Characteristic of chronic myelogenous leukemia (CML) is the presence of the chimeric p210(bcr-abl) protein possessing elevated protein tyrosine kinase activity relative to normal c-abl tyrosine kinase. Hematopoietic progenitors isolated from CML patients in the chronic phase contain a constitutively tyrosine-phosphorylated protein that migrates at 62 kDa by SDS-PAGE and associates with the p120 ras GTPase-activating protein (GAP). We have purified p62(dok) from a hematopoietic cell line expressing p210(bcr-abl). p62(dok) is a novel protein with features of a signaling molecule. Association of p62(dok) with GAP correlates with its tyrosine phosphorylation. p62(dok) is rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases.

Many receptor and nonreceptor tyrosine kinases activate phosphoinositide 3-kinases (PI3Ks). To assess the role of the ␦ isoform of the p110 catalytic subunit of PI3Ks, we derived enzyme-deficient mice. The mice are viable but have decreased numbers of mature B cells, a block in pro-B-cell differentiation, and a B1 B-cell deficiency. Both immunoglobulin M receptor-induced Ca 2؉ flux and proliferation in response to B-cell mitogens are attenuated. Immunoglobulin levels are decreased substantially. The ability to respond to T-cellindependent antigens is markedly reduced, and the ability to respond to T-cell-dependent antigens is completely eliminated. Germinal center formation in the spleen in response to antigen stimulation is disrupted. These results define a nonredundant signaling pathway(s) utilizing the ␦ isoform of p110 PI3K for the development and function of B cells.Lymphocyte development and function are regulated through the coordinated action of receptors of the cytokine receptor superfamily and the B-cell antigen-specific receptor (BCR) or T-cell antigen-specific receptors (TCR). Engagement of either receptor complex initiates tyrosine phosphorylation of a variety of intracellular substrates, including receptor chains, resulting in the initiation of cellular responses. Members of the cytokine receptor superfamily utilize JAK family cytoplasmic kinases (14), while the BCR and TCR complexes utilize members of the Src, Tec, and Zap70/Syk families of tyrosine kinases. In BCR signal transduction, the Tec family kinase Btk plays a critical role as evidenced by the loss of a proliferative response to BCR engagement in Btk-deficient B cells (15,16). Among the substrates typically phosphorylated and recruited to hematopoietic receptor complexes are the regulatory subunits for phosphoinositide 3-kinases (PI3Ks) (9,10,30). In mammals there are three genes that encode adapter proteins for PI3K catalytic subunits, including p85␣, p85␤, and p55␥. The adapter proteins facilitate association of the catalytic subunits with the receptor complex and are proposed to regulate enzyme activity. The disruption of the p85␣ gene, in a manner that deletes the p85 isoform as well as two splice variants of p55 and p50, results in defective BCR signaling comparable to that seen with Btk deficiency (11,26). This strongly suggests that PI3K activity is critical in BCR signal transduction.Three of the known PI3Ks, i.e., PI3K␣, PI3K␤, and PI3K␦, are regulated through their interaction with regulatory subunits (30). The fourth PI3K, PI3K␥, functions in the context of heterotrimeric G-protein-coupled receptors and is essential for leukocyte function (12). The critical, nonredundant role that PI3K␣ plays in cellular responses has been demonstrated through the derivation of mice lacking the gene. This deficiency results in an embryonic lethality at E9.5 to E10.5 due to a severe proliferative defect in many tissues (2). Similarly, deletion of the PI3K␤ gene alone results in a very early embryonic lethality (1, 2). In contrast to PI3K␣ and PI...

T cells play a central role in the recognition and elimination of foreign pathogens. Signals through the T cell receptor (TCR) control the extent and duration of the T cell response. To ensure that T cells are not inappropriately activated, signaling pathways downstream of the TCR are subject to multiple levels of positive and negative regulation. Herein, we describe two related proteins, Sts-1 and Sts-2, that negatively regulate TCR signaling. T cells from mice lacking Sts-1 and Sts-2 are hyperresponsive to TCR stimulation. The phenotype is accompanied by increased Zap-70 phosphorylation and activation, including its ubiquitinylated forms. Additionally, hyperactivation of signaling proteins downstream of the TCR, a marked increase in cytokine production by Sts1/2(-/-) T cells, and increased susceptibility to autoimmunity in a mouse model of multiple sclerosis is observed. Therefore, Sts-1 and Sts-2 are critical regulators of the signaling pathways that regulate T cell activation.