Functional involvement of a novel Nedd4-like ubiquitin ligase on retrovirus budding

In this study, we have identified a novel Nedd4-like ubiquitin ligase, BUL1, as the host factor involved in budding of type D retrovirus Mason-Pfizer monkey virus (M-PMV). Overexpression of BUL1 enhanced virus particle release, while a BUL1 mutant in which a W to G substitution was introduced into a WW domain, W791G, lost the ability to bind to the viral Gag protein and abolished its ability to mediate virus budding. In addition, a fragment of BUL1 containing only the WW domains inhibited virus budding in a dominant negative manner. These results, together with previous findings, indicate that the M-PMV Gag L domain interacts with the BUL1 WW domain and that this interaction is essential for virus budding. Our observations provide new insights into the mechanism of virus budding, and could be useful in establishing new antiviral strategies targeted at progeny virus release from a host cell.