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

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Abstract
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 Igα and Igβ 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 Igα and Igβ 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.