Biosynthesis of glycoproteins E and I of feline herpesvirus: gE-gI interaction is required for intracellular transport.

The biosynthesis of glycoproteins E and I of feline herpesvirus was studied by using the vaccinia virus vTF7-3 expression system. gE and gI were synthesized as N-glycosylated, endoglycosidase H (EndoH)-sensitive precursors with Mrs of 83,000 and 67,000, respectively. When coexpressed, gE and gI formed sodium dodecyl sulfate-sensitive hetero-oligomeric complexes that were readily transported from the endoplasmic reticulum (ER). Concomitantly, the glycoproteins acquired extensive posttranslational modifications, including O glycosylation, leading to an increase in their apparent molecular weights to 95,000 and 80,000 to 100,000 for gE and gI, respectively. In the absence of gE, most gI remained EndoH sensitive. Only a minor population became EndoH resistant, but these molecules were processed aberrantly as indicated by their Mrs (100,000 to 120,000). By immunofluorescence microscopy, gI was detected primarily in the ER but also at the plasma membrane. gE, when expressed by itself, remained EndoH sensitive and was found only in the ER and the nuclear envelope. These results were corroborated by studying the biosynthesis of gE in feline herpesvirus (FHV)-infected cells. In cells infected with wild-type FHV, gE acquired the same co- and posttranslational modifications as during vTF7-3-driven expression. However, an FHV mutant lacking gI failed to produce mature gE. We conclude that gE is retained in the ER, presumably by associating with molecular chaperones, and becomes transport competent only when in a complex with gI.