Absence of N-linked glycans from the F2 subunit of the major baculovirus envelope fusion protein F enhances fusogenicity.

The F protein is the major glycoprotein present in the envelopes of budded virus (BV) of members of the family Baculoviridae. The F protein mediates low-pH-activated fusion with insect cell membranes. Baculovirus F proteins are synthesized as a precursor (F(0)) and cleaved post-translationally into two disulfide-bonded subunits, F(1) (C-terminal, large subunit) and F(2) (N-terminal, small subunit). Recently, N-linked glycosylation of the F(1) and F(2) subunits of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) was demonstrated [Long, G., Westenberg, M., Wang, H., Vlak, J. M. & Hu, Z. (2006). J Gen Virol 87, 839-846]. Sequence analysis frequently predicts that one or more N-linked glycosylation sites are present in the F(2) subunit of baculovirus F proteins. N-glycans on envelope fusion proteins are usually required for proper conformational integrity and biological function, such as infectivity. This study examined the importance of N-linked glycosylation of the F(2) subunit of HearNPV by site-directed mutagenesis. The only putative N-linked glycosylation site in F(2) was eliminated by mutating asparagine (N(104)) to glutamine (Q), resulting in the mutant HearNPV(fN104Q). When inserted into an f-null HearNPV and a gp64-null bacmid of Autographa californica multiple nucleopolyhedrovirus, infectious BV could be retrieved that contained unglycosylated F(2). The virulence of HearNPV(fN104Q) was enhanced, as BV was produced earlier after infection and yielded larger plaques than f-null HearNPV repaired with the wild-type f gene. HearNPV(fN104Q) BV also induced much more efficient low-pH-activated syncytium formation. These results indicate that N-linked glycosylation of the HearNPV baculovirus F(2) subunit is not essential for viral infectivity and suggest that it is involved in BV production and fusogenicity.