Group-specific antigen

Abstract: The host restriction factor TRIM5α mediates species-specific, early blocks to retrovirus infection; susceptibility to these blocks is determined by viral capsid sequences. Here we demonstrate that TRIM5α variants from Old World monkeys specifically associate with the HIV type 1 (HIV-1) capsid and that this interaction depends on the TRIM5α B30.2 domain. Human and New World monkey TRIM5α proteins associated less efficiently with the HIV-1 capsid, accounting for the lack of restriction in cells of these species. After infection, the expression of a restricting TRIM5α in the target cells correlated with a decrease in the amount of particulate capsid in the cytosol. In some cases, this loss of particulate capsid was accompanied by a detectable increase in soluble capsid protein. Inhibiting the proteasome did not abrogate restriction. Thus, TRIM5α restricts retroviral infection by specifically recognizing the capsid and promoting its rapid, premature disassembly.

Abstract: Transfer of the RNA genome of a retrovirus from one cell to another requires its assembly within the structure of an infectious virion. Although most of the components of the retroviral particle have been identified (see Fig. 1 and Chapter 2), the molecular details of the assembly mechanisms are poorly understood. This may seem surprising since the number of distinct proteins contained in a retrovirus is small. What makes the process of assembly both difficult and interesting to study is its highly dynamic nature.An examination of the mature retroviral particle cannot fully explain the steps in assembly because these components are not those from which the particle was made. Rather, all of the structural proteins of the virion, with few exceptions, are derived from three polyproteins: Gag, Gag-Pro-Pol, and Env (Fig. 2). Each of these precursor proteins has special characteristics needed for specific steps in the assembly process, and each undergoes extensive changes along the way. In brief, the surface (SU) and transmembrane (TM) proteins found on the surface of the virion are initially synthesized as a single polypeptide, the Env glycoprotein, which is assembled into oligomeric complexes in the rough endoplasmic reticulum (RER), extensively modified, and then cleaved by a cell-encoded protease during transport to the surface of the cell. In some instances, further proteolytic processing of TM occurs after the particle is released. Likewise, the proteins found on the inside the virion (matrix, MA; capsid, CA; nucleocapsid, NC; protease, PR; reverse transcriptase, RT; integrase, IN) are initially linked within the Gag and Gag-Pro-Pol proteins. The Gag protein is sufficient for directing budding at the plasma membrane, and the Pro-Pol polyproteins are incorporated into the resulting particle because they are linked to Gag. Subsequent cleavage of the Gag and Gag-Pro-Pol proteins by the viral protease brings about new shapes and arrangements inside the nascent virion as the immature particle undergoes a metamorphosis into the mature, infectious retrovirus. Even the structure of the viral RNA, packaged into the virion through an interaction with Gag, changes during the budding and maturation process.

Abstract: A critical early event in the HIV type 1 (HIV-1) particle assembly pathway is the targeting of the Gag protein to the site of virus assembly. In many cell types, assembly takes place predominantly at the plasma membrane. Cellular factors that regulate Gag targeting remain undefined. The phosphoinositide phosphatidylinositol (4,5) bisphosphate [PI(4,5)P2] controls the plasma membrane localization of a number of cellular proteins. To explore the possibility that this lipid may be involved in Gag targeting and virus particle production, we overexpressed phosphoinositide 5-phosphatase IV, an enzyme that depletes cellular PI(4,5)P2, or overexpressed a constitutively active form of Arf6 (Arf6/Q67L), which induces the formation of PI(4,5)P2-enriched endosomal structures. Both approaches severely reduced virus production. Upon 5-phosphatase IV overexpression, Gag was no longer localized on the plasma membrane but instead was retargeted to late endosomes. Strikingly, in cells expressing Arf6/Q67L, Gag was redirected to the PI(4,5)P2-enriched vesicles and HIV-1 virions budded into these vesicles. These results demonstrate that PI(4,5)P2 plays a key role in Gag targeting to the plasma membrane and thus serves as a cellular determinant of HIV-1 particle production.

Abstract: Here, a protein known as MX2 is shown to be a major effector of interferon-α-mediated resistance to HIV-1 infection: susceptibility of the HIV-1 virus to inhibition by MX2 is dictated by the Capsid region of the viral Gag protein, and inhibition occurs at a late post-entry step of infection. Two groups report in this issue of Nature that the human interferon-induced GTP-binding protein MX2 is a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) and a number of other lentiviruses. For some years it had been known that the related protein MX1 can inhibit HIV-1 replication in humans, but MX2 was thought to be devoid of antiviral activity. The anti-HIV-1 action of MX2 is much less dependent on GTPase activity than is the broader antiviral activity of MX1, pointing to possible mechanistic differences between them. Animal cells harbour multiple innate effector mechanisms that inhibit virus replication. For the pathogenic retrovirus human immunodeficiency virus type 1 (HIV-1), these include widely expressed restriction factors1, such as APOBEC3 proteins2, TRIM5-α3, BST2 (refs 4, 5) and SAMHD1 (refs 6, 7), as well as additional factors that are stimulated by type 1 interferon (IFN)8,9,10,11,12,13,14. Here we use both ectopic expression and gene-silencing experiments to define the human dynamin-like, IFN-induced myxovirus resistance 2 (MX2, also known as MXB) protein as a potent inhibitor of HIV-1 infection and as a key effector of IFN-α-mediated resistance to HIV-1 infection. MX2 suppresses infection by all HIV-1 strains tested, has equivalent or reduced effects on divergent simian immunodeficiency viruses, and does not inhibit other retroviruses such as murine leukaemia virus. The Capsid region of the viral Gag protein dictates susceptibility to MX2, and the block to infection occurs at a late post-entry step, with both the nuclear accumulation and chromosomal integration of nascent viral complementary DNA suppressed. Finally, human MX1 (also known as MXA), a closely related protein that has long been recognized as a broadly acting inhibitor of RNA and DNA viruses, including the orthomyxovirus influenza A virus15,16, does not affect HIV-1, whereas MX2 is ineffective against influenza virus. MX2 is therefore a cell-autonomous, anti-HIV-1 resistance factor whose purposeful mobilization may represent a new therapeutic approach for the treatment of HIV/AIDS.