Mechanical Quantification of Molecular Interactions Reveals Spatiotemporal Fluctuations on the Cell Surface

Abstract The cell membrane hosts initial interactions between receptors and ligands for subsequent cellular functions. However, how the membrane spatiotemporally regulates binding mechanics in living cells remains unclear. Using magnetic tweezers, we developed a single-molecule platform to reveal that dissociation forces of integrin–RGD (arginylglycylaspartic acid), nucleolin–AS1411 (a nucleolin-binding DNA aptamer), and membrane–cholesterol complexes correlated with their respective binding affinities on HeLa cell surfaces. By quantifying the mechanical force that breaks molecular interactions on the live cell surface, we showed that lipid rafts have a stronger retainment of cholesterol than the rest of the membrane. Similarly, integrin–RGD binding is strengthened in lipid rafts, likely due to the preferential location of different integrin subtypes. Significantly, cell extensions show weaker integrin–RGD binding than the main bodies, consistent with the role of cell protrusions in migration, while increased integrin–RGD binding on cell main bodies indicates their critical role in cell anchoring. In addition, integrin–RGD binding is more strengthened during the S than the G1 phase of cell cycles, suggesting a higher priority for cells to focus on DNA synthesis in the S phase unperturbed while reducing cell migration in search of resources, an energy-costly process frequently adopted in the G1 phase. Our work provides a broadly applicable platform to interrogate nanomechanics of receptor–ligand interactions on the cell surface. These nanomechanical results indicate that the strength of molecular interactions on the cell surface fluctuates spatiotemporally to accommodate cell activities.