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

To test the consensus view that HBsAg monomers are assembled and bud as lipoprotein particles in the ER, we needed specific markers for this compartment. Fig. 1 shows a Western blot with four different antibodies specific for proteins residing in the ER against SV24 cell lysate. 1D3 is a mouse monoclonal IgG raised against the peptide KDDDQKAVKDEL, corresponding to the carboxy terminus of PDI. It recognizes a major protein band at a relative molecular mass of '~55 kD corresponding to PDI and another protein, calreticulin, which is one of the major calcium binding proteins in the ER lumen and migrates slightly more slowly on SDS-PAGE Vaux et al., 1990 ; Buck, P., D. J. Vaux, J. Tooze, R. Hendriks, and S. D. Fuller, manuscript in preparation). Anti-PDI is a polyclonal rabbit antiserum raised against purified bovine liver PDI which reacts specifically with PDI on Western blots of SV24 lysate and does not recognize calreticulin. 10C3, raised against the peptide KSEKDEL, which contains the carboxyterminal six amino acids of BiP, recognizes three major bands at "~99, 75, and 47 kD. The band at an apparent molecular mass of 75 kD is immunoglobulin heavy chain binding protein (BiP). BiP (Haas and Wabl, 1983) , also known as GRP78 (Shiu et al., 1977) , is an abundant soluble resident specifically recognize soluble proteins of the ER will be published elsewhere (Buck, P., D. J. Vaux, J. Tooze, R. Hendriks, and S. D. Fuller, manuscript in preparation) .