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

¹ State Key Laboratory of Virology, Key Laboratory of Molecular Virology and Joint Laboratory of Invertebrate Virology, Wuhan Institute of Virology, Chinese Academy of Sciences, Wuhan 430071, People's Republic of China
² Laboratory of Virology, Wageningen University, Binnenhaven 11, 6709 PD Wageningen, The Netherlands

Correspondence
Just M. Vlak
just.vlak{at}wur.nl

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 (F₀) and cleaved post-translationally into two disulfide-bonded subunits, F₁ (C-terminal, large subunit) and F₂ (N-terminal, small subunit). Recently, N-linked glycosylation of the F₁ and F₂ 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 F₂ 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 F₂ subunit of HearNPV by site-directed mutagenesis. The only putative N-linked glycosylation site in F₂ was eliminated by mutating asparagine (N¹⁰⁴) 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 F₂. 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 F₂ subunit is not essential for viral infectivity and suggest that it is involved in BV production and fusogenicity.

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