [63, 118, 188] . In cell culture, MARV particles are preferentially released at filopodia, filamentous cellular protrusions [60, 188, 189] . Filopodia are used by cells to explore the extracellular environment, which includes neighboring cells, and it is believed that viral particles can bud directly into adjacent cells via filopodia-mediated cell-to-cell contact [188, 190] . Budding at filopodia depends on actin and is not sensitive to the depolymerization of microtubules [188] . MARV budding was observed at the basolateral membrane of polarized epithelial cells and hepatocytes [109, 191] , whereas viral particles were predominantly released from the apical membrane of infected endothelial cells [64] , suggesting that cell-type specific components determine the sites of virus release.