Protein composition and morphology of human foamy virus intracellular cores and extracellular particles

Characterization of human foamy virus (HFV) gag-encoded precursors and the search for a Gag-Pol polyprotein and mature proteins derived from proteolytic processing were carried out in HFV-infected cells and with purified preassembled cores and extracellular virus by Western blotting and radioimmunoprecipitation using antisera against synthetic peptides corresponding to putative Gag and protease proteins. Precursor proteins, Pr78gag/74gag and Pr135pol, were found in the nucleus of epithelial and fibroblast cells 3-4 days after HFV infection. Kinetic analysis of HFV Pr78gag and Pr74gag indicated that Pr78gag is a precursor to Pr74gag. South-Western blot analysis indicated that Pr78gag and Pr74gag have properties associated with nucleic acid binding protein although they lack the typical zinc-finger motifs found in retroviral nucleocapsid proteins. Western blot analyses of preassembled HFV cores isolated from the cytoplasm of infected cells and purified by sucrose gradient centrifugation demonstrated the presence of Pr78gag/74gag and Pr135pol, but no proteolytically processed Gag proteins were observed. The majority of extracellular HFV particles were found to have pentagon-shaped cores, as observed intracellularly, and are believed to be the immature extracellular form of the virus. The highest concentration of extracellular particles, estimated by EM, Western blot, and reverse transcriptase assays were found in sucrose gradient fractions having a density of 1.21-1.24 g/cm3. Western blot analysis revealed that Pr78gag/74gag and Pr135pol were the major viral proteins associated with these extracellular particles, as only small amounts of putative proteolytically cleaved capsid (p32) were observed. Our results support the notion that Pol is translated independent of Gag in HFV-infected cells.