Serine Phosphorylation of HIV-1 Vpu and Its Binding to Tetherin Regulates Interaction with Clathrin Adaptors.

HIV-1 Vpu prevents incorporation of tetherin (BST2/ CD317) into budding virions and targets it for ESCRT-dependent endosomal degradation via a clathrin-dependent process. This requires a variant acidic dileucine-sorting motif (ExxxLV) in Vpu. Structural studies demonstrate that recombinant Vpu/tetherin fusions can form a ternary complex with the clathrin adaptor AP-1. However, open questions still exist about Vpu's mechanism of action. Particularly, whether endosomal degradation and the recruitment of the E3 ubiquitin ligase SCFßTRCP1/2 to a conserved phosphorylated binding site, DSGNES, are required for antagonism. Re-evaluation of the phenotype of Vpu phosphorylation mutants and naturally occurring allelic variants reveals that the requirement for the Vpu phosphoserine motif in tetherin antagonism is dissociable from SCFßTRCP1/2 and ESCRT-dependent tetherin degradation. Vpu phospho-mutants phenocopy ExxxLV mutants, and can be rescued by direct clathrin interaction in the absence of SCFßTRCP1/2 recruitment. Moreover, we demonstrate physical interaction between Vpu and AP-1 or AP-2 in cells. This requires Vpu/tetherin transmembrane domain interactions as well as the ExxxLV motif. Importantly, it also requires the Vpu phosphoserine motif and adjacent acidic residues. Taken together these data explain the discordance between the role of SCFßTRCP1/2 and Vpu phosphorylation in tetherin antagonism, and indicate that phosphorylation of Vpu in Vpu/tetherin complexes regulates promiscuous recruitment of adaptors, implicating clathrin-dependent sorting as an essential first step in tetherin antagonism.

Differential sensitivities of tetherin isoforms to counteraction by primate lentiviruses.

UNLABELLED: The mammalian antiviral membrane protein tetherin (BST2/CD317) can be expressed as two isoforms derived from differential translational initiation. The shorter isoform of the human protein (S-tetherin) lacks the first 12 amino acids of the longer (L-tetherin) cytoplasmic tail, which includes a tyrosine motif that acts as both an endocytic recycling signal and a determinant of virus-induced NF-κB activation. S-tetherin is also reported to be less sensitive to the prototypic viral antagonist human immunodeficiency virus type 1 (HIV-1) Vpu. Here we analyzed the relative sensitivities of L- and S-tetherins to primate lentiviral countermeasures. We show that the reduced sensitivity of S-tetherin to HIV-1 Vpu is a feature of all group M proteins, including those of transmitted founder viruses, primarily because it cannot be targeted for endosomal degradation owing to the truncation of its cytoplasmic tail. In contrast, both isoforms of the human and rhesus macaque tetherins display the same sensitivity to nondegradative lentiviral countermeasures of HIV-2 and SIVmac, respectively. Surprisingly, however, the Vpu proteins encoded by simian immunodeficiency viruses (SIVs) of African guenons, as well as that from recently isolated highly pathogenic HIV-1 group N, do not discriminate between tetherin isoforms. Together, these data suggest that the group M HIV-1 Vpu primarily adapted to target L-tetherin upon zoonotic transmission from chimpanzees, and further, we speculate that functions specifically associated with this isoform, such as proinflammatory signaling, play key roles in human tetherin's antiviral function in vivo. IMPORTANCE: The ability of HIV-1 and related viruses to counteract a host antiviral protein, tetherin, is strictly maintained. The adaptation of the HIV-1 Vpu protein to counteract human tetherin is thought to have been one of the key events in the establishment of the HIV/AIDS pandemic. Recent evidence shows that tetherin is expressed as two isoforms and that Vpu preferentially targets the longer form. Here we show that unlike other virus-encoded countermeasures, such as those from primate viruses related to HIV-1, the enhanced ability to counteract the long tetherin isoform is conserved among HIV-1 strains that make up the majority of the human pandemic. This correlates with the ability of Vpu to induce long tetherin degradation. We speculate that functions associated with the human version of this isoform, such as an inflammatory signaling capacity, selected for Vpu's enhanced targeting of long tetherin during its adaptation to humans.

Cell Surface Proteomic Map of HIV Infection Reveals Antagonism of Amino Acid Metabolism by Vpu and Nef.

Critical cell surface immunoreceptors downregulated during HIV infection have previously been identified using non-systematic, candidate approaches. To gain a comprehensive, unbiased overview of how HIV infection remodels the T cell surface, we took a distinct, systems-level, quantitative proteomic approach. >100 plasma membrane proteins, many without characterized immune functions, were downregulated during HIV infection. Host factors targeted by the viral accessory proteins Vpu or Nef included the amino acid transporter SNAT1 and the serine carriers SERINC3/5. We focused on SNAT1, a ß-TrCP-dependent Vpu substrate. SNAT1 antagonism was acquired by Vpu variants from the lineage of SIVcpz/HIV-1 viruses responsible for pandemic AIDS. We found marked SNAT1 induction in activated primary human CD4+ T cells, and used Consumption and Release (CoRe) metabolomics to identify alanine as an endogenous SNAT1 substrate required for T cell mitogenesis. Downregulation of SNAT1 therefore defines a unique paradigm of HIV interference with immunometabolism.