Proteolytic cleavage of HIV-1 GFP-Vpr fusions at novel sites within virions and living cells : concerns for intracellular trafficking studies

Fluorescent labelling of the highly conserved HIV-1 accessory protein Vpr (Viral Protein R) with GFP or variants thereof has proved a valuable approach to track Vpr and/or HIV-1 subcellular localisation in vivo. Our analysis in transfected mammalian cells expressing GFP-Vpr fusion protein, as well as within virus derived there from, documents site-specific proteolytic cleavage of the GFP-Vpr fusion protein. Western analysis revealed that transfected mammalian cells harbour a C-terminally truncated variant of Vpr in addition to full-length GFP-Vpr. Further, virions derived from these GFP-Vpr expressing cells show protein in which the GFP-tag has been additionally cleaved from the Vpr protein. Endogenous HIV protease (PR) activity was shown to be responsible for the latter, as addition of Saquinavir™, a potent PR inhibitor abolished the cleavage. Since many previous studies have relied on imaging the GFP fluorescence of GFP-Vpr, it would appear that the results may not reflect intact GFP-Vpr.