Viruses are notorious for manipulating host cells in ways that favor viral replication, and HIV is no exception. The HIV protein Nef has a well-described capacity to subvert normal cellular biology, including down-regulating cell surface class I HLA molecules that might otherwise allow CD8 T cells to recognize that the cell is infected. Last week saw the publication of three independent studies that all report the discovery of another important Nef function.
Two papers in Nature describe the use different techniques to converge on the finding that Nef diverts cellular proteins named serine incorporator (SERINC) 5 and SERINC3 away from the cell membrane, thus preventing their incorporation into budding HIV virions. In the absence of this diversionary Nef activity, the proteins—particularly SERINC5—dramatically impair HIV infectivity. The mechanism by which incorporation of the SERINC proteins compromises HIV is now under investigation. The phenomenon appears relevant to a broad range of enveloped retroviruses, because occlusion of SERINC5 was found to be a conserved function of Nef proteins encoded by multiple primate immunodeficiency viruses and was also accomplished by the structurally distinct Gag protein of murine leukemia virus.
The third paper, an open access publication in Cell Host & Microbe, also identified SERINC3/5 as targets of Nef using a proteomic analysis of alterations to the T cell surface caused by HIV infection. Additionally, the researchers discovered that HIV’s Vpu protein significantly downregulates SNAT1 (Sodium Coupled Neutral Amino Acid Transporter 1), a protein involved in amino acid metabolism, thereby impairing T cell proliferation. Interestingly, this capacity of Vpu appears to have been acquired relatively recently in evolutionary time as it is restricted to viral variants from the SIVcpz/HIV-1 lineage. Given that this lineage is responsible for the HIV pandemic, the researchers note that Vpu’s acquisition of the ability to downregulate SNAT1 may have contributed to the spread of the virus in the human population.
The apparent importance of these host cell manipulations in the HIV life cycle suggests that targeted inhibition of Nef and/or Vpu might have therapeutic potential, and further studies are planned.
Cell Host & Microbe 18, 1–15
Nicholas J. Mathesonl, Jonathan Sumner, Kim Wals, Radu Rapiteanu, Michael P. Weekes, Raphael Vigan, Julia Weinelt, Michael Schindler, Robin Antrobus, Ana S.H. Costa, Christian Frezza, Clary B. Clish, Stuart J.D. Neil, Paul J. Lehner
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 β-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.
Nature. 2015 Sep 30. doi: 10.1038/nature15400. [Epub ahead of print]
Usami Y, Wu Y, Göttlinger HG.
HIV-1 Nef and the unrelated mouse leukaemia virus glycosylated Gag (glycoGag) strongly enhance the infectivity of HIV-1 virions produced in certain cell types in a clathrin-dependent manner. Here we show that Nef and glycoGag prevent the incorporation of the multipass transmembrane proteins serine incorporator 3 (SERINC3) and SERINC5 into HIV-1 virions to an extent that correlates with infectivity enhancement. Silencing of both SERINC3 and SERINC5 precisely phenocopied the effects of Nef and glycoGag on HIV-1 infectivity. The infectivity of nef-deficient virions increased more than 100-fold when produced in double-knockout human CD4+ T cells that lack both SERINC3 and SERINC5, and re-expression experiments confirmed that the absence of SERINC3 and SERINC5 accounted for the infectivity enhancement. Furthermore, SERINC3 and SERINC5 together restricted HIV-1 replication, and this restriction was evaded by Nef. SERINC3 and SERINC5 are highly expressed in primary human HIV-1 target cells, and inhibiting their downregulation by Nef is a potential strategy to combat HIV/AIDS.
Nature. 2015 Sep 30. doi: 10.1038/nature15399. [Epub ahead of print]
Rosa A, Chande A, Ziglio S, De Sanctis V, Bertorelli R, Goh SL, McCauley SM, Nowosielska A, Antonarakis SE, Luban J, Santoni FA, Pizzato M.
HIV-1 Nef, a protein important for the development of AIDS, has well-characterized effects on host membrane trafficking and receptor downregulation. By an unidentified mechanism, Nef increases the intrinsic infectivity of HIV-1 virions in a host-cell-dependent manner. Here we identify the host transmembrane protein SERINC5, and to a lesser extent SERINC3, as a potent inhibitor of HIV-1 particle infectivity that is counteracted by Nef. SERINC5 localizes to the plasma membrane, where it is efficiently incorporated into budding HIV-1 virions and impairs subsequent virion penetration of susceptible target cells. Nef redirects SERINC5 to a Rab7-positive endosomal compartment and thereby excludes it from HIV-1 particles. The ability to counteract SERINC5 was conserved in Nef encoded by diverse primate immunodeficiency viruses, as well as in the structurally unrelated glycosylated Gag from murine leukaemia virus. These examples of functional conservation and convergent evolution emphasize the fundamental importance of SERINC5 as a potent anti-retroviral factor.