Revious studies demonstrated that Vpu retention in the RER by the addition of a putative retrieval motif prevents downmodulation of tetherin at the PM [44,47]. We found that R7227 placement of the KKDQ ER-retention motif on the Cterminus of Vpu, exactly as previously described [47], reduced its ability to restrict either target, though the effect on tetherin restriction was more severe. These data are consistent with direct or indirect interactions between Vpu and both target proteins in a post-ER region.Conserved amino acid features in Vpu cytoplasmic tail are required for activityNext, we generated truncation mutations in Vpu to determine the minimal sequence required for modulation of the two targets in this system. For both tetherin and GaLV Env, truncation CY5-SE chemical information beyond 13 C-terminal amino acids (D13) resulted in a decrease in Vpu function, although for tetherin this decrease was progressive (Figure 2). To identify critical regions upstream of D13, we mutated two residues at a time to alanine and assayed for activity. For both targets, Vpu was most sensitive to mutations within the conserved hinge region while upstream regions were less sensitive (Figure 3A). Unlike reported findings for BH10 Vpu R30A,K31A [48], mutations located within the YRKIL trafficking motif, we did not observe a decrease in infectivity in the presence of tetherin with our HXB2 Vpu system. Although both are subtype B and almost identical in amino acid sequence, we cannot exclude thatsubtle variation between the two strains may explain observed differences. We then sought to identify specific amino acids required in the CT by scanning individual point mutants through substitution of alanine for individual amino acids, with the exception of alanine which was substituted with serine. Interestingly, almost all amino acids within the Vpu-hinge region between amino acids 51?0, not solely the serines 53, 57, were sensitive to disruption (Figure 3B). A recent report suggested that a putative trafficking motif, ExxxLV, located between residues 60?5 is required for tetherin antagonism [49]. In agreement with this report, we found that E60A disrupted tetherin activity, but the effects of L64A, and V65A alone were more modest. These results demonstrate Vpu’s requirement for conservation of the hinge region for antagonism of two distinct protein targets. Because alanine substitution should not affect physical accessibility of the hinge region by proteins such as b-TrCP, we presume that modification of the conserved features, such as the acidic amino acids, disrupts recognition of Vpu by cellular factors or Vpu’s ability to interact with targets.DiscussionHere we have identified shared critical features in Vpu required for restriction of two distinct proteins, tetherin and the glycoprotein GaLV Env. With the exception of the TMD region, Vpu requires similar features to counteract both targets. Our Vpu screen raises the question: why are similar features in Vpu required for modulation of two disparate target proteins? We propose that Vpu utilizes multiple regions for three somewhat overlapping steps in both restriction pathways: i) retention through interaction, ii) modification and redirection, and iii) degradation. In the case of tetherin, interaction occurs between the TMDs and for CD4 interaction occurs in the CTs and is absolutely requiredVpu Modulation of Distinct TargetsFigure 3. Alanine mutagenic scan of Vpu reveals antagonistic regions for downmodulation of tetherin (dark bars).Revious studies demonstrated that Vpu retention in the RER by the addition of a putative retrieval motif prevents downmodulation of tetherin at the PM [44,47]. We found that placement of the KKDQ ER-retention motif on the Cterminus of Vpu, exactly as previously described [47], reduced its ability to restrict either target, though the effect on tetherin restriction was more severe. These data are consistent with direct or indirect interactions between Vpu and both target proteins in a post-ER region.Conserved amino acid features in Vpu cytoplasmic tail are required for activityNext, we generated truncation mutations in Vpu to determine the minimal sequence required for modulation of the two targets in this system. For both tetherin and GaLV Env, truncation beyond 13 C-terminal amino acids (D13) resulted in a decrease in Vpu function, although for tetherin this decrease was progressive (Figure 2). To identify critical regions upstream of D13, we mutated two residues at a time to alanine and assayed for activity. For both targets, Vpu was most sensitive to mutations within the conserved hinge region while upstream regions were less sensitive (Figure 3A). Unlike reported findings for BH10 Vpu R30A,K31A [48], mutations located within the YRKIL trafficking motif, we did not observe a decrease in infectivity in the presence of tetherin with our HXB2 Vpu system. Although both are subtype B and almost identical in amino acid sequence, we cannot exclude thatsubtle variation between the two strains may explain observed differences. We then sought to identify specific amino acids required in the CT by scanning individual point mutants through substitution of alanine for individual amino acids, with the exception of alanine which was substituted with serine. Interestingly, almost all amino acids within the Vpu-hinge region between amino acids 51?0, not solely the serines 53, 57, were sensitive to disruption (Figure 3B). A recent report suggested that a putative trafficking motif, ExxxLV, located between residues 60?5 is required for tetherin antagonism [49]. In agreement with this report, we found that E60A disrupted tetherin activity, but the effects of L64A, and V65A alone were more modest. These results demonstrate Vpu’s requirement for conservation of the hinge region for antagonism of two distinct protein targets. Because alanine substitution should not affect physical accessibility of the hinge region by proteins such as b-TrCP, we presume that modification of the conserved features, such as the acidic amino acids, disrupts recognition of Vpu by cellular factors or Vpu’s ability to interact with targets.DiscussionHere we have identified shared critical features in Vpu required for restriction of two distinct proteins, tetherin and the glycoprotein GaLV Env. With the exception of the TMD region, Vpu requires similar features to counteract both targets. Our Vpu screen raises the question: why are similar features in Vpu required for modulation of two disparate target proteins? We propose that Vpu utilizes multiple regions for three somewhat overlapping steps in both restriction pathways: i) retention through interaction, ii) modification and redirection, and iii) degradation. In the case of tetherin, interaction occurs between the TMDs and for CD4 interaction occurs in the CTs and is absolutely requiredVpu Modulation of Distinct TargetsFigure 3. Alanine mutagenic scan of Vpu reveals antagonistic regions for downmodulation of tetherin (dark bars).
