Identification of structural determinants of the first transmembrane domain of the small envelope protein of duck hepatitis B virus essential for particle morphogenesis

Role of transmembrane domains of hepatitis B virus small surface proteins in subviral-particle biogenesis

The hepatitis B virus (HBV) surface proteins not only are incorporated into the virion envelope but in addition form subviral particles (SVP) consisting solely of surface proteins and lipids. Heterologous expression of the small HBV envelope protein S produces secreted spherical SVP 20 nm in diameter, with approximately 100 S molecules per particle. The pathway leading from the initial S translation product as a multispanning transmembrane protein to the final SVP is largely unknown. To investigate the role of the four transmembrane domains (TM) of S in this process, we introduced mutations in these regions and characterized their effects on SVP formation in transfected Huh7 cells. We found that the insertion of one amino acid in the center of the α-helix of TM1 or the exchange of TM1 with a heterologous TM blocked SVP release and SVP formation by coexpressed wild-type S chains in a transdominant negative fashion. Surprisingly, this effect was partially neutralized when the mutations were expressed in the background of the HBV surface protein M, suggesting that mutations in TM1 could partially be complemented by the pre-S2 domain. The exchange of TM2 with heterologous TMs that form α-helices of the same lengths was also incompatible with SVP formation. However, these mutants no longer blocked SVP formation by coexpressed wild-type S. We conclude that TM2 is essential for the stable assembly of S chains by establishing intramembrane interactions.

Hepatitis B virus morphogenesis

The hepatitis B virus (HBV) particle consists of an envelope containing three related surface proteins and probably lipid and an icosahedral nucleocapsid of approximately 30 nm diameter enclosing the viral DNA genome and DNA polymerase. The capsid is formed in the cytosol of the infected cell during packaging of an RNA pregenome replication complex by multiple copies of a 21-kDa C protein. The capsid gains the ability to bud during synthesis of the viral DNA genome by reverse transcription of the pregenome in the lumen of the particle. The three envelope proteins S, M, and L shape a complex transmembrane fold at the endoplasmic reticulum, and form disulfide-linked homo- and heterodimers. The transmembrane topology of a fraction of the large envelope protein L changes post-translationally, therefore, the N terminal domain of L (preS) finally appears on both sides of the membrane. During budding at an intracellular membrane, a short linear domain in the cytosolic preS region interacts with binding sites on the capsid surface. The virions are subsequently secreted into the blood. In addition, the surface proteins can bud in the absence of capsids and form subviral lipoprotein particles of 20 nm diameter which are also secreted.

A hydrophobic domain in the large envelope protein is essential for fusion of duck hepatitis B virus at the late endosome

The duck hepatitis B virus (DHBV) envelope is comprised of two transmembrane (TM) proteins, the large (L) and the small (S), that assemble into virions and subviral particles. Secondary-structure predictions indicate that L and S have three alpha-helical, membrane-spanning domains, with TM1 predicted to act as the fusion peptide following endocytosis of DHBV into the hepatocyte. We used bafilomycin A1 during infection of primary duck hepatocytes to show that DHBV must be trafficked from the early to the late endosome for fusion to occur. Alanine substitution mutations in TM1 of L and S, which lowered TM1 hydrophobicity, were used to examine the role of TM1 in infectivity. The high hydrophobicity of the TM1 domain of L, but not of S, was shown to be essential for virus infection at a step downstream of receptor binding and virus internalization. Using wild-type and mutant synthetic peptides, we demonstrate that the hydrophobicity of this domain is required for the aggregation and the lipid mixing of phospholipid vesicles, supporting the role of TM1 as the fusion peptide. While lipid mixing occurred at pH 7, the kinetics of insertion of the fusion peptide was increased at pH 5, consistent with the location of DHBV in the late-endosome compartment and previous studies of the nonessential role of low pH for infectivity. Exchange of the TM1 of DHBV with that of hepatitis B virus yielded functional, infectious DHBV particles, suggesting that TM1 of all of the hepadnaviruses act similarly in the fusion mechanism.

St, a truncated envelope protein derived from the S protein of duck hepatitis B virus, acts as a chaperone for the folding of the large envelope protein

Envelope proteins of hepadnaviruses undergo a unique folding mechanism which results in the posttranslational translocation of 50% of the large envelope protein (L) chains across the endoplasmic reticulum. This mechanism is essential for the eventual positioning of the receptor-binding domain on the surface of the virus particle and in duck hepatitis B virus (DHBV) is dependent on the small (S) envelope protein as part of the assembly process. In this study, we report the identification of a third envelope protein, St, derived from the S protein and carrying functions previously attributed to S. Antibody mapping and mutagenesis studies indicated St to be C terminally truncated, spanning the N-terminal transmembrane domain (TM1) plus the adjacent cysteine loop. We have previously shown that the mutation of two conserved polar residues in TM1 of S (SAA) eliminates L translocation and assembly. A plasmid expressing a functional equivalent of St was able to rescue assembly, demonstrating that this assembly defect is due to mutations of the corresponding residues in St and not in S per se. Immunofluorescence analysis showed that St directly affects L protein cellular localization. These results indicate that St acts as a viral chaperone for L folding, remaining associated with the DHBV envelope upon secretion. The presence of St at a molar ratio of half that of L suggests that it is St which regulates L translocation to 50%.

Envelopment of the hepatitis B virus nucleocapsid

The hepatitis B virus (HBV) is an enveloped DNA virus with an icosahedral capsid replicating via reverse transcription. The crystal structure of the capsid is known. It has a diameter of 36 nm and is formed by one protein species (C protein). The viral envelope contains three different coterminal proteins (S, M, and L proteins) spanning the membrane several times. These proteins are not only released from infected cells as components of the viral envelope but in 10,000-fold excess as subviral lipoprotein particles with a diameter of 22 nm containing no capsid. Assembly of the capsid occurs in the cytosol and results in packaging of a 3.5 kb RNA molecule together with viral and cellular factors. This newly formed capsid cannot be enveloped. Rather, synthesis of the viral DNA genome in the lumen of the capsid by reverse transcription is required to induce a budding competent state. Envelopment then takes place at an intracellular membrane of the pre-Golgi compartment. The S and the L protein, but not the M protein, is required for this process. The L protein forms two different transmembrane topologies. The isoform exposing the N-terminal part at the cytosolic side of the membrane is essential for budding. In this domain, a 22 amino acid (aa) long linear stretch has been mapped genetically to play a vital role in the morphogenetic process. This domain probably mediates the contact to the capsid. A second matrix domain was mapped to the cytosolic loop of the S protein. A similar genetic approach identified two small areas on the capsid surface, which might interact with the envelope proteins during envelopment.

Assessment of determinants affecting the dual topology of hepadnaviral large envelope proteins

For functional diversity, the large (L) envelope protein of hepatitis B virus (HBV) acquires a dual transmembrane topology via co-translational membrane integration of the S region and partial post-translational translocation of the preS subdomain. Because each process requires the second transmembrane segment (TM2), we explored the action of this determinant by using protease protection analysis of mutant L proteins. We demonstrated that neither the disruption of a leucine zipper-like motif by multiple alanine substitutions nor the flanking charges of TM2 affected the topological reorientation of L. The dispensability of both putative subunit interaction modules argues against a link between preS post-translocation and envelope assembly. Phenotypic mixing experiments revealed that the preS and S protein domains of the related duck HBV L polypeptide failed to substitute functionally for the topogenic elements of HBV in directing the correct L topogenesis, implicating different translocation mechanisms used by the two hepadnavirus genera.