Abstract
BACKGROUND: Increases in ligand binding to integrins (activation) play critical roles in platelet and leukocyte function. Integrin activation requires talin and kindlin binding to integrin β cytoplasmic tails. Research has focused on the conserved GFFKR motif in integrin αII b tails, integrin β cytoplasmic tails and the binding partners of β tails. However, the roles of αII b tail distal of GFFKR motif are unexplored. OBJECTIVE: To investigate the role of αII b tail distal of GFFKR in talin-mediated inside-out integrin signaling. METHODS: We used model cell systems to examine the role of αII b tail distal of GFFKR in bidirectional αII b β3 signaling and αII b β3 -talin interactions. RESULTS: Deletion of amino acid residues after the GFFKR motif in αII b tail moderately decreased β3 (D723R)-induced activation, abolished talin-induced αII b β3 activation in model cells, and inhibited agonist-induced αII b β3 activation in megakaryocytic cells. Furthermore, residues in αII b tail distal of GFFKR did not affect outside-in αII b β3 signaling or αII b β3 -talin interaction. Addition of non-homologous or non-specific amino acids to the GFFKR motif restored αII b β3 activation in model cells and in megakaryocytic cells. Molecular modeling indicates that β3 -bound talin sterically clashes with the αII b tail in the αII b β3 complexes, potentially disfavoring the α-β interactions that keep αII b β3 inactive. CONCLUSION: The αII b tail sequences distal of GFFKR participate in talin-mediated inside-out αII b β3 activation through its steric clashes with β3 -bound talin.