Assembly and budding of a hepatitis B virus is mediated by a novel type of intracellular vesicles

Abstract

Formation of enveloped viruses involves assembly and budding at cellular membranes. In this study, we elucidated the morphogenesis of hepadnaviruses on the ultrastructural and biochemical level using duck hepatitis B virus (DHBV) as a model system. Formation of virus progeny initiates at the endoplasmic reticulum (ER) and is conserved both in vitro and in vivo. The morphogenesis proceeds via membrane-surrounded vesicles containing both virions and subviral particles, indicating a common morphogenetic pathway. The virus particle-containing vesicles (VCVs) are generated and maintained by reorganization of endomembranes accompanied by a striking disorganization of the rough ER (rER). DHBV-infected hepatocytes produce not only complete virions, but also large quantities of subviral particles (SVPs). SVPs are devoid of the nucleocapsid and viral DNA, 4,5 but contain the envelope proteins. The extracellular DHB virion has a spherical structure of approximately 45 to 65 nm and contains an electron-dense nucleocapsid of about 27 nm. 6 Little is known about the morphogenesis of this virus family, but several of the following features indicate that this process is likely to be unique and complex: the DHB viral envelope proteins are multispanning transmembrane proteins, encoded by a single open reading frame, whose differential transcription results in the large (L) and the small (S) surface protein. 7 Both proteins share an identical carboxyterminal region, representing the S protein, with an additional Nterminal extension forming the preS-region of L. The ratio between the L and S proteins in the viral particles is about 1:4. 8 As the major structural component of the envelope, the S protein determines envelope curvature and is indispensable for both budding and secretion of viral particles. 9 Both surface proteins are synthesized at the rough endoplasmic reticulum (ER), or rER, and then bud into the ER lumen, forming SVPs. This budding event is autonomous and independent of all other viral components (e.g., nucleocapsid). 10,11 The intrinsic membrane-deforming activity of hepadnaviral envelope proteins is host cell-independent and conserved from yeast to

Hepatocytes Traffic and Export Hepatitis B Virus Basolaterally by Polarity-Dependent Mechanisms

“…While it has been suggested that subviral particles in nonhepatocyte (MDCK) cells traffic by different routes to HBV particles (18), this would appear to be a function of the cells rather than the antigen because it is contrary to the mass export of both HBsAg and HBV virions into plasma from hepatocytes. In hepatocytes, synthesized DHBV and subviral particles have been shown to enter the same type of vesicular bodies after the endoplasmic reticulum (20), which reinforces the need to use hepatocyte-derived cells for the current studies.…”

Ultrastructural Evidences of Hepatitis B Infection in Human Liver Biopsies Disclose Complex Assembly and Morphogenesis Pathways for Hepatitis B Virus

“…Thus, they may represent newly forming intermediates in either the degradation pathway or the HBV virion maturation. This is in agreement with a recent report showing assembly and budding of virion particles into a novel type of intracellular vesicles using duck HBV as a model system [23] . These vesicles derived from ER having unique identity and properties of ER, intermediate compartment, endosomes and MVB.…”

“…Some of these particles consist of empty particles with no electron-dense nucleocapsid possibly representing either defective interfering particles [27] or SVP [23] . In addition, enveloped particles with an electron-dense nucleocapsid likely corresponding to virions were observed.…”

Hepatitis B virus infection: An insight into infection outcomes and recent treatment options

“…The S protein plays a major role in the secretion and the budding of virus particles. Budding of surface proteins without the nucleocapsids and virus genome into the ER lumen results in the production of sub-viral particles (SVPs) [24].…”

“…Mature nucleocapsids containing rcDNA genomes are then recycled to the nucleus to form a pool of cccDNA ( Fig. 1), ranging from 30 to 50 copies of cccDNA per cell [21], or are exported from the cell via the endoplasmic reticulum (ER), where nucleocapsids are enveloped in surface proteins embedded in host-derived lipid [22][23][24].…”