Molecular Insights into Crimean-Congo Hemorrhagic Fever Virus

Zivcec, Marko; Scholte, Florine E. M.; Spiropoulou, Christina F.; Spengler, Jessica R.; Bergeron, Éric

Viruses 2016, 8, 106 5 of 24 and the transmembrane domains of the precursor protein span the ER membrane five times [31] . Before GPC is completely translated, it is cleaved into the Gn precursor (PreGn), the Gc precursor (PreGc), and the NSM. This step is thought to require the signal peptidase and the intramembrane cleaving proteases (I-CLiPs), as generation of these three proteins requires cleavage after the signal peptide and near or within the transmembrane domains-2 and -4 ( Figure 3 ) [31, 52] . The productive maturation in the ER is followed by PreGn and PreGc transport to the Golgi complex [52, 54] , where the PreGn mucin-like domain (MLD) acquire numerous O-linked glycans [51] . PreGn can traffic to the Golgi complex, the proposed site of CCHFV assembly, in the absence of PreGc [37] , but PreGc requires PreGn to exit the ER [37] . Mutating the N577 glycosylation site of PreGn, located in the mature Gn region, blocks PreGn from exiting the ER and prevents secretion of GP160, GP85, and GP38, suggesting a critical role for the glycosylation of N577 in folding and normal trafficking to the Golgi complex [55] . The GPC is synthesized in the ER where N-glycosylation occurs (N-glycan). Numbers indicate amino acids positions. The signal peptidase and/or the intramembrane cleaving proteases (iCLIPs) co-translationally cleave GPC close to or within transmembrane domain-2 and -4. These cleavages yield PreGn, non-structural M protein (NS M ) and PreGc. These proteins traffic to the Golgi where the mucin-like domain of PreGn is O-glycosylated (O-glycan), and is cleaved by subtilisin kexin isozyme-1/site-1 protease (SKI-1/S1P) at the RRLL motif. PreGc at the RKPL motif by a protease with similar specificity to SKI-1/SIP (SKI-1/S1P-like?). PreGn cleavage liberates an N-terminal fragment with an apparent total molecular weight on SDS-PAGE of 160 kDa (GP160) and 85 kDa (GP85). GP160/85 is later cleaved by furin in the trans-Golgi network (TGN). (B) GPC products. Processing of GPC yields non-structural products associated with the cell (Non-structural intracellular), associated with the virions (Structural), secreted but not part of the virions (Non-structural secreted), and inferred products that remain uncharacterized or have yet to be detected Undectected/uncharacterized). The GPC contains an N-terminal signal peptide directing its synthesis to the secretory pathway [31, 52] . Upon elongation and translocation into the endoplasmic reticulum (ER), the signal peptide is removed, the GPC is N-glycosylated, folded, intra-molecular disulfide bridges are formed, and the transmembrane domains of the precursor protein span the ER membrane five times [31] . Before GPC is completely translated, it is cleaved into the Gn precursor (PreGn), the Gc precursor (PreGc), and the NS M. This step is thought to require the signal peptidase and the intramembrane cleaving proteases (I-CLiPs), as generation of these three proteins requires cleavage after the signal peptide and near or within the transmembrane domains-2 and -4 ( Figure 3 ) [31, 52] .