Mediation by the Protein-tyrosine Kinase Tec of Signaling between the B Cell Antigen Receptor and Dok-1

Abstract

The protein-tyrosine kinase (PTK) 1 Tec was initially isolated from mouse liver (1) and was subsequently shown to be expressed in many tissues, including spleen, lung, brain, and kidney (2). Four Tec-related PTKs, including Btk (3, 4), Itk (also known as Emt or Tsk) (5-7), Bmx (8), and Txk (or Rlk) (9, 10), have since been molecularly cloned. With the exception of Txk, Tec and the Tec-related PTKs possess a relatively long NH 2 -terminal region that consists of a pleckstrin homology (PH) domain (11) and a Tec homology (TH) domain (12). The PH domain is thought to mediate protein binding to various phospholipids or phospholipid-derived molecules; for example, the Tec PH domain binds to phosphatidylinositol (PI) 3,4,5-trisphosphate (13), and the Btk PH domain binds to inositol 1,3,4,5-tetrakisphosphate (14) and PI 3,4,5-trisphosphate (15). These PH domain-phospholipid interactions are thought to mediate the conditional tethering of Tec family kinases to the cell membrane, suggesting that these enzymes might act downstream of PI 3-kinase. The TH domain of Tec PTKs contains proline-rich sequences that interact with the Src homology (SH) 3 domain of these same proteins (16) and, probably, also with the SH3 domains of other proteins. The intramolecular interaction between the TH and SH3 domains results in the bending of Tec proteins, which likely serve to mask their catalytic centers and to inhibit kinase activity.Several Tec proteins are abundant in hematopoietic tissues and are therefore thought to play important roles in the development or maintenance of the hematopoietic system. Indeed, Tec PTKs are activated in blood cells by stimulation of cytokine receptors, lymphocyte surface antigens, G protein-coupled receptors, receptor type PTKs, or integrins (17). Furthermore, a functional Btk is indispensable for the maturation of B lymphocytes and the subsequent production of immunoglobulins. However, the downstream effectors of Tec family kinases remain largely unknown. Tec, Btk, and Itk each phosphorylate and activate phospholipase C (PLC)-␥2 (18). Candidate substrates for Btk also include BAP-135 (or TFII-I) (19) and WASP (20) and those for Tec include Grb10 (21), BRDG1 (22), and Sak kinase. 2Stimulation of cell surface receptors often induces the tyrosine phosphorylation of two unidentified Tec-binding proteins in addition to that of Tec. One of these Tec-binding phosphoproteins, p62, gives rise to a broad blurred band of ϳ62-66 kDa on immunoblot analysis with antibodies to phosphotyrosine, suggesting that the protein is phosphorylated on multiple tyrosine residues. The other Tec-binding phosphoprotein migrates at a position corresponding to a molecular size of ϳ56 kDa. Tyrosine phosphorylation of p62 is induced by cross-linking of the B cell antigen receptor (BCR) in B lymphocytes (23) and by activation of c-Kit in myeloid cells (24). Immunoblot analysis indicates that p62 is not identical to Sam68 or SHC. The protein p62Dok-1 was isolated as a major substrate for activated Abl tyrosine kinases (25,26). Dok-1 conta...

Gα12 activates Rho GTPase through tyrosine-phosphorylated leukemia-associated RhoGEF

“…We also examined tyrosine phosphorylation of LARG in cells. A Tec construct with an N-terminal myristoylation signal (mHTec) was targeted to the plasma membrane and exhibited constitutive activity (19). As shown in Fig.…”

“…LARG (1-1543), ⌬PDZ-LARG (307-1543), ⌬N-LARG (617-1543), PDZ-RhoGEF, p115RhoGEF, Tec (1-629), and kinase domain-deleted Tec (Tec-KD) (1-358) were subcloned into pcDNA-myc vector with N-terminal myc-tag. cDNAs for Tec lacking TH domain (⌬TH-Tec) and the constitutively active form of Tec (mHTec), which has N-terminal myristoylation signal, were subcloned into pSR␣ mammalian expression vector (17,19). cDNAs encoding the constitutively active G␣12 (G␣12Q229L) and G␣13 (G␣13Q226L) were subcloned into pCMV5 vector.…”

Signaling network of the Btk family kinases

“…This protein was recently shown to be associated with Tec (but not Btk or Itk) and is a substrate of Tec (Yoshida et al, 2000). Indeed, the tyrosine phosphorylation of Dok-1 by Tec, enables Dok-1 to bind the SH2 domain of Tec, which in turn hyperphosphorylates Dok-1.…”

“…First, in a cell type that lacks PTEN, a lipid phosphatase which counteracts PI3K, Itk is constitutively localized on the plasma membrane and hypersensitive to T cell receptor activation (Shan et al, 2000). Second, the attachment of a myristylation site or the extracellular and transmembrane domain of cytokine receptor to Tec lead to constitutive activation and membrane localization, in the absence of PI3K activation (Li et al, 1997a;Yoshida et al, 2000). Third, the importance of unfolding the PH domain during activation is underscored by the ®nding that deletion of the PH domain leads to constitutive activation of Etk/Bmx in either prostate or lung epithelial cells (Qiu et al, 1998b;Wen et al, 1999).…”

p62dokNegatively Regulates CD2 Signaling in Jurkat Cells

“…Depending on the transduction pathway examined, Dok proteins can act as a positive or negative regulator. In B cells, phosphorylation of p62 dok is involved in the Fc␥RIIB-mediated inhibition of B cell receptor signaling (13,14) most likely by negatively regulating the activity of Ras (15), thereby inhibiting the Ras-dependent activation of extracellular signal-regulated kinase 1/2 (Erk1/2) (13,14). By contrast, transient overexpression of p62 dok in 293 cells (8) and in Chinese hamster ovary cells expressing insulin receptors (12) has been reported to have no effect on v-abldependent and insulin-dependent mitogen-activated protein kinase activation, respectively.…”