Structural specializations of α4β7, an integrin that mediates rolling adhesion

The lymphocyte homing receptor integrin alpha(4)beta(7) is unusual for its ability to mediate both rolling and firm adhesion. alpha(4)beta(1) and alpha(4)beta(7) are targeted by therapeutics approved for multiple sclerosis and Crohn's disease. Here, we show by electron microscopy and crystallography how two therapeutic Fabs, a small molecule (RO0505376), and mucosal adhesion molecule-1 (MAdCAM-1) bind alpha(4)beta(7). A long binding groove at the alpha(4)-beta(7) interface for immunoglobulin superfamily domains differs in shape from integrin pockets that bind Arg-Gly-Asp motifs. RO0505376 mimics an Ile/Leu-Asp motif in alpha(4) ligands, and orients differently from Arg-Gly-Asp mimics. A novel auxiliary residue at the metal iondependent adhesion site in alpha(4)beta(7) is essential for binding to MAdCAM-1 in Mg2+ yet swings away when RO0505376 binds. A novel intermediate conformation of the alpha(4)beta(7) headpiece binds MAdCAM-1 and supports rolling adhesion. Lack of induction of the open headpiece conformation by ligand binding enables rolling adhesion to persist until integrin activation is signaled.