Ubiquitin in avian leukosis virus particles

We have identified unconjugated ubiquitin as a component of avian leukosis virus (ALV). Quantitation both by immunoblotting and by protein staining showed that ubiquitin makes up about 0.5% of total viral protein, corresponding to 100 molecules per virion. This level is about fivefold higher than the level of unconjugated ubiquitin in the cytosol, when expressed as a fraction of total protein. Other abundant low molecular weight proteins in the cytosol were not detected in virions, indicating that packaging occurs in a specific manner. A naturally occurring ALV mutant that lacks the env gene was found to package normal levels of ubiquitin, ruling out involvement of the viral glycoproteins as carriers of ubiquitin. We examined disrupted virus particles as well as lysates of infected cells for the presence of gag protein-ubiquitin conjugates. No conjugates could be detected, suggesting that ubiquitin does not enter virions linked to gag protein.