Mutational and functional analysis of N-linked glycosylation of envelope fusion protein F of Helicoverpa armigera nucleopolyhedrovirus

The envelope fusion (F) protein of baculoviruses is a heavily N-glycosylated protein that plays a
significant role in the virus infection cycle. N-Linked glycosylation of virus envelope glycoprotein
is important for virus envelope glycoprotein folding and its function in general. There are six
predicted N-glycosylation sites in the F (HaF) protein of Helicoverpa armigera
nucleopolyhedrovirus (HearNPV). The N-glycosylation site located in the F2 subunit (N104) of
HaF has been identified and functionally characterized previously (Long et al., 2007). In this
study, the other five potential N-glycosylation sites located in the HaF1 subunit, namely, N293,
N361, N526, N571 and N595, were analysed extensively to examine their N-glycosylation and
relative importance to the function of HaF. The results showed that four of these five potential
glycosylation sites in the F1 subunit, N293, N361, N526 and N571, were N-glycosylated in
F proteins of mature HearNPV budded viruses (BVs) but that N595 was not. In general, the
conserved site N526 was critical to the functioning of HaF, as absence of N-glycosylation of
N526 reduced the efficiency of HaF folding and trafficking, consequently decreased
fusogenicity and modified the subcellular localization of HaF proteins, and thus impaired virus
production and infectivity. The absence of N-glycosylation at other individual sites was found to
have different effects on the fusogenicity and subcelluar distribution of HaF proteins in HzAM1
cells. In summary, N-glycosylation plays comprehensive roles in HaF function and virus
infectivity, which is further discussed.