B-CELL ANTIGEN RECEPTOR MOTIFS HAVE REDUNDANT SIGNALING CAPABILITIES AND BIND THE TYROSINE KINASES PTK72, LYN AND FYN

Background: The B-cell antigen receptor (BCR) is a multimeric protein complex consisting of an antigen-recognition structure (membrane immunoglobulin) and two associated proteins, Ig-alpha and Ig-beta. It has been proposed that signalling through the BCR involves Ig-alpha and Ig-beta. Both of these proteins contain within their cytoplasmic domains an amino-acid motif that is present in a number of immune recognition receptors, including the BCR, T-cell antigen receptor and Fc receptor complexes. This motif, termed the antigen-receptor homology motif (ARH1), appears to have signal transduction ability. Results: We now show that the presence of cytoplasmic regions containing the ARH1 motif from either Ig-alpha or Ig-beta is sufficient to confer signalling capability on an otherwise non-functional fusion protein. Both Ig-alpha and Ig-beta-containing chimeras induced, in an apparently redundant fashion, signalling events seen upon membrane immunoglobulin crosslinking, including tyrosine phosphorylation of particular proteins, phosphoinositide breakdown and calcium mobilization. Furthermore, crosslinking of the chimeras resulted in tyrosine phosphorylation of the Ig-alpha and Ig-beta tails and their association with the tyrosine kinases PTK72, p53/56 (lym) and p59 (fyn). Conclusions: These observations indicate that Ig-alpha and Ig-beta are responsible for coupling membrane immunoglobulin to intracellular signalling components. Moreover, they demonstrate that a number of tyrosine kinases associate directly with the cytoplasmic domains of both Ig-alpha and Ig-beta. Stimulation of the chimeras, which results in tyrosine phosphorylation of the Ig-alpha and Ig-beta tails, is a prerequisite for some of these associations. The implications of these findings for the mechanism by which the BCR initiates the signalling reactions are discussed.