Function(s) of the Ubiquitin—Proteasome System in Retrovirus Budding

Several retroviruses contain mono-ubiquitinylated Gag protein, which is modified adjacent to the viral late asembly domain (L, a sequence required for efficient virus release). Furthermore, it has been demonstrated for HIV that inhibition of proteasome activity blocks virus budding and processing of the virus Gag polyproteins without direct inhibition of viral protease. Two mutually exclusive hypotheses have been investigated, that proteasome inhibition acts by causing (1) depletion of free ubiquitin and thus preventing mono-ubiquitinylation of the L-domain containing Gag proteins, or (2) accumulation of defective, possibly misfolded Gag proteins that interfere with assembly, budding, and maturation of progeny virions. The ubiquitin-depletion hypothesis originates from the observation that extended proteasome inhibition significantly reduces the levels of free ubiquitin in HIV-1-infected cells and prevents the mono-ubiquitinylation of Gag proteins. A potential role of ubiquitin in virus budding is, however, inconsistent with the finding that mutants of HIV-1 carrying mutations of ubiquitin acceptor sites in Gag exhibit wild-type replication and maintain sensitivity to proteasome inhibitors. In contrast, the defective Gag hypothesis originates from the finding that poly-ubiquitinylated and misfolded forms of defective ribosomal products (DRiPs) of HIV-1 Gag accumulate immediately following proteasome inhibition and that such Gag-DRiPs ultimately interfere with the folding and processing of viral structure proteins. Regardless of the mechanism, the phenomenon suggests a novel pharmaceutical strategy for interfering with retrovirus replication.