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

Abstract

AbstracL Expression of hepatitis B surface antigen (HBsAg), the major envelope protein of the virus, in the absence of other viral proteins leads to its secretion as oligomers in the form of disk-like or tubular lipoprotein particles. The observation that these lipoprotein particles are heavily disulphide crosslinked is paradoxical since HBsAg assembly is classically believed to occur in the ER, and hence in the presence of high levels of protein disulphide isomerase (PDI) which should resolve these higher intermolecular crosslinks. Indeed, incubation of mature, highly disulphide crosslinked HBsAg with recombinant PDI causes the disassembly of HBsAg to dimers. We have used antibodies against resident ER proteins in double immunofluorescence studies to study the stages of the conversion of the HBsAg from individual protein subunits to the secreted, crosslinked, oligomer. We show that HBsAg is rapidly sorted to a post-ER, preGolgi compartment which excludes PDI and other major soluble resident ER proteins although it overlaps with the distribution of rab2, an established marker of an intermediate compartment. Kinetic studies showed that disulphide-linked HBsAg dimers began to form during a short (2 rain) pulse, increased in concentration to peak at 60 rain, and then decreased as the dimers were crosslinked to form higher oligomers. These higher oligomers are the latest identifiable intracellular form of HBsAg before its secretion (h/2 = 2 h). Brefeldin A treatment does not alter the localization of HBsAg in this PDI excluding compartment, however, it blocks the formation of new oligomers causing the accumulation of dimeric HBsAg. Hence this oligomerization must occur in a pre-Golgi compartment. These data support a model in which rapid dimer formation, catalyzed by PDI, occurs in the ER, and is followed by transport of dimers to a pre-Golgi compartment where the absence of PDI and a different lumenal environment allow the assembly process to be completed.NVELOPED animal viruses have been major tools in the characterization of the secretory pathway and in advancing our understanding of the mechanisms of protein sorting. Frequently, particular features of the pathways of protein traffic are exploited in the maturation pathways of different viruses so that a study of virus assembly leads to unexpected rewards in terms of an understanding of intracellular transport. This paper presents a study of the assembly of the lipoprotein particle formed by the transmembrane glycoprotein of hepatitis B virus. This particle has the unusual feature of being stabilized by extensive intermolecutax crosslinks and here we show that this reflects its assembly in an intermediate compartment between the ER and the Golgi.Hepatitis B virus (HBV) ~ is a small (440/~) DNA virus which is the type virus of the hepadnaviridae (hepatotropic DNA viruses). HBV consists of a nucleoprotein core containing a relatively small DNA genome (3,200 bp) surrounded by a protein capsid composed of the core protein, hepatitis B core antigen. The nucleocapsid i...

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