Integrin α4β7 mediates rolling and firm adhesion of lymphocytes pre- and post-activation, which is distinct from most integrins only mediating firm cell adhesion upon activation. This two-phase cell adhesion suggests a unique molecular basis for the dynamic interaction of α4β7 with its ligand, mucosal addressin cell adhesion molecule 1 (MAdCAM-1). Here we report that a disulfide bond-stabilized W1 β4-β1 loop in α4 β-propeller domain plays critical roles in regulating integrin α4β7 affinity and signaling. Either breaking the disulfide bond or deleting the disulfide bond-occluded segment in the W1 β4-β1 loop inhibited rolling cell adhesion supported by the low-affinity interaction between MAdCAM-1 and inactive α4β7 but negligibly affected firm cell adhesion supported by the high-affinity interaction between MAdCAM-1 and Mn(2+)-activated α4β7. Additionally, disrupting the disulfide bond or deleting the disulfide bond-occluded segment not only blocked the conformational change and activation of α4β7 triggered by talin or phorbol-12-myristate-13-acetate via inside-out signaling but also disrupted integrin-mediated outside-in signaling and impaired phosphorylation of focal adhesion kinase and paxillin. Thus, these findings reveal a particular molecular basis for α4β7-mediated rolling cell adhesion and a novel regulatory element of integrin affinity and signaling.
Keywords: Affinity Regulation; Cell Adhesion; Cell Signaling; Cell Surface Receptor; Integrin; Integrin α4β7; Ligand Binding Protein; W1 β4-β1 Loop.