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

The finding that assembly of HBsAg dimers into the secreted lipoprotein particle occurs in an intermediate compartrnent between the ER and the Golgi ascribes a new role to this compartment in biosynthesis. Saraste and Kuismanen (1984) first identified this compartment by showing that at 15~ the transport of viral membrane glycoproteins from the ER to the Golgi complex in BHK cells was blocked reversibly. The protein could leave the ER but failed to enter the Golgi, accumulating in a tubulovesicular compartment distinct from the ER and the Golgi. An important and, to many workers, defining function is the retrieval of the soluble resident ER proteins containing the KDEL retention sequence at their carboxy termini, such as PDI and BiP (Munro and Pelham, 1987) . Munro and Pelham (1987) hypothesized the existence of a salvage compartment from which escaped KDEL terminated proteins could be retrieved by recycling to the ER. Since the extensive disulphide crosslinking is a consequence of the exclusion of PDI it is likely that the compartment in which oligomer crosslinks form is either identical to or later than the salvage compartment. We have also shown here that rab2, an established marker of an intermediate compartment (Chavier et al., 1990) is found in the HBsAg enriched, PDI excluding regions. This salvage compartment has been proposed to be the same as the 15~ compartment (Lippincott-Schwartz, 1990; Pelham, 1988 Pelham, , 1989 Vaux et al., 1990 , Warren, 1987 , although colocalization with a viral glycoprotein under conditions of 15~ block has not yet been published. Although a number of authors refer to "the" ER-Golgi intermediate compartment, the diversity of functions that have been attributed to it open the possibility that this compartment is heterogeneous. An ER-Golgi intermediate compartment has been identified as the site of the interaction of MHC class I with antigen (Cox et al., 1989; Nuchtern et al., 1989; Yewdell et al., 1989) , and several proteins including p53 and p58, two transmembrane proteins of still unknown function (Schweizer et al., 1988; Saraste and Svensson, 1991) , have been localized to it. Tooze et al. (1988) showed that the first step of the O-glycosylation of a coronavirus glycoprotein, the addition of N-acetylgalactosamine, occurred in the same ER-Golgi intermediate compartment as budding of viral particles. Palmitoylation of proteins (Bonatti et al., 1989 ) and the initial step in the formation of the lysosomal mannose-6-phosphate targeting signal, the transfer of N-acetylglucosamine-l-phosphate from UDP-N-acetylgalactosamine-l-phosphate to newly synthesized lysosomal enzymes have also been shown to occur in a post-ER pre-Golgi compartment (Rizzolo et al., 1985; Kornfeld, 1988, Komfeld and Mellman, 1989; Lazzarino and Gabel, 1988; Pelham, 1988) . Recent work shows that these functions may be in distinct compartments between the ER and the Golgi complex. Schweizer et al. (1991) isolated a fraction from Veto cells that is enriched in p53 and has properties different from either the ER or the cis-Golgi. Further, Bonatti et al. (1989) demonstrated that protein palmitoylation occurs in a compartrnent which is later than the 15~ block but before mannosidase I trimming of glycans. We are pursuing experiments to further characterize the compartment which is involved in the maturation of HBsAg lipoprotein particles and to relate it to other markers of this region.