RNA and Protein Requirements for Incorporation of the Pol Protein into Foamy Virus Particles

Abstract

Foamy viruses (FVs) generate their Pol protein precursor molecule independently of the Gag protein from a spliced mRNA. This mode of expression raises the question of the mechanism of Pol protein incorporation into the viral particle (capsid). We previously showed that the packaging of (pre)genomic RNA is essential for Pol encapsidation (M. Heinkelein, C. Leurs, M. Rammling, K. Peters, H. Hanenberg, and A. Rethwilm, J. Virol. 76: [10069][10070][10071][10072][10073] 2002). Here, we demonstrate that distinct sequences in the RNA, which we termed Pol encapsidation sequences (PES), are required to incorporate Pol protein into the FV capsid. Two PES were found, which are contained in the previously identified cis-acting sequences necessary to transfer an FV vector. One PES is located in the U5 region of the 5 long terminal repeat and one at the 3 end of the pol gene region. Neither element has any significant effect on RNA packaging. However, deletion of either PES resulted in a significant reduction in Pol encapsidation. On the protein level, we show that only the Pol precursor, but not the individual reverse transcriptase (RT) and integrase (IN) subunits, is incorporated into FV particles. However, enzymatic activities of the protease (PR), RT, or IN are not required. Our results strengthen the view that in FVs, (pre)genomic RNA functions as a bridging molecule between Gag and Pol precursor proteins.Spumaretroviruses, or foamy viruses (FVs), are one of two subfamilies of retroviruses (30). A hallmark of the replication strategy of FVs is the Gag-independent expression of a Pol precursor protein from a spliced RNA (3,6,18,25,40). The consequences of this highly unusual strategy for the generation of a retroviral Pol protein are poorly understood. It has been shown that FV reverse transcriptase (RT) is much more processive than that of orthoretroviruses (31). Furthermore, it has been suggested that a very few molecules of Pol precursor are packaged into the FV capsid (4, 31). The FV means of expressing and encapsidating Pol protein has to be viewed in the context of reverse transcription, which appears to occur to a large extent in the virus-producing cell prior to budding (26,32,41). This leads to linear DNA, which is probably the virion nucleic acid and more relevant for infection than RNA (30).The actual mechanism of FV Pol protein particle incorporation despite lack of a Gag-Pol precursor has remained obscure. It is evident that FVs must have found a means of Pol encapsidation that acknowledges that a Gag-Pol precursor protein, which in orthoretroviruses facilitates Pol incorporation into the viral particle, is not generated (35). We previously showed that the incorporation of (pre)genomic RNA is essential for Pol protein encapsidation (12). Here, we took the experiments a step further and asked whether specific sequences on the RNA which allow Pol protein incorporation can be identified and whether such sequences are different from signals needed to package RNA. Furthermore, we wanted to know what requirements e...

Novel Functions of Prototype Foamy Virus Gag Glycine- Arginine-Rich Boxes in Reverse Transcription and Particle Morphogenesis

“…The actual details of this mechanism are still largely unknown. However, it appears to be linked to the packaging of another essential component of the infectious viral particle, the viral genomic RNA (29,32). Current data suggest that protein-RNA interaction of both Gag and Pol with viral genomic RNA as well as protein-protein interactions between Gag and Pol are involved in the assembly process (14,21,29).…”

Prototype Foamy Virus Protease Activity Is Essential for Intraparticle Reverse Transcription Initiation but Not Absolutely Required for Uncoating upon Host Cell Entry

“…Previously it was reported that virion encapsidation of mature PFV Pol subunits, when expressed as separate proteins from individual packaging constructs, could not be detected (9,14). We demonstrate in this study that mature PFV Pol subunits can be functionally incorporated into PFV capsids, either individually or in combination, when intracellular protein levels are highly elevated, achieved here by using corresponding expression-optimized subunit packaging constructs.…”

“…Previous investigations using expression systems employing the authentic PFV pol ORF demonstrated that PFV Pol is packaged only in its precursor state (9,14). Coexpression of individual mature PR-RT or IN subunits in the context of such packaging systems in 293T cells did not result in any detectable encapsidation of the mature Pol subunits or in any measurable infectivity of viral supernatants.…”

“…There are conflicting results related to the requirement of additional protein-protein interactions between both proteins for PFV Pol incorporation (11)(12)(13). Interestingly, only the unprocessed Pol precursor protein appears to be efficiently packaged into assembling PFV particles (9,14).…”

“…Thus, the FV Pol precursor cannot be packaged into newly forming capsids in an orthoretroviral-like fashion as part of a Gag-Pol fusion protein; instead, FVs directly incorporate the Pol protein into assembling virions (5). Although the mechanism of FV Pol encapsidation is only partially understood, it seems clear that viral RNA (vRNA) is necessary for efficient prototype FV (PFV) Pol particle incorporation (8)(9)(10). Furthermore, it is assumed that the vRNA serves as a bridging molecule between Gag and Pol during assembly.…”