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Background
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Mammalian cells have developed multiple strategies to limit retroviral infection including numerous proteins termed restriction factors that restrict retrovirus replication and infection. One such protein is TRIM5, a member of a broad family of otherwise unrelated proteins whose longest isoform, TRIM5?±, enables resistance to infection by HIV-1 through rapid degradation of HIV-1 Gag polyproteins. Another protein, APOBEC3G (and to a lesser extent APOBEC3F) can be incorporated into HIV-1 virions and induce hypermutation in the newly synthesized viral DNA and thus destabilize the viral genome. This innate mechanism of retroviral resistance is counteracted by the HIV-1 Vif protein by inducing the ubiquitization and degradation of APOBEC3G, a single amino acid substitution (D128K) blocks APOBEC3G depletion without affecting its inhibitory activity. The human uracil-DNA glycosylase UNG2 can also be incorporated into the HIV-1virion, indicating that it is required to remove uracils from the viral genome. It has been suggested that the UNG2 contributes to the APOBEC3G-mediated loss of infectivity by generating abasic sites in the viral genome. UNG1, the mitochondrial form of UNG, is transcribed from the same gene as UNG2 through differentially regulated promoters and alternative splicing, but does not appear to have anti-retroviral properties. AID, a protein related to APOBEC3 also possesses cytidine deaminase activity that can be blocked by the HIV-1 Vif protein in E. coli, but so far it appears unlikely that AID deaminates dC to dU residues in HIV cDNA as does APOBEC3G.
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