Novel activating and inactivating mutations in the integrin β1 subunit A domain

The ligand-binding activity of integrins is regulated by shape changes that convert these receptors from a resting (or inactive) state to an active state. However, the precise conformational changes that take place in head region of integrins (the site of ligand binding) during activation are not well understood. The portion of the integrin β subunit involved in ligand recognition contains a von Willebrand factor type A domain, which comprises a central β-sheet surrounded by seven a helices (α1-α7). Using site-directed mutagenesis, we show here that point mutation of hydrophobic residues in the α1 and α7 helices (which would be predicted to increase the mobility of these helices) markedly increases the ligand-binding activity of both integrins α5β1 and α4β1. In contrast, mutation of a hydrophilic residue near the base of the α1 helix decreases activity and also suppresses exposure of activation epitopes on the underlying hybrid domain. Our results provide new evidence that shifts of the α1 and α7 helices are involved in activation of the A domain. Although these changes are grossly similar to those defined in the A domains found in some integrin α subunits, movement of the α1 helix appears to play a more prominent role in βA domain activation.