ISG15 inhibits Ebola VP40 VLP budding in an L-domain-dependent manner by blocking Nedd4 ligase activity

Okumura A, Pitha PM, and Harty RN.
Proc Natl Acad Sci U S A. 2008 Mar 11;105(10):3974-9.

Ebola virus budding is mediated by the VP40 matrix protein. VP40
can bud from mammalian cells independent of other viral proteins, and efficient
release of VP40 virus-like particles (VLPs) requires interactions with host
proteins such as tsg101 and Nedd4, an E3 ubiquitin ligase. Ubiquitin itself is
thought to be exploited by Ebola virus to facilitate efficient virus egress.
Disruption of VP40 function and thus virus budding remains an attractive target
for the development of novel antiviral therapies. Here, we investigate the
effect of ISG15 protein on the release of Ebola VP40 VLPs. ISG15 is an
IFN-inducible, ubiquitin-like protein expressed after bacterial or viral
infection. Our results show that expression of free ISG15, or the ISGylation
system (UbE1L and UbcH8), inhibits budding of Ebola virus VP40 VLPs. Addressing
the molecular mechanism of this inhibition, we show that ISG15 interacts with
Nedd4 ubiquitin ligase and inhibits ubiquitination of VP40. Furthermore, the
L-domain deletion mutant of VP40 (DeltaPT/PY), which does not interact with
Nedd4, was insensitive to ISG15-mediated inhibition of VLP release. These data
provide evidence of antiviral activity of ISG15 against Ebola virus and suggest
a mechanism of action involving disruption of Nedd4 function and subsequent
ubiquitination of VP40.