FIG. 2.

Detection of virion-like particles and naked capsids in culture medium. Medium of Huh7 cells transiently transfected with the plasmids (Fig. 1) indicated on the right was fractionated by isopycnic CsCl gradient centrifugation. HBV capsids in the fractions were precipitated after the addition of mild detergent, separated through an agarose gel, and depicted by Western blotting. Naked capsids appeared in fractions 7 to 9, and enveloped virions appeared in fractions 3 to 5. −, no cotransfection; +, cotransfection of plasmids pE and pS (Fig. 1) for envelope protein expression; st inserted in panel A, 1 ng of purified capsids expressed in E. coli.