Foamy Virus Pol Protein Expressed as a Gag–Pol Fusion Retains Enzymatic Activities, Allowing for Infectious Virus Production

Foamy viruses (FV) synthesize Pol from a spliced polmRNA independently of Gag, unlike orthoretroviruses, which synthesize Pol as a Gag-Pol protein that coassembles with Gag.We found that prototype FV (PFV) mutants expressing Gag and Pol only as a Gag-Pol protein without the spliced Pol contain protease activity equivalent to that of wild-type (WT) Pol. Regardless of the presence or absence of the spliced Pol, the PFV Gag-Pol proteins can assemble into virus-like particles (VLPs), in contrast to the orthoretroviral Gag-Pol proteins, which cannot form VLPs. However, the PFV Gag-Pol VLPs have aberrant morphologies and are not infectious. In the absence of the spliced Pol, coexpression of a PFV Gag-Pol protein with Gag can produce infectious viri-ons. Our results suggest that enzymes encoded by PFV pol (protease, reverse transcriptase, and integrase) are enzymatically ac-tive if they are synthesized as part of a Gag-Pol protein. The foamy virus (FV) genome encodes the three major viralproteins Gag, Pol, and Env that are common to all other ret-roviruses, in addition to two regulatory proteins, Bet and Tas. However, FVs are classified as one of two subfamilies of the Ret-roviridae, the Spumaretrovirinae, whose viral replication pathway is distinct from that of Orthoretrovirinae. Major differences in-clude the mode of Pol expression and its encapsidation into viri-ons. Orthoretroviruses, such as human immunodeficiency viru