HIV-1 capsid is involved in post-nuclear entry steps

BACKGROUND: HIV-1 capsid influences viral uncoating and nuclear import. Some capsid is detected in the nucleus but it is unclear if it has any function. We reported that the antibiotic Coumermycin-A1 (C-A1) inhibits HIV-1 integration and that a capsid mutation confers resistance to C-A1, suggesting that capsid might affect post-nuclear entry steps. RESULTS: Here we report that C-A1 inhibits HIV-1 integration in a capsid-dependent way. Using molecular docking, we identify an extended binding pocket delimited by two adjacent capsid monomers where C-A1 is predicted to bind. Isothermal titration calorimetry confirmed that C-A1 binds to hexameric capsid. Cyclosporine washout assays in Jurkat CD4+ T cells expressing engineered human TRIMCyp showed that C-A1 causes faster and greater escape from TRIMCyp restriction. Sub-cellular fractionation showed that small amounts of capsid accumulated in the nuclei of infected cells and C-A1 reduced the nuclear capsid. A105S and N74D capsid mutant viruses did not accumulate capsid in the nucleus, irrespective of C-A1 treatment. Depletion of Nup153, a nucleoporin located at the nuclear side of the nuclear pore that binds to HIV-1 capsid, made the virus less susceptible to TRIMCyp restriction, suggesting that Nup153 may help maintain some integrity of the viral core in the nucleus. Furthermore C-A1 increased binding of CPSF6, a nuclear protein, to capsid. CONCLUSIONS: Our results indicate that capsid is involved in post-nuclear entry steps preceding integration

Restriction of HIV-1 Requires the N-Terminal Region of MxB as a Capsid-Binding Motif but Not as a Nuclear Localization Signal

International audienceThe interferon alpha (IFN-α)-inducible restriction factor MxB blocks HIV-1 infection after reverse transcription but prior to integration. Fate-of-capsid experiments have correlated the ability of MxB to block HIV-1 infection with stabilization of viral cores during infection. We previously demonstrated that HIV-1 restriction by MxB requires capsid binding and oligomerization. Deletion and gain-of-function experiments have mapped the HIV-1 restriction ability of MxB to its N-terminal 25 amino acids. This report reveals that the N-terminal 25 amino acids of MxB exhibit two separate functions: (i) the ability of MxB to bind to HIV-1 capsid and (ii) the nuclear localization signal of MxB, which is important for the ability of MxB to shuttle into the nucleus. To understand whether MxB restriction of HIV-1 requires capsid binding and/or nuclear localization, we genetically separated these two functions and evaluated their contributions to restriction. Our experiments demonstrated that the (11)RRR(13) motif is important for the ability of MxB to bind capsid and to restrict HIV-1 infection. These experiments suggested that capsid binding is necessary for the ability of MxB to block HIV-1 infection. Separately from the capsid binding function of MxB, we found that residues (20)KY(21) regulate the ability of the N-terminal 25 amino acids of MxB to function as a nuclear localization signal; however, the ability of the N-terminal 25 amino acids to function as a nuclear localization signal was not required for restriction. IMPORTANCE MxB/Mx2 blocks HIV-1 infection in cells from the immune system. MxB blocks infection by preventing the uncoating process of HIV-1. The ability of MxB to block HIV-1 infection requires that MxB binds to the HIV-1 core by using its N-terminal domain. The present study shows that MxB uses residues (11)RRR(13) to bind to the HIV-1 core during infection and that these residues are required for the ability of MxB to block HIV-1 infection. We also found that residues (20)KY(21) constitute a nuclear localization signal that is not required for the ability of MxB to block HIV-1 infection

Effects of T592 phosphomimetic mutations on tetramer stability and dNTPase activity of SAMHD1 can not explain the retroviral restriction defect

Open access article. Creative Commons Attribution 4.0 International License (CC BY 4.0) appliesSAMHD1, a dNTP triphosphohydrolase, contributes to interferon signaling and restriction of retroviral replication. SAMHD1-mediated retroviral restriction is thought to result from the depletion of cellular dNTP pools, but it remains controversial whether the dNTPase activity of SAMHD1 is sufficient for restriction. The restriction ability of SAMHD1 is regulated in cells by phosphorylation on T592. Phosphomimetic mutations of T592 are not restriction competent, but appear intact in their ability to deplete cellular dNTPs. Here we use analytical ultracentrifugation, fluorescence polarization and NMR-based enzymatic assays to investigate the impact of phosphomimetic mutations on SAMHD1 tetramerization and dNTPase activity in vitro. We find that phosphomimetic mutations affect kinetics of tetramer assembly and disassembly, but their effects on tetramerization equilibrium and dNTPase activity are insignificant. In contrast, the Y146S/Y154S dimerization-defective mutant displays a severe dNTPase defect in vitro, but is indistinguishable from WT in its ability to deplete cellular dNTP pools and to restrict HIV replication. Our data suggest that the effect of T592 phosphorylation on SAMHD1 tetramerization is not likely to explain the retroviral restriction defect and we hypothesize that enzymatic activity of SAMHD1 is subject to additional cellular regulatory mechanisms that have not yet been recapitulated in vitro.Ye

Cyclophilin A Prevents HIV-1 Restriction in Lymphocytes by Blocking Human TRIM5α Binding to the Viral Core

International audienceDisruption of cyclophilin A (CypA)-capsid interactions affects HIV-1 replication in human lymphocytes. To understand this mechanism, we utilize human Jurkat cells, peripheral blood mononuclear cells (PBMCs), and CD4+ T cells. Our results show that inhibition of HIV-1 infection caused by disrupting CypA-capsid interactions is dependent on human tripartite motif 5α (TRIM5αhu), showing that TRIM5αhu restricts HIV-1 in CD4+ T cells. Accordingly, depletion of TRIM5αhu in CD4+ T cells rescues HIV-1 that fail to interact with CypA, such as HIV-1-P90A. We found that TRIM5αhu binds to the HIV-1 core. Disruption of CypA-capsid interactions fail to affect HIV-1-A92E/G94D infection, correlating with the loss of TRIM5αhu binding to HIV-1-A92E/G94D cores. Disruption of CypA-capsid interactions in primary cells has a greater inhibitory effect on HIV-1 when compared to Jurkat cells. Consistent with TRIM5α restriction, disruption of CypA-capsid interactions in CD4+ T cells inhibits reverse transcription. Overall, our results reveal that CypA binding to the core protects HIV-1 from TRIM5αhu restriction

Functionality of redox-active crysteines is required for restriction of retroviral replication by SAMHD1

Open access article. Creative Commons 4.0 International License (CC BY 4.0) appliesSAMHD1 is a dNTP triphosphohydrolase (dNTPase)that impairs retroviral replication in a subset of non-cycling immune cells. Here we show that SAMHD1is a redox-sensitive enzyme and identify threeredox-active cysteines within the protein: C341,C350, and C522. The three cysteines reside nearone another and the allosteric nucleotide bindingsite. Mutations C341S and C522S abolish the abilityof SAMHD1 to restrict HIV replication, whereas theC350S mutant remains restriction competent. TheC522S mutation makes the protein resistant to inhibi-tion by hydrogen peroxide but has no effect onthe tetramerization-dependent dNTPase activity ofSAMHD1in vitroor on the ability of SAMHD1 todeplete cellular dNTPs. Our results reveal that enzy-matic activation of SAMHD1 via nucleotide-depen-dent tetramerization is not sufficient for the estab-lishment of the antiviral state and that retroviralrestriction depends on the ability of the protein to un-dergo redox transformations.Ye