Abstract
The minimum oligosaccharide structure required for binding to the potent HIV-inactivating protein cyanovirin-N (CV-N) was determined by saturation-transfer difference (STD) NMR spectroscopy. Despite the low molecular mass of the protein (11 kDa), STD-NMR spectroscopy allowed the precise atomic mapping of the interactions between CV-N and various di- and trimannosides, substructures of Man-9, the predominant oligosaccharide on the HIV viral surface glycoprotein gp120. Contacts with mannosides containing the terminal Manalpha(1-->2)Manalpha unit of Man-9 were observed, while (1-->3)- and (1-6)-linked di- and trimannosides showed no interactions, demonstrating that the terminal Manalpha(1-->2)Manalpha structure plays a key role in the interaction. Precise epitope mapping revealed that, for Manalpha(1-->2)ManalphaOMe, Manalpha(1-->2)Manalpha(1-->3)ManalphaOMe, and Manalpha(1-->2)Manalpha(1-->6)ManalphaOMe, the protein is in close contact with H2, H3, and H4 of the nonreducing terminal mannose unit. In contrast, the STD-NMR spectrum of the CV-N/trisaccharide Manalpha(1-->2)Manalpha(1-->2)ManalphaOMe complex was markedly different, with resonances on all sugar units displaying equal enhancements, suggesting that CV-N is able to discriminate between the three structurally related trisaccharides.
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