ICTV Virus Taxonomy Profile: Filoviridae.

Members of the family Filoviridae produce variously shaped, often filamentous, enveloped virions containing linear non-segmented, negative-sense RNA genomes of 15-19 kb. Several filoviruses (e.g., Ebola virus) are pathogenic for humans and are highly virulent. Several filoviruses infect bats (e.g., Marburg virus), whereas the hosts of most other filoviruses are unknown. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on Filoviridae, which is available at www.ictv.global/report/filoviridae.

Virion

Virions are enveloped and diverse in shape and can appear as branched, toroid, U- or 6-shaped, and long filamentous forms (Table 1, Fig. 1). Virions contain ribonucleoprotein (RNP) complexes composed of genomic RNA and, typically, the structural proteins nucleoprotein (NP), polymerase co-factor (VP35), transcriptional activator (VP30), RNP-associated protein (VP24) and RNA-dependent RNA polymerase (L). The matrix protein (VP40) forms a regular layer beneath the viral envelope. Surface spikes formed by glycoproteins (GP1,2) are approximately 7 nm in diameter and cover the virion surface at approximately 10-nm intervals [1-3]. Some filoviruses do not have discernable glycoproteins and may have different RNP complexes [4].

Table 1.

Characteristics of members of the family Filoviridae

Typical member:	Marburg virus (DQ217792), species Marburg marburgvirus, genus Marburgvirus
Virion	Enveloped, variously shaped but predominantly filamentous, typically with a single nucleocapsid
Genome	Approximately 15–19 kb of linear, negative-sense, non-segmented RNA
Replication	Antigenomic RNA is a replication intermediate. The genome and antigenome form ribonucleoprotein complexes, which serve as templates
Translation	From multiple 5′-capped and 3′-polyadenylated mRNAs
Host range	Primates (e.g., ebolaviruses, marburgviruses), bats (e.g., marburgviruses), domestic pigs (e.g., Reston virus) and probably fish (e.g., striaviruses, thamnoviruses) become naturally infected
Taxonomy	Realm Riboviria, phylum Negarnaviricota, subphylum Haploviricotina, class Monjiviricetes, order Mononegavirales; family includes multiple genera

Fig. 1.

Electron microscopic images of Marburg virus particles budding from infected Vero E6 cells, (a) scanning EM, (b) transmission EM. Images are colourized for clarity. Courtesy of John G. Bernbaum and Jiro Wada, IRF-Frederick.

Genome

Filovirus genomes are approximately 15–19 kb (Fig. 2) without a 5′-cap or 3′-poly(A). Terminal leader and trailer sequences contain the replication and transcription promoters. Marburgvirus genomes contain seven separate, continuous open reading frames (ORFs) flanked by 3′- and 5′-terminal non-coding regions that contain transcription initiation and termination sites. These ORFs encode the virion structural proteins. Cuevavirus and ebolavirus genomes encode homologues of the marburgvirus structural proteins. However, the marburgvirus GP gene encodes only GP1,2, whereas the primary expression product of cuevavirus and ebolavirus GP gene transcription is a soluble glycoprotein. Co-transcriptional editing is used to express GP1,2 and additional non-structural proteins. Striaviruses and thamnoviruses encode some, but not all, marburgvirus protein homologues and several proteins of unknown function [4, 5].

Fig. 2.

Schematic representation of the genome organization of Marburg virus. Courtesy of Jiro Wada, IRF-Frederick.

Replication

Virus proteins are translated from mRNAs that are synthesized by successive, polar transcription from RNP complexes containing genomic RNA. Replication occurs in the cytoplasm through the synthesis of RNP complexes containing antigenomes that are templates for genomic RNA production. Replication and transcription enzymes include L and VP35. VP30 serves as a transcription enhancer for ebolaviruses and probably cuevaviruses, but its function in marburgvirus infection is less defined. Virion assembly, including acquisition of the GP1,2-containing lipid envelope, occurs by VP40-mediated budding at the plasma membrane [4, 5].

Taxonomy

Filoviruses form a family in the haploviricotine order Mononegavirales. Within this order, filoviruses are most closely related to members of the families Paramyxoviridae, Pneumoviridae and Sunviridae. The family Filoviridae includes multiple genera for viruses that differ in geographic and host range and genomic organization.

Resources

Full ICTV Report on the family Filoviridae: www.ictv.global/report/filoviridae.