Biology Reference
In-Depth Information
Tsg101 (mediated by the P(T/S)AP) motif), as has been shown for
Gag.
37
This hypothesis is supported by the finding that Ebola VP40 is
ubiquitinated by Nedd4-like ubiquitin ligases;
38
however, it remains to
be seen whether this modification affects its interaction with Tsg101.
Nucleocapsid Assembly
In contrast to the extensive work conducted on the budding process,
research on nucleocapsid formation has been limited to date. Filovirus
nucleocapsids are thought to consist of viral RNA and the NP, VP30,
VP35, and L (and possibly also the VP24) proteins. Three of these
proteins (NP, VP35 and L) are necessary and sufficient for replication
of a monocistronic genome-like Marburg virus replicon,
39,40
while
VP30 acts as a transcription regulator.
40,41
By contrast, all four of these
proteins are required for the replication of Ebola virus genome-like
replicons.
40
All four proteins are also essential for the generation of
Ebola virus
42,43
and Marburg
56
virus from cloned cDNAs (“reverse
genetics”). An earlier study demonstrated that VP24 was not
completely removed from purified nucleocapsids under isotonic con-
ditions,
44
suggesting that it could also be a constituent of the nucleo-
capsids, although it is not required for transcription and replication of
the viral genome.
Expression of Marburg virus NP in insect cells leads to the for-
mation of NP-RNA complexes,
57
as had been reported with the rabies
virus and measles virus NP proteins.
45-47
Purified NP-RNA complexes
of Marburg virus show loose coils that differ from authentic Marburg
virus nucleocapsids. By contrast, NP-RNA complexes of rabies and
measles viruses resemble those found in infected cells. Therefore,
additional components are necessary for the formation of Marburg
virus nucleocapsids.
Electron microscopic
48
and tomographic studies
49
identified the
components of filovirus nucleocapsids (Fig. 5). Simultaneous expression
of Ebola NP, VP35, and VP24 in mammalian cells results in the forma-
tion of nucleocapsid-like structures indistinguishable from authentic
nucleocapsids.
48
Of the three proteins, VP24 seems to be a minor com-
ponent.
48
NP and VP35 appear to build up the framework of the
