Tyrosine phosphorylation of IgM- and IgD-associated molecules of a human B lymphoma cell line B104

We investigated tyrosine phosphorylation and structural properties of the IgM-associated molecules in comparison with IgD-associated molecules in a recently established human surface IgM+, IgD+ B lymphoma cell line, B104, the growth of which was irreversibly inhibited by anti-IgM mAbs but not by anti-IgD mAbs. Tyrosine kinase activity and tyrosine phosphorylated proteins were detected in anti-IgM and anti-IgD immunoprecipitates from digitonin lysates of B104 cells with the use of an in vitro kinase assay followed by a re-immunoprecipitation experiment with anti-phosphotyrosine mAbs. Tyrosine phosphorylated proteins of 74, 58-44, 41, and 39 kDa were detected in anti-IgM immunoprecipitates, whereas tyrosine phosphorylated proteins of 74, 58-44, and 39 kDa, but not 41 kDa, were detected in anti-IgD immunoprecipitates. Crosslinking of surface IgM and surface IgD stimulated rapid tyrosine phosphorylation of different sets of proteins which included tyrosine-phosphorylated proteins of the same or similar molecular weights as those detected in the anti-IgM and anti-IgD immunoprecipitates respectively. After deglycosylation by N-glycosidase, both the IgM- and IgD-associated phosphoproteins (pp58-pp39) gave rise to the same three bands of 29, 27, and 26 kDa. Proteolytic peptide mapping of these three deglycosylated proteins showed that the primary structures of the IgM- and IgD-associated molecules are identical, suggesting that the IgM- and IgD-associated phosphoproteins (pp58-pp39) are the products of the same or closely related genes. One of the products, pp41, may be associated with IgM, but not with IgD, although the same gene product may be associated with IgD in a different glycosylation pattern.