The large hepatitis B virus (HBV) surface protein (L) forms two isomers which display their N-terminal pre-S domain at the internal and external side of the viral envelope, respectively. The external pre-S domain has been implicated in binding to a virus receptor. To investigate functions of the internal pre-S domain, a secretion signal sequence was fused to the N terminus of L (sigL), causing exclusive expression of external pre-S domains. A fusion construct with a nonfunctional signal (s25L), which corresponds in its primary sequence to sigL cleaved by signal peptidase, was used as a control. SigL was N glycosylated in transfected COS cells at both potential sites in pre-S in contrast to s25L or wild-type L, confirming the expected transmembrane topologies of sigL and s25L. Phenotypic characterization revealed the following points. (i) SigL lost the inhibitory effect of L or s25L on secretion of subviral hepatitis B surface antigen particles, suggesting that the retention signal mapped to the N terminus of L is recognized in the cytosol and not in the lumen of the endoplasmic reticulum. (ii) SigL was secreted into the culture medium even in the absence of the major HBV surface protein (S), while release of an L mutant lacking the retention signal was still dependent on S coexpression. (iii) s25L but not sigL could complement an L-negative HBV genome defective for virion secretion in cotransfections. This suggests that the cytosolic pre-S domain, like a matrix protein, is involved in the interaction of the viral envelope with preformed cytosolic nucleocapsids during virion assembly.

The hepatitis B virus (HBV) envelope and the subviral lipoprotein particles contain three viral surface proteins (L, M, and S) which are expressed from one open reading frame by the usage of three start codons and a common stop codon. The largest surface protein L has some unusual properties. It adopts two different transmembrane topologies due to a posttranslational switch of the folding in approximately half of the L proteins. L molecules which expose their N-terminal preSl domain on the viral particle surface are probably ligands for a putative virus receptor and determine the species specificity and liver tropism of this virus. L chains with internal preSl domains are required in virion morphogenesis and mediate the contact to the nucleocapsid like a matrix protein. Overexpression of this form of the L protein is also responsible for the inhibition of viral particle release. This short review summarizes our knowledge on the biosynthesis and maturation of the HBV surface proteins and their functions in viral particle morphogenesis with special emphasis on the L protein.

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