Ebolaviruses: New roles for old proteins

Abstract

In 2014, the world witnessed the largest Ebolavirus outbreak in recorded history. The subsequent humanitarian effort spurred extensive research, significantly enhancing our understanding of ebolavirus replication and pathogenicity. The main functions of each ebolavirus protein have been studied extensively since the discovery of the virus in 1976; however, the recent expansion of ebolavirus research has led to the discovery of new protein functions. These newly discovered roles are revealing new mechanisms of virus replication and pathogenicity, whilst enhancing our understanding of the broad functions of each ebolavirus viral protein (VP). Many of these new functions appear to be unrelated to the protein's primary function during virus replication. Such new functions range from bystander T-lymphocyte death caused by VP40-secreted exosomes to new roles for VP24 in viral particle formation. This review highlights the newly discovered roles of ebolavirus proteins in order to provide a more encompassing view of ebolavirus replication and pathogenicity.

Fig 1. Genome organisation of Filoviridae family members: MARV, EBOV, and RESTV.

Each box represents the open-reading frames that produce the VPs. RNA editing by addition of Us occurs on the GP transcript in order to produce GP and the ssGP. Post-translational modification of the GP protein results in two GP subunits after cleavage by host furin. Slight differences in gene overlaps and the presence of intergenic regions are evident between the different ebolavirus species, though the functional consequence of these differences is not clear at present. EBOV, Ebola virus; GP, glycoprotein; L, L-polymerase; MARV, Marburg virus; NP, nucleoprotein; RESTV, Reston virus; sGP; soluble glycoprotein; ssGP, small soluble glycoprotein; U, uridine; VP, viral protein.

Fig 2. Multiple roles of VP35 during virus replication.

VP35 inhibits the type-I IFN response through several different mechanisms. VP35 can bind to dsRNA, preventing the activation of RIG-I signalling. In addition, VP35 blockade of IRF3 and IRF7 phosphorylation inhibits the production of IFN-β. Recent studies have also highlighted the importance of VP35 in regulating NP–RNA association. During viral genome replication, the VP35 N-terminal peptide binds to NP, enabling the vRNA to associate with the RdRp complex for replication. During virus assembly, VP35 disassociates, enabling NP to oligomerise, bind RNA, and form the nucleocapsid. 5’PPP, 5’ triphosphate; dsRNA, double-stranded RNA; IFN, interferon; IKK, inhibitor of nuclear factor kappa B kinase subunit epsilon; IRF, interferon regulatory factor; MAVS, mitochondrial antiviral-signalling protein; MDA5, melanoma differentiation-associated protein 5; NP, nucleoprotein; PACT, protein activator of the interferon-induced protein kinase; RdRp, RNA-dependent RNA polymerase; RIG-I, retinoic acid-inducible gene I; TANK, tumour necrosis factor–receptor-associated factor family member–associated nuclear factor kappa B activator; TBK1, tumour necrosis factor–receptor-associated factor family member–associated nuclear factor kappa B activator binding kinase 1; TRAF3, tumour necrosis factor–receptor-associated factor 3; VP, viral protein; vRNA, viral RNA.

Fig 3. Ebolavirus proteins VP30, VP35, and VP40 are suppressors of RNA silencing.

Cellular RNA interference requires the assembly of the Dicer:TRBP:PACT complex. VP30 inhibits RNAi by interacting with Dicer, preventing TRBP binding and complex activity. VP35 also inhibits complex assembly by binding TRBP and PACT, preventing their association with Dicer. VP40 suppresses RNAi during infection or when transferred to bystander immune cells through exosomes, though the mechanism by which VP40 inhibits the Dicer machinery is currently unknown. PACT, protein activator of the interferon-induced protein kinase; RNAi, RNA interference; TRBP, Trans-activation response RNA binding protein; VP, viral protein.