Dual topology of the hepatitis B virus large envelope protein: determinants influencing post-translational pre-S translocation.
The large (L) envelope protein of the hepatitis B virus (HBV) has the peculiar capacity to form two transmembrane topologies via an as yet uncharacterized process of partial post-translational translocation of its pre-S domain across membranes. In view of a current model that predicts an HBV-specific channel generated during virion envelope assembly to enable pre-S translocation, we have examined parameters influencing L topogenesis by using protease protection analysis of wild-type and mutant L proteins synthesized in transfected cells. We demonstrate that contrary to expectation, all determinants, thought to be responsible for channel formation, are dispensable for pre-S reorientation. In particular, we observed that this process does not require (i) the helper function of the HBV S (small) and M (middle) envelope proteins, (ii) covalent dimer formation of envelope chains, or (iii) either of the three amphipathic transmembrane segments of L. Rather, the most hydrophobic transmembrane segment 2 of L was identified as a vital topogenic determinant, essential and sufficient for post-translational pre-S translocation. Cell fractionation studies revealed that pre-S refolding and thus the dual topology of L is established at the endoplasmic reticulum (ER) membrane rather than at a post-ER compartment as originally supposed. Together our data provide evidence to suggest that the topological reorientation of L is facilitated by a host cell transmembrane transport machinery such as the ER translocon.
Lambert
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C
Department of Medical Microbiology and Hygiene, Johannes Gutenberg-Universität Mainz, D-55101 Mainz, Germany.
Prange
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R
D000818:Animals; D019556:COS Cells; D019281:Dimerization; D004220:Disulfides / Q000737:chemistry; D011487:Protein Conformation; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D014759:Viral Envelope Proteins / Q000737:chemistry* / Q000378:metabolism*
D004220:Disulfides; C096362:L protein, hepatitis B virus; C077433:Pre-S protein, Duck hepatitis B virus; D014759:Viral Envelope Proteins