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

BfA is a fungal metabolite which has been shown to have profound effects on the distribution of Golgi components. All characterized medial-and cis-Golgi markers have been shown to redistribute to the ER in the presence of BfA and to return to their normal distribution upon its removal (Doms et al., 1989; Fujiwara et al., 1988; Lippincott-Schwartz et al., 1989; Takatsuki and Tamura, 1985) . Immunofluorescent F) . Some of the structures in which I-IBsAg is concentrated and which exclude PDI are marked by arrowheads. The fraction of HBsAg that is found in the PDI-excluding compartment increases during the incubation. For comparison, the half time for secretion is ,~ 2 h (see Fig. 7 ). Bar, 10/zm. staining with 1D3 and a polyclonal antibody that recognizes mannosidase II (obtained from Brian Burke, Harvard Medical School) shows that BfA has a similar effect on the redistribution of Golgi markers in SV24 cells (data not shown). Double immunofluorescence on BfA-treated SV24 cells (Fig. 8) shows that BfA does not cause HBsAg in the PDI excluding compartment to recycle back to the ER, indicating that the HBsAg maturation compartment is not part of the Golgi complex. This behavior is reminiscent of the effect of BfA on the region of the intermediate compartment marked by the 72-kD KDEL binding protein (Vaux et al., 1990) or by rab2 (Chavrier et al., 1990) . In combination with the ex- shows the time course of intermolecular disulphide bond formation. The data plotted are optical densities of the scanned autoradiograph in arbitrary units (r~OD) normalized by the total counts taken from a reduced sample of the lysate at the same time point. The decrease in monomers (--*-) is followed by an increase in dimers (--~-) which then form oligomers which do not enter the gel. The half-time for particle secretion (2 h) is indicated by an arrow. Pulse-chase experiments, immunoprecipitation, and SDS-PAGE autoradiography were performed as described in Materials and Methods. The mobilities of the molecular weight markers are indicated.