Surface-bound selectin–ligand binding is regulated by carrier diffusion

Two-dimensional (2D) kinetics of receptor–ligand interactions governs cell adhesion in many biological processes. While the dissociation kinetics of receptor–ligand bond is extensively investigated, the association kinetics has much less been quantified. Recently receptor–ligand interactions between two surfaces were investigated using a thermal fluctuation assay upon biomembrane force probe technique (Chen et al. in Biophys J 94:694–701, 2008). The regulating factors on association kinetics, however, are not well characterized. Here we developed an alternative thermal fluctuation assay using optical trap technique, which enables to visualize consecutive binding–unbinding transition and to quantify the impact of microbead diffusion on receptor–ligand binding. Three selectin constructs (sLs, sPs, and PLE) and their ligand P-selectin glycoprotein ligand 1 were used to conduct the measurements. It was indicated that bond formation was reduced by enhancing the diffusivity of selectin-coupled carrier, suggesting that carrier diffusion is crucial to determine receptor–ligand binding. It was also found that 2D forward rate predicted upon first-order kinetics was in the order of sPs > sLs > PLE and bond formation was history-dependent. These results further the understandings in regulating association kinetics of surface-bound receptor–ligand interactions.