Vpu Downmodulates Two Distinct Targets, Tetherin and Gibbon Ape Leukemia Virus Envelope, through Shared Features in the Vpu Cytoplasmic Tail

During human immunodeficiency virus-1 (HIV-1) assembly, the host proteins CD4 (the HIV-1 receptor) and tetherin (an interferon stimulated anti-viral protein) both reduce viral fitness. The HIV-1 accessory gene Vpu counteracts both of these proteins, but it is thought to do so through two distinct mechanisms. Modulation of CD4 likely occurs through proteasomal degradation from the endoplasmic reticulum. The exact mechanism of tetherin modulation is less clear, with possible roles for degradation and alteration of protein transport to the plasma membrane. Most investigations of Vpu function have used different assays for CD4 and tetherin. In addition, many of these investigations used exogenously expressed Vpu, which could result in variable expression levels. Thus, few studies have investigated these two Vpu functions in parallel assays, making direct comparisons difficult. Here, we present results from a rapid assay used to simultaneously investigate Vpu-targeting of both tetherin and a viral glycoprotein, gibbon ape leukemia virus envelope (GaLV Env). We previously reported that Vpu modulates GaLV Env and prevents its incorporation into HIV-1 particles through a recognition motif similar to that found in CD4. Using this assay, we performed a comprehensive mutagenic scan of Vpu in its native proviral context to identify features required for both types of activity. We observed considerable overlap in the Vpu sequences required to modulate tetherin and GaLV Env. We found that features in the cytoplasmic tail of Vpu, specifically within the cytoplasmic tail hinge region, were required for modulation of both tetherin and GaLV Env. Interestingly, these same regions features have been determined to be critical for CD4 downmodulation. We also observed a role for the transmembrane domain in the restriction of tetherin, as previously reported, but not of GaLV Env. We propose that Vpu may target both proteins in a mechanistically similar manner, albeit in different cellular locations.