Dual role of the tyrosine activation motif of the Ig-alpha protein during signal transduction via the B cell antigen receptor.

The B cell antigen receptor (BCR) is a multimeric protein complex consisting of the ligand binding immunoglobulin molecule and the Ig-alpha/beta heterodimer that mediates intracellular signalling by coupling the receptor to protein tyrosine kinases (PTKs). Transfection of the Ig-alpha deficient myeloma cell line J558L microns with expression vectors coding for mutated Ig-alpha allowed us to test the function of the tyrosines in the cytoplasmic region of Ig-alpha in the context of the BCR. Furthermore we expressed Ig-alpha mutations as chimeric CD8-Ig-alpha molecules on K46 B lymphoma cells and tested their signalling capacity in terms of PTK activation and release of calcium. We show here that the conserved tyrosine residues in the cytoplasmic portion of Ig-alpha have a dual role. First, they are required for efficient activation of PTKs during signal induction and second, one of them is subject to phosphorylation by activated src-related PTKs. Phosphorylation on tyrosine in the cytoplasmic portion of Ig-alpha is discussed as a possible mechanism to couple the BCR to SH2 domain-carrying molecules.