Retrovirus Biology

Division Of Basic Sciences - Nci

Investigators

Linked publications & trials

Abstract

We have performed detailed structure-function studies of the HIV-1 Rev Response Element and of an analogous element in the RNA of the HERV-K human endogenous virus. The results of these studies have important implications for the origin of specificity in the interactions of these RNA elements with virus-coded regulatory proteins. These interactions are critical for virus replication, as they make it possible for the export of both spliced and unspliced viral RNAs from the nucleus. It has recently been found that the cellular protein Serinc5 somehow interferes with the ability of HIV-1 particles assembled in Serinc5-containing cells to successfully enter new host cells. The HIV-1 Nef protein counteracts the effects of Serinc5. We have found that Serinc5 also blocks the entry of many other retroviruses; both murine leukemia viruses (MLVs, which are members of the gammaretrovirus genus) and equine infectious anemia virus (like HIV-1, a lentivirus) encode "accessory proteins" that counter the effects of Serinc5. The mechanism of Serinc5 action is not understood. We have systematically surveyed retroviruses for their sensitivity to Serinc5; the purpose of these studies is to try to identify a common feature that might be the target for Serinc5 action. We found that retroviruses differ widely with respect to Serinc5 sensitivity, but we have not yet discerned a pattern underlying these differences. We have also collaborated with Dr. Alex Compton (HIV DRP) in a study of the effects of another antiviral protein, i.e., interferon-induced transmembrane protein 3 (IFITM3). Interestingly, the same MLV accessory protein counteracts the effects of both Serinc5 and IFITM3. We are also analyzing the antiviral action of mouse Apobec3 (mA3), which has strikingly different effects on HIV-1 and MLVs. In collaboration with Dr. John Briggs and others, we found that the structures of mature HIV-1 and MLV particles are remarkably different; it seems possible that these structural differences are responsible for the differences in their response to mA3. We are exploring this possibility in several ways, including testing the effects of chimeras between mA3 and the well-characterized human APOBEC3G. ______We are also studying the properties of MuERV-L, a family of retrotransposons expressed in very early mouse embryogenesis. The functional significance of this expression is unknown. We have continued our studies on the breadth of antiviral activity of Serinc5. We found that, in addition to retroviruses, this cellular protein restricts infection by the orthomyxovirus Influenza A Virus (IAV). On one hand, this discovery shows that the activity of Serinc5 is not limited to retroviruses and raises the possibility that it evolved in response to selective pressure by other viruses. Moreover, it provides some novel information regarding the mechanism of Serinc5 antiviral activity. IAV entry is mediated by the surface protein hemagglutinin (HA); while the fusogenic action of the Env proteins of HIV-1 and MLV is activated simply by contact with the receptors on the plasma membrane and occurs at neutral pH, that of HA is induced by the low pH in endosomes. Using single-molecule FRET (Forster resonance energy transfer) analysis, we showed that the conformational dynamics of HA at different pH's was altered by Serinc5. Serinc5 appears to interact directly with HA, since it co-immunoprecipitates with HA when they are expressed in the same cell, even in the absence of other viral constituents. Further experiments showed that Serinc5 is naturally expressed in the host cells of IAV, including human small airway epithelial cells and cell lines derived from either lung or intestinal tissue, and that shRNA knockdown of Serinc5 enhanced IAV replication in these cells. These observations suggest that Serinc5 provides us with some protection against IAV infection and pathogenicity. We have also contributed to a multi-laboratory study of the biochemical properties of Serinc5. Using cryo-electron microscopy, Mark Yeager (University of Miami) performed a detailed analysis of the structure of Serinc3, a close relative of Serinc5. Based on the novel structure of this protein, he proposed that it is a "lipid scramblase", an enzyme that redistributes fatty acids between the two leaflets in a membrane bilayer and reduces any asymmetry in their placement. As phosphatidyl serine (PS) is frequently found in the inner, but not the outer, leaflet, the presence of a scramblase in a membrane tends to lead to its exposure on the outer surface. Indeed, this activity was demonstrated biochemically in proteliposomes in Dr. Yeager's laboratory. Such exposure can also be detected by staining with fluorescent Annexin V. Using this approach, our laboratory showed that Serinc5 induces PS exposure in intact MLV virions, while that of Walther Mothes (Yale University) performed analogous experiments on HIV-1 particles.