Forty-Five Years of Marburg Virus Research

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

As a peripheral membrane protein, VP40 coats the inner side of the virion's membrane ( Figure 5 ) [114] . Cryo-EM tomography studies suggest that VP40 associates with the nucleocapsid through flexible interactions [60] . It can be easily removed from the nucleocapsid by salt dissociation, indicating that it is only loosely connected to the nucleocapsid [115] . After synthesis in the cytoplasm of the infected cell, VP40 associates rapidly with cellular membranes and accumulates in membranous structures of the late endosomal compartment, the multivesicular bodies. A minor portion of VP40 is also found in association with viral nucleocapsids and in inclusions. Additionally, VP40 appears in patches beneath the plasma membrane where it is transported via the retrograde late endosomal pathway [62, 114, 116] . Similar to EBOV VP40, MARV VP40 is the major factor in particle formation and budding. Expression of VP40 in the absence of other viral proteins leads to the formation and release of filamentous virus-like particles (VLPs) resembling authentic virions. This process is enhanced in the presence of GP [113, [117] [118] [119] . The role of VP40 during budding is described in more detail below (see 8.3. Budding). Compared to EBOV VP40, little is known about the structure of MARV VP40. The N-terminal domain of MARV VP40 folds into ring-like structures, which have the tendency to polymerize into rod-like structures. While EBOV VP40 has been shown to form hexamers and octamers, the stoichiometry of MARV VP40 oligomers is not known [120] . MARV VP40 is phosphorylated at several tyrosine residues located in the N-terminal region of the protein. A non-phosphorylatable mutant of VP40 is impaired in its ability to recruit nucleocapsids to the sites of budding, but is still able to efficiently induce particle release [112] . VP40 also possesses a PPPY late domain motif in its amino terminus which is important for its interaction with components of the Endosomal Sorting Complex Required for Transport (ESCRT) machinery in order to mediate budding, including Tumor susceptibility gene 101 (Tsg101) and the membrane-bound E3 ubiquitin ligase Nedd4.1 [119, 121, 122] . Besides the PPPY motif, other motifs and single amino acids have been found to be important for particle release [123, 124] .