N-linked glycans of Bombyx mori nucleopolyhedrovirus fibroblast growth factor are crucial for its secretion

Bombyx mori nucleopolyhedrovirus (BmNPV) fibroblast growth factor (BmFGF) is a glycosylated protein that is efficiently secreted into the medium. Here, we constructed mutant NPVs expressing His-tagged wild-type (wt) or mutant BmFGFs and showed that the two residues, asparagine 44 and 171, are the glycosylation sites of BmFGF. Also, removal of N-linked glycans from BmFGF resulted in almost complete inhibition of the secretion into the medium, suggesting that N-linked glycans of BmFGF are required for its secretion. These residues are not conserved in closely related Autographa californica NPV (AcMNPV)-encoded vFGF (AcFGF). Western blot analysis suggested that AcFGF is not glycosylated and is poorly secreted. A mutant AcFGF possessing two N-linked glycosylation sites was secreted into the medium more abundantly than that which occurred for wt AcFGF. This is the first direct evidence showing the role of N-linked glycans in the secretion process of a baculovirus protein.