Association with B‐cell‐antigen receptor with protein‐tyrosine kinase p72syk and activation by engagement of membrane IgM

Abstract

We have demonstrated that a 72-kDa non-receptor-type protein-tyrosine kinase (p72^syk) was coimmunoprecipitated with membrane IgM in digitonin lysates of porcine tonsillar cells and was rapidly activated following the engagement of membrane IgM. This activation was occurred within 5 s, even in the presence of EGTA and 5,5′-dimethyl-bis-(O-aminophenoxy)-ethane-N,N,N′,N′-tetraacetic acid as extracellular and intracellular Ca²⁺-chelating agents, respectively, as well as in the presence of the protein-kinase-C inhibitor, H-7. Additionally, genistein, a potent protein-tyrosine kinase inhibitor, was capable of reducing both IgM-stimulated Ca²⁺ mobilization and p72^syk activation in a dose-dependent manner. These results indicate that p72^syk is physically associated with the B-cell-antigen receptor, participating in antigen-mediated signal transduction in both a Ca²⁺-independent and protein-kinase-C-independent manners.