FOAMY VIRUS PARTICLE FORMATION

Subgenomic expression plasmids for the so-called human foamy, virus (H FV) structural gag, gag/pol, and env genes were constructed and used t o analyze foamy virus particle formation by electron microscopy. Expre ssion of an R-US-gag-pol construct under control of the human cytomega lovirus immediate-early enhancer-promoter resulted in the formation of viral cores with a homogeneous size of approximately 50 nm located in the cytoplasm. Upon coexpression of an envelope construct, particles were observed budding into cytoplasmic vesicles and from the plasma me mbrane, Expression of the Gag protein precursor pr74 alone led to aber rantly formed viral particles of heterogeneous size and with open core s, Normal-shaped cores were seen after transfection of a construct exp ressing the p70(gag) cleavage product, indicating that p70(gag) is abl e to assemble into capsids, Coexpression of p70(gag) and Env resulted in budding virions, ruling out a requirement of the reverse transcript ase for capsid or virion formation, In sharp contrast to other retrovi ruses, the HFV cores did not spontaneously bud from cellular membranes . Radiochemical labeling followed by protein gel electrophoresis also revealed the intracellular retention of Env-deprived HFV capsids.