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 αIIb tails, integrin β cytoplasmic tails and the binding partners of β tails. However, the roles of αIIb tail distal of GFFKR motif are unexplored.Objective:
To investigate the role of αIIb tail distal of GFFKR in talin-mediated inside-out integrin signaling.Methods:
We used model cell systems to examine the role of αIIb tail distal of GFFKR in bidirectional αIIbβ3 signaling and αIIbβ3–talin interactions.Results:
Deletion of amino acid residues after the GFFKR motif in αIIb tail moderately decreased β3(D723R)-induced activation, abolished talin-induced αIIbβ3 activation in model cells, and inhibited agonist-induced αIIbβ3 activation in megakaryocytic cells. Furthermore, residues in αIIb tail distal of GFFKR did not affect outside-in αIIbβ3 signaling or αIIbβ3–talin interaction. Addition of non-homologous or non-specific amino acids to the GFFKR motif restored αIIbβ3 activation in model cells and in megakaryocytic cells. Molecular modeling indicates that β3-bound talin sterically clashes with the αIIb tail in the αIIbβ3 complexes, potentially disfavoring the α–β interactions that keep αIIbβ3 inactive.Conclusion:
The αIIb tail sequences distal of GFFKR participate in talin-mediated inside-out αIIbβ3 activation through its steric clashes with β3-bound talin.