RESUMO
A mannose-binding lectin (Narcissus tazetta lectin [NTL]) with potent antiviral activity was isolated and purifi ed from the bulbs of the Chinese daffodil Narcissus tazetta var. chinensis, using ion exchange chromatography on diethylaminoethyl (DEAE)-cellulose, affinity chromatography on mannose–agarose and fast protein liquid chromatography (FPLC)-gel filtration on Superose 12. The purified lectin was shown to have an apparent molecular mass of 26 kDa by gel filtration and 13 kDa by SDS–PAGE, indicating that it is probably a dimer with two identical subunits. The cDNA-derived amino acid sequence of NTL as determined by molecular cloning also reveals that NTL protein contains a mature polypeptide consisting of 105 amino acids and a C-terminal peptide extension. Three-dimensional modelling study demonstrated that the NTL primary polypeptide contains three subdomains, each with a conserved mannose-binding site. It shows a high homology of about 60%–80% similarity with the existing monocot mannose-binding lectins. NTL could signifi cantly inhibit plaque formation by the human respiratory syncytial virus (RSV) with an IC50 of 2.30 μg/ml and exhibit strong antiviral properties against infl uenza A (H1N1, H3N2, H5N1) and infl uenza B viruses with IC50 values ranging from 0.20 μg/ml to 1.33 μg/ml in a dose-dependent manner. It is worth noting that the modes of antiviral action of NTL against RSV and infl uenza A virus are signifi cantly different. NTL is effective in the inhibition of RSV during the whole viral infection cycle, but the antiviral activity of NTL is mainly expressed at the early stage of the viral cycle of infl uenza A (H1N1) virus. NTL with a high selective index (SI=CC50/IC50 >141) resulting from its potent antiviral activity and low cytotoxicity demonstrates a potential for biotechnological development as an antiviral agent.