Site-specific Discrimination by Cyanovirin-N for α-Linked Trisaccharides Comprising the Three Arms of Man8 and Man9

Cyanovirin-N (CVN) is a novel cyanobacterial protein that selectively binds with nanomolar affinities the mammalian oligosaccharides Man8 and Man9. Consequently, CVN potently blocks HIV entry through highly avid carbohydrate-mediated interactions with the HIV-envelope glycoprotein gp120, and is under preclinical investigation as an anti-HIV microbicide. CVN contains two non-overlapping carbohydrate-binding sites that bind the disaccharide Manα(1-2)Manα (which represents the terminal disaccharide of all three arms of Man9) with low to sub-micromolar affinities. The solution structure of a 1:2 CVN:Manα(1-2)Manα complex revealed that CVN recognizes the stacked conformation of Manα(1-2)Manα through a deep hydrophilic-binding pocket on one side of the protein (site 2) and a semi-circular cleft on the other (site 1). With the prominent exception of the C1 hydroxyl group of the reducing mannopyranose ring, the bound disaccharide is positioned so that each hydroxyl group is involved in a direct or water-mediated hydrogen bond to the polar or charged side-chains comprising the binding pocket. Thus, to determine whether the next-most reducing mannopyranose ring will augment CVN affinity and selectivity, we have characterized by NMR and ITC the binding of CVN to three synthetic trisaccharides representing the full-length D1, D2 and D3 arms of mammalian oligomannosides. Our findings demonstrate that site 1 is able to discriminate between the three related trisaccharides methyl Manα(1-2)Manα(1-2)Man, methyl Manα(1-2)Manα(1-3)Man and methyl Manα(1-2)Manα(1-6)Man with remarkable selectivity, and binds these trisaccharides with KA values ranging from 8.1×103 M−1 to 6.6×106 M−1. Site 2 is less selective in that it binds all three trisaccharides with similar KA values ranging from 1.7 to 3.7(±0.3)×105 M−1, but overall binds these trimannosides with higher affinities than site 1. The diversity of pathogenic organisms that display α(1-2)-linked mannosides on their cell surfaces suggests a broad defensive role for CVN in its cyanobacterial source.