Measuring ligand-receptor unbinding forces with magnetic beads: Molecular leverage

We describe a new method for the quantitative measurement of weak forces be tween adhesion domains formed by receptor-ligand pairs. The method relies o n the microinterferometric analysis of adhering vesicles subjected to an ex ternal force produced by a magnetic bead adhered to the membrane. The force s exerted on pinning centers can be deduced using classical elasticity theo ry. The method is applied to measure the binding strengths between solubili zed solid supported integrin receptors alpha (IIb)beta (3) Of blood platele ts and lipid-coupled cyclic peptides containing arginine-glycine-aspartate (RGD) tripetide ligands, embedded in the vesicle, that are recognized selec tively by the integrins. We find that unbinding takes place at force levels far below those found in single-molecule studies. We interpret our finding s in terms of the fracturing of the receptor-ligand pairs by the torque gen erated by the force which is much more effective than the traction.