Activation of Syk protein tyrosine kinase through interaction with integrin β cytoplasmic domains

Syk protein tyrosine kinase is essential for immune system development and function [1] and for the maintenance of vascular integrity [2, 3]. In leukocytes, Syk is activated by binding to diphosphorylated immune receptor tyrosine-based activation motifs (pITAMs) [1]. Syk can also be activated by integrin adhesion receptors [4, 5], but the mechanism of its activation is unknown. Here we report a novel mechanism for Syk's recruitment and activation, which requires that Syk bind to the integrin β3 cytoplasmic tail. We found that both Syk and the related kinase ZAP-70 bound the β3 cytoplasmic tail through their tandem SH2 domains. However, unlike Syk binding to pITAMs, this interaction was independent of tyrosine phosphorylation and of the phosphotyrosine binding function of Syk's tandem SH2 domains. Deletion of the four C-terminal residues of the β3 cytoplasmic tail [β3(759X)] decreased Syk binding and disrupted its physical association with integrin αIIbβ3. Furthermore, cells expressing αIIbβ3(759X) failed to exhibit Syk activation or lamellipodia formation upon cell adhesion to the αIIbβ3 ligand, fibrinogen. In contrast, FAK phosphorylation and focal adhesion formation were unimpaired by this mutation. Thus, the direct binding of Syk kinase to the integrin β3 cytoplasmic tail is a novel and functionally significant mechanism for the regulation of this important non-receptor tyrosine kinase.