Characterization of early hepatitis B virus surface protein oligomers

The small surface protein (S) of the hepatitis B virus (HBV) is synthesized as unglycosylated p24 and N-glycosylated gp27 and forms disulfide linked dimers. Former models proposed that these complexes consist preferentially of p24-gp27 heterodimers. Furthermore, cell free in vitro experiments suggested that p24 has a transmembrane topology different from gp27. We tested these models by expressing the HBV surface proteins in transfected cell cultures and characterizing early maturation products after short pulse labelings. Two dimensional unreduced-reduced polyacrylamide gel electrophoresis demonstrated that p24 and gp27 dimerized without preference for a specific pairing. Protease protection experiments showed that both, p24 and gp27, had identical transmembrane topologies in cell culture. The middle sized (M) and large HBV surface proteins formed mixed dimers with the S protein. Mutant M and S protein in which all 10 cysteine residues in the ectodomain and transmembrane regions were replaced by serine residues formed no intermolecular S-S bridges but were secreted like wild type M and S protein.