Identification of the feline foamy virus Bet domain essential for APOBEC3 counteraction

Abstract

BackgroundAPOBEC3 (A3) proteins restrict viral replication by cytidine deamination of viral DNA genomes and impairing reverse transcription and integration. To escape this restriction, lentiviruses have evolved the viral infectivity factor (Vif), which binds A3 proteins and targets them for proteolytic degradation. In contrast, foamy viruses (FVs) encode Bet proteins that allow replication in the presence of A3, apparently by A3 binding and/or sequestration, thus preventing A3 packaging into virions and subsequent restriction. Due to a long-lasting FV-host coevolution, Bet proteins mainly counteract restriction by A3s from their cognate or highly related host species.ResultsThrough bioinformatics, we identified conserved motifs in Bet, all localized in the bel2 exon. In line with the localization of these conserved motifs within bel2, this part of feline FV (FFV) Bet has been shown to be essential for feline A3 (feA3) inactivation and feA3 protein binding. To study the function of the Bet motifs in detail, we analyzed the ability of targeted deletion, substitution, and chimeric FFV-PFV (prototype FV) Bet mutants to physically bind and/or inactivate feA3. Binding of Bet to feA3Z2b is sensitive to mutations in the first three conserved motifs and N- and C-terminal deletions and substitutions across almost the complete bel2 coding sequence. In contrast, the Bel1 (also designated Tas) domain of Bet is dispensable for basal feA3Z2b inactivation and binding but mainly increases the steady state level of Bet. Studies with PFV Bel1 and full-length FFV Bel2 chimeras confirmed the importance of Bel2 for A3 inactivation indicating that Bel1 is dispensable for basal feA3Z2b inactivation and binding but increases Bet stability. Moreover, the bel1/tas exon may be required for expression of a fully functional Bet protein from a spliced transcript.ConclusionsWe show that the Bel2 domain of FV Bet is essential for the inactivation of APOBEC3 cytidine deaminase restriction factors. The Bel1/Tas domain increases protein stability and can be exchanged by related sequence. Since feA3 binding and inactivation by Bet are highly correlated, the data support the view that FV Bet prevents A3-mediated restriction of viral replication by creating strong complexes with these proteins.

Simian Foamy Virus Infection of Rhesus Macaques in Bangladesh: Relationship of Latent Proviruses and Transcriptionally Active Viruses

“…SFV have been shown in vitro to be susceptible to APOBEC3G mutation (38,39), but there was little published evidence of this in the natural host prior to this study. It is unclear whether the SFV accessory protein Bet can counter APOBEC3G activity (38,40,41), although evidence is increasing that Bet is important in protecting FV from restriction by APOBEC (42,43). Recently, APOBEC3 proteins from rhesus macaque PBMC have been characterized (19).…”

APOBECs and virus restriction

“…Feline FV Bet has been shown to bind to feline A3 (Lochelt et al, 2005), and prototype FV Bet can prevent human A3G dimerization and function (Jaguva Vasudevan et al, 2013; Perkovic et al, 2009; Russell et al, 2005). Bioinformatic analysis has identified six conserved motifs encoded within the bel2 portion of the bet mRNA, and these motifs appear to be required for creating high affinity complexes between Bet and A3 (Lukic et al, 2013). Interestingly, Bet is expressed at high levels in infected cells, both in culture and in animals, consistent with inactivation of A3 by Bet binding or sequestration (Alke et al, 2001; Lukic et al, 2013).…”

“…Bioinformatic analysis has identified six conserved motifs encoded within the bel2 portion of the bet mRNA, and these motifs appear to be required for creating high affinity complexes between Bet and A3 (Lukic et al, 2013). Interestingly, Bet is expressed at high levels in infected cells, both in culture and in animals, consistent with inactivation of A3 by Bet binding or sequestration (Alke et al, 2001; Lukic et al, 2013). In addition, A3s may be able to inhibit FV replication in both producer as well as target cells (Lochelt et al, 2005), which may be linked to the fact that spumaviruses can initiate reverse transcription in producer cells (Moebes et al, 1997).…”

Evolution of Foamy Viruses: The Most Ancient of All Retroviruses

“…Both virus genera generate a viral protein to antagonize APOBEC3. While in most lentiviruses, the Vif protein prevents APOBEC3 (mainly APOBEC3G and 3F) protein packaging by delivering them to the cellular proteasome for degradation [106,128], the foamy viral accessory Bet protein prevents encapsidation by blocking APOBEC3 dimerization and targeting them to form insoluble complexes incapable of incorporation into nascent FV capsids [129,130]. FV Bet proteins are not initiating APOBEC3 protein destruction [57,58,59].…”

“…Domains of physical interaction on the side of effector molecules and targets have been mapped [57,130]. Recent results from the feline FV system indicate that ORF-2 encodes the APOBEC3-interacting sequences [130].…”