Distinct binding kinetics of E-, P- and L-selectin to CD44.

Molecular-level selectin-CD44 interactions are far from clear because of the complexity and diversity of CD44 glycosylation and isoforms expressed on various kinds of cells. By combining experimental measurements and simulation predictions, the binding kinetics of three selectin members to the recombinant CD44 were quantified and the corresponding micro-structural mechanisms were explored, respectively. Experimental results showed that the E-selectin-CD44 interactions mainly mediated the firm adhesion of microbeads under shear flow with the strongest rupture force. P- and L-selectins had similar interaction strength but different association and dissociation rates by respectively mediating stable rolling and transient adhesions of microbeads. Molecular docking and molecular dynamics (MD) simulations predicted that the binding epitopes of CD44 to selectins are all located at the side face of each selectin, but the interfaces denoted as the hinge region are between Lectin and EGF domains of E-selectin, Lectin domain side of P-selectin, and EGF domain side of L-selectin, respectively. The lowest binding free energy, the largest rupture force and the longest lifetime for E-selectin, as well as the comparable values for P- and L-selectins, demonstrated in both equilibration and steered MD simulations, supported the above experimental results. These results offer basic data for understanding the functional differences of selectin-CD44 interactions.