The role of the downstream signal sequences in the maturation of the HBV middle surface glycoprotein: Development of a novel therapeutic vaccine candidate

The signal sequences that mediate entry of the hepatitis B virus (HBV) envelope proteins into the endoplasmic reticulum (ER) are located within the S domain at positions 11–32 and at positions 80–98 (from the start of the S domain). In addition, hydrophobic patches at positions 160–184 and 189–210 of the S domain may also be involved in entry into the ER. The role of each of these domains in the entry of the HBV M glycoprotein into the ER was studied by deletion mutations of each of the signal sequences. Glycosylation of proteins was used as a marker of entry into the ER. Our results indicate that association with the ER could not be prevented by the deletion of either individual or combinations of the HBV signal sequences. M protein lacking signal sequence I was able to enter the ER and had limited secretion. In contrast, M protein lacking signal sequence II could not be secreted but still entered the ER. M protein lacking signal sequences I and II, while still associated with the ER, was rapidly degraded by the cytosolic proteasome. The potential use of such a vector as a CTL vaccine was tested through an in vitro antigen presentation assay. In this assay, a DNA vaccine candidate lacking signal sequences I and II lead to a > 6-fold increase in CTL activation, as compared to the vector expressing wild type M protein. These results suggest that increased degradation of the HBV envelope proteins can lead to enhanced antigen presentation.