Structure of filovirus particles

Filoviruses are enveloped, non-segmented, negative-sense (NNS) RNA viruses and constitute a separate family within the order Mononegavirales. The family consists of the genera Marburgvirus (MARV) and Ebolavirus (EBOV). The genus Ebolavirus is further subdivided into four distinct species: Ivory Coast ebolavirus (ICEBOV), Reston ebolavirus (REBOV), Sudan ebolavirus (SEBOV) and Zaire ebolavirus (ZEBOV) [18]. Filoviral particles are bacilliform in shape, but can also appear as branched, circular, U-shaped, 6-shaped, and long filamentous forms (Fig. 1A). They display a uniform diameter of approximately 80 nm, but vary greatly in length. Negatively contrasted particles, regardless of serotype or host cell, contain an electron-dense central axis (19-25 nm in diameter) surrounded by

Marburg Virus Structure


II... I

■ J

1 NP |

|35 J

1 GP ★

HI 301


Fig. 1. Structure of filovirus particles. (A) Electron micrograph. Marburg virus (MARV) particles shown here demonstrate a characteristic bacilliform shape. The electron-dense central axis, formed by the ribonucleoprotein complex and the surrounding lipid envelope are clearly visible. Additionally, the glycoprotein (GP) can be observed as projections on the surface of the particles. [altered from reference #19] (B) Genome organization. The gene orders of fully sequenced filovirus genomes are presented. The intergenic regions are shown in white, the non-coding regions in dark gray boxes and the open reading frames in light gray boxes. Steps indicate the positions of the gene overlaps. Key: REBOV Reston ebolavirus; ZEBOV Zaire ebolavirus; GP glycoprotein; kb kilobase; L RNA-dependent RNA polymerase; MARV Marburgvirus; NP nucleoprotein; 24, 30, 35, 40 virion proteins (number indicates the molecular weight in kDa); * RNA editing site. [altered from reference #31]

an outer helical layer (45-50 nm in diameter) with cross-striations at 5 nm intervals. This central core is formed by the RNP complex, which is surrounded by a lipid envelope derived from the host cell plasma membrane. Spikes of approximately 7 nm in diameter and spaced at about 5-10 nm intervals are seen as globular structures on the surface of virions (Fig. 1A) [29, 50, 60].

Virus particles are made up of seven structural proteins with presumed identical functions for the different viruses. Four proteins make up the RNP complex [nucleoprotein (NP), virion protein (VP) 35, VP30 and RNA-dependent RNA polymerase (L)] together with the viral RNA, while the remaining three proteins are membrane-associated [glycoprotein (GP), VP40, VP24]. The single type I transmembrane GP is inserted in the envelope as a homotrimer and functions in receptor binding and fusion; VP40 has been identified and characterized as the matrix protein but the function of VP24 is yet unknown. EBOV expresses a nonstructural soluble glycoprotein (sGP) as the primary gene product of the glycoprotein gene, which is efficiently secreted from infected cells; its functions remain unknown [19, 21, 49, 53, 67, 77, 85].

The single negative-sense linear RNA genome of filoviruses does not contain a poly(A) tail and is noninfectious. Upon entry into the cytoplasm of host cells it is transcribed by the viral polymerase to generate polyadenylated sub-genomic mRNA species. Filovirus genomes are approximately 19 kb in length and genes are organized in the following linear order: 3' leader - NP - VP35 - VP40 -GP - VP30 - VP24 - L - 5' trailer (Fig. 1B). Genes are delineated by transcriptional signals at their 3; and 5; ends that have been identified by their conservation and by sequence analysis of mRNA species. The sequences 3'-CUNCNUNUAAUU-5' and 3'-UNAUUCUUUUU-5' represent the consensus motifs for transcriptional start and stop signals, respectively. Filoviral genes are usually separated from each other by intergenic regions that vary in length and nucleotide composition, but some gene overlaps occur at characteristic positions (Fig. 1B). The length of the overlaps is limited to five highly conserved nucleotides within the transcriptional signals (3'-UAAUU-5') that are found at the internal ends of the conserved sequences. Most genes tend to possess long non-coding sequences at their 3; and/or 5; ends, which contribute to the increased length of the genome. Extragenic sequences are found at the 3;-leader and 5;-trailer ends of the genome. The leader and trailer sequences are complementary to each other at the extreme ends; a feature that is shared by many NNS RNA viruses [19, 49,67].

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