Low affinity integrin states have faster binding kinetics than the high affinity state

Integrin conformational ensembles contain two low-affinity states, bent-closed and extended-closed, and an active, high-affinity, extended-open state. It is widely thought that integrins must be activated before they bind ligand; however, one model holds that activation follows ligand binding. As ligand-binding kinetics are not only rate limiting for cell adhesion but also have important implications for the mechanism of activation, we measure them here for integrins 4{beta}1 and 5{beta}1 and show that the low-affinity states bind substantially faster than the high-affinity state. On and off-rate measurements are similar for integrins on cell surfaces and ectodomain fragments. Although the extended-open conformations on-rate is [~]20-fold slower, its off-rate is [~]25,000-fold slower, resulting in a large affinity increase. The tighter ligand-binding pocket in the open state may slow its on-rate. These kinetic measurements, together with previous equilibrium measurements of integrin conformational state affinity and relative free energy on intact cells, are key to a definitive understanding of the mechanism of integrin activation.