The β8 integrin EGF domains support a constitutive extended conformation, and the cytoplasmic domain impairs outside-in signaling.

Abstract

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 β8 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 β8 domains affect its unique conformation and signaling. We swapped different domains of integrin β3 with those of β8 and investigated how they affected integrin ligand binding, global conformation, and outside-in signaling. We found that the β8 epidermal growth factor (EGF) domains increased integrin ligand binding ability and contributed to its extended conformation. By comparison, the β8 transmembrane and cytoplasmic domains had little effect on ligand binding or global conformation. The β8 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 β8 play various roles on its unique conformation, ligand binding, and signaling, which are considered atypical among integrin members.