Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics

Abstract

Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin α and β subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin α IIb b β3 . Allostery in the β 3 I domain alters three metal binding sites, associated loops and a α1-and α7-helices. Piston-like displacement of the a 7-helix causes a 62° reorientation between the β 3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper β 3 leg causes a 70Å separation between the knees of the α and β legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.Integrins are adhesion receptors that transmit signals bidirectionally across the plasma membrane 1-4 . Rearrangements in integrin extracellular, transmembrane and cytoplasmic domains underlie diverse biological processes, including cell migration, morpho-genesis, immune responses and vascular haemostasis. The platelet-specific integrin α IIb β 3 is important in both the arrest of bleeding at sites of vascular injury and pathological thrombosis leading to heart attacks and stroke. Loss of the vascular endothelium results in platelet deposition, and receptors for collagen, thrombin and other agonists then initiate

Clinical utility gene card for: Glanzmann thrombasthenia

“…Molecular modeling on the basis of the known crystal structure of the aIIbb3 and avb3 extracellular domains can help to assess the effect of missense mutations on integrin structure. 6,7 Expression of recombinant mutated integrin in heterologous cell lines can be informative on expression and function, but is only available on a research basis in specialized laboratories. RT-PCR may be used to confirm potential splicing defects of RNA expression.…”

Structural and mechanical functions of integrins

“…The α-and β-subunits are both type I transmembrane receptors and share structural similarities, such as a large extracellular domain, a single transmembrane domain and a cytoplasmic tail (Xiao et al 2004). …”

“…The βI domain also contains a MIDAS site similar to the αI domain, and this site is important in mediating ligand binding to negatively charged amino acid residues. In addition to the MIDAS site, the βI domain contains two other metal ion-binding sites called the adjacent metal ion-dependent adhesion site (ADMIDAS) and the synergistic metal ion-binding site (SyMBS) (Xiao et al 2004). The βI domain functions to either bind ligands directly in integrins lacking the inserted αI domain or to regulate the binding activity of α-subunits containing the αI domain (Xiong et al 2001;Xiao et al 2004).…”

“…extended with a closed headpiece (high affinity); (3) ligand occupied, i.e. extended with an open headpiece (Xiong et al 2001(Xiong et al , 2002Shimaoka et al 2003;Xiao et al 2004). At the present time, two models are proposed to describe the process of integrin activation: the 'deadbolt' model and the 'switchblade' model (Xiong et al 2003;Luo et al 2007).…”

“…The function of the cytoplasmic tails during integrin activation is to facilitate the binding of integrin adaptor proteins, such as talin and kindlin, via NxxY motifs (Calderwood et al 1999). Upon adaptor protein binding, the cytoplasmic tails separate along with the TM domains, which destabilises the tail-head interface and facilitates the 'switchblade'-like opening, causing the hybrid domain to swing out and integrin to enter the high-affinity or active conformation Vinogradova et al 2002;Xiao et al 2004;Luo et al 2007). In this 'active' extended conformation with open headpiece, integrins are able to bind extracellular ligands, which further stabilises the integrin heterodimer and eventually leads to integrin clustering, intracellular kinase recruitment and activation of downstream signalling pathways ).…”