Apolipoprotein B mRNA-editing Enzyme Catalytic Polypeptide-like 3 Research Articles

Inhibition of Uracil DNA Glycosylase Alters Frequency and Spectrum of Action-at-a-Distance Mutations Induced by 8-Oxo-7,8-dihydroguanine.

The generation of DNA damage causes mutations and consequently cancer. Reactive oxygen species are important sources of DNA damage and some mutation signatures found in human cancers. 8-Oxo-7,8-dihydroguanine (GO, 8-hydroxyguanine) is one of the most abundant oxidized bases and induces a G→T transversion mutation at the modified site. The damaged G base also causes untargeted base substitution mutations at the G bases of 5'-GpA-3' dinucleotides (action-at-a-distance mutations) in human cells, and the cytosine deaminase apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3) is involved in the mutation process. The deaminated cytosine, i.e., uracil, bases are expected to be removed by uracil DNA glycosylase. Most of the substitution mutations at the G bases of 5'-GpA-3' might be caused by abasic sites formed by the glycosylase. In this study, we expressed the uracil DNA glycosylase inhibitor from Bacillus subtilis bacteriophage PBS2 in human U2OS cells and examined the effects on the GO-induced action-at-a-distance mutations. The inhibition of uracil DNA glycosylase increased the mutation frequency, and in particular, the frequency of G→A transitions. These results indicated that uracil DNA glycosylase, in addition to APOBEC3, is involved in the untargeted mutation process induced by GO.

Action-at-a-distance mutations induced by 8-oxo-7,8-dihydroguanine are dependent on APOBEC3.

DNA oxidation is a serious threat to genome integrity and is involved in mutations and cancer initiation. The G base is most frequently damaged, and 8-oxo-7,8-dihydroguanine (GO, 8-hydroxyguanine) is one of the predominant damaged bases. In human cells, GO causes a G:C→T:A transversion mutation at the modified site, and also induces untargeted substitution mutations at the G bases of 5'-GpA-3' dinucleotides (action-at-a-distance mutations). The 5'-GpA-3' sequences are complementary to the 5'-TpC-3' sequences, the preferred substrates for apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3) cytosine deaminases, and thus their contribution to mutagenesis has been considered. In this study, APOBEC3B, the most abundant APOBEC3 protein in human U2OS cells, was knocked down in human U2OS cells, and a GO-shuttle plasmid was then transfected into the cells. The action-at-a-distance mutations were reduced to ~25% by the knockdown, indicating that GO-induced action-at-a-distance mutations are highly dependent on APOBEC3B in this cell line.

Upregulation of the APOBEC3 Family Is Associated with a Poor Prognosis and Influences Treatment Response to Raf Inhibitors in Low Grade Glioma.

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) has been identified as a group of enzymes that catalyze cytosine deamination in single-stranded (ss) DNA to form uracil, causing somatic mutations in some cancers. We analyzed the APOBEC3 family in 33 TCGA cancer types and the results indicated that APOBEC3s are upregulated in multiple cancers and strongly correlate with prognosis, particularly in low grade glioma (LGG). Then we constructed a prognostic model based on family expression in LGG where the APOBEC3 family signature is an accurate predictive model (AUC of 0.85). Gene mutation, copy number variation (CNV), and a differential gene expression (DEG) analysis were performed in different risk groups, and the weighted gene co-expression network analysis (WGCNA) was employed to clarify the role of various members in LGG; CIBERSORT algorithm was deployed to evaluate the landscape of LGG immune infiltration. We found that upregulation of the APOBEC3 family expression can strengthen Ras/MAPK signaling pathway, promote tumor progression, and ultimately reduce the treatment benefits of Raf inhibitors. Moreover, the APOBEC3 family was shown to enhance the immune response mediated by myeloid cells and interferon gamma, as well as PD-L1 and PD-L2 expression, implying that they have immunotherapy potential. Therefore, the APOBEC3 signature enables an efficient assessment of LGG patient survival outcomes and expansion of clinical benefits by selecting appropriate individualized treatment strategies.

Specific Deubiquitinating Enzymes Promote Host Restriction Factors Against HIV/SIV Viruses.

Hijacking host ubiquitin pathways is essential for the replication of diverse viruses. However, the role of deubiquitinating enzymes (DUBs) in the interplay between viruses and the host is poorly characterized. Here, we demonstrate that specific DUBs are potent inhibitors of viral proteins from HIVs/simian immunodeficiency viruses (SIVs) that are involved in viral evasion of host restriction factors and viral replication. In particular, we discovered that T cell-functioning ubiquitin-specific protease 8 (USP8) is a potent and specific inhibitor of HIV-1 virion infectivity factor (Vif)-mediated apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3)G (A3G) degradation. Ectopic expression of USP8 inhibited Vif-induced A3G degradation and suppressed wild-type HIV-1 infectivity even in the presence of Vif. In addition, specific DUBs repressed Vpr-, Vpu-, and Vpx-triggered host restriction factor degradation. Our study has revealed a previously unrecognized interplay between the host’s DUBs and viral replication. Enhancing the antiviral activity of DUBs therefore represents an attractive strategy against HIVs/SIVs.

Action-at-a-distance mutations at 5'-GpA-3' sites induced by oxidised guanine in WRN-knockdown cells.

G:C sites distant from 8-oxo-7,8-dihydroguanine (GO, 8-hydroxyguanine) are frequently mutated when the lesion-bearing plasmid DNA is replicated in human cells with reduced Werner syndrome (WRN) protein. To detect the untargeted mutations preferentially, the oxidised guanine base was placed downstream of the reporter supF gene and the plasmid DNA was introduced into WRN-knockdown cells. The total mutant frequency seemed higher in the WRN-knockdown cells as compared to the control cells. Mutation analyses revealed that substitution mutations occurred at the G:C pairs of 5'-GpA-3'/5'-TpC-3' sites, the preferred sequence for the apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3)-family cytosine deaminases, in the supF gene in both control and knockdown cells. These mutations were observed more frequently at G sites than C sites on the DNA strand where the GO base was originally located. This tendency was promoted by the knockdown of the WRN protein. The present results imply the possible involvement of APOBEC3-family cytosine deaminases in the action-at-a-distance (untargeted) mutations at G:C (or G) sites induced by GO and in cancer initiation by oxidative stress.

Coronavirus genomes carry the signatures of their habitats.

Coronaviruses such as SARS-CoV-2 regularly infect host tissues that express antiviral proteins (AVPs) in abundance. Understanding how they evolve to adapt or evade host immune responses is important in the effort to control the spread of infection. Two AVPs that may shape viral genomes are the zinc finger antiviral protein (ZAP) and the apolipoprotein B mRNA editing enzyme-catalytic polypeptide-like 3 (APOBEC3). The former binds to CpG dinucleotides to facilitate the degradation of viral transcripts while the latter frequently deaminates C into U residues which could generate notable viral sequence variations. We tested the hypothesis that both APOBEC3 and ZAP impose selective pressures that shape the genome of an infecting coronavirus. Our investigation considered a comprehensive number of publicly available genomes for seven coronaviruses (SARS-CoV-2, SARS-CoV, and MERS infecting Homo sapiens, Bovine CoV infecting Bos taurus, MHV infecting Mus musculus, HEV infecting Sus scrofa, and CRCoV infecting Canis lupus familiaris). We show that coronaviruses that regularly infect tissues with abundant AVPs have CpG-deficient and U-rich genomes; whereas those that do not infect tissues with abundant AVPs do not share these sequence hallmarks. Among the coronaviruses surveyed herein, CpG is most deficient in SARS-CoV-2 and a temporal analysis showed a marked increase in C to U mutations over four months of SARS-CoV-2 genome evolution. Furthermore, the preferred motifs in which these C to U mutations occur are the same as those subjected to APOBEC3 editing in HIV-1. These results suggest that both ZAP and APOBEC3 shape the SARS-CoV-2 genome: ZAP imposes a strong CpG avoidance, and APOBEC3 constantly edits C to U. Evolutionary pressures exerted by host immune systems onto viral genomes may motivate novel strategies for SARS-CoV-2 vaccine development.

Deaminase-Independent Mode of Antiretroviral Action in Human and Mouse APOBEC3 Proteins.

Apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3 (APOBEC3) proteins (APOBEC3s) are deaminases that convert cytosines to uracils predominantly on a single-stranded DNA, and function as intrinsic restriction factors in the innate immune system to suppress replication of viruses (including retroviruses) and movement of retrotransposons. Enzymatic activity is supposed to be essential for the APOBEC3 antiviral function. However, it is not the only way that APOBEC3s exert their biological function. Since the discovery of human APOBEC3G as a restriction factor for HIV-1, the deaminase-independent mode of action has been observed. At present, it is apparent that both the deaminase-dependent and -independent pathways are tightly involved not only in combating viruses but also in human tumorigenesis. Although the deaminase-dependent pathway has been extensively characterized so far, understanding of the deaminase-independent pathway remains immature. Here, we review existing knowledge regarding the deaminase-independent antiretroviral functions of APOBEC3s and their molecular mechanisms. We also discuss the possible unidentified molecular mechanism for the deaminase-independent antiretroviral function mediated by mouse APOBEC3.

The interesting relationship between APOBEC3 deoxycytidine deaminases and cancer: a long road ahead.

Cancer is considered a group of diseases characterized by uncontrolled growth and spread of abnormal cells and is propelled by somatic mutations. Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) family of enzymes are endogenous sources of somatic mutations found in multiple human cancers. While these enzymes normally act as an intrinsic immune defence against viruses, they can also catalyse ‘off-target’ cytidine deamination in genomic single-stranded DNA intermediates. The deamination of cytosine forms uracil, which is promutagenic in DNA. Key factors to trigger the APOBEC ‘off-target’ activity are overexpression in a non-normal cell type, nuclear localization and replication stress. The resulting uracil-induced mutations contribute to genomic variation, which may result in neutral, beneficial or harmful consequences for the cancer. This review summarizes the functional and biochemical basis of the APOBEC3 enzyme activity and highlights their relationship with the most well-studied cancers in this particular context such as breast, lung, bladder, and human papillomavirus-associated cancers. We focus on APOBEC3A, APOBEC3B and APOBEC3H haplotype I because they are the leading candidates as sources of somatic mutations in these and other cancers. Also, we discuss the prognostic value of the APOBEC3 expression in drug resistance and response to therapies.

Immune Cytolytic Activity for Comprehensive Understanding of Immune Landscape in Hepatocellular Carcinoma.

Cytolytic activity score (CYT), defined by granzyme A and perforin expression, is a useful marker for underlying immunity. We hypothesized that CYT-high hepatocellular carcinomas (HCCs) have stronger immunogenicity and favorable tumor microenvironments, which would result in better clinical outcomes, using the cancer genome atlas (TCGA) cohort with 371 patients with HCC. We found CYT-high HCCs were associated with higher expressions of the apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3), well-known mutagenic enzymes. Further, higher numbers of anti-cancer immune cells, such as CD8+ T cells and M1 macrophages, were infiltrated in CYT-high HCCs. Major T cell exhaustion markers were expressed significantly higher in CYT-high HCCs, likely as a negative feedback loop. Additionally, CYT-high HCCs strongly enriched gene sets related with enhanced immune activity. With strong immunity, patients with CYT-high HCCs had significantly longer disease-specific survival (DSS) and overall survival (OS) (p = 0.03 and <0.01). Furthermore, when the OS is stratified by exhaustion marker expressions, the CYT-high/exhaustion-low group had the best and CYT-low/exhaustion-high groups had the worst OS. Lastly, high CYT was an independent protective factor for prognosis. In conclusion, CYT-high HCCs were associated with enhanced immunity and better survival. Our findings suggest that proper identification of tumor-immune microenvironments could stratify the patients for appropriate treatments.

A Multivariate Data Analytics Approach Revealed No Footprint of APOBEC3 Proteins in Hepatitis B Virus Genome

The human genome encodes for a family of editing enzymes known as APOBEC3 (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like3). Several member of this family such as APOBEC3G, APOBEC3F and APOBEC3H haplotype II are active against viruses such as HIV. These enzymes induce C-to-U mutations in the negative strand of viral genomes, which leads to multiple G-to-A changes, referred to as ‘hypermutation’. Mutation by these enzymes are sequence context dependent in HIV genome, in which APOBEC3G preferentially mutates G within GG, TGG and TGGG content while other members mutate G within GA, TGA and TGAA. However, the same context between these enzymes and HBV virus have never been explored. In this study, we aim to identify mutational footprint of APOBEC3 enzymes in HBV genome. To do this, we perform a multivariable data analytics technique to investigate the motif preference and possible sequence hierarchy of mutation by APOBEC3 enzymes in a large number of full genome HBV sequences from a wide range of naturally infected patients. This enables us to differentiate between normal and hypermutated sequences in terms of the representation of mono- to tetra-nucleotide motifs and allows us to identify motifs associated with hypermutation by different APOBEC3 enzymes in the HBV genomes. Our analyses reveal that either APOPEC3 enzymes are not active against HBV virus or inducing G-to-A mutations by these enzymes are not sequence context dependent in HBV genome. Funding Statement: This work was supported by grants to HAR. HAR is currently supported by the UNSW Scientia Fellowship. Declaration of Interests: The authors declare that they have no financial or non-financial competing interests. Ethics Approval Statement: The authors stated: Not applicable.

Abstract 3675: Elucidating the role and function of APOBEC3 DNA deaminases in myeloproliferative neoplasms

Abstract The prevalence of upregulated apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3 (APOBEC3) transcripts as well as a distinct mutational signature has been identified and presented extensively in numerous cancer types. Previously, this family of cytidine deaminases has been studied in innate immunity, as they are active in the conversion of cytosine to uracil to restrict retroviral replication. Recent studies employing whole exome sequencing of multiple cancer types have shown a significant increase in both APOBEC3 transcript level as well as an increase in mutations indicated as a result of APOBEC3 driven mutagenesis. However, the expression and role of these enzymes in cancer initiation and progression as well as the mechanisms by which the APOBEC3 enzymes elicit a response in the cancer microenvironment remains unknown, especially in hematopoietic malignancies. Here, we elucidate the APOBEC3 mutation phenotype in Myeloproliferative Neoplasms (MPNs) as well as the naïve cell populations of CD34+ cord blood and normal aged samples. Through RNA sequencing we have found a cell type and context specific nature of these enzymes, notably the upregulation of APOBEC3G (A3G) in the Myelofibrosis (MF) stem and progenitor cell population as compared to normal aged counterparts. There is also significant differential expression of APOBEC3C (A3C), APOBEC3D (A3D), and APOBEC3F (A3F) in MF disease states compared to normal controls. By cloning the APOBEC3 enzymes into lentiviral expression vectors, we can now study the physiological effects of changes in APOBEC3 transcript level in relation to the known changes in expression seen in many cancers, focusing on the upregulation of A3G through lentiviral overexpression. Using these techniques, we will also elucidate the mutation signature of APOBEC3G in CD34+ cord blood as compared to the known mutagenesis pattern derived from the editing signature of APOBEC3B. Upon APOBEC3G overexpression we can connect the prevalence and proposed activity of the enzymes to the physiological deregulation present in hematopoietic malignancies. We have found that A3G induces a proliferative burst in normal CD34+ cord blood, which leads us to study the effects of APOBEC3 upregulation in regards to proliferation, differentiation and self-renewal in normal and malignant cells as well as their potential function in disease initiation and progression in MPNs. Citation Format: Jane Isquith, Qingfei Jiang, Raymond Diep, Jessica Pham, Frida Holm, Catriona Jamieson. Elucidating the role and function of APOBEC3 DNA deaminases in myeloproliferative neoplasms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3675.

The effects of APOBEC3 proteins on Hepatitis B virus replication

Hepatitis B virus causes chronic liver infection in 257 million people worldwide. Current treatments against HBV can control, but not cure HBV infection. Therefore, new treatments for chronic HBV infection need to be developed. The APOBEC3 (apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3) proteins are cellular restriction factors, which have been shown to restrict viral replication for a number of viruses (e.g. HIV) and for retrotransposons. The aim of this study is to elucidate the role of APOBEC3 proteins in inhibiting Hepatitis B virus replication. Quantitative polymerase chain reaction (qPCR) was used to evaluate the impact of APOBEC3 family members on the Hepatitis B virus replication in HepG2.2.15 cells. The highest inhibition of intracellular capsid associated HBV DNA, extracellular virion associated HBV DNA was induced by APOBEC3DE, APOBEC3F, and APOBEC3G as compared to the other APOBEC3 proteins. However, APOBEC3DE showed no inhibition of HBV total RNA, whereas the highest inhibition of HBV total RNA was induced by APOBEC3F and APOBEC3G. The sub-cellular localisation of APOBEC3 proteins was determined by immunofluorescence using confocal microscopy. It was found that APOBEC3DE, APOBEC3F, and APOBEC3G localise to the cytoplasm, suggesting a crucial role of these proteins in HBV replication in the cytoplasm. Nevertheless, APOBEC3A and APOBEC3B localise to the nucleus. The expression and co-localisation of APOBEC3 proteins and viral and host proteins (Uracil DNA Glycosylase UNG and ATP-dependent RNA helicase DDX3) in HepG2.2.15 and HEK293T cells are being investigated in order to determine the interaction between these proteins.

Characterization of APOBEC3 variation in a population of HIV-1 infected individuals in northern South Africa

BackgroundThe apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3) genes A3D, A3F, A3G and A3H have all been implicated in the restriction of human immunodeficiency virus type 1 (HIV-1) replication. Polymorphisms in these genes are likely to impact viral replication and fitness, contributing to viral diversity. Currently, only a few studies indicate that polymorphisms in the A3 genes may be correlated with infection risk and disease progression.MethodsTo characterize polymorphisms in the coding regions of these APOBEC3 genes in an HIV-1 infected population from the Limpopo Province of South Africa, APOBEC3 gene fragments were amplified from genomic DNA of 192 HIV-1 infected subjects and sequenced on an Illumina MiSeq platform. SNPs were confirmed and compared to SNPs in other populations reported in the 1000 Genome Phase III and HapMap databases, as well as in the ExAC exome database. Hardy-Weinberg Equilibrium was calculated and haplotypes were inferred using the LDlink 3.0 web tool. Linkage Disequilibrium (LD) for these SNPS were calculated in the total 1000 genome and AFR populations using the same tool.ResultsKnown variants compared to the GRCh37 consensus genome sequence were detected at relatively high frequencies (> 5%) in all of the APOBEC3 genes. A3H showed the most variation, with several of the variants present in both alleles in almost all of the patients. Several minor allele variants (< 5%) were also detected in A3D, A3F and A3G. In addition, novel R6K, L221R and T238I variants in A3D and I117I in A3F were observed. Four, five, four, and three haplotypes were identified for A3D, A3F, A3G, and A3H respectively.ConclusionsThe study showed significant polymorphisms in the APOBEC3D, 3F, 3G and 3H genes in our South African HIV1-infected cohort. In the case of all of these genes, the polymorphisms were generally present at higher frequencies than reported in other 1000 genome populations and in the ExAC exome consortium database .

APOBEC3G/3A Expression in Human Immunodeficiency Virus Type 1-Infected Individuals Following Initiation of Antiretroviral Therapy Containing Cenicriviroc or Efavirenz.

Apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3) family members are cytidine deaminases that play crucial roles in innate responses to retrovirus infection. The mechanisms by which some of these enzymes restrict human immunodeficiency virus type 1 (HIV-1) replication have been extensively investigated in vitro. However, little is known regarding how APOBEC3 proteins affect the pathogenesis of HIV-1 infection in vivo and how antiretroviral therapy influences their expression. In this work, a longitudinal analysis was performed to evaluate APOBEC3G/3A expression in peripheral blood mononuclear cells of antiretroviral-naive HIV-1-infected individuals treated with cenicriviroc (CVC) or efavirenz (EFV) at baseline and 4, 12, 24, and 48 weeks post-treatment follow-up. While APOBEC3G expression was unaffected by therapy, APOBEC3A levels increased in CVC but not EFV arm at week 48 of treatment. APOBEC3G expression correlated directly with CD4+ cell count and CD4+/CD8+ cell ratio, whereas APOBEC3A levels inversely correlated with plasma soluble CD14. These findings suggest that higher APOBEC3G/3A levels may be associated with protective effects against HIV-1 disease progression and chronic inflammation and warrant further studies.

The cytidine deaminase under-representation reporter (CDUR) as a tool to study evolution of sequences under deaminase mutational pressure

BackgroundActivation induced deaminase (AID) and apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3 (APOBEC3) are deaminases that mutate C to U on single-stranded DNA (ssDNA). AID is expressed primarily in germinal center B-cells, where it facilitates affinity maturation and class-switch recombination. APOBEC3 are a family of anti-viral proteins that act as part of the intrinsic immune response. In both cases, there are particular sequence motifs, also known as “mutation motifs”, to which these deaminases prefer to bind and mutate.ResultsWe present a program, the cytidine deaminase under-representation reporter (CDUR) designed to statistically determine whether a given sequence has an under/over-representation of these mutation motifs. CDUR shows consitency with other studies of mutation motifs, as we show by analyzing sequences from the adeno-associated virus 2 (AAV2) and human papillomavirus (HPV).ConclusionUsing various shuffling mechanisms to generate different null model distributions, we can tailor CDUR to correct for metrics such as GC-content, dinucleotide frequency, and codon bias.

A naturally occurring feline APOBEC3 variant that loses anti-lentiviral activity by lacking two amino acid residues.

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) is a mammalian protein that restricts lentiviral replication. Various polymorphisms of mammalian APOBEC3 genes have been observed in humans, Old World monkeys and domestic cats; however, the genetic diversity of APOBEC3 genes in other mammals remains unaddressed. Here we identify a novel haplotype of the feline APOBEC3Z3 gene, an APOBEC3 gene that restricts feline immunodeficiency virus (FIV) replication, in a Eurasian lynx (Lynx lynx). Compared to the previously identified lynx APOBEC3Z3 (haplotype I), the new sequence (haplotype II) harbours two amino acid deletions (Q16 and H17) and a nonsynonymous substitution (R68Q). Interestingly, lynx APOBEC3Z3 haplotype II does not suppress FIV infectivity, whereas haplotype I does. Mutagenesis experiments further revealed that deleting two amino acids (Q16 and H17) causes anti-FIV activity loss. This report demonstrates that a naturally occurring APOBEC3 variant loses anti-lentiviral activity through the deletion of two amino acid residues.

Feline APOBEC3s, Barriers to Cross-Species Transmission of FIV?

The replication of lentiviruses highly depends on host cellular factors, which defines their species-specific tropism. Cellular restriction factors that can inhibit lentiviral replication were recently identified. Feline immunodeficiency virus (FIV) was found to be sensitive to several feline cellular restriction factors, such as apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3) and tetherin, but FIV evolved to counteract them. Here, we describe the molecular mechanisms by which feline APOBEC3 restriction factors inhibit FIV replication and discuss the molecular interaction of APOBEC3 proteins with the viral antagonizing protein Vif. We speculate that feline APOBEC3 proteins could explain some of the observed FIV cross-species transmissions described in wild Felids.

Influence of the DNA sequence/length and pH on deaminase activity, as well as the roles of the amino acid residues around the catalytic center of APOBEC3F.

APOBEC3F (A3F), an apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) family protein, catalyzes cytosine-to-uracil conversion in single-stranded (ss) DNA. A3F acts as an inhibitor of retrovirus replication and exhibits antiviral activity against viral infectivity factor (Vif)-deficient human immunodeficiency virus 1 (HIV-1). Previous studies have mostly been focused on the interaction between A3F and Vif, and the studies on A3F's deamination properties are limited. Here, we report comprehensive characterization of the deaminase activity and ssDNA binding of the C-terminal domain (CTD) of A3F. It was shown that the deaminase activity of A3F-CTD is affected by the nucleic acid residues adjacent to the target sequence, TC, and that TTCA/G are the most preferred sequences. A3F-CTD deaminates the target sequence in longer ssDNAs most efficiently. Mutation analysis identified the amino acid residues that are responsible for the deaminase activity and ssDNA binding in the loops surrounding the catalytic center. The functions of these residues were rationally interpreted on the basis of the co-crystal structure of A3A-ssDNA and the known roles of the equivalent amino acid residues found in other A3s. Furthermore, we demonstrated that the deaminase activity of A3F-CTD could be regulated through phosphorylation of a putative site, S216. Finally, A3F-CTD was found to be active in a wide pH range (5.5 to 9.5) with similar activity. Interestingly, the A3F-CTD N214H mutant exhibited a dramatic increase in activity at pH 5.5.

Feline Immunodeficiency Virus Evolutionarily Acquires Two Proteins, Vif and Protease, Capable of Antagonizing Feline APOBEC3

ABSTRACTThe interplay between viral and host proteins has been well studied to elucidate virus-host interactions and their relevance to virulence. Mammalian genes encode apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins, which act as intrinsic restriction factors against lentiviruses. To overcome APOBEC3-mediated antiviral actions, lentiviruses have evolutionarily acquired an accessory protein, viral infectivity factor (Vif), and Vif degrades host APOBEC3 proteins via a ubiquitin/proteasome-dependent pathway. Although the Vif-APOBEC3 interaction and its evolutionary significance, particularly those of primate lentiviruses (including HIV) and primates (including humans), have been well investigated, those of nonprimate lentiviruses and nonprimates are poorly understood. Moreover, the factors that determine lentiviral pathogenicity remain unclear. Here, we focus on feline immunodeficiency virus (FIV), a pathogenic lentivirus in domestic cats, and the interaction between FIV Vif and feline APOBEC3 in terms of viral virulence and evolution. We reveal the significantly reduced diversity of FIV subtype B compared to that of other subtypes, which may associate with the low pathogenicity of this subtype. We also demonstrate that FIV subtype B Vif is less active with regard to feline APOBEC3 degradation. More intriguingly, we further reveal that FIV protease cleaves feline APOBEC3 in released virions. Taken together, our findings provide evidence that a lentivirus encodes two types of anti-APOBEC3 factors, Vif and viral protease.IMPORTANCE During the history of mammalian evolution, mammals coevolved with retroviruses, including lentiviruses. All pathogenic lentiviruses, excluding equine infectious anemia virus, have acquired the vif gene via evolution to combat APOBEC3 proteins, which are intrinsic restriction factors against exogenous lentiviruses. Here we demonstrate that FIV, a pathogenic lentivirus in domestic cats, antagonizes feline APOBEC3 proteins by both Vif and a viral protease. Furthermore, the Vif proteins of an FIV subtype (subtype B) have attenuated their anti-APOBEC3 activity through evolution. Our findings can be a clue to elucidate the complicated evolutionary processes by which lentiviruses adapt to mammals.

Core Binding Factor β Protects HIV, Type 1 Accessory Protein Viral Infectivity Factor from MDM2-mediated Degradation

HIV, type 1 overcomes host restriction factor apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins by organizing an E3 ubiquitin ligase complex together with viral infectivity factor (Vif) and a host transcription cofactor core binding factor β (CBFβ). CBFβ is essential for Vif to counteract APOBEC3 by enabling the recruitment of cullin 5 to the complex and increasing the steady-state level of Vif protein; however, the mechanisms by which CBFβ up-regulates Vif protein remains unclear. Because we have reported previously that mouse double minute 2 homolog (MDM2) is an E3 ligase for Vif, we hypothesized that CBFβ might protect Vif from MDM2-mediated degradation. Co-immunoprecipitation analyses showed that Vif mutants that do not bind to CBFβ preferentially interact with MDM2 and that overexpression of CBFβ disrupts the interaction between MDM2 and Vif. Knockdown of CBFβ reduced the steady-state level of Vif in MDM2-proficient cells but not in MDM2-null cells. Cycloheximide chase analyses revealed that Vif E88A/W89A, which does not interact with CBFβ, degraded faster than wild-type Vif in MDM2-proficient cells but not in MDM2-null cells, suggesting that Vif stabilization by CBFβ is mainly caused by impairing MDM2-mediated degradation. We identified Vif R93E as a Vif variant that does not bind to MDM2, and the virus with this substitution mutation was more resistant to APOBEC3G than the parental virus. Combinatory substitution of Vif residues required for CBFβ binding and MDM2 binding showed full recovery of Vif steady-state levels, supporting our hypothesis. Our data provide new insights into the mechanism of Vif augmentation by CBFβ.