Interactions of Lyn with the antigen receptor during B cell activation.

Signaling through the B cell antigen receptor requires a complex set of interactions involving transmembrane components of the IgM receptor complex and cytosolic protein-tyrosine kinases. We have focused on the nature of these protein-protein interactions, the requirements for their occurrence, as well as the temporal sequence of events during the activation process. We found that cross-linking B cell antigen receptors at 0 degree C resulted in the rapid association of the Src-family protein-tyrosine kinase, Lyn, with the antigen receptor complex as judged by the presence of Lyn in anti-IgM and anti-phosphotyrosine immune complexes and the presence of MB-1 in anti-Lyn immune complexes. Receptor engagement also resulted in the rapid association of Lyn with the phosphotyrosine phosphatase, CD45. This association of Lyn with receptor components was stable in the detergent Brij 96, but was readily disrupted by Nonidet P-40, suggesting the involvement of hydrophobic interactions in stabilizing formation of the Lyn-receptor complex. The protein-tyrosine kinase, Syk, was also found associated with activated receptor complexes. This association of Syk with components of the antigen receptor complex was stable to Nonidet P-40. Antibodies directed against the carboxyl teminus of Syk, but not against the amino-terminal SH2 domain, co-immunoprecipitated MB-1 from activated cells, consistent with the binding of Syk through an SH2 domain-phosphotyrosine interaction.