Polyvalent Glycan-Quantum Rods as Multifunctional Mechanistic Probes for Multivalent Glycan-Lectin Recognitions

Multivalent lectin-glycan interactions (MLGIs) are widespread and vital for biology, and also hold the key to many therapeutic applications. However, the underlying structural and biophysical mechanisms for many MLGIs remain poorly understood, limiting our ability to design glycoconjugates that can potently target specific MLGIs for therapeutic intervention. Glycosylated nanoparticles have recently emerged as a powerful biophysical probe for MLGIs, although how nanoparticle shape affects MLGI mechanisms remain largely unexplored. Herein, we have prepared fluorescent quantum rods (QRs), densely coated with -1,2-manno-biose ligands (denoted as QR-DiMan), as a new multifunctional probe to investigate how scaffold geometry affects the MLGI of a pair of closely-related, important tetrameric viral receptors, DC-SIGN and DC-SIGNR. We have previously shown that a DiMan-capped spherical quantum dot (QD-DiMan) gives weak crosslinking interactions with DC-SIGNR but strong simultaneous binding with DC-SIGN. Against the elongated QR scaffold, DC-SIGN retains a similarly strong simultaneous binding of all four binding sites with a single QR-DiMan (apparent K_d ~0.5 nM, ~1.8 million fold stronger than the corresponding monovalent binding), while DC-SIGNR is able to achieve both weak crosslinking and strong individual binding interactions, resulting in a larger binding affinity enhancement than that with QD-DiMan. S/TEM analysis of QR-DiMan-lectin assemblies reveals that DC-SIGNR’s different binding modes arise from the different surface curvatures of the QR scaffold. The glycan display at the spherical ends present too high a steric barrier for DC-SIGNR to bind with all four binding sites, thus it crosslinks between two QR-DiMan to maximize binding multivalency, whereas the more planar character of the cylindrical center allows the glycans to bridge all binding sites in DC-SIGNR. This work thus establishes glycosylated QRs as a powerful new biophysical probe for MLGIs, not only to provide quantitative binding affinities and binding modes, but also to demonstrate the specificity of multivalent lectins in discriminating different glycan displays dictated by the scaffold curvature. This work thus highlights the importance of nano-scaffold shape on MLGIs.