The crystal structure of the plexin-semaphorin-integrin domain/hybrid domain/I-EGF1 segment from the human integrin β₂ subunit at 1.8-Å resolution

Integrins are modular (αβ) heterodimeric proteins that mediate cell adhesion and convey signals across the plasma membrane. Interdomain motions play a key role in signal transduction by propagating structural changes through the molecule, thus controlling the activation state and adhesive properties of the integrin. We expressed a soluble fragment of the human integrin β₂ subunit comprising the plexin-semaphorin-integrin domain (PSI)/hybrid domain/I-EGF1 fragment and present its crystal structure at 1.8-Å resolution. The structure reveals an elongated molecule with a rigid architecture stabilized by nine disulfide bridges. The PSI domain is located centrally and participates in the formation of extended interfaces with the hybrid domain and I-EGF1 domains, respectively. The hybrid domain/PSI interface involves the burial of an Arg residue, and contacts between PSI and I-EGF1 are mainly mediated by well conserved Arg and Trp residues. Conservation of key interacting residues across the various integrin β subunits sequences suggests that our structure represents a good model for the entire integrin family. Superposition with the integrin β₃ receptor in its bent conformation suggests that an articulation point is present at the linkage between its I-EGF1 and I-EGF2 modules and underlines the importance of this region for the control of integrin-mediated cell adhesion.