PDCD4 restricts PRRSV replication in an eIF4A-dependent manner and is antagonized by the viral nonstructural protein 9

As obligate parasites, viruses have evolved multiple strategies to evade the host immune defense. Manipulation of the host proteasome system to degrade specific detrimental factors is a common viral countermeasure. To identify host proteins targeted for proteasomal degradation by porcine reproductive and respiratory syndrome virus (PRRSV), we conducted a quantitative proteomics screen of PRRSV-infected Marc-145 cells under the treatment with proteasome inhibitor MG132. The data revealed that the expression levels of programmed cell death 4 (PDCD4) were strongly downregulated by PRRSV and significantly rescued by MG132. Further investigation confirmed that PRRSV infection induced the translocation of PDCD4 from the nucleus to the cytoplasm, and the viral nonstructural protein 9 (Nsp9) promoted PDCD4 proteasomal degradation in the cytoplasm by activating the Akt-mTOR-S6K1 pathway. The C-terminal domain of Nsp9 was responsible for PDCD4 degradation. As for the role of PDCD4 during PRRSV infection, we demonstrated that PDCD4 knockdown favored viral replication, while its overexpression significantly attenuated replication, suggesting that PDCD4 acts as a restriction factor for PRRSV. Mechanistically, we discovered eukaryotic translation initiation factor 4A (eIF4A) was required for PRRSV. PDCD4 interacted with eIF4A through four sites (E249, D253, D414, and D418) within its two MA3 domains, disrupting eIF4A-mediated translation initiation in the 5'-untranslated region of PRRSV, thereby inhibiting PRRSV infection. Together, our study reveals the antiviral function of PDCD4 and the viral strategy to antagonize PDCD4. These results will contribute to our understanding of the immune evasion strategies employed by PRRSV and offer valuable insights for developing new antiviral targets.IMPORTANCEPorcine reproductive and respiratory syndrome virus (PRRSV) infection results in major economic losses in the global swine industry and is difficult to control effectively. Here, using a quantitative proteomics screen, we identified programmed cell death 4 (PDCD4) as a host protein targeted for proteasomal degradation by PRRSV. We demonstrated that PDCD4 restricts PRRSV replication by interacting with eukaryotic translation initiation factor 4A, which is required for translation initiation in the viral 5'-untranslated region. Additionally, four sites within two MA3 domains of PDCD4 are identified to be responsible for its antiviral function. Conversely, PRRSV nonstructural protein 9 promotes PDCD4 proteasomal degradation in the cytoplasm by activating the Akt-mTOR-S6K1 pathway, thus weakening the anti-PRRSV function. Our work unveils PDCD4 as a previously unrecognized host restriction factor for PRRSV and reveals that PRRSV develops countermeasures to overcome PDCD4. This will provide new insights into virus-host interactions and the development of new antiviral targets.

Host factor MxA restricts Dabie bandavirus infection by targeting the viral NP protein to inhibit NP-RdRp interaction and ribonucleoprotein activity

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with high case mortality rates, which is caused by Dabie bandavirus (DBV), a novel pathogen also termed as SFTS virus (SFTSV). Currently, no specific therapeutic drugs or vaccines are available for SFTS. Myxovirus resistance protein A (MxA) has been shown to inhibit multiple viral pathogens; however, the role of MxA in DBV infection is unknown. Here, we demonstrated that DBV stimulates MxA expression which, in turn, restricts DBV infection. Mechanistic target analysis revealed that MxA specifically interacts with the viral nucleocapsid protein (NP) in a manner independent of RNA. Minigenome reporter assay showed that in agreement with its targeting of NP, MxA inhibits DBV ribonucleoprotein (RNP) activity. In detail, MxA interacts with the NP N-terminal and disrupts the interaction of NP with the viral RNA-dependent RNA polymerase (RdRp) but not NP multimerization, the critical activities of NP for RNP formation and function. Furthermore, MxA N-terminal domain was identified as the functional domain inhibiting DBV infection, and, consistently, then was shown to interact with NP and obstruct the NP-RdRp interaction. Additionally, threonine 103 within the N-terminal domain is important for MxA inhibition to DBV, and its mutation (T103A) attenuates MxA binding to NP and obstruction of the NP-RdRp interaction. This study uncovers MxA inhibition of DBV with a series of functional and mechanistical analyses, providing insights into the virus-host interactions and probably helping inform the development of antiviral agents in the future.IMPORTANCEDBV/SFTSV is an emerging high-pathogenic virus. Since its first identification in China in 2009, cases of DBV infection have been reported in many other countries, posing a significant threat to public health. Uncovering the mechanisms of DBV-host interactions is necessary to understand the viral pathogenesis and host response and may advance the development of antiviral therapeutics. Here, we found that host factor MxA whose expression is induced by DBV restricts the virus infection. Mechanistically, MxA specifically interacts with the viral NP and blocks the NP-RdRp interaction, inhibiting the viral RNP activity. Further studies identified the key domain and amino acid residue required for MxA inhibition to DBV. Consistently, they were then shown to be important for MxA targeting of NP and obstruction of the NP-RdRp association. These findings unravel the restrictive role of MxA in DBV infection and the underlying mechanism, expanding our knowledge of the virus-host interactions.

A New Cellular Interactome of SARS-CoV-2 Nucleocapsid Protein and Its Biological Implications

There is still much to uncover regarding the molecular details of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. As the most abundant protein, coronavirus nucleocapsid (N) protein encapsidates viral RNAs, serving as the structural component of ribonucleoprotein and virion, and participates in transcription, replication, and host regulations. Virus-host interaction might give clues to better understand how the virus affects or is affected by its host during infection and identify promising therapeutic candidates. Considering the critical roles of N, we here established a new cellular interactome of SARS-CoV-2 N by using a high-specific affinity purification (S-pulldown) assay coupled with quantitative mass spectrometry and immunoblotting validations, uncovering many N-interacting host proteins unreported previously. Bioinformatics analysis revealed that these host factors are mainly involved in translation regulations, viral transcription, RNA processes, stress responses, protein folding and modification, and inflammatory/immune signaling pathways, in line with the supposed actions of N in viral infection. Existing pharmacological cellular targets and the directing drugs were then mined, generating a drug-host protein network. Accordingly, we experimentally identified several small-molecule compounds as novel inhibitors against SARS-CoV-2 replication. Furthermore, a newly identified host factor, DDX1, was verified to interact and colocalize with N mainly by binding to the N-terminal domain of the viral protein. Importantly, loss/gain/reconstitution-of-function experiments showed that DDX1 acts as a potent anti-SARS-CoV-2 host factor, inhibiting the viral replication and protein expression. The N-targeting and anti-SARS-CoV-2 abilities of DDX1 are consistently independent of its ATPase/helicase activity. Further mechanism studies revealed that DDX1 impedes multiple activities of N, including the N-N interaction, N oligomerization, and N-viral RNA binding, thus likely inhibiting viral propagation. These data provide new clues to better depiction of the N-cell interactions and SARS-CoV-2 infection and may help inform the development of new therapeutic candidates.

DDX3-mediated miR-34 expression inhibits autophagy and HBV replication in hepatic cells

HBV entry to the host cells and its successful infection depends on its ability to modulate the host restriction factors. DEAD box RNA helicase, DDX3, is shown to inhibit HBV replication. However, the exact mechanism of inhibition still remains unclear. DDX3 is involved in multitude or RNA metabolism processes including biogenesis of miRNAs. In this study, we sought to determine the mechanism involved in DDX3-mediated HBV inhibition. First, we observed that HBx protein of HBV downregulated DDX3 expression in HBV-infected cells. Overexpression of DDX3 inhibited HBx, HBsAg and total viral load, while its knockdown reversed the result in Hep G2.2.15 cells. Expression of miR-34 was downregulated in HBV-infected cells. Overexpression of pHBV_1.3 further confirmed that HBV downregulates miR-34 expression. Consistent with the previous finding that DDX3 is involved in miRNA biogenesis, we observed that expression of miR-34 positively corelated with DDX3 expression. miRNA target prediction tools showed that miR-34 can target autophagy pathway which is hijacked by HBV for the benefit of its own replication. Indeed, transfection with miR-34 oligos downregulated the expression of autophagy marker proteins in HBV-expressing cells. Overexpression of DDX3 in HBV-expressing cells, downregulated expression of autophagy proteins while silencing of DDX3 reversed the results. These results led us to conclude that DDX3 upregulates miR-34 expression and thus inhibits autophagy in HBV-expressing cells while HBx helps HBV evade DDX3-mediated inhibition by downregulating DDX3 expression in HBV-infected cells.

Hepatitis B Virus X Protein Inhibits the Expression of Barrier To Autointegration factor1 via Upregulating miR-203 Expression in Hepatic Cells

Hepatitis B virus (HBV) infection targets host restriction factors that inhibit its replication and survival. Previous studies have shown that barriers to autointegration factor1 (BANF1) inhibited the replication of herpes simplex virus and vaccinia virus by binding to phosphate backbone of dsDNA. To date, no reports are available for the interplay between BANF1 and HBV. In this study, we elucidated the mechanisms by which HBV inhibit BANF1. First, the effect of HBV on BANF1 was observed in Huh-7, Hep G2, and Hep G2.2.15 cells. Huh-7 cells were transfected with pHBV_1.3 or HBx plasmids. The results showed that there was a decreased expression of BANF1 in Hep G2.2.15 cells (P ≤ 0.005) or in HBV/HBx expressing Huh-7 cells (P ≤ 0.005), whereas BANF1 overexpression decreased viral replication (P ≤ 0.05). To study whether phosphorylation/dephosphorylation of BANF1 was responsible for antiviral activity, mutants were created, and it was found that inhibition due to mutants was less significant compared to BANF1 wild type. Previous studies have shown that HBV, at least in part, could regulate the expression of host miRNAs via HBx. It was found that miR-203 expression was high in Hep G2.2.15 cells (P ≤ 0.005) compared to Hep G2 cells. Next, the effect of HBx on miR-203 expression was studied and result showed that HBx upregulated miR-203 expression (P ≤ 0.005). Overexpression of miR-203 downregulated BANF1 expression (P ≤ 0.05) and viral titer was upregulated (P ≤ 0.05), while inhibition of miR-203, reversed these changes. In conclusion, BANF1 downregulated HBV, whereas HBV inhibited BANF1, at least in part, via HBx-mediated miR-203 upregulation in hepatic cells. IMPORTANCE In this study, for the first time, we found that BANF1 inhibited HBV replication and restricted the viral load. However, as previously reported for other viruses, the results in this study showed that BAF1 phosphorylation/dephosphorylation is not involved in its antiviral activity against HBV. HBV infection inhibited the intracellular expression of BANF1, via HBx-mediated upregulation of miR-203 expression. Overexpression of miR-203 downregulated BANF1 and increased the viral titer, while inhibition of miR-203 reversed these changes. This study helped us to understand the molecular mechanisms by which HBV survives and replicates in the host cells.

Cell Entry of Animal Coronaviruses

Coronaviruses (CoVs) are a group of enveloped positive-sense RNA viruses and can cause deadly diseases in animals and humans. Cell entry is the first and essential step of successful virus infection and can be divided into two ongoing steps: cell binding and membrane fusion. Over the past two decades, stimulated by the global outbreak of SARS-CoV and pandemic of SARS-CoV-2, numerous efforts have been made in the CoV research. As a result, significant progress has been achieved in our understanding of the cell entry process. Here, we review the current knowledge of this essential process, including the viral and host components involved in cell binding and membrane fusion, molecular mechanisms of their interactions, and the sites of virus entry. We highlight the recent findings of host restriction factors that inhibit CoVs entry. This knowledge not only enhances our understanding of the cell entry process, pathogenesis, tissue tropism, host range, and interspecies-transmission of CoVs but also provides a theoretical basis to design effective preventive and therapeutic strategies to control CoVs infection.

Aromatic Side Chain at Position 412 of SERINC5 Exerts Restriction Activity toward HIV-1 and Other Retroviruses

The host transmembrane protein SERINC5 is incorporated into viral particles and restricts infection by certain retroviruses. However, what motif of SERINC5 mediates this process remains elusive. By conducting mutagenesis analyses, we found that the substitution of phenylalanine with alanine at position 412 (F412A) resulted in a >75-fold reduction in SERINC5's restriction function. The F412A substitution also resulted in the loss of SERINC5's function to sensitize HIV-1 neutralization by antibodies recognizing the envelope's membrane proximal region. A series of biochemical analyses revealed that F412A showed steady-state protein expression, localization at the cellular membrane, and incorporation into secreted virus particles to a greater extent than in the wild type. Furthermore, introduction of several amino acid mutations at this position revealed that the aromatic side chains, including phenylalanine, tyrosine, and tryptophan, were required to maintain SERINC5 functions to impair the virus-cell fusion process and virion infectivity. Moreover, the wild-type SERINC5 restricted infection of lentiviruses pseudotyped with envelopes of murine leukemia viruses, simian immunodeficiency virus, and HIV-2, and F412A abrogated this function. Taken together, our results highlight the importance of the aromatic side chain at SERINC5 position 412 to maintain its restriction function against diverse retrovirus envelopes. IMPORTANCE The host protein SERINC5 is incorporated into progeny virions of certain retroviruses and restricts the infectivity of these viruses or sensitizes the envelope glycoprotein to a class of neutralizing antibodies. However, how and which part of SERINC5 engages with the diverse array of retroviral envelopes and exerts its antiretroviral functions remain elusive. During mutagenesis analyses, we eventually found that the single substitution of phenylalanine with alanine, but not with tyrosine or tryptophan, at position 412 (F412A) resulted in the loss of SERINC5's functions toward diverse retroviruses, whereas F412A showed steady-state protein expression, localization at the cellular membrane, and incorporation into progeny virions to a greater extent than the wild type. Results suggest that the aromatic side chain at position 412 of SERINC5 plays a critical role in mediating antiviral functions toward various retroviruses, thus providing additional important information regarding host and retrovirus interaction.

Antiviral Activity of Feline BCA2 Is Mainly Dependent on Its Interference With Proviral Transcription Rather Than Degradation of FIV Gag

Human BCA2/RNF115/Rabring7 (hBCA2) is a RING type E3 ubiquitin ligase with the ability of autoubiquitination or promoting protein ubiquitination. It also acts as a host restriction factor has BST2-dependent and BST2-independent antiviral activity to inhibit the release of HIV-1. In a previous study, we demonstrated that feline BCA2 (fBCA2) also has E3 ubiquitin ligase activity, although its antiviral mechanism remained unclear. In this study, we showed that fBCA2 can interact with feline BST2 (fBST2) and exhibits an fBST2-independent antiviral function, and the RING domain is necessary for the antiviral activity of fBCA2. fBCA2 could degrade HIV-1 Gag and restrict HIV-1 transcription to counteract HIV-1 but not promote the degradation of HIV-1 through lysosomal. Furthermore, for both fBCA2 and hBCA2, restricting viral transcription is the main anti-FIV mechanism compared to degradation of FIV Gag or promoting viral degradation. Consequently, transcriptional regulation of HIV or FIV by BCA2 should be the primary restriction mechanism, even though the degradation mechanism is different when BCA2 counteracts HIV or FIV. This may be due to BCA2 has a special preference in antiviral mechanism in the transmission of primate or non-primate retroviruses.

Identification of CP77 as the Third Orthopoxvirus SAMD9 and SAMD9L Inhibitor with Unique Specificity for a Rodent SAMD9L

Orthopoxviruses (OPXVs) have a broad host range in mammalian cells, but Chinese hamster ovary (CHO) cells are nonpermissive for vaccinia virus (VACV). Here, we revealed a species-specific difference in host restriction factor SAMD9L as the cause for the restriction and identified orthopoxvirus CP77 as a unique inhibitor capable of antagonizing Chinese hamster SAMD9L (chSAMD9L). Two known VACV inhibitors of SAMD9 and SAMD9L (SAMD9&L), K1 and C7, can bind human and mouse SAMD9&L, but neither can bind chSAMD9L. Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 knockout of chSAMD9L from CHO cells removed the restriction for VACV, while ectopic expression of chSAMD9L imposed the restriction for VACV in a human cell line, demonstrating that chSAMD9L is a potent restriction factor for VACV. In contrast to K1 and C7, cowpox virus CP77 can bind chSAMD9L and rescue VACV replication in cells expressing chSAMD9L, indicating that CP77 is yet another SAMD9L inhibitor but has a unique specificity for chSAMD9L. Binding studies showed that the N-terminal 382 amino acids of CP77 were sufficient for binding chSAMD9L and that both K1 and CP77 target a common internal region of SAMD9L. Growth studies with nearly all OPXV species showed that the ability of OPXVs to antagonize chSAMD9L correlates with CP77 gene status and that a functional CP77 ortholog was maintained in many OPXVs, including monkeypox virus. Our data suggest that a species-specific difference in rodent SAMD9L poses a barrier for cross-species OPXV infection and that OPXVs have evolved three SAMD9&L inhibitors with different specificities to overcome this barrier.IMPORTANCE Several OPXV species, including monkeypox virus and cowpox virus, cause zoonotic infection in humans. They are believed to use wild rodents as the reservoir or intermediate hosts, but the host or viral factors that are important for OPXV host range in rodents are unknown. Here, we showed that the abortive replication of several OPXV species in a Chinese hamster cell line was caused by a species-specific difference in the host antiviral factor SAMD9L, suggesting that SAMD9L divergence in different rodent species poses a barrier for cross-species OPXV infection. While the Chinese hamster SAMD9L could not be inhibited by two previously identified OPXV inhibitors of human and mouse SAMD9&L, it can be inhibited by cowpox virus CP77, indicating that OPXVs encode three SAMD9&L inhibitors with different specificities. Our data suggest that OPXV host range in broad rodent species depends on three SAMD9&L inhibitors with different specificities.

HIV-1 Envelope Glycoprotein at the Interface of Host Restriction and Virus Evasion

Without viral envelope proteins, viruses cannot enter cells to start infection. As the major viral proteins present on the surface of virions, viral envelope proteins are a prominent target of the host immune system in preventing and ultimately eliminating viral infection. In addition to the well-appreciated adaptive immunity that produces envelope protein-specific antibodies and T cell responses, recent studies have begun to unveil a rich layer of host innate immune mechanisms restricting viral entry. This review focuses on the exciting progress that has been made in this new direction of research, by discussing various known examples of host restriction of viral entry, and diverse viral countering strategies, in particular, the emerging role of viral envelope proteins in evading host innate immune suppression. We will also highlight the effective cooperation between innate and adaptive immunity to achieve the synergistic control of viral infection by targeting viral envelope protein and checking viral escape. Given that many of the related findings were made with HIV-1, we will use HIV-1 as the model virus to illustrate the basic principles and molecular mechanisms on host restriction targeting HIV-1 envelope protein.

Analysis of Competing HIV-1 Splice Donor Sites Uncovers a Tight Cluster of Splicing Regulatory Elements within Exon 2/2b

The HIV-1 accessory protein Vif is essential for viral replication by counteracting the host restriction factor APOBEC3G (A3G), and balanced levels of both proteins are required for efficient viral replication. Noncoding exons 2/2b contain the Vif start codon between their alternatively used splice donors 2 and 2b (D2 and D2b). For vif mRNA, intron 1 must be removed while intron 2 must be retained. Thus, splice acceptor 1 (A1) must be activated by U1 snRNP binding to either D2 or D2b, while splicing at D2 or D2b must be prevented. Here, we unravel the complex interactions between previously known and novel components of the splicing regulatory network regulating HIV-1 exon 2/2b inclusion in viral mRNAs. In particular, using RNA pulldown experiments and mass spectrometry analysis, we found members of the heterogeneous nuclear ribonucleoparticle (hnRNP) A/B family binding to a novel splicing regulatory element (SRE), the exonic splicing silencer ESS2b, and the splicing regulatory proteins Tra2/SRSF10 binding to the nearby exonic splicing enhancer ESE2b. Using a minigene reporter, we performed bioinformatics HEXplorer-guided mutational analysis to narrow down SRE motifs affecting splice site selection between D2 and D2b. Eventually, the impacts of these SREs on the viral splicing pattern and protein expression were exhaustively analyzed in viral particle production and replication experiments. Masking of these protein binding sites by use of locked nucleic acids (LNAs) impaired Vif expression and viral replication.IMPORTANCE Based on our results, we propose a model in which a dense network of SREs regulates vif mRNA and protein expression, crucial to maintain viral replication within host cells with varying A3G levels and at different stages of infection. This regulation is maintained by several serine/arginine-rich splicing factors (SRSF) and hnRNPs binding to those elements. Targeting this cluster of SREs with LNAs may lead to the development of novel effective therapeutic strategies.

Inhibition of Avian Influenza A Virus Replication in Human Cells by Host Restriction Factor TUFM Is Correlated with Autophagy

Avian influenza A viruses generally do not replicate efficiently in human cells, but substitution of glutamic acid (Glu, E) for lysine (Lys, K) at residue 627 of avian influenza virus polymerase basic protein 2 (PB2) can serve to overcome host restriction and facilitate human infectivity. Although PB2 residue 627 is regarded as a species-specific signature of influenza A viruses, host restriction factors associated with PB2₆₂₇E have yet to be fully investigated. We conducted immunoprecipitation, followed by differential proteomic analysis, to identify proteins associating with PB2₆₂₇K (human signature) and PB2₆₂₇E (avian signature) of influenza A/WSN/1933(H1N1) virus, and the results indicated that Tu elongation factor, mitochondrial (TUFM), had a higher binding affinity for PB2₆₂₇E than PB2₆₂₇K in transfected human cells. Stronger binding of TUFM to avian-signature PB2₅₉₀G/₅₉₁Q and PB2₆₂₇E in the 2009 swine-origin pandemic H1N1 and 2013 avian-origin H7N9 influenza A viruses was similarly observed. Viruses carrying avian-signature PB2₆₂₇E demonstrated increased replication in TUFM-deficient cells, but viral replication decreased in cells overexpressing TUFM. Interestingly, the presence of TUFM specifically inhibited the replication of PB2₆₂₇E viruses, but not PB2₆₂₇K viruses. In addition, enhanced levels of interaction between TUFM and PB2₆₂₇E were noted in the mitochondrial fraction of infected cells. Furthermore, TUFM-dependent autophagy was reduced in TUFM-deficient cells infected with PB2₆₂₇E virus; however, autophagy remained consistent in PB2₆₂₇K virus-infected cells. The results suggest that TUFM acts as a host restriction factor that impedes avian-signature influenza A virus replication in human cells in a manner that correlates with autophagy.

An understanding of the mechanisms that influenza A viruses utilize to shift host tropism and the identification of host restriction factors that can limit infection are both critical to the prevention and control of emerging viruses that cross species barriers to target new hosts. Using a proteomic approach, we revealed a novel role for TUFM as a host restriction factor that exerts an inhibitory effect on avian-signature PB2₆₂₇E influenza virus propagation in human cells. We further found that increased TUFM-dependent autophagy correlates with the inhibitory effect on avian-signature influenza virus replication and may serve as a key intrinsic mechanism to restrict avian influenza virus infection in humans. These findings provide new insight regarding the TUFM mitochondrial protein and may have important implications for the development of novel antiviral strategies.

Differential Contributions of Ubiquitin-Modified APOBEC3G Lysine Residues to HIV-1 Vif-Induced Degradation

Apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (A3G) is a host restriction factor that impedes HIV-1 replication. Viral integrity is salvaged by HIV-1 virion infectivity factor (Vif), which mediates A3G polyubiquitination and subsequent cellular depletion. Previous studies have implied that A3G polyubiquitination is essential for Vif-induced degradation. However, the contribution of polyubiquitination to the rate of A3G degradation remains unclear. Here, we show that A3G polyubiquitination is essential for degradation. Inhibition of ubiquitin-activating enzyme E1 by PYR-41 or blocking the formation of ubiquitin chains by over-expressing the lysine to arginine mutation of ubiquitin K48 (K48R) inhibited A3G degradation. Our A3G mutagenesis study showed that lysine residues 297, 301, 303, and 334 were not sufficient to render lysine-free A3G sensitive to Vif-mediated degradation. Our data also confirm that Vif could induce ubiquitin chain formation on lysine residues interspersed throughout A3G. Notably, A3G degradation relied on the lysine residues involved in polyubiquitination. Although A3G and the A3G C-terminal mutant interacted with Vif and were modified by ubiquitin chains, the latter remained more resistant to Vif-induced degradation. Furthermore, the A3G C-terminal mutant, but not the N-terminal mutant, maintained potent antiviral activity in the presence of Vif. Taken together, our results suggest that the location of A3G ubiquitin modification is a determinant for Vif-mediated degradation, implying that in addition to polyubiquitination, other factors may play a key role in the rate of A3G degradation.

Natural Polymorphisms and Oligomerization of Human APOBEC3H Contribute to Single-stranded DNA Scanning Ability

APOBEC3H is a deoxycytidine deaminase that can restrict the replication of HIV-1 in the absence of the viral protein Vif that induces APOBEC3H degradation in cells. APOBEC3H exists in humans as seven haplotypes (I-VII) with different cellular stabilities. Of the three stable APOBEC3H haplotypes (II, V, and VII), haplotypes II and V occur most frequently in the population. Despite APOBEC3H being a bona fide restriction factor, there has been no comparative biochemical characterization of APOBEC3H haplotypes. We characterized the ssDNA scanning mechanisms that haplotypes II and V use to search their ssDNA substrate for cytosine-containing deamination motifs. APOBEC3H haplotype II was able to processively deaminate multiple cytosines in a single enzyme-substrate encounter by using sliding, jumping, and intersegmental transfer movements. In contrast, APOBEC3H haplotype V exhibited diminished sliding and intersegmental transfer abilities but was able to jump along ssDNA. Due to an Asp or Glu at amino acid 178 differentiating these APOBEC3H haplotypes, the data indicated that this amino acid on helix 6 contributes to processivity. The diminished processivity of APOBEC3H haplotype V did not result in a reduced efficiency to restrict HIV-1 replication in single-cycle infectivity assays, suggesting a redundancy in the contributions of jumping and intersegmental transfer to mutagenic efficiency. Optimal processivity on ssDNA also required dimerization of APOBEC3H through the β2 strands. The findings support a model in which jumping can compensate for deficiencies in intersegmental transfer and suggest that APOBEC3H haplotypes II and V induce HIV-1 mutagenesis efficiently but by different mechanisms.