B cell development, selection and activation require appropriate signal transduction initiated by the surface B cell antigen receptor (BCR); balancing and tuning activation and inhibitory signals is essential for maintaining physiological B cell biology. Engagement of the BCR results in the activation of protein tyrosine kinases and the tyrosine phosphorylation of components of the BCR, Ig-α and Ig-β. Experiments performed with Ig-α (CD79a) and Ig-β (CD79b) gene targeted mice suggest that Ig-α and Ig-β amino acid residues other than tyrosines contribute to the modulation of BCR signals. The occurrence of serine and threonine phosphorylation of Ig-α and Ig-β has been previously reported, and was suggested to inhibit BCR signals in cell lines. Therefore, mutations leading to abnormal phosphorylation of Ig-α/Ig-β serines and threonines may result in B cell hyper-activation. However, the functional consequence of Ig-α/Ig-β serine/threonine phosphorylation in the context of BCR signaling and B cell function remains largely unexplored. We hypothesisize that the phosphorylation of serine and threonine residues of Ig-α, and specifically Ig-β inhibits Ig-α and Ig-β tyrosine phosphorylation, which serves to inhibit constitutive and BCR aggregation initiated activation signaling.

While the identity of exactly which of the multiple residues undergo modification is still unknown, the lack of specific reagents provides an additional hurdle to investigating these events. To identify the phosphorylation of these residues we have employed thin layer chromatography, mass spectrometry, and western blotting with custom phospho-Ig-β antibody approaches. Our results show that recombinant and native Ig-β molecules are subject to serine and threonine phosphorylation and specifically S220 and T214 are phosphorylated. Also, in support of previous findings, we have found that the serine and threonine residues of Ig-β inhibit BCR activation induced tyrosine phosphorylation and calcium mobilization.