Biology Reference
In-Depth Information
order,
Mononegavirales
, such as paramyxoviruses and rhabdoviruses,
have stimulated research on filovirus assembly, and vice versa. Such
studies have shown that viruses co-opt cellular transport machinery
for their assembly and budding processes, and that even diverse
viruses can exploit these pathways.
3
Virion Budding
Electron microscopic observations suggest that filovirus virions typi-
cally bud from the plasma membrane,
2
where viral components such
as GP, VP40, and nucleocapsids are assembled. The egress of virions
occurs at cytoplasmic vacuoles in monocytes/macrophages,
4
although
it remains unclear whether virions that accumulate in intracellular vac-
uoles are released normally to extracellular spaces. Some electronmi-
crographs suggest that filamentous virions bud both vertically and
horizontally from the plasma membranes of virus-infected cells.
2
Whether differences exist between these two forms and the biological
significance of this finding remain unknown.
Matrix proteins play a central role in the virion formation of many
enveloped negative-strand RNA viruses.
5
In general, these proteins
intrinsically associate with cellular membranes and are believed to
form a bridge between the nucleocapsids and the cytoplasmic tails of
transmembrane glycoproteins.
5
The Ebola virus and Marburg virus
VP40 proteins interact with the plasma membrane and with intracel-
lular membranes.
6-8
Coexpression of VP40 and GP results in the for-
mation of virus-like particles (VLPs) that contain orderly arranged GP
spikes,
9
which suggests that VP40 and GP interact. A recent study
also found an interaction between VP40 and VP35, a component of
the viral nucleocapsid.
10
Thus, VP40 appears to connect the nucleo-
capsid to the viral shell, similar to the function of the matrix proteins
of other negative-strand RNA viruses.
The VP40 Protein
The VP40 matrix protein is the most abundant protein in filovirus
particles. The X-ray crystallographic structure of monomeric VP40
