Hepatitis B surface antigen assembles in a post-ER, pre-Golgi compartment

The role of this intermediate compartment in the biosynthesis of the HBsAg lipoprotein particle is indicated in the model (Fig. 10) which extends that of Ganem and co-workers (Eble et al., 1986; Simon et al., 1988) by identifying the compartments in which events occur more precisely. Newly synthesized HBsAg is translocated across to the lumen of the ER (Eble et al., 1986; Simon et al., 1988) where disulphidelinked dimers are formed rapidly (<2 min) in the presence of PDI. The concentration of dimers continues to increase until 60 min after the pulse. Our cycloheximide data indicates that the bulk of the HBsAg is segregated to the PDI excluding region at the time that dimers become the majority of the population. After the transport of dimers to a post-ER compartment (intermediate compartment), the absence of PDI allows the formation of oligomer crosslinks. Concomi- Figure 9 . Incubation of HBsAg with PDI in vitro. The autoradiograph in a shows the effect of incubating secreted, 35S-labeled HBsAg particles with increasing amounts of Sf9 cell lysate from cells infected with control virus (wiM type) or PDI-cDNA bearing virus. The plots in b (control virus-w/M type) and c (rat PDI--cDNA bearing-PDI) show the amounts of the different forms of HBsAg after incubation with varying amounts of cell lysate. The monomer (-A-) and dimer (--~-) components are defined as in Fig. 6 . The oligomer (--m-) components are the combination of the material that did not enter the separating gel (Wells) and the very high molecular weight material that enters the gel and disappears after DTT reduction. The specific effect of increased PDI expression in the lysate is the conversion of the oligomeric forms of HBsAg to dimers. tant with oligomer formation, HBsAg becomes resistant to protease digestion from the cytosolic side and becomes carbonate extractable (Simon et al., 1988) reflecting the budding of the oligomer into the lumen. Host membrane proteins are excluded from these oligomers, perhaps as a result of the tight interaction between HBsAg dimers. After budding into this compartment, the crosslinked, oligomeric, lipoprotein particles pass rapidly through the Golgi where they acquire endoglycosidase H resistance before their secretion into the medium.