Transforming growth factor (TGF)-β is stored in the extracellular matrix as a latent complex with its prodomain. Activation of TGF-β1 requires the binding of αv integrin to an RGD sequence in the prodomain and exertion of force on this domain, which is held in the extracellular matrix by latent TGF-β binding proteins. Crystals of dimeric porcine proTGF-β1 reveal a ring-shaped complex, a novel fold for the prodomain, and show how the prodomain shields the growth factor from recognition by receptors and alters its conformation. Complex formation between αvβ6 integrin and the prodomain is insufficient for TGF-β1 release. Force-dependent activation requires unfastening of a ‘straitjacket’ that encircles each growth-factor monomer at a position that can be locked by a disulphide bond. Sequences of all 33 TGF-β family members indicate a similar prodomain fold. The structure provides insights into the regulation of a family of growth and differentiation factors of fundamental importance in morphogenesis and homeostasis.

The complete ectodomain of integrin αIIbβ3 reveals a bent, closed, low-affinity conformation, the β-knee, and a mechanism for linking cytoskeleton attachment to high affinity for ligand. Ca and Mg ions in the recognition site, including the synergistic metal ion binding site (SyMBS), are loaded prior to ligand binding. Electrophilicity of the ligand-binding Mg ion is increased in the open conformation. The β3 knee passes between the β3-PSI and αIIb-knob to bury the lower β-leg in a cleft, from which it is released for extension. Different integrin molecules in crystals and EM reveal breathing that appears on pathway to extension. Tensile force applied to the extended ligand-receptor complex stabilizes the closed, low-affinity conformation. By contrast, an additional lateral force applied to the β subunit to mimic attachment to moving actin filaments stabilizes the open, high-affinity conformation. This mechanism propagates allostery over long distances and couples cytoskeleton attachment of integrins to their high affinity state.

We report the structure of an integrin with an aI domain, a X b 2 , the complement receptor type 4. It was earlier expected that a fixed orientation between the aI domain and the b-propeller domain in which it is inserted would be required for allosteric signal transmission. However, the aI domain is highly flexible, enabling two bI domain conformational states to couple to three aI domain states, and greater accessibility for ligand recognition. Although a X b 2 is bent similarly to integrins that lack aI domains, the terminal domains of the a-and b-legs, calf-2 and b-tail, are oriented differently than in aI-less integrins. Linkers extending to the transmembrane domains are unstructured. Previous mutations in the b 2 -tail domain support the importance of extension, rather than a deadbolt, in integrin activation. The locations of further activating mutations and antibody epitopes show the critical role of extension, and conversion from the closed to the open headpiece conformation, in integrin activation. Differences among 10 molecules in crystal lattices provide unprecedented information on interdomain flexibility important for modelling integrin extension and activation.