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Journal of Virology, May 2005, p. 5346-5352, Vol. 79, No. 9
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.9.5346-5352.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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

Elizabeth V. L. Grgacic* and David A. Anderson

Macfarlane Burnet Institute for Medical Research and Public Health and Australian Centre for Hepatitis Virology, Melbourne, Australia

Received 23 August 2004/ Accepted 14 December 2004

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%.

* Corresponding author. Mailing address: Macfarlane Burnet Institute for Medical Research and Public Health, Melbourne 3004, Australia. Phone: 61 3 9282 2109. Fax: 61 3 9282 2100. E-mail: grgacic{at}burnet.edu.au.

Journal of Virology, May 2005, p. 5346-5352, Vol. 79, No. 9
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.9.5346-5352.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.





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