The antiretroviral activity of APOBEC3 is inhibited by the foamy virus accessory Bet protein

Abstract

Genome hypermutation of different orthoretroviruses by cellular cytidine deaminases of the APOBEC3 family during reverse transcription has recently been observed. Lentiviruses like HIV-1 have acquired proteins preventing genome editing in the newly infected cell. Here we show that feline foamy virus (FFV), a typical member of the foamy retrovirus subfamily Spumaretrovirinae, is also refractory to genome deamination. APOBEC3-like FFV genome editing in APOBEC3-positive feline CRFK cells only occurs when the accessory FFV Bet protein is functionally inactivated. Editing of bet-deficient FFV genomes is paralleled by a strong decrease in FFV titer. In contrast to lentiviruses, cytidine deamination already takes place in APOBEC3-positive FFV-producing cells, because edited proviral DNA genomes are consistently present in released particles. By cloning the feline APOBEC3 orthologue, we found that its homology to the second domain of human APOBEC3F is 48%. Expression of feline APOBEC3 in APOBEC3-negative human 293T cells reproduced the effects seen in homologous CRFK cells: Betdeficient FFV displayed severely reduced titers, high-level genome editing, reduced particle release, and suppressed Gag processing. Although WT Bet efficiently preserved FFV infectivity and genome integrity, it sustained particle release and Gag processing only when fe3 was moderately expressed. Similar to lentiviral Vif proteins, FFV Bet specifically bound feline APOBEC3. In particles from Bet-deficient FFV, feline APOBEC3 was clearly present, whereas its foamy viral antagonist Bet was undetectable in purified WT particles. This is the first report that, in addition to lentiviruses, the foamy viruses also developed APOBEC3-counteracting proteins.cytidine deamination ͉ virion infectivity factor ͉ spumaretrovirus ͉ restriction factor ͉ zoonosis

Restriction of Equine Infectious Anemia Virus by Equine APOBEC3 Cytidine Deaminases

“…The human A3G and muA3 expression constructs (human A3Z2g-Z1c and muA3Z2-Z3) were provided by N. R. Landau (49). Human A3B, -C, -F, and -H (A3Z2A-Z1b, A3Z2b, A3Z2e-Z2f, and A3Z3); feline A3; and canine A3 expression plasmids were previously described (33,44,57,58). Porcine A3 (A3Z2-Z3) was a gift of Eva Dörrschuck and Ralf Tönjes (GenBank accession no.EU871587; E. Dörrschuck et al, unpublished data).…”

“…In contrast to the well-characterized A3-Vif interaction, still little is known about A3-neutralizing strategies used by retroviruses that do not encode a Vif protein. It has been reported that foamy retroviruses use the accessory protein Bet, and Human T-cell leukemia virus type I has evolved a unique nucleocapsid protein to counteract the packaging of cognate A3 proteins (12,44,58,71). The debate over the mechanism of resistance to murine A3 (muA3) of the rodent gammaretrovirus Moloney murine leukemia virus (Mo-MLV) has not come up with a generally convincing model, despite many studies (5, 9, 13, 32, 35, 49).…”

Construction and characterization of efficient, stable and safe replication-deficient foamy virus vectors

“…29,30 In addition, we did not find a significant effect of Bet on the vector titer, as the 293T cells used for vector production are APOBEC3-deficient and thus, the deaminase-counteracting function of Bet is not required. 22 Additional effects of Bet on FFV vector particle release were not observed in this study.…”

“…The bel genes (Figure 1a) encode the viral Bel1/Tas transactivator and Bet that counteracts APOBEC3-mediated host restriction. 21,22 Importantly, only the env-bel-deleted vectors carrying SIN-deleted long terminal repressor (LTR) promoters turned out to be suited for the safe and long-term transduction of target cells.…”

APOBEC3 Cytidine Deaminases: Distinct Antiviral Actions along the Retroviral Life Cycle

“…The 3Ј regions in the minus-strand remain single-stranded for the longest time as they await plus-strand synthesis. This antiviral activity of human A3G also extends to other lentiviruses including simian immunodeficiency virus (SIV), equine infectious anemia virus (EIAV), and even distantly related retroviruses such as murine leukemia virus (MLV) and foamy viruses (6,29,31,34,35) Other Antiviral APOBEC3 Proteins Intriguingly, other APOBEC3 family members also exert antiviral activity. A3F induces dG to dA transitions in viral plus-strand DNAs, and its action is blocked by HIV-1 Vif (18, 36 -38).…”