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

Our conclusion that the assembly of HBsAg dimers into crosslinked oligomeric, lipoprotein particles occurs in a separate compartment rests on several assumptions about the nature of compartments along the secretory pathway. We use the word "compartment" to mean a region that lies on the secretory pathway, is distinct in composition from the compartments which precede and follow it, and is morphologically distinct from other compartments. Clearly, the compartment in which oligomer crosslinks are formed in the HBsAg lipoprotein particle lies on the secretory pathway since the overwhelming majority of the HBsAg is eventually secreted from cells. The composition of this compartment is distinct from that of the ER since it lacks soluble ER proteins and from that of the Golgi since it lacks the enzymes responsible for conferring endoglycosidase H resistance. The morphological separation of the compartment is demonstrated by our immunocytochemistry which shows that the region from which PDI is excluded is far larger than that corresponding to an HBsAg particle and by the observation that HBsAg segregation is unaffected by BfA treatment.