Talin activates integrins by altering the topology of the β transmembrane domain

Talin binding to integrin beta tails increases ligand binding affinity (activation). Changes in beta transmembrane domain (TMD) topology that disrupt alpha-beta TMD interactions are proposed to mediate integrin activation. In this paper, we used membrane-embedded integrin beta 3 TMDs bearing environmentally sensitive fluorophores at inner or outer membrane water interfaces to monitor talin-induced beta 3 TMD motion in model membranes. Talin binding to the beta 3 cytoplasmic domain increased amino acid side chain embedding at the inner and outer borders of the beta 3 TMD, indicating altered topology of the beta 3 TMD. Talin's capacity to effect this change depended on its ability to bind to both the integrin. tail and the membrane. Introduction of a flexible hinge at the midpoint of the beta 3 TMD decoupled the talin-induced change in intracellular TMD topology from the extracellular side and blocked talin-induced activation of integrin alpha Ilb beta 3. Thus, we show that talin binding to the integrin beta TMD alters the topology of the TMD, resulting in integrin activation.