THE ROLES OF POL AND ENV IN THE ASSEMBLY PATHWAY OF HUMAN FOAMY VIRUS

Human foamy virus (HFV) is the prototype of the Spumavirus genus of re troviruses. These viruses have a genomic organization close to that of other complex retroviruses but have similarities to hepadnaviruses su ch as human hepatitis B virus (HBV). Both HFV and HBV express their Po l protein independently of their structural proteins. Retroviruses and hepadnaviruses differ in their requirements for particle assembly and genome packaging, Assembly of retroviral particles containing RNA gen omes requires only the Gag structural protein. The Pol protein is not required for capsid assembly, and the Env surface glycoprotein is not required for release of virions from the cell. In contrast, assembly o f extracellular HBV particles containing DNA requires core structural protein and polymerase (P protein) for assembly of nucleocapsids and r equires surface glycoproteins for release from the cell. We investigat ed the requirements for synthesis of extracellular HFV particles by co nstructing mutants with either the pol or env gene deleted. We found t hat the Pol protein is dispensable for production of extracellular par ticles containing viral nucleic acid. In the absence of Env, intracell ular particles are synthesized but few or no extracellular particles c ould be detected. Thus, foamy virus assembly is distinct from that of other reverse transcriptase-encoding mammalian viruses.