Evidence that N-linked glycosylation is necessary for hepatitis B virus secretion.
Human hepatitis B virus (HBV) envelopes contain three distinct glycoproteins called L, M, and S HBsAg. Each is posttranslationally modified to contain N-linked oligosaccharides. N-linked oligosaccharides, after attachment to a polypeptide backbone, are processed by enzymes within the endoplasmic reticulum (ER). There is uncertainty about what role, if any, these N glycans and their modification in the ER play in the function of the HBV envelope proteins. By treating hepatoblastoma cultures which secrete HBV (HepG 2.2.15 cells) with inhibitors of different steps of the glycosylation and glycan modifying pathway, we provide evidence that glycosylation and the first step in the processing pathway are necessary for virion, but not subviral particle, secretion. That is, using a highly sensitive immunoprecipitation/polymerase chain reaction system, enveloped HBV could not be detected in the medium of HepG2.2.15 cells incubated with tunicamycin. However, HBV subviral particle secretion was not prevented by tunicamycin. Moreover, inhibitors of alpha-glucosidase I (the first step in the glycan processing pathway) also prevented virion secretion. Inhibitors of mannose trimming (a later step) and glycolipid synthesis, did not prevent virion secretion, defining the limits of the glycosylation requirements in secretion. These results demonstrate a requirement for N-glycosylation and glucosidase processing in the secretion of virions and further distinguish between the requirements for virion and subviral particle secretion.