Cation-Pi Interaction Regulates Ligand-Binding Affinity And Signaling Of Integrin Alpha(4)Beta(7)

Integrin alpha(4)beta(7) mediates rolling and firm adhesion of leucocytes, two of the critical steps in leukocyte migration and tissue specific homing. Affinity of alpha(4)beta(7) for ligand is dynamically regulated by three interlinked metal ion-binding sites in beta(7)-subunit I domain. In this study, we found that Phe185 (F185), a highly conserved aromatic residue in beta(7)-subunit, links the specificity-determining loop and the synergistic metal ion-binding site (SyMBS) through cation-pi interaction. Mutations of F185 that disrupted the SyMBS cation-F185 interaction led to deficient firm cell adhesion mediated by high affinity alpha(4)beta(7), and only slightly affected rolling adhesion mediated by low affinity alpha(4)beta(7). Disruption of SyMBS cation-F185 interaction induced partial extension of integrin ectodomain and separation of cytoplasmic tails, and impaired alpha(4)beta(7)-mediated bidirectional signaling. In addition, loss of SyMBS cation-F185 interaction increased paxillin expression and promoted paxillin-integrin binding, leading to deficient cell spreading. Furthermore, integrin alpha(4)beta(7)-mediated cell migration was decreased by the abolishment of SyMBS cation-F185 interaction. Thus, these findings reveal a cation-pi interaction playing vital roles in the regulation of integrin affinity, signaling, and biological functions.