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

The conclusion that lipoprotein particle assembly occurs in an intermediate compartment is based on a combination of immunocytochemical and biochemical results. Using a cell line that stably expresses HBsAg, we showed that a fraction of the HBsAg was segregated to a region depleted of soluble ER proteins such as BiP and PDI, although it contained an established marker of an intermediate compartment, rab2. Quantitation of confocal microscope data showed that the exclusion of PDI was abrupt and suggested that the HBsAg enriched regions were distinct from the ER. The observation that the fraction of HBsAg segregated to this region increases during incubations with cycloheximide while the amount in the ER decreases shows that this region corresponds to a later and rate limiting step in HBsAg maturation. In agreement with previous work on other cell lines (Patzer et al., 1984) , our biochemical data showed that intracellular HBsAg remained endoglycosidase H sensitive and hence must be in a premedial Golgi compartment. This is reinforced by the immunocytochemical observation that treatment of cells with BfA has no effect on the morphology of the PDI excluding region in which HBsAg is concentrated. Since BfA is known to have a dramatic effect on the distribution of Golgi markers including those of the cis-Golgi, this shows that HBsAg is concentrated in a pre-Golgi compartment.