Distinct p53/56lyn and p59fyn domains associate with nonphosphorylated and phosphorylated Ig-alpha

Among the earliest detectable events in B-cell antigen receptor-mediated signal transduction are the activation of receptor-associated Src-family tyrosine kinases and the tyrosine phosphorylation of Ig-alpha and Ig-beta receptor subunits. These kinases appear to interact with resting B-cell antigen receptor complexes primarily through the Ig-alpha chain antigen receptor homology 1 (ARH1) motif. Recent studies showed a dramatic increase in the amount of Src-family kinase p59fyn bound to Ig-alpha when ARH1 motif tyrosines were phosphorylated. To explore the submolecular basis of these interactions, we conducted mutational analysis to localize sites in p53/56lyn and p59fyn that bind nonphosphorylated and phosphorylated Ig-alpha. Here we report that distinct regions within these kinases bind nonphosphorylated and phosphorylated Ig-alpha ARH1 motifs. The N-terminal 10 residues mediate binding to the nonphosphorylated Ig-alpha ARH1 motif. Association with the phosphorylated Ig-alpha ARH1 motif is mediated by Src homology 2 domains. These findings suggest a mechanism whereby ligand-induced Ig-alpha tyrosine phosphorylation initiates a change in the orientation of an associated kinase that may alter its activity and/or access to substrates and other effectors.