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

Previous studies have shown that foamy virus (FV) particle budding, especially the involvement of the viral env glycoprotein is different from that of other (ortho) retroviruses: the N-terminal Env leader protein Elp is a constituent of released FV particles. A defined sequence in Elp required for particle budding binds to the MA domain of Gag. To extend these findings, we show that feline FV Elp is a membrane-anchored protein with the N-terminus located inside the particle. Thus, the internal/cytoplasmic domain of Elp has the correct topology for interacting with Gag during budding. In addition to Elp, an Elp-related protein of about 9 kDa was shown to be virion associated and is probably generated by cellular signal peptidases. Besides the function of Elp binding, the N-terminal domain of Gag was shown to be required for proper localization of feline FV Gag to the cytoplasm and the perinuclear/nuclear region.

The molecular biology of spuma or foamy retroviruses is different from that of the other members of the Retroviridae. Among the distinguishing features, the N-terminal domain of the foamy virus Env glycoprotein, the 16-kDa Env leader protein Elp, is a component of released, infectious virions and is required for particle budding. The transmembrane protein Elp specifically interacts with N-terminal Gag sequences during morphogenesis. In this study, we investigate the mechanism of Elp release from the Env precursor protein. By a combination of genetic, biochemical, and biophysical methods, we show that the feline foamy virus (FFV) Elp is released by a cellular furin-like protease, most likely furin itself, generating an Elp protein consisting of 127 amino acid residues. The cleavage site fully conforms to the rules for an optimal furin site. Proteolytic processing at the furin cleavage site is required for full infectivity of FFV. However, utilization of other furin proteases and/or cleavage at a suboptimal signal peptidase cleavage site can partially rescue virus viability. In addition, we show that FFV Elp carries an N-linked oligosaccharide that is not conserved among the known foamy viruses.In retroviruses, proteolytic processing of the structural Gag and enzymatic Pol proteins is executed by the virus-encoded protease (6). In contrast, the Env glycoprotein precursors are considered to be processed by two cellular proteases (8,13,19). The first cleavage occurring in the endoplasmic reticulum (ER) removes the signal peptide (SP). In analogy to cellular secretory or transmembrane (TM) proteins, proteolytic removal of the SP which targets the Env precursor to the secretory pathway via the ER and Golgi apparatus is considered to be performed by cellular signal peptidases (SPases) (for a review, see references 17 and 19). The second cleavage, which separates the surface (SU) and TM domains, is catalyzed by a member of the furin convertase protease family, often the furin protease itself (8,28). Related glycoprotein processing mechanisms also apply to a wide variety of other membrane viruses such as influenza virus and Ebola virus (4).The processing of cellular type I membrane proteins (or even polyprotein precursors) by ER-resident SPases has been exploited by many viruses leading to SPs of about 15 to more than 50 amino acid residues independent of whether the protein is of viral or cellular origin (17). In addition, the SP is rapidly degraded by signal peptide peptidases, a process which is often directly linked to the presentation of the corresponding cleavage products by the major histocompatibility complex I for purposes of immune surveillance (17).However, for two distinct foamy retroviruses, namely human and feline foamy virus (HFV and FFV), the 14-to 18-kDa N-terminal Env cleavage product was clearly detectable as a component of released virus particles (11,14,15,35). We showed that the N terminus of FFV Env, termed the Env leader protein (Elp), is located inside particles, confirming that specific interact...