Host and Viral Proteins Modulating Ebola and Marburg Virus Egress

Abstract

The filoviruses Ebolavirus and Marburgvirus are among the deadliest viral pathogens known to infect humans, causing emerging diseases with fatality rates of up to 90% during some outbreaks. The replication cycles of these viruses are comprised of numerous complex molecular processes and interactions with their human host, with one key feature being the means by which nascent virions exit host cells to spread to new cells and ultimately to a new host. This review focuses on our current knowledge of filovirus egress and the viral and host factors and processes that are involved. Within the virus, these factors consist of the major matrix protein, viral protein 40 (VP40), which is necessary and sufficient for viral particle release, and nucleocapsid and glycoprotein that interact with VP40 to promote egress. In the host cell, some proteins are hijacked by filoviruses in order to enhance virion budding capacity that include members of the family of E3 ubiquitin ligase and the endosomal sorting complexes required for transport (ESCRT) pathway, while others such as tetherin inhibit viral egress. An understanding of these molecular interactions that modulate viral particle egress provides an important opportunity to identify new targets for the development of antivirals to prevent and treat filovirus infections.

Keywords: ESCRT; Ebola virus; Marburg virus; VP40; budding; egress; filovirus; ubiquitination; viral inhibition.

Figure 1

Ebolavirus glycoprotein. (A) The Ebolavirus genome comprises 7 genes that encode the nucleoprotein (NP), viral proteins 35 (VP35), 40 (VP40), 30 (VP30), and 24 (VP24), the polymerase (L), and four glycoproteins (GP), each transcribed from monocistronic RNA. (B) The GP gene encodes four products, three of which are generated by transcriptional stuttering at a centrally located polyuridine (poly-U) domain that results in the use of three different open reading frames (ORF1, ORF2 and ORF3). These products (sGP, GP_1,2, GP_TACE, ssGP) are produced at the frequencies noted next to the arrows in part B.

Figure 2

Filovirus replication cycle showing host and viral proteins that either promote (green boxes) or block (red boxes) virion assembly and release. Following viral attachment and entry into the host cell (1), the nucleocapsid, containing genomic RNA, is released into the host cell cytoplasm (2) and transcription of filovirus genes (3) and genome replication occurs. Replication is catalysed by the polymerase and viral cofactors NP, VP30 and VP35 (4). In the magnified diagram (5), nucleocapsids are transported to the plasma membrane by hijacking endosomal sorting complexes required for transport (ESCRT) machinery and the multivesicular body (MVB) trafficking pathway. Simultaneously, the major matrix protein, VP40, oligomerises as it moves towards the plasma membrane, while the full length glycoprotein is sent to the endoplasmic reticulum (ER) and Golgi apparatus (GA) for processing and is subsequently transported in vesicles to the plasma membrane in its trimeric form. Nascent virions assemble at lipid rafts and bud from the host cell. In the case of Marburg virus, budding occurs from actin filopodia. Host factors that promote viral egress (green boxes) include actin and myosin, calcium ion channels, neural precursor cell expressed developmentally down-regulated protein 4 (Nedd4), Itchy E3 ubiquitin protein ligase (ITCH), WW domain-containing E3 ubiquitin protein ligase 1 (WWP1), suppressor of cytokine signaling 3 (SOCS3), tumor susceptibility gene 101 (Tsg101), ALG-2-interacting protein X (Alix), and vacuolar protein sorting 4 (Vps4). Host factors that interfere with viral egress (red boxes) include tetherin, interferon stimulated gene 15 (ISG15) and BCL2 associated athanogene 3 (Bag3).

Figure 3

Domain organisation of EBOV and MARV VP40. VP40 is composed of a C-terminal membrane binding domain and an N-terminal oligomerisation domain. The late (L) domain PTAP and PPEY motifs are denoted in pink. Motifs were obtained from EBOV VP40 (eVP40) and MARV VP40 (mVP40) uniprot database entries (Q05128 and Q1PD51, respectively).