Energy landscape differences among integrins establish the framework for understanding activation.

Why do integrins differ in basal activity, and how does affinity for soluble ligand correlate with cellular adhesiveness? We show that basal conformational equilibrium set points for integrin α4 β1 are cell type specific and differ from integrin α5 β1 when the two integrins are coexpressed on the same cell. Although α4 β1 is easier to activate, its high-affinity state binds vascular cell adhesion molecule and fibronectin 100- to 1,000-fold more weakly than α5 β1 binds fibronectin. Furthermore, the difference in affinity between the high- and low-affinity states is more compressed in α4 β1 (600- to 800-fold) than in α5 β1 (4,000- to 6,000-fold). α4 β1 basal conformational equilibria differ among three cell types, define affinity for soluble ligand and readiness for priming, and may reflect differences in interactions with intracellular adaptors but do not predict cellular adhesiveness for immobilized ligand. The measurements here provide a necessary framework for understanding integrin activation in intact cells, including activation of integrin adhesiveness by application of tensile force by the cytoskeleton, across ligand-integrin-adaptor complexes.