The Direct Recruitment of BLNK to Immunoglobulin α Couples the B-Cell Antigen Receptor to Distal Signaling Pathways

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Abstract
Following B-cell antigen receptor (BCR) ligation, the cytoplasmic domains of immunoglobulin α (Igα) and Igβ recruit Syk to initiate signaling cascades. The coupling of Syk to several distal substrates requires linker protein BLNK. However, the mechanism by which BLNK is recruited to the BCR is unknown. Using chimeric receptors with wild-type and mutant Igα cytoplasmic tails we show that the non-immunoreceptor tyrosine-based activation motif (ITAM) tyrosines, Y176 and Y204, are required to activate BLNK-dependent pathways. Subsequent analysis demonstrated that BLNK bound directly to phospho-Y204 and that fusing BLNK to mutated Igα reconstituted downstream signaling events. Moreover, ligation of the endogenous BCR induced Y204 phosphorylation and BLNK recruitment. These data demonstrate that the non-ITAM tyrosines of Igα couple Syk activation to BLNK-dependent pathways.