Fig. 2. Recombinant HBV SVPs containing all the HBV surface proteins. CHO cells stably transfected with the AL26 plasmid express 22 nm HBV subparticles. (A) Purified particles were fractionated on a CsCl gradient and the resulting fractions were assayed for HBsAg by radioimmunoassay using ¹²⁵I-labeled anti-HBsAg antibodies. The density of HBsAg SVPs was calculated to be 1.22 g/ml, characteristic of the 22 nm spherical HBsAg SVPs. The insert shows 22 nm particles that were negatively stained by uranyl acetate followed by TEM. (B) The composition of the various surface proteins was determined by [³⁵S]methionine labeling on SDS–PAGE and by western analysis with MA 18/7, the antibody specific for preS1 (αS1), MA Q19/10 (for preS2), and with polyclonal anti-S antibodies (αS). The various HBV surface proteins are indicated. (C) Schematic presentation of the different (S, L and M) HBsAg proteins with the corresponding antibody epitopes. The gray boxes represent the transmembrane regions.