Functions as a central linker protein, downstream of the B-cell receptor (BCR), bridging the SYK kinase to a multitude of signaling pathways and regulating biological outcomes of B-cell function and development. Plays a role in the activation of ERK/EPHB2, MAP kinase p38 and JNK. Modulates AP1 activation. Important for the activation of NF-kappa-B and NFAT. Plays an important role in BCR-mediated PLCG1 and PLCG2 activation and Ca(2+) mobilization and is required for trafficking of the BCR to late endosomes. However, does not seem to be required for pre-BCR-mediated activation of MAP kinase and phosphatidyl-inositol 3 (PI3) kinase signaling. May be required for the RAC1-JNK pathway. Plays a critical role in orchestrating the pro-B cell to pre-B cell transition (By similarity). Plays a critical role in B-cell development in the bursa. Plays an important role in BCR-induced apoptosis. Associates with PLCG1, VAV1 and NCK1 in a B-cell antigen receptor-dependent fashion. Interacts through its SH2 domain with CD79A (By similarity). Interacts with VAV3, PLCG2 and GRB2. BCR activation results in the translocation to membrane fraction. Highly expressed in the bursa, very low expression in ovary and spleen. Expression was variable among B-cell lines. Highly expressed in immature B-cell lines such as DT40 and CL18, low expression was seen in relatively mature B-cell lines, such as 293B9 and 249L4. No expression was seen in T-cell lines. Following BCR activation, phosphorylated on tyrosine residues by SYK and LYN. When phosphorylated, serves as a scaffold to assemble downstream targets of antigen activation, including PLCG1, VAV1, GRB2 and NCK1. Phosphorylation is required for both Ca(2+) and MAPK signaling pathways (By similarity). Phosphorylation of Tyr-103, Tyr-194 and Tyr-205 facilitates PLCG1 binding. Phosphorylation of Tyr-115 facilitates BTK binding. Phosphorylation of Tyr-91 facilitates VAV1 and NCK1 binding.