The β 8 integrin EGF domains support a constitutive extended conformation, and the cytoplasmic domain impairs outside-in signaling

Integrins are transmembrane proteins that transmit bi-directional signals across the cell membrane through global structural rearrangement among three different conformational states: bent, extended- closed, and extended-open conformations. However, the β integrin is distinctive and may adopt only one conformation, that is, extended-closed conformation, with high affinity for ligands under physiological conditions, and may not transmit bi-directional signals like other integrin members. It is unclear how different β domains affect its unique conformation and signaling. We swapped different domains of integrin β with those of β and investigated how they affected integrin ligand binding, global conformation, and outside-in signaling. We found that the β epidermal growth factor (EGF) domains increased integrin ligand binding ability and contributed to its extended conformation. By comparison, the β transmembrane and cytoplasmic domains had little effect on ligand binding or global conformation. The β EGF and transmembrane domains did not affect integrin-mediated cell adhesion, cell spreading, focal adhesion formation, or colocalization of integrin with other proteins, but the cytoplasmic domain had a defective effect on outside-in signaling. Our results showed that different domains of β play various roles on its unique conformation, ligand binding, and signaling, which are considered atypical among integrin members.