The N-terminal SH2 domains of Syk and ZAP-70 mediate phosphotyrosine-independent binding to integrin β cytoplasmic domains

Syk and ZAP-70 form a subfamily of nonreceptor tyrosine kinases that contain tandem SH2 domains at their N termini. Engagement of these SH2 domains by tyrosine-phosphorylated immunoreceptor tyrosine-based activation motifs leads to kinase activation and downstream signaling. These kinases are also regulated by beta(3) integrin-dependent cell adhesion via a phosphorylation-independent interaction with the beta(3) integrin cytoplasmic domain. Here, we report that the interaction of integrins with Syk and ZAP-70 depends on the N-terminal SH2 domain and the interdomain A region of the kinase. The N-terminal SH2 domain alone is sufficient for weak binding, and this interaction is independent of tyrosine phosphorylation of the integrin tail. Indeed, phosphorylation of tyrosines within the two conserved NXXY motifs in the integrin beta(3) cytoplasmic domain blocks Syk binding. The tandem SH2 domains of these kinases bind to multiple integrin 13 cytoplasmic domains with varying affinities (beta(3), (K-d = 24 n(M)) > beta(2) (K-d = 38 n(M)) > beta(1) (K-d = 71 n(M))) as judged by both affinity chromatography and surface plasmon resonance. Thus, the binding of Syk and ZAP-70 to integrin beta cytoplasmic domains represents a novel phosphotyrosine-independent interaction mediated by their N-terminal SH2 domains.