Activation of phosphatidylinositol-3' kinase by Src-family kinase SH3 binding to the p85 subunit

The CD19/CD21 signal transduction complex of B lymphocytes

CD19 and CD21 are B-cell surface molecules that associate with each other and with CD81 and Leu-13 to generate a signal transduction complex that is independent of the antigen receptor. Current studies, reviewed here by Thomas Tedder, Liang-Ji Zhou and Pablo Engel, indicate an important biological role for this protein complex in the regulation of B-cell development and activation.

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.

Tec homology (TH) adjacent to the PH domain

The pleckstrin homology (PH) domain is extended in the Btk kinase family by a region designated the TH (Tec homology) domain, which consists of about 80 residues preceding the SH3 domain. The TH domain contains a conserved 27 amino acid stretch designated the Btk motif and a proline-rich region. Sequence similarity was found to a putative Ras GTPase activating protein and a human interferon-gamma binding protein both in the PH domain and the Btk motif region. SLK1/SSP31 protein kinase and a non-catalytic p85 subunit of PI-3 kinase had similarity only with the proline rich region. The identification of a PH domain extension in some signal transduction proteins in different species suggests that this region is involved in protein-protein interactions.

Deletion within the Src homology domain 3 of Bruton's tyrosine kinase resulting in X-linked agammaglobulinemia (XLA)

The gene responsible for X-linked agammaglobulinemia (XLA) has been recently identified to code for a cytoplasmic tyrosine kinase (Bruton's agammaglobulinemia tyrosine kinase, BTK), required for normal B cell development. BTK, like many other cytoplasmic tyrosine kinases, contains Src homology domains (SH2 and SH3), and catalytic kinase domain. SH3 domains are important for the targeting of signaling molecules to specific subcellular locations. We have identified a family with XLA whose affected members have a point mutation (g-->a) at the 5' splice site of intron 8, resulting in the skipping of coding exon 8 and loss of 21 amino acids forming the COOH-terminal portion of the BTK SH3 domain. The study of three generations within this kinship, using restriction fragment length polymorphism and DNA analysis, allowed identification of the mutant X chromosome responsible for XLA and the carrier status in this family. BTK mRNA was present in normal amounts in Epstein-Barr virus-induced B lymphoblastoid cell lines established from affected family members. Although the SH3 deletion did not alter BTK protein stability and kinase activity of the truncated BTK protein was normal, the affected patients nevertheless have a severe B cell defect characteristic for XLA. The mutant protein was modeled using the normal BTK SH3 domain. The deletion results in loss of two COOH-terminal beta strands containing several residues critical for the formation of the putative SH3 ligand-binding pocket. We predict that, as a result, one or more crucial SH3 binding proteins fail to interact with BTK, interrupting the cytoplasmic signal transduction process required for B cell differentiation.

Phosphatidylinositol 3-kinase

Currently, a central question in biology is how signals from the cell surface modulate intracellular processes. In recent years phosphoinositides have been shown to play a key role in signal transduction. Two phosphoinositide pathways have been characterized, to date. In the canonical phosphoinositide turnover pathway, activation of phosphatidylinositol-specific phospholipase C results in the hydrolysis of phosphatidylinositol 4,5-bisphosphate and the generation of two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. The 3-phosphoinositide pathway involves protein-tyrosine kinase-mediated recruitment and activation of phosphatidylinositol 3-kinase, resulting in the production of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. The 3-phosphoinositides are not substrates of any known phospholipase C, are not components of the canonical phosphoinositide turnover pathway, and may themselves act as intracellular mediators. The 3-phosphoinositide pathway has been implicated in growth factor-dependent mitogenesis, membrane ruffling and glucose uptake. Furthermore the homology of the yeast vps34 with the mammalian phosphatidylinositol 3-kinase has suggested a role for this pathway in vesicular trafficking. In this review the different mechanisms employed by protein-tyrosine kinases to activate phosphatidylinositol 3-kinase, and its involvement in the signaling cascade initiated by tyrosine phosphorylation, are examined.

Distinct p53/56lyn and p59fyn domains associate with nonphosphorylated and phosphorylated Ig-alpha

Among the earliest detectable events in B-cell antigen receptor-mediated signal transduction are the activation of receptor-associated Src-family tyrosine kinases and the tyrosine phosphorylation of Ig-alpha and Ig-beta receptor subunits. These kinases appear to interact with resting B-cell antigen receptor complexes primarily through the Ig-alpha chain antigen receptor homology 1 (ARH1) motif. Recent studies showed a dramatic increase in the amount of Src-family kinase p59fyn bound to Ig-alpha when ARH1 motif tyrosines were phosphorylated. To explore the submolecular basis of these interactions, we conducted mutational analysis to localize sites in p53/56lyn and p59fyn that bind nonphosphorylated and phosphorylated Ig-alpha. Here we report that distinct regions within these kinases bind nonphosphorylated and phosphorylated Ig-alpha ARH1 motifs. The N-terminal 10 residues mediate binding to the nonphosphorylated Ig-alpha ARH1 motif. Association with the phosphorylated Ig-alpha ARH1 motif is mediated by Src homology 2 domains. These findings suggest a mechanism whereby ligand-induced Ig-alpha tyrosine phosphorylation initiates a change in the orientation of an associated kinase that may alter its activity and/or access to substrates and other effectors.

The hetero-oligomeric antigen receptor complex and its coupling to cytoplasmic effectors

T-cell and B-cell antigen receptors are representative of a family of multisubunit receptors that utilize Src-family kinases as proximal cytoplasmic effectors in signal transduction. Recent studies have shown that distinct receptor subunits mediate ligand and effector interactions and demonstrate that physical interaction with effectors, and their activation, is a function of a 26 amino acid motif found in multiple receptor subunits. Further, receptor ligation induces tyrosine phosphorylation of this motif, and this initiates SH2-mediated association and activation of Src-family kinases and, apparently, ZAP70 kinases. Finally, this association triggers SH3-mediated binding of Lyn and Fyn to PI3-K, resulting in PI3-K activation. An integrated model of signal transduction is presented.

The GRB family of SH2 domain proteins

The cloning of SH2 domain proteins based on their binding to growth factor receptors is a powerful technique to elucidate new signaling pathways. In some cases the function of these proteins has been quickly ascertained while in others the answers still elude us. However the major power of the technique is its ability to identify novel signaling cascades that can emanate from tyrosine kinases. The challenge is to define the nature of these signaling cascades.