A Naturally Occurring Extracellular α−β Clasp Contributes to Stabilization of β3 Integrins in a Bent, Resting Conformation

Control of αIIbβ3 and αvβ3 integrin activation is critical for cardiovascular homeostasis. Mutations that perturb association of integrin α and β subunits in their transmembrane and cytoplasmic regions activate the integrin heterodimer, suggesting that a low-affinity or “off” conformation is the default state, likely corresponding to the bent conformation seen in the crystal structure of αvβ3. In this bent structure, a segment of αv (301−308) and β3 (560−567) are juxtaposed. Here we provide evidence that these regions of αv/αIIb and β3 function as a novel extracellular clasp to restrain activation. Synthetic peptides representing the αIIb and β3 clasp regions promote integrin activation as judged by cell adhesion, cell spreading, and exposure of epitopes for three β3 LIBS antibodies. Mutation of the clasp region of αv or β3 results in a constitutively activated integrin, confirming the role of the extracellular clasp in restraining integrin activation. Molecular dynamics simulations of the αvβ3 structure yield a refined model for the αvβ3 clasp and provide plausible explanations for the effects of the activating mutations.