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

The demonstrated ability of BfA treatment to block exit of newly synthesized proteins from the ER while allowing ER functions to continue (Lippincott-Schwartz et al., 1989; Klausner et al., 1992) allowed us to separate the formation of the stages of HBsAg maturation which occur in the ER from later ones. The time course of dimer formation was followed by pulse-chase analysis of cells in the presence of BfA (Fig. 8, c and d) . In the presence of BfA, newly synthesized HBsAg is converted from monomers to dimers, however, the dimers accumulate and do not proceed to form oligomer crosslinks. The rate of dimer formation is approximately the same as in the non-BfA-treated cells represented in Fig. 6 . Hence the formation of oligomer crosslinks in HBsAg requires exit from the ER. This distinguishes the case of HBsAg from that of smaller disulphide-linked oligomers such as influenza haemagglutinin in which all disulphide bond formation is completed in the ER and required for exit from it.