Forty-Five Years of Marburg Virus Research

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

The MARV genome encodes seven structural proteins listed in Table 2 . MARV has a single surface protein, GP, which is encoded by the fourth gene and mediates attachment to target cells and virus entry [76] . GP is a Type I transmembrane protein which is inserted into the viral envelope in the form of homotrimeric spikes [65] . In contrast to ebolaviruses, which use transcriptional editing to express the membrane-bound GP and at least two nonstructural glycoproteins [77] [78] [79] [80] , the MARV GP gene contains a single open reading frame (ORF) encoding the full-length GP. During its transport from the endoplasmic reticulum (ER) to the plasma membrane via the secretory pathway, the precursor GP is the target of various posttranslational modifications including glycosylation [65, 81] , acylation [82] , and phosphorylation [83] . GP is heavily glycosylated by complex and high mannose-type N-linked glycans as well as by mucin-type O-linked glycans, with the carbohydrates contributing about 50% of the apparent molecular weight of the protein [65, 84, 85] . Similar to EBOV GP, the O-linked glycans and many of the N-linked oligosaccharides are clustered in a mucin-like domain [76] . After synthesis in the ER, the precursor GP is cleaved at amino acid 435 by furin or a furin-like protease in the trans Golgi network, resulting in two disulfide-linked subunits, GP 1 (160 kD) and GP 2 (38 kD) [86] . While the ectodomain, which is mainly formed by GP 1 , mediates binding to entry factors and receptors [87] [88] [89] [90] [91] [92] [93] [94] [95] , the transmembrane subunit GP 2 contains the fusion peptide and is presumed to mediate fusion of the viral and the cellular membrane based on similarity to EBOV GP 2 both at the amino acid and structural level [96, 97] . The 30 amino acid long transmembrane domain of GP 2 is required for the incorporation of GP into virions [98] . In addition, the cytoplasmic tail of GP 2 is involved in enhancing the efficiency of viral entry by maintaining the structure of the ectodomain [99] . The receptor binding domain of MARV GP was mapped to the aminoterminal region of GP 1 spanning amino acids 38 to 188 [100] , whereas the highly glycosylated mucin-like domain is not essential for virus entry [101] . An important step in MARV entry is the proteolytic activation of GP 1 by endosomal proteases, facilitating binding of the receptor binding region to the endosomal entry factor Niemann-Pick C1 protein (see below, 8.1. Entry) [94, 95] .