Glycosylated Extracellular Domains of Membrane Immunoglobulin M Contribute to its Association with MB-1/B29 Gene Products and the B Cell Receptor Complex

It has recently become clear that the B cell antigen receptor, membrane immunoglobulin (mlg) is part of a complex composed of a number of different polypeptides. In a manner analogous to the T cell receptor, mlg has been found to be associated with several tyrosine kinases, and other proteins, which although not kinases themselves become targets of kinase activity upon binding of mlg to antigen. Thus the B cell receptor complex appears to be a structure whose function during signal transduction is to facilitate the interaction of tyrosine kinases with their proper substrates, and to coordinate the phosphorylation of these proteins with the binding of antigen to mlg. In an effort to understand the nature of the interactions which mediate the organization of the B cell receptor complex, we have explored binding of components of the complex including Ig-α and Ig-β to IgM. Previous results have indicated that binding was mediated by transmembrane domains. Our results indicate that extracellular domains of IgM may also contribute to its association with Ig α and β and other members of the B cell receptor complex.