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...

The data provide a molecular explanation for HCV genotype 3-specific lipid accumulation. This difference between genotypes may be due to phenylalanine having a higher affinity for lipids than tyrosine (Y). These observations provide useful information for further studies of the mechanisms involved in HCV-induced steatosis.