Structural analysis of the varicella-zoster virus gp98-gp62 complex: posttranslational addition of N-linked and O-linked oligosaccharide moieties

Abstract

Varicella-zoster virus specifies the formation of several glycoproteins, including the preponderant gp98-gp62 glycoprotein complex in the outer membranes of virus-infected cells. These viral glycoproteins are recognized and precipitated by a previously described monoclonal antibody designated monoclone 3B3. When an immunoblot analysis was performed, only gp98 was reactive with monoclone 3B3 antibody; likewise, titration in the presence of increased concentrations of sodium dodecyl sulfate during antigen-antibody incubations caused selective precipitation of gp98 but not gp62. Further structural analyses of gp98 were performed by using the glycosidases endo-beta-N-acetylglucosaminidase H (endoglycosidase H) and neuraminidase and two inhibitors of glycosylation (tunicamycin and monensin). In addition to gp98, antibody 3B3 reacted with several intermediate products, including gp90, gp88, gp81, and a nonglycosylated polypeptide, p73. Since gp98 was completely resistant to digestion with endoglycosidase H, it contained only complex carbohydrate moieties; conversely, gp81 contained mainly high-mannose residues. Polypeptide p73 was immunodetected in the presence of tunicamycin and designated as a nascent recipient of N-linked sugars, whereas gp88 was considered to contain O-linked oligosaccharides because its synthesis was not affected by tunicamycin. The ionophore monensin inhibited production of mature gp98, but other intermediate forms, including gp90, were detected. Since the latter product was similar in molecular weight to the desialated form of gp98, one effect of monensin treatment of varicella-zoster virus-infected cells was to block the addition of N-acetylneuraminic acid. Monensin also blocked insertion of gp98 into the plasma membrane and, as determined by electron microscopy, inhibited envelopment of the nucleocapsid and its transport within the cytoplasm. On the basis of this study, we reached the following conclusions: the primary antibody 3B3-binding epitope is located on gp98, gp98 is a mature product of viral glycoprotein processing, gp98 contains both N-linked and O-linked oligosaccharide side chains, gp90 is the desialated penultimate form of gp98, gp88 is an O-linked intermediate of gp98, gp81 is the high-mannose intermediate of gp98, and p73 is the unglycosylated precursor of gp98.