Article

Newly synthesized APOBEC3G is incorporated into HIV virions, inhibited by HIV RNA, and subsequently activated by RNase H

National Institutes of Health, United States of America
PLoS Pathogens (Impact Factor: 8.06). 03/2007; 3(2):e15. DOI: 10.1371/journal.ppat.0030015
Source: PubMed

ABSTRACT Author Summary

APOBEC3G (A3G) is a cellular enzyme that promotes DNA mutagenesis and can restrict infection by HIV-1. However, HIV counters the antiviral effects of A3G through the action of its Vif protein. In the absence of Vif, A3G is effectively incorporated into virions, where it mutagenizes the first DNA copy (cDNA) generated during reverse transcription of the viral RNA genome. A3G also appears to be able to inhibit HIV via nonenzymatic mechanisms. A3G and related deoxycytidine deaminases can also inhibit the growth of retroviruses other than HIV and protect the cellular genome from endogenous mobile retroelements. In this study, we analyzed the recruitment and enzymatic activity of A3G incorporated into HIVΔVif virions. Unexpectedly, we found that the binding of A3G to viral genomic RNA led to inactivation of the enzyme. However, latent A3G was ultimately activated through the action of HIV RNase H, which degrades the RNA genome during reverse transcription. These findings highlight an unexpected interplay between a host enzyme and HIV, where the antiviral enzymatic activity of the host factor (A3G) is dependent on the action of an essential HIV enzyme (RNase H). The strong interaction with viral RNA also suggests a potential mechanism by which A3G could exert antiviral activity in the absence of enzymatic activity, by physically impeding reverse transcription.

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Available from: Wes Yonemoto, Jul 31, 2015
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    • "Vif was not associated with these high molecular mass A3G forms, but was required for their formation (Goila-Gaur et al., 2008). Although A3G regularly forms high molecular mass complexes in cells, which are less likely to be packaged into virions, Vif can induce an even higher molecular weight form of A3G (Soros et al., 2007; Goila-Gaur et al., 2008). Moreover, studies with an A3G C97A mutant that is resistant to Vif-mediated degradation suggested that Vif-mediated degradation and inhibition of packaging are two distinct properties of A3G since the A3G C97A mutant was encapsidated less well in the presence of Vif (Opi et al., 2007). "
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    ABSTRACT: The APOBEC3 restriction factors are a family of deoxycytidine deaminases that are able to suppress replication of viruses with a single-stranded DNA intermediate by inducing mutagenesis and functional inactivation of the virus. Of the seven human APOBEC3 enzymes, only APOBEC3-D, -F, -G, and -H appear relevant to restriction of HIV-1 in CD4+ T cells and will be the focus of this review. The restriction of HIV-1 occurs most potently in the absence of HIV-1 Vif that induces polyubiquitination and degradation of APOBEC3 enzymes through the proteasome pathway. To restrict HIV-1, APOBEC3 enzymes must be encapsidated into budding virions. Upon infection of the target cell during reverse transcription of the HIV-1 RNA into (-)DNA, APOBEC3 enzymes deaminate cytosines to form uracils in single-stranded (-)DNA regions. Upon replication of the (-)DNA to (+)DNA, the HIV-1 reverse transcriptase incorporates adenines opposite to the uracils thereby inducing C/G to T/A mutations that can functionally inactivate HIV-1. APOBEC3G is the most studied APOBEC3 enzyme and it is known that Vif attempts to thwart APOBEC3 function not only by inducing its proteasomal degradation but also by several degradation-independent mechanisms, such as inhibiting APOBEC3G virion encapsidation, mRNA translation, and for those APOBEC3G molecules that still become virion encapsidated, Vif can inhibit APOBEC3G mutagenic activity. Although most Vif variants can induce efficient degradation of APOBEC3-D, -F, and -G, there appears to be differential sensitivity to Vif-mediated degradation for APOBEC3H. This review examines APOBEC3-mediated HIV restriction mechanisms, how Vif acts as a substrate receptor for a Cullin5 ubiquitin ligase complex to induce degradation of APOBEC3s, and the determinants and functional consequences of the APOBEC3 and Vif interaction from a biological and biochemical perspective.
    Frontiers in Microbiology 08/2014; 5:450. DOI:10.3389/fmicb.2014.00450 · 3.94 Impact Factor
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    • "How, then, is the virion-incorporated A3G ultimately activated so that it mediate mutation of the newly synthesized minus-strand DNA? It appears that the action of the viral RNase H during reverse transcription frees the enzyme from its RNA inhibitor, allowing full expression of its deaminase activity (Soros et al., 2007). These findings highlight a rather unexpected hostpathogen interaction in which the anti-HIV activity of A3G depends on its activation by an HIV enzyme. "
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    ABSTRACT: The Vif protein of HIV is essential for the effective propagation of this pathogenic retrovirus in vivo. Vif acts by preventing virion encapsidation of two potent antiviral factors, the APOBEC3G and APOBEC3F cytidine deaminases. Decreased encapsidation in part involves Vif-mediated recruitment of a ubiquitin E3 ligase complex that promotes polyubiquitylation and proteasome-mediated degradation of APOBEC3G/F. The resultant decline in intracellular levels of these enzymes leads to decreased encapsidation of APOBECG/F into budding virions. This review discusses recent advances in our understanding of the dynamic interplay of Vif with the antiviral APOBEC3 enzymes.
    Molecular Aspects of Medicine 10/2010; 31(5):383-97. DOI:10.1016/j.mam.2010.06.001 · 10.30 Impact Factor
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    • "A possibility exists that the interaction is not dependent on RNA. These results are consistent with reports that the interaction between A3G and HIV-1 Gag is RNA-independent (Soros et al., 2007). HBc is present in the nucleus and cytoplasm of infected hepatocytes . "
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    ABSTRACT: APOBEC3G (A3G) is an intrinsic antiretroviral factor which can inhibit Hepatitis B virus (HBV) replication. This antiviral activity mainly depends on A3G incorporation into viral particles. However, the mechanisms of A3G packaging into HBV particles have not been well characterized. In this paper, we demonstrated that A3G interacted with the HBV core protein (HBc) directly in co-transfected HepG2 cells using the fluorescence resonance energy transfer (FRET) approach. In addition, we further found that this interaction did not require other factors in vitro using surface plasmon resonance (SPR) technology on BIAcore 3000. While cellular RNA or viral RNA was added to A3G protein solution before flow through the BIAcore chip, the interaction was not affected. In conclusion, these results suggest the possibility that A3G is incorporated into HBV viral particles via direct binding with HBc protein.
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