• The Antiviral Activity of the Cellular Glycoprotein LGALS3BP/90K Is Species Specific.

      Lodermeyer, Veronika; Ssebyatika, George; Passos, Vânia; Ponnurangam, Aparna; Malassa, Angelina; Ewald, Ellen; Stürzel, Christina M; Kirchhoff, Frank; Rotger, Margalida; Falk, Christine S; et al. (American Society for Microbiology (ASM), 2018-07-15)
      Cellular antiviral proteins interfere with distinct steps of replication cycles of viruses. The galectin 3 binding protein (LGALS3BP, also known as 90K) was previously shown to lower the infectivity of nascent human immunodeficiency virus type 1 (HIV-1) virions when expressed in virus-producing cells. This antiviral effect was accompanied by impaired gp160Env processing and reduced viral incorporation of mature Env glycoproteins. Here, we examined the ability of 90K orthologs from primate species to reduce the particle infectivity of distinct lentiviruses. We show that 90K's ability to diminish the infectivity of lentiviral particles is conserved within primate species, with the notable exception of 90K from rhesus macaque. Comparison of active and inactive 90K orthologs and variants uncovered the fact that inhibition of processing of the HIV-1 Env precursor and reduction of cell surface expression of HIV-1 Env gp120 are required, but not sufficient, for 90K-mediated antiviral activity. Rather, 90K-mediated reduction of virion-associated gp120 coincided with antiviral activity, suggesting that 90K impairs the incorporation of HIV-1 Env into budding virions. We show that a single "humanizing" amino acid exchange in the BTB (broad-complex, tramtrack, and bric-à-brac)/POZ (poxvirus and zinc finger) domain is sufficient to fully rescue the antiviral activity of a shortened version of rhesus macaque 90K, but not that of the full-length protein. Comparison of the X-ray structures of the BTB/POZ domains of 90K from rhesus macaques and humans point toward a slightly larger hydrophobic patch at the surface of the rhesus macaque BTB domain that may modulate a direct interaction with either a second 90K domain or a different protein.IMPORTANCE The cellular 90K protein has been shown to diminish the infectivity of nascent HIV-1 particles. When produced in 90K-expressing cells, particles bear smaller amounts of the HIV-1 Env glycoprotein, which is essential for attaching to and entering new target cells in the subsequent infection round. However, whether the antiviral function of 90K is conserved across primates is unknown. Here, we found that 90K orthologs from most primate species, but, surprisingly, not from rhesus macaques, inhibit HIV-1. The introduction of a single amino acid exchange into a short version of the rhesus macaque 90K protein, consisting of the two intermediate domains of 90K, resulted in full restoration of antiviral activity. Structural elucidation of the respective domain suggests that the absence of antiviral activity in the rhesus macaque factor may be linked to a subtle change in protein-protein interaction.
    • Characterization of Endogenous SERINC5 Protein as anti-HIV-1 Factor.

      Passos, Vânia; Zillinger, Thomas; Casartelli, Nicoletta; Wachs, Amelie S; Xu, Shuting; Malassa, Angelina; Steppich, Katja; Schilling, Hildegard; Franz, Sergej; Todt, Daniel; et al. (American Society of Microbiology, 2019-10-09)
      When expressed in virus-producing cells, the cellular multipass-transmembrane protein SERINC5 reduces the infectivity of HIV-1 particles and is counteracted by HIV-1 Nef. Due to the unavailability of an antibody of sufficient specificity and sensitivity, investigation of SERINC5 protein expression and subcellular localization has been limited to heterologously expressed SERINC5. We generated, via CRISPR/Cas9-assisted gene editing, Jurkat T-cell clones expressing endogenous SERINC5 bearing an extracellularly exposed HA epitope (Jurkat SERINC5(iHA-knock-in) T-cells). This modification enabled quantification of endogenous SERINC5 protein levels and demonstrated a predominant localization in lipid rafts. IFN-α treatment enhanced cell surface levels of SERINC5 in a Ruxolitinib-sensitive manner in the absence of modulation of mRNA and protein quantities. Parental and SERINC5(iHA-knock-in) T-cells shared the ability to produce infectious wildtype HIV-1, but not HIV-1 Δnef SERINC5-imposed reduction of infectivity involved a modest reduction of virus fusogenicity. Association of endogenous SERINC5 protein with HIV-1 Δnef virions was consistently detectable as a 35 kDa species, as opposed to heterologous SERINC5 that presented as 51 kDa species. Nef-mediated functional counteraction did not correlate with virion exclusion of SERINC5, arguing for the existence of additional counteractive mechanisms of Nef that act on virus-associated SERINC5. In HIV-1-infected cells, Nef triggered internalization of SERINC5 in the absence of detectable changes of steady-state protein levels. These findings establish new properties of endogenous SERINC5 expression and subcellular localization, challenge existing concepts of HIV-1 Nef-mediated antagonism of SERINC5 and uncover an unprecedented role of IFN-α in modulating SERINC5 through accumulation at the cell surface.IMPORTANCE SERINC5 is the long-searched antiviral factor that is counteracted by the HIV-1 accessory gene product Nef. Here, we engineered, via CRISPR/Cas9 technology, T-cell lines that express endogenous SERINC5 alleles tagged with a knocked-in HA epitope. This genetic modification enabled us to study basic properties of endogenous SERINC5 and to verify proposed mechanisms of HIV-1 Nef-mediated counteraction of SERINC5. Using this unique resource, we identified the susceptibility of endogenous SERINC5 protein to posttranslational modulation by type I IFNs and suggest uncoupling of Nef-mediated functional antagonism from SERINC5 exclusion from virions.