KINASE-INDEPENDENT POTENTIATION OF B-CELL ANTIGEN RECEPTOR-MEDIATED SIGNAL-TRANSDUCTION BY THE PROTEIN-TYROSINE KINASE SRC

Signal transduction mediated by the B cell Ag receptor involves the ac tivation of multiple protein tyrosine kinases that are members of the Src family (i.e., Fyn, Lyn, Blk, Lck). To determine whether members of the Src family possess common physical and/or enzymatic properties th at enable them to potentiate signal transduction via the B cell Ag rec eptor, we expressed the protein tyrosine kinase Src in the B lymphoma cell line K46-17 mu m lambda. Based on coprecipitation analysis and tw o-color immunofluorescence, this heterologous Src family kinase was ob served to physically associate with the B cell Ag receptor, Additional experiments demonstrated that B cell Ag receptor cross-linking result s in increased tyrosine phosphorylation and activation of Src, Several parameters of B cell activation, including tyrosine phosphorylation o f intracellular substrates, calcium mobilization, and transcription fa ctor activation, were potentiated in cells that expressed Src when com pared with control cells. To determine whether potentiation of Ag rece ptor-mediated signaling by Src was dependent on its catalytic activity , a kinase-deficient form of Src was expressed in K46-17 mu m lambda c ells. Transfectants expressing kinase-deficient Src exhibited an enhan ced responsiveness to stimulation through the B cell Ag receptor that was comparable with transfectants expressing wild-type Src. Additional ly, kinase-deficient Src was observed to associate with the endogenous kinase Lyn in an activation-dependent manner. These findings indicate that members of the Src family may potentiate Ag receptor-mediated si gnaling via a kinase-independent mechanism(s) that involves amplificat ion of kinase recruitment to the Ag receptor activation complex.