Foamy virus reverse transcriptase is expressed independently from the Gag protein.

Sequence motifs (L domains) have been described in viral structural proteins. Mutations in these lead to a defect at a late stage in virus assembly and budding. For several viruses, recruitment of an endosomal sorting complexes required for transport 1 subunit (Tsg101), a component of the class E vacuolar protein sorting (EVPS) machinery, is a prerequisite for virion budding. To effect this, Tsg101 interacts with the PT/SAP L domain. We have identified candidate L-domain motifs, PSAP, PPPI, and YEIL, in the prototypic foamy virus (PFV) Gag protein, based on their homology to known viral L domains. Mutation of the PSAP and PPPI motifs individually reduced PFV egress, and their combined mutation had an additive effect. When PSAP was mutated, residual infectious PFV release was unaffected by dominant negative Vps4 (an ATPase involved in the final stages of budding), and sensitivity to dominant negative Tsg101 was dramatically reduced, suggesting that the PSAP motif functions as a conventional class E VPS-dependent L domain. Consistent with this notion, yeast two-hybrid analysis showed a PSAP motif-dependent interaction between PFV Gag and Tsg101. Surprisingly, PFV release which is dependent on the PPPI motif was Vps4-independent and was partially inhibited by dominant negative Tsg101, suggesting that PPPI functions by an unconventional mechanism to facilitate PFV egress. Mutation of the YEIL sequence completely abolished particle formation and also reduced the rate of Gag processing by the viral protease, suggesting that the integrity of YEIL is required at an assembly step prior to budding and YEIL is not acting as an L domain.

mentioning

confidence: 91%

Identification of Domains in Gag Important for Prototypic Foamy Virus Egress

Patton

Morris

Chung

et al. 2005

“…The genomic organization of the FVs, including the prototype molecular cloned simian foamy virus SFVcpz(hu), is similar to that of other complex retroviruses, with several additional open reading frames located 3Ј of the canonical gag, pol, and env genes, including the transcriptional transactivator gene, tas (23,29,36). Unlike in retroviruses, however, the Pol protein is expressed from a unique spliced mRNA, not as a Gag-Pol fusion, and therefore must be specifically incorporated into newly forming capsids (14,28,47). In addition, reverse transcription of the genome is initiated early, during budding of capsids, viral egress, or prior to infection of new cells, suggesting a novel coordination of morphogenesis (50).…”

mentioning

confidence: 98%

Identification of a Conserved Residue of Foamy Virus Gag Required for Intracellular Capsid Assembly

Eastman

Linial

2001

In contrast to all retroviruses but similar to the hepatitis B virus, foamy viruses (FV) require expression of the envelope protein for budding of intracellular capsids from the cell, suggesting a specific interaction between the Gag and Env proteins. Capsid assembly occurs in the cytoplasm of infected cells in a manner similar to that for the B-and D-type viruses; however, in contrast to these retroviruses, FV Gag lacks an N-terminal myristylation signal and capsids are not targeted to the plasma membrane (PM). We have found that mutation of an absolutely conserved arginine (Arg) residue at position 50 to alanine (R50A) of the simian foamy virus SFV cpz(hu) inhibits proper capsid assembly and abolishes viral budding even in the presence of the envelope (Env) glycoproteins. Particle assembly and extracellular release of virus can be restored to this mutant with the addition of an N-terminal Src myristylation signal (Myr-R50A), presumably by providing an alternate site for assembly to occur at the PM. In addition, the strict requirement of Env expression for capsid budding can be bypassed by addition of a PM-targeting signal to Gag. These results suggest that intracellular capsid assembly may be mediated by a signal akin to the cytoplasmic targeting and retention signal CTRS found in Mason-Pfizer monkey virus and that FV Gag has the inherent ability to assemble capsids at multiple sites like conventional retroviruses. The necessity of Env expression for particle egress is most probably due to the lack of a membrane-targeting signal within FV Gag to direct capsids to the PM for release and indicates that Gag-Env interactions are essential to drive particle budding.

“…Certain intrinsic features of FV gene expression and replication-for instance, the high genetic stability, the presence of a functionally active internal promoter, and the expression of FV Pol proteins from a spliced transcript-are advantageous for the construction of viral vectors for the targeted expression of therapeutic proteins (4,5,8,14,18,19,25,26,29,41) Recently, we have generated and characterized replicationcompetent FFV vectors for the expression of heterologous proteins: for instance, the green fluorescent protein (GFP) (29). The utilization of corresponding replication-competent viruses expressing therapeutic proteins circumvents problems associated with the generation of packaging cells and vector genomes (discussed in reference 29).…”

mentioning

confidence: 99%

Application of Chimeric Feline Foamy Virus-Based Retroviral Vectors for the Induction of Antiviral Immunity in Cats

Schwantes

Truyen

Weikel

et al. 2003

In order to define the potential and applicability of replication-competent foamy virus-based vaccine vectors, recombinant feline foamy virus (FFV) vectors encoding defined segments of the feline calicivirus (FCV) capsid protein E domain were constructed. In cell cultures, these FFV-FCV vectors efficiently transduced and expressed a hybrid fusion protein consisting of the essential FFV Bet protein and the attached FCV E domains. The stability of the vectors in vitro was inversely correlated to the size of the heterologous insert. The deletion of a part of the FFV U3 sequence in these FFV-FCV vectors did not interfere with replication and titer in cell cultures but increased the genetic stability of the hybrid vectors. Selected chimeric vectors were injected into immunocompetent cats and persisted in the transduced host concomitant with a strong and specific humoral immune response against vector components. In a substantial number of cats, antibodies directed against the FCV E domain were induced by the FFV-FCV vectors, but no FCV-neutralizing activities were detectable in vitro. When the vaccinated cats were challenged with a high-titer FCV dose, sterile immunity was not induced by any of the hybrid FFV-FCV vectors. However, the FFV-FCV vector with a truncated U3 region of the long terminal repeat promoter significantly reduced the duration of FCV shedding after challenge and suppressed the appearance of FCV-specific ulcers. Possible mechanisms contributing to the partial protection will be discussed.The great success of applied virology during the last century has been mainly due to the development and application of efficient and safe antiviral vaccines. These prevent virus-mediated disease or spread of the infectious agent and have even resulted in the eradication of poxvirus (reviewed in reference 7). These achievements are on one hand based on the injection of defined virus-derived proteins, as in the case of the hepatitis B virus vaccine or inactivated viruses, which are both capable of inducing a stable and broadly reactive humoral immunity. Alternatively, attenuated, apathogenic virus variants and/or related viruses inducing a stable cross-protection are used. Modified live virus vaccines have the great advantage of mimicking the natural route and mode of virus replication, inducing not only a humoral, mainly immunoglobulin G (IgG)-mediated immunity, but also stimulating the cell-mediated and/or mucosal arm of the adaptive immune response (7). Although these strategies have been efficient in controlling various viruses pathogenic to humans and livestock animals, vaccines against some virus infections are presently either not available or display only a limited degree of protection (23).Over the last several years, novel vaccination strategies have been developed based on recombinant, chimeric viruses used to deliver and efficiently express heterologous vaccine antigens in the recipient (28). These novel strategies include not only prophylactic preexposure vaccination but also therapeutic postexposure im...