Vpx proteins of SIVmac239 and HIV-2ROD interact with the cytoskeletal protein alpha-actinin 1

Purified recombinant lentiviral Vpx proteins maintain their SAMHD1 degradation efficiency in resting CD4+ T cells

Different protein purification methods exist. Yet, they need to be adapted for specific downstream applications to maintain functional integrity of the recombinant proteins. This study established a purification protocol for lentiviral Vpx (viral protein X) and test its ability to degrade sterile alpha motif and histidine-aspartate domain-containing protein 1 (SAMHD1) ex vivo in resting CD4⁺ T cells. For this purpose, we cloned a novel eukaryotic expression plasmid for Vpx including C-terminal 10x His- and HA-tags and confirmed that those tags did not alter the ability to degrade SAMHD1. We optimized purification conditions for Vpx produced in HEK293T cells with CHAPS as detergent and Co-NTA resins yielding the highest solubility and protein amounts. Size exclusion chromatography (SEC) further enhanced the purity of recombinant Vpx proteins. Importantly, nucleofection of resting CD4⁺ T cells demonstrated that purified recombinant Vpx protein efficiently degraded SAMHD1 in a proteasome-dependent manner. In conclusion, this protocol is suitable for functional downstream applications of recombinant Vpx and might be transferrable to other recombinant proteins with similar functions/properties as lentiviral Vpx.

Interaction of influenza A virus NS1 and cytoskeleton scaffolding protein α-actinin 4

NS1 (Non-structural protein 1) is a non-structural protein that can highly express when the avian influenza virus infects the host cells. NS1 can interact with various proteins to alter the intracellular distribution of host proteins and regulate the virulence and pathogenicity of the avian influenza virus. To further study the role of NS1 protein in replication and pathogenesis of avian influenza virus, Glutathione S-transferase (GST) Pull-down was used for screening more proteins interacting with NS1 in human lung adenocarcinoma cell line A549. By mass spectrometry, a potential interacted protein is identified as α-actinin 4 and its interaction with NS1 has not been reported yet. The interaction between NS1 and α-actinin 4 in vitro was confirmed by enzyme-linked immunosorbent assay experiments, and the results showed that the absorbance value of OD_450nm in the experimental group was positively correlated with the concentration of NS1-GST protein compared to the negative control group. The co-immunoprecipitation and immunofluorescence results further confirmed the interaction between NS1 and α-actinin 4 at the cellular level. The interaction between NS1 and α-actinin 4 provided a new target for pathogenic mechanism studying and drug screening.

Incomplete Suppression of HIV-1 by SAMHD1 Permits Efficient Macrophage Infection

Background:

Sterile alpha motif and histidine/aspartic acid domain-containing protein (SAMHD1) is a dNTP triphosphorylase that reduces cellular dNTP levels in non-dividing cells, such as macrophages. Since dNTPs are required for reverse transcription, HIV-2 and most SIVs encode a Vpx protein that promotes proteasomal degradation of SAMHD1. It is unclear how HIV-1, which does not appear to harbor a SAMHD1 escape mechanism, is able to infect macrophages in the face of SAMHD1 restriction.

Methods:

To assess whether HIV-1 had a mechanism to negate SAMHD1 activity, we compared SAMHD1 and dNTP levels in macrophages infected by HIV-1 and SIV. We examined whether macrophages infected by HIV-1 still harbored antiviral levels of SAMHD1 by assessing their susceptibility to superinfection by vpx-deleted SIV. Finally, to assess whether HIV-1 reverse transcriptase (RT) has adapted to a low dNTP environment, we evaluated SAMHD1 sensitivity of chimeric HIV-1 and SIV variants in which the RT regions were functionally exchanged.

Results:

Here, we demonstrate that HIV-1 efficiently infects macrophages without modulating SAMHD1 activity or cellular dNTP levels, and that macrophages permissive to HIV-1 infection remained refractory to superinfection by vpx-deleted SIV. Furthermore, through the use of chimeric HIV/SIV, we demonstrate that the differential sensitivity of HIV-1 and SIV to SAMHD1 restriction is not dictated by RT.

Conclusions:

Our study reveals fundamental differences between HIV-1 and SIV in the strategy used to evade restriction by SAMHD1 and suggests a degree of resistance of HIV-1 to the antiviral environment created by SAMHD1. Understanding how these cellular restrictions antagonize viral replication will be important for the design of novel antiviral strategies.

Evidence that Processing of the Severe Fever with Thrombocytopenia Syndrome Virus Gn/Gc Polyprotein Is Critical for Viral Infectivity and Requires an Internal Gc Signal Peptide

The severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging, highly pathogenic bunyavirus against which neither antivirals nor vaccines are available. The SFTSV glycoproteins, Gn and Gc, facilitate viral entry into host cells. Gn and Gc are generated from a precursor protein, Gn/Gc, but it is currently unknown how the precursor is converted into the single proteins and whether this process is required for viral infectivity. Employing a rhabdoviral pseudotyping system, we demonstrate that a predicted signal sequence at the N-terminus of Gc is required for Gn/Gc processing and viral infectivity while potential proprotein convertase cleavage sites in Gc are dispensable. Moreover, we show that expression of Gn or Gc alone is not sufficient for host cell entry while particles bearing both proteins are infectious, and we provide evidence that Gn facilitates Golgi transport and virion incorporation of Gc. Collectively, these results suggest that signal peptidase liberates mature Gc from the Gn/Gc precursor and that this process is essential for viral infectivity and thus constitutes a potential target for antiviral intervention.

Influenza A viral nucleoprotein interacts with cytoskeleton scaffolding protein α-actinin-4 for viral replication

Influenza A virus (IAV), similar to other viruses, exploits the machinery of human host cells for its survival and replication. We identified α‐actinin‐4, a host cytoskeletal protein, as an interacting partner of IAV nucleoprotein (NP). We confirmed this interaction using co‐immunoprecipitation studies, first in a coupled in vitro transcription‐translation assay and then in cells either transiently co‐expressing the two proteins or infected with whole IAV. Importantly, the NP–actinin‐4 interaction was observed in several IAV subtypes, including the 2009 H1N1 pandemic virus. Moreover, immunofluorescence studies revealed that both NP and actinin‐4 co‐localized largely around the nucleus and also in the cytoplasmic region of virus‐infected A549 cells. Silencing of actinin‐4 expression resulted in not only a significant decrease in NP, M2 and NS1 viral protein expression, but also a reduction of both NP mRNA and viral RNA levels, as well as viral titers, 24 h post‐infection with IAV, suggesting that actinin‐4 was critical for viral replication. Furthermore, actinin‐4 depletion reduced the amount of NP localized in the nucleus. Treatment of infected cells with wortmannin, a known inhibitor of actinin‐4, led to a decrease in NP mRNA levels and also caused the nuclear retention of NP, further strengthening our previous observations. Taken together, the results of the present study indicate that actinin‐4, a novel interacting partner of IAV NP, plays a crucial role in viral replication and this interaction may participate in nuclear localization of NP and/or viral ribonucleoproteins.

Structured digital abstract

•http://www.uniprot.org/uniprot/P03466 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9512541, http://www.ebi.ac.uk/intact/interaction/EBI-9512553)•http://www.uniprot.org/uniprot/Q8JR21 and http://www.uniprot.org/uniprot/O43707 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0403 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0416 (http://www.ebi.ac.uk/intact/interaction/EBI-9514040)•http://www.uniprot.org/uniprot/Q91U50 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9514006)•http://www.uniprot.org/uniprot/Q5L4H4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 to http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0007 (http://www.ebi.ac.uk/intact/interaction/EBI-9512166, http://www.ebi.ac.uk/intact/interaction/EBI-9512219)•http://www.uniprot.org/uniprot/C3W6D7 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9513951)•http://www.uniprot.org/uniprot/Q5L4H4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0007 (http://www.ebi.ac.uk/intact/interaction/EBI-9512237)•http://www.uniprot.org/uniprot/Q6DPG0 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9513984) •http://www.uniprot.org/uniprot/B2BU63 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9513930) •http://www.uniprot.org/uniprot/Q5L4H4 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0018 (http://www.ebi.ac.uk/intact/interaction/EBI-9512145, http://www.ebi.ac.uk/intact/interaction/EBI-9512095) •http://www.uniprot.org/uniprot/C9S3S8 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0915 with http://www.uniprot.org/uniprot/O43707 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0006 (http://www.ebi.ac.uk/intact/interaction/EBI-9513909)

Vpx is critical for SIVmne infection of pigtail macaques

BACKGROUND

Viral protein X (Vpx) of SIV has been reported to be important for establishing infection in vivo. Vpx has several different activities in vitro, promoting preintegration complex import into the nucleus in quiescent lymphocytes and overcoming a block in reverse transcription in macrophages. Vpx interacts with the DDB1-CUL4-DCAF1 E3 ligase complex, which may or may not be required for the ascribed functions. The goal of the current study was to determine whether these activities of Vpx are important in vivo.

RESULTS

An infectious, pathogenic clone of SIVmne was used to examine correlations between Vpx functions in vitro and in vivo. Three previously described HIV-2 Vpx mutants that were shown to be important for nuclear import of the preintegration complex in quiescent lymphocytes were constructed in SIVmne: A vpx-deleted virus, a truncation of Vpx at amino acid 102 that deletes the C-terminal proline-rich domain (X(102)), and a mutant with tyrosines 66, 69, and 71 changed to alanine (X(y-a)). All mutant viruses replicated similarly to wild type SIVmne027 in primary pigtail macaque PBMCs, and were only slightly retarded in CEMx174 cells. However, all the vpx mutant viruses were defective for replication in both human and pigtail monocyte-derived macrophages. PCR assays demonstrated that the efficiency of reverse transcription and the levels of viral integration in macrophages were substantially reduced for the vpx mutant viruses. In vitro, the X(y-a) mutant, but not the X(102) mutant lost interaction with DCAF1. The wild type SIVmne027 and the three vpx mutant SIVs were inoculated by the intra-rectal route into pigtail macaques. Peak levels of plasma viremia of the vpx mutant SIVs were variable, but consistently lower than that observed in macaques infected with wild type SIVmne. In situ hybridization for SIV demonstrated that compared to wild type SIVmne infected macaques five of the six animals inoculated with the vpx mutant SIVs had only low levels of SIV-expressing cells in the rectum, most intestinal epithelial tissues, spleen, and mesenteric and peripheral nodes.

CONCLUSIONS

This work demonstrates that the activities of Vpx to overcome restrictions in culture in vitro are also likely to be important for establishment of infection in vivo and suggest that both the nuclear localization and DCAF1-interaction functions of Vpx are critical in vivo.

HIV-2 viral protein X (Vpx) ubiquitination is dispensable for ubiquitin ligase interaction and effects on macrophage infection

HIV-2 Vpx, a virus-associated accessory protein, is critical for infection of non-dividing myeloid cells. To understand the function of Vpx ubiquitination, interaction with an E3 ubiquitin ligase complex, and ability to overcome an inhibition of reverse transcription, we analyzed Vpx lysine mutants for their function and replication capability in macrophages. Both Wt Vpx and Vpx TA (lysine-less Vpx) localized to the cytoplasm and nucleus in HeLa cells. All HIV-2 Vpx lysine mutants were functional in virion packaging. However, ubiquitination was absent with Vpx TA and Vpx K84A mutants, indicating a lack of ubiquitin on positions K68 and K77. Mutants Vpx K68A and K77A were unable to infect macrophages due to impaired reverse transcription from loss of interaction with the ubiquitin substrate receptor, DCAF1. Even though Vpx K84A lacked ubiquitination, it bound DCAF1, and infected macrophages comparable to Wt Vpx.

Hsp40 facilitates nuclear import of the human immunodeficiency virus type 2 Vpx-mediated preintegration complex

Human immunodeficiency virus type 2 (HIV-2) Vpx is required for nuclear translocation of the viral preintegration complex (PIC) in quiescent cells. In order to decipher the mechanism of action of Vpx, a cDNA library was screened with the yeast two-hybrid assay, resulting in the identification of heat shock protein 40, Hsp40/DnaJB6, as a Vpx-interactive protein. Interaction with Vpx was confirmed by glutathione S-transferase (GST) pull-down and coimmunoprecipitation assays. Overexpression of Hsp40/DnaJB6 enhanced Vpx nuclear import, whereas overexpression of a nuclear localization mutant of Hsp40/DnaJB6 (H31Q) or down-regulation of Hsp40/DnaJB6 by small interfering RNA (siRNA) reduced the nuclear import of Vpx. Hsp40/DnaJB6 competed with the Pr55(Gag) precursor protein for the binding of Vpx and incorporation into virus-like particles. Overexpression of Hsp40/DnaJB6 promoted viral PIC nuclear import, whereas siRNA down-regulation of Hsp40/DnaJB6 inhibited PIC nuclear import. These results demonstrate a role for Hsp40/DnaJB6 in the regulation of HIV-2 PIC nuclear transport.

SIVSM/HIV-2 Vpx proteins promote retroviral escape from a proteasome-dependent restriction pathway present in human dendritic cells

Background

Vpx is a non-structural protein coded by members of the SIV_SM/HIV-2 lineage that is believed to have originated by duplication of the common vpr gene present in primate lentiviruses. Vpx is incorporated into virion particles and is thus present during the early steps of viral infection, where it is thought to drive nuclear import of viral nucleoprotein complexes. We have previously shown that Vpx is required for SIV_MAC-derived lentiviral vectors (LVs) infection of human monocyte-derived dendritic cells (DCs). However, since the requirement for Vpx is specific for DCs and not for other non-dividing cell types, this suggests that Vpx may play a role other than nuclear import.

Results

Here, we show that the function of Vpx in the infection of DCs is conserved exclusively within the SIV_SM/HIV-2 lineage. At a molecular level, Vpx acts by promoting the accumulation of full length viral DNA. Furthermore, when supplied in target cells prior to infection, Vpx exerts a similar effect following infection of DCs with retroviruses as divergent as primate and feline lentiviruses and gammaretroviruses. Lastly, the effect of Vpx overlaps with that of the proteasome inhibitor MG132 in DCs.

Conclusion

Overall, our results support the notion that Vpx modifies the intracellular milieu of target DCs to facilitate lentiviral infection. The data suggest that this is achieved by promoting viral escape from a proteasome-dependent pathway especially detrimental to viral infection in DCs.

Analysis of HIV-2 Vpx by modeling and insertional mutagenesis

Vpx facilitates HIV-2 nuclear localization by a poorly understood mechanism. We have compared Vpx to an NMR structure HIV-1 Vpr in a central helical domain and probed regions of Vpx by insertional mutagenesis. A predicted loop between helices two and three appears to be unique, overlapping with a known novel nuclear localization signal. Overall, Vpx was found to be surprisingly flexible, tolerating a series of large insertions. We found that insertion within the polyproline-containing C-terminus destabilizes nuclear localization, whereas mutating a second helix in the central domain disrupts viral packaging. Other insertional mutants in the predicted loop and in a linker region between the central domain and the C-terminus may be useful as sites of intramolecular tags as they could be packaged adequately and retained preintegration complex associated integration activity in a serum starvation assay. An unexpected result was found within a previously defined nuclear localization motif near aa 71. This mutant retained robust nuclear localization in a GFP fusion assay and was competent for preintegration complex associated nuclear import. In summary, we have modeled helical content in Vpx and assessed potential sites of intramolecular tags which may prove useful for protein-protein interactions studies.

Nuclear actin-binding proteins as modulators of gene transcription

Dynamic transformations in the organization of the cellular microfilament system are the driving force behind fundamental biological processes such as cellular motility, cytokinesis, wound healing and secretion. Eukaryotic cells express a plethora of actin-binding proteins (ABPs) allowing cells to control the organization of the actin cytoskeleton in a flexible manner. These structural proteins were, not surprisingly, originally described as (major) constituents of the cytoplasm. However, in recent years, there has been a steady flow of reports detailing not only translocation of ABPs into and out of the nucleus but also describing their role in the nuclear compartment. This review focuses on recent developments pertaining to nucleocytoplasmic transport of ABPs, including their mode of translocation and nuclear function. In particular, evidence that structurally and functionally unrelated cytoplasmic ABPs regulate transcription activation by various nuclear (steroid hormone) receptors is steadily accruing. Furthermore, the recent finding that actin is a necessary component of the RNA polymerase II-containing preinitiation complex opens up new opportunities for nuclear ABPs in gene transcription regulation.