Current models of lymphocyte activation suggest that formation of a signaling complex, or "signalosome", composed of Syk, Bruton's tyrosine kinase (Btk), phospholipase gamma2 and the adaptor protein B cell linker protein (BLNK) is critical for transmission of signals from the BCR. However, impaired B cell development in mice lacking each individual signalosome component has made it difficult to study the functional consequences of the formation of this complex in mature B cells. Sensitized genetic systems, commonly used in Drosophila, define signaling pathways by combining partial loss of function mutations in the components of interest. This allows genetic interactions to be observed in the absence of pleiotropic or lethal effects of complete deficiency of either gene. We used this approach to demonstrate that Btk and BLNK are limiting components of a common signaling pathway that mediates the mitogenic response of mature B cells to antigen. B cells from transgenic mice expressing a limiting dosage of Btk (Btk(lo)) have normal numbers of mature B cells that have reduced, but measurable, responses to BCR cross-linking. Haploinsufficiency of BLNK did not affect the development of Btk(lo) B cells. However, it exacerbated their defects in BCR-induced Ca(2+) flux, IkappaB degradation, and up-regulation of cyclin D2, bcl-x(L) and A1 leading to dramatic impairment of B cell mitogenic responses. In contrast, no effect of reduced Btk and BLNK dosage was observed on extracellular signal-regulated kinase activation. These results suggest that the signals regulating the maintenance and activation of mature B cells are differentially sensitive to the strength of the signal emanating from the signalosome.