Ubiquitin is part of the retrovirus budding machinery

Retroviruses contain relatively large amounts of ubiquitin, but the signifi cance of this finding has been unknown. Here, we show that drugs that are k nown to reduce the level of free ubiquitin in the cell dramatically reduced the release of Rous sarcoma virus, an avian retrovirus, This effect was su ppressed by overexpressing ubiquitin and also by directly fusing ubiquitin to the C terminus of Gag, the viral protein that directs budding and partic le release. The block to budding was found to be at the plasma membrane, an d electron microscopy revealed that the reduced level of ubiquitin results in a failure of mature virus particles to separate from each other and from the plasma membrane during budding. These data indicate that ubiquitin is actually part of the budding machinery.