Hepatitis B Virus Subviral Envelope Particle Morphogenesis and Intracellular Trafficking

Abstract

Hepatitis B virus (HBV) is unusual in that its surface proteins (small [S], medium, and large [L]) are not only incorporated into the virion envelope but they also bud into empty subviral particles in great excess over virions. The morphogenesis of these subviral envelope particles remains unclear, but the S protein is essential and sufficient for budding. We show here that, in contrast to the presumed model, the HBV subviral particle formed by the S protein self-assembles into branched filaments in the lumen of the endoplasmic reticulum (ER). These long filaments are then folded and bridged for packing into crystal-like structures, which are then transported by ER-derived vesicles to the ER-Golgi intermediate compartment (ERGIC). Within the ERGIC, they are unpacked and relaxed, and their size and shape probably limits further progression through the secretory pathway. Such progression requires their conversion into spherical particles, which occurred spontaneously during the purification of these filaments by affinity chromatography. Small branched filaments are also formed by the L protein in the ER lumen, but these filaments are not packed into transport vesicles. They are transported less efficiently to the ERGIC, potentially accounting for the retention of the L protein within cells. These findings shed light on an important step in the HBV infectious cycle, as the intracellular accumulation of HBV subviral filaments may be directly linked to viral pathogenesis.The human hepatitis B virus (HBV) is the prototype of the mammalian Hepadnaviridae (genus Orthohepadnavirus), a family of small hepatotropic DNA viruses causing acute and chronic liver disease (16). Despite the existence of an effective HBV vaccine, HBV remains a major health problem worldwide, as there is no generally effective treatment for the estimated 350 million chronic carriers who have a high risk of liver cirrhosis and hepatocellular carcinoma. A thorough understanding of HBV morphogenesis and life cycle is thus required for the development of innovative antiviral treatments. The virion (or Dane particle) is a spherical particle, 42 nm in diameter, consisting of an icosahedral nucleocapsid of approximately 30 nm in diameter and an envelope composed of three surface proteins and, presumably, lipids of host cell origin. The nucleocapsid and envelope are synthesized and mature separately in different cellular compartments, subsequently interacting to form the virion (5, 26, 34). The three HBV envelope proteins are encoded by a single open reading frame, using three in-frame start codons (21). The large surface protein (L, or p39) is the translation product of the entire open reading frame (389 to 400 amino acid [aa] residues, depending on HBV genotype). The middle surface protein (M, or p30) lacks the N-terminal 119 aa of L (the pre-S1 sequence), and the small surface protein (S, or p24) lacks the N-terminal 55 aa of M (the pre-S2 sequence). These proteins are synthesized at the endoplasmic reticulum (ER) membrane and have a complex transmembra...

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

“…The protein envelope present on HBV surface consists of three protein types: Small (S), Medium (M) and Large (L) proteins. The S protein (HBsAg) represents the main component of viral envelope and of subviral particles (SVP) [4,5] . SVP are seen in the form of spherical or elongated tubular structures of various lengths and 20-22 nm in diameter [4,5] .…”

Hepatitis B Surface Antigen-based Vaccines for Human Neoplastic Disease

“…HBsAg-S alone is sufficient for the morphogenesis of (ii) filamentous rods and (iii) spherical 20nm subviral particles, or VLPs (Patient et al, 2007).…”

“…HBsAg contains signal sequences which insert HBsAg into the ER where it is targeted to the secretory pathway (Bruss, 2004;Patient et al, 2007). Such targeting has been previously suggested to be a cause of the enhanced immunogenicity of HBsAg molecules (Minev et al, 2000;Sherritt et al, 2001;Xu et al, 2005a;Isaji et al, 2009).…”

“…The filamentous rods of HBsAg-S were suggested to be a source of spherical 22nm VLPs (Patient et al, 2007). Filamentous HBsAg particles formed at the ER and budded into vesicles for transport to large cisternae, which were assumed to be ERGIC.…”