Crimean-Congo Hemorrhagic Fever Virus Glycoprotein Precursor Is Cleaved by Furin-Like and SKI-1 Proteases To Generate a Novel 38-Kilodalton Glycoprotein

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Abstract
Crimean-Congo hemorrhagic fever virus (genus Nairovirus, family Bunyaviridae) genome M segment encodes an unusually large (in comparison to members of other genera) polyprotein (1,684 amino acids in length) containing the two major structural glycoproteins, Gn and Gc, that are posttranslationally processed from precursors PreGn and PreGc by SKI-1 and SKI-1-like proteases, respectively. The characteristics of the N-terminal 519 amino acids located upstream of the mature Gn are unknown. A highly conserved furin/proprotein convertase (PC) cleavage site motif (RSKR247) is located between the variable N-terminal region that is predicted to have mucin-like properties and the rest of PreGn. Mutational analysis of the RSKR247 motif and use of a specific furin/PC inhibitor and brefeldin A demonstrate that furin/PC cleavage occurs at the RSKR247 motif of PreGn as the protein transits the trans Golgi network and generates a novel glycoprotein designated GP38. Immunoprecipitation analysis identified two additional proteins, GP85 and GP160, which contain both mucin and GP38 domain regions, and whose generation does not involve furin/PC cleavage. Consistent with glycosylation predictions, heavy O-linked glycosylation and moderate levels of N-glycans were detected in the GP85 and GP160 proteins, both of which contain the mucin domain. GP38, GP85, and GP160 are likely soluble proteins based on the lack of predicted transmembrane domains, their detection in virus-infected cell supernatants, and the apparent absence from virions. Analogy with soluble glycoproteins and mucin-like proteins encoded by other hemorrhagic fever-associated RNA viruses suggests these proteins could play an important role in viral pathogenesis.