Effects of the association of the α β lower legs on integrin ligand binding.

Many integrins transmit signals through global conformational changes. However, it is unclear whether integrin α β adopts a similar mechanism during integrin activation and signaling on the cell surface. Here, we showed that disulfide-bonded mutants, which prevented integrin α β lower leg dissociation, bound ligands with similar level as the wild-type protein, suggesting that α β ligand binding did not require lower leg disassociation. We further showed that the N-glycosylation mutant at the interface between the β I and hybrid domains did not affect ligand binding, suggesting that the α β open headpiece was not present on the cell surface. We proposed that α β integrin may adopt only one state, that is, the extended conformation with a closed headpiece. Our results showed that two lower legs retained heterodimeric interfaces, and this association might be important for stabilizing integrin in the extended conformation. Therefore, α β may not transmit bidirectional signals across the plasma membrane but instead may serve as an anchoring site with high affinity and high accessibility for extracellular ligands.