Virus-like particles: passport to immune recognition.

Virus-like particles (VLPs) are formed by the self-assembly of envelope and/or capsid proteins from many viruses. In many cases such VLPs have structural characteristics and antigenicity similar to the parental virus, and some have already proven successful as vaccines against the cognate virus infection. The structural components of some VLPs have also proven amenable to the insertion or fusion of foreign antigenic sequences, allowing the production of chimeric VLPs exposing the foreign antigen on their surface. Other VLPs have been used as carriers for foreign antigens, including non-protein antigens, via chemical conjugation. This review outlines some of the advantages, disadvantages, and technical considerations for the use of a wide range of VLP systems in vaccine development.

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%.

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

The envelope of duck hepatitis B virus (DHBV) consists of the small (S) and large (L) envelope proteins, which share a common C-terminal multispanning transmembrane region but differ by the long N-terminal pre-S domain of L, which is essential for interactions with both the receptor and nucleocapsid. To achieve these dual functions, L acquires mixed topologies through S-dependent post-translational translocation of its pre-S domain. This study has examined the role of S in this unusual mechanism of translocation by analysis of the alpha-helical transmembrane domains and their potential to engage in lateral interactions for envelope assembly. Through mutagenesis in constructs expressing the S and L envelope proteins independently, transmembrane domain 1 was identified as an essential structural determinant in S. Two polar residues in this helix were identified as contributing to L protein translocation through the assembly of S into particles, implying that the topological switch of L is part of the assembly and maturation process. The same domain in L was shown to be dispensable for L translocation and assembly, suggesting that transmembrane domain 1 of L and S have different functional roles and structural arrangements on the assembled particle. The conservation in all hepadnavirus envelope proteins of two polar residues at positions 24 and 27 of transmembrane domain 1, the former positively charged, points to this being a common determinant in particle morphogenesis for all hepadnaviruses.