Vaccinia Virus BTB-Kelch Proteins and the Ubiquitin-Proteasome System during poxvirus infection.
Vaccinia virus (VACV)—the live-virus vaccine used to eradicate smallpox (Variola virus)—encodes three BTB-Kelch protein (BBK) orthologues, a family of cellular proteins that have demonstrated roles in the Ubiquitin-Proteasome System (UPS). The UPS is the common mechanism by which specific proteins are degraded at specific times inside the host cell. Substrate proteins are multiply-ligated with ubiquitin and are thus flagged for degradation by the 26S Proteasome. If an invading virus were to commandeer such a system it may be rewarded with a unique and powerful solution to avoid the intrinsic cellular defences. BBKs function as UPS substrate adaptors, acting as a link between the ubiquitination machinery and the ubiquitin-ligated substrates themselves.
By encoding BBKs VACV can hijack the UPS and selectively degrade a wide range of host proteins to its advantage; preventing the establishment of an antiviral immune response, transforming the cell into a virus-production factory or enhancing viral spread. Manipulation of the UPS is a phenomena known to play a role in mediating infection in many other viral contexts. The identification of VACV BBK substrates may highlight new mechanisms by which VACV and other viruses overcome the intrinsic cellular defences to mediate infection.
We have previously shown, using fluorescently tagged BBKs, partial colocalisation with the ubiquitinylation machinery, indicating that these proteins act via a common UPS-based mechanism. These results are consistent with partial redundancy observed in BBK mutants and the obstructive effect of UPS inhibitors on poxvirus replication. We are now attempting to further elucidate any potential interactions and dissect the global implications of poxviral interactions with the UPS.