Restriction of retroviral replication by APOBEC3G/F and TRIM5alpha

Abstract

Pathogenic viral infections have exerted selection pressure on their hosts to evolve cellular antiviral inhibitors referred to as restriction factors. Examples of such molecules are APOBEC3G, APOBEC3F and TRIM5alpha. APOBEC3G and APOBEC3F are cytidine deaminases that are able to strongly inhibit retroviral replication by at least two mechanisms. They are counteracted by the lentiviral Vif protein. TRIM5alpha binds to sensitive, incoming retroviruses via its C-terminal PRY/SPRY domain and rapidly recruits them to the proteasome before significant viral DNA synthesis can occur. Both of these proteins robustly block retroviral replication in a species-specific way. It remains an open but important question as to whether innate restriction factors such as these can be harnessed to inhibit HIV-1 replication in humans.

Figure 1

Model of APOBEC antiviral function. (a) Effect of hA3G on the life cycle of wild-type HIV-1. The integrated HIV-1 provirus (blue) is transcribed and translated like a normal cellular gene. The viral Vif protein (purple) acts as an adaptor protein, connecting hA3G (green) to an E3 ubiquitin ligase complex comprising ElonginB, ElonginC, Cullin5 and RING-box-1 (rbx1), thereby inducing the polyubiquitylation and proteasomal degradation of hA3G. Viral proteins (turquoise) and genomic RNA (red) assemble and bud through the plasma membrane, releasing viral particles. Virions then enter target cells and, after reverse transcription, uncoating and nuclear entry have taken place, the viral DNA is integrated into the host genome. Formation of this provirus thus completes the replication cycle. (b) Effect of hA3G on the life cycle of HIV-1/Δvif. The HIV-1 provirus is transcribed and translated as before, but no Vif is synthesized. Cellular hA3G is therefore not degraded but instead is packaged into nascent budding virions. On infection of target cells, the presence of hA3G leads to a block in infection by any one or more of the following mechanisms. (1) hA3G might impede the formation of reverse transcripts through an unknown mechanism. (2) Cytidine deamination of nascent reverse transcripts by hA3G could be sufficient to hinder ensuing rounds of infectious virion production owing to inactivating mutations in viral genes and/or proteins. (3) Edited reverse transcripts might be recognized by cellular DNA repair pathways, leading to the degradation of these transcripts. (4) It is also possible that hA3G induces the degradation of HIV-1 reverse transcripts through an editing-independent mechanism, perhaps by recruiting a cellular endonuclease. (Reproduced with permission from [6]).

Figure 2

NMR structure of APOBEC3G and a model for substrate DNA binding. (a) Ribbon representation of the NMR structure of A3G [11]. Beta strands are shown in yellow and are numbered 1–5 (2′ indicates a short disrupted β strand); α helices are shown in red and are numbered 1–5. A purple sphere indicates the position of the zinc ion. (b) Model of the interaction of APOBEC3G with a trinucleotide 5′-C1-C2-T3–3′ DNA substrate. C2 represents the nucleotide that is being edited and is flipped out from the phosphodiester backbone for positioning towards the catalytic zinc ion (purple) and H257 (yellow). W285 (green) can facilitate positioning of C2 through stacking interactions. Arginines are shown in blue, and several conserved arginines of APOBEC3G have the potential to interact with the DNA. R213, R215 and R320 are shown in close proximity to the DNA backbone phosphates, whereas R313 could interact with C1.

Figure 3

Summary of the antiviral activity of TRIM5α. During unrestricted, productive infection, the retrovirus enters the cell, synthesises DNA by reverse transcription and enters the nucleus (N) where it integrates into a host chromosome (1). TRIM5α is found in dynamic structure in the cytoplasm called cytoplasmic bodies (CB) [64]. If sensitive incoming virions encounter TRIM5α they are rapidly recruited to the proteasome and degraded (2). If the proteasome is inhibited by MG132, then the virus does not get degraded but the TRIM5α-associated virus remains uninfectious (3) [63].