Characterization of molecular components associated with surface immunoglobulin M in human B lymphocytes: presence of tyrosine and serine/threonine protein kinases.

To characterize the signal transduction through the antigen receptor (AgR) on human B lymphocytes, we analyzed its association with other molecular components. The surface IgM (sIgM) complex isolated in digitonin contains two surface expressed polypeptides--the previously described Ig alpha and Ig beta proteins--covalently linked to each other in a 48/39-kDa heterodimer. We show herein that the human sIgM complex isolated from the Burkitt's lymphoma cell line, Ramos, or from dense tonsillar B cells contains additional molecules--160 kDa and 75 kDa in size--and enzymatic activities able to phosphorylate on tyrosine as well as serine/threonine residues the 39-, 48-, 75- and 160-kDa polypeptides. By specific immunoprecipitation with antibodies to src-family kinases, we consistently detected p56lyn in the sIgM complex. In the Ramos cell line, both p56lck and p59fyn activity were also observed, although to a much lesser extent than p56lyn. These kinases are associated with sIgM before cell stimulation. As shown by two-dimensional electrophoresis, they interact in a tight complex with multimeric forms of the Ig alpha and Ig beta components. The kinases are active in vitro but must be highly regulated in vivo: Western blotting with anti-phosphotyrosine antibodies revealed that stimulation of the AgR on viable B cells increased detectable phosphotyrosine residues on the components present in the sIgM complex. Based on these phosphorylation changes, the 39-, 48-, 75- and 160-kDa molecules are likely to be functionally active elements in an IgM complex crucial for the transduction of the antigenic signal.