Vif, Vpr & PR


Project Leaders:  John Gross & Charles Craik

The accessory protein Vif inactivates cellular cytidine deaminases A3F and A3G (members of the APOBEC3 family) that would otherwise cause extensive incorporation of deoxyuridines into the viral genome, leading to a high mutation rate and destroying key viral functions. Vif recruits A3G and A3F to a cellular ubiquitin protein ligase (E3 ligase) that includes Cullin-5, Ring-box 1 and Elongins B and C, ultimately leading to ubiquitination of A3G and A3F and their destruction by the proteosome. The HARC Center Vif project, headed by John Gross aims to elucidate the structural nature of Vif-mediated interactions that target APOBEC cytidine deaminases for destruction by the proteasome, as well as to better understand the antiviral activity of the APOBEC proteins themselves.

Figure 1. Research in the HARC Center led to the discovery of the transcription factor CBF-β as a key Vif cofactor that stabilizes Vif in Cullin ligase complexes (Figure 1; Jager et al., Nature, 2012). Vif is depicted hijacking cellular CBF-β to the E3 ubiquitin ligase complex required for A3G polyubiquitination and degradation

Vpr is a conserved multi-functional protein that is incorporated into virions and drives early (transport of pre-integration complex) and late (apoptosis) viral life cycle steps.  Despite more than 20 years of research, the mechanism by which Vpr accelerates virus replication is not well understood. A wide range of molecular functions has been ascribed to Vpr, including roles in G2 cell cycle arrest and pre-integration complexes, perhaps reflecting a multi-purpose adaptor protein. The HARC Proteomics/Genomics Core has compiled the most extensive Vpr-host interaction data set described to date. Encouragingly, these novel interactions reveal host complexes and partners functionally linked with many Vpr activities. By evaluating the functional significance and ability to form structured complexes, the Vpr project, headed by Charles Craik, will develop and test new hypotheses about Vpr-host interactions.

HIV-1 Protease (PR) processes the viral Gag and GagPol polyproteins during virion maturation, but its role in affecting host functions is unclear. In vitro, PR cleaves several mammalian proteins, but it is not known if cleavage occurs during HIV infection or how much active enzyme exists in host cells during the viral life cycle. The HARC Center Proteomics/Genomics Core uncovered novel host proteins that interact with an inactive version of PR, including proteins involved in immune responses, splicing, translation, cell growth, and apoptosis. The PR project, headed by Charles Craik, aims to perform structural studies of such PR-host complexes, which will be the first of PR with a macromolecular substrate, providing new knowledge on substrate recognition. Analyses of the roles of potential host PR substrates in HIV replication have the potential to establish new paradigms for PR function, link PR to the roles of Vif, Vpu, and Vpr, and define the structures of novel PR-host complexes.