Conformational Frustration at the Protein–Glycan Interface of a Nonmitogenic Anti-HIV Lectin Results in Altered Quaternary Structure

Lectins are carbohydrate-binding proteins that have enormous therapeutic value because of their potent antiviral activity. However, the design of lectins for targeted intervention is marred by our poor understanding of protein-glycan recognition. Here, we focus on the mannose-specific lectin horcolin, which is nonmitogenic and shows dose-dependent inhibition of HIV infection. Saturation transfer and relaxation dispersion NMR experiments reveal that the lectin-glycan interface is conformationally frustrated, resulting in the formation of a minor state with a millisecond time-scale lifetime. There is a rearrangement of the quaternary structure of horcolin in this minor state that manifests as a noncanonical tetramer. The glycan-binding site is sequestered at the tetrameric interface, suggesting that the tetramer could serve as an autoinhibitory conformation. However, glycan recognition itself occurs via the major dimeric conformation through a "ground-state conformational selection" mechanism. We also demonstrate that the tetramer is destabilized by mannose and that conformational frustration is alleviated in the lectin-glycan complex. Our work illustrates how the architecture of biomolecular assemblies is molded in response to conflicting evolutionary signals such as folding and recognition. The work also provides insights into protein-glycan recognition that could have potential implications for deploying lectins as antiviral agents.