Functional analysis of Csk in signal transduction through the B-cell antigen receptor

CD45 modulates phosphorylation of both autophosphorylation and negative regulatory tyrosines of Lyn in B cells

CD45 is a tyrosine phosphatase that is required for normal B cell receptor (BCR)-mediated signaling. It has been shown that Src-family tyrosine kinases such as Lyn could be a potential substrate for CD45. In vitro studies indicate that activities of Src family tyrosine kinases are regulated by tyrosine phosphorylation; C-terminal phosphorylation is inhibitory, and autophosphorylation is stimulatory. We report here that both autophosphorylation and C-terminal negative regulatory tyrosines of Lyn were hyperphosphorylated in CD45-deficient DT40 B cells. In this mutant cell, BCR-induced protein-tyrosine phosphorylation and calcium mobilization were severely compromised, as seen in Lyn-deficient cells. Consistent with this observation, Lyn activation upon receptor ligation was profoundly decreased in CD45-deficient cells. Taken together, our results suggest that dephosphorylation of tyrosine residues at both autophosphorylation and negative regulatory sites is mediated by CD45 in vivo, and that dephosphorylation of C-terminal tyrosine is a prerequisite for participation of Lyn in BCR signaling.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Csk is constitutively associated with a 60-kDa tyrosine-phosphorylated protein in human T cells

The protein-tyrosine kinase Csk is one of the main down-regulators of the Src family of kinases. Csk may be involved in the down-regulation of T cell receptor (TCR) signaling by C-terminal tyrosine phosphorylation of Lck and Fyn; however, it is not known how Csk activity is regulated or how it targets these Src family members. We used Jurkat T cells and normal human T cells to examine proteins that bind to the SH2 domain of Csk. In both Jurkat and normal T cells, the Src homology 2 (SH2) domain of Csk bound constitutively to a tyrosine-phosphorylated protein of 60 kDa (p60). The 60-kDa protein was detected in Csk immunoprecipitates from both unstimulated and CD3-stimulated cells. In addition to p60, a protein of 190 kDa coprecipitated with Csk, and both proteins were phosphorylated on tyrosine residues by the immunocomplex. Small amounts of GTPase-activating protein (GAP) were detected in anti-Csk immunoprecipitates, suggesting that p60 may be a GAP-associated protein. Our data demonstrate that the SH2 domain of Csk specifically associates with at least two tyrosine-phosphorylated proteins in normal human T cells, that this association is independent of TCR/CD3 activation, and that Csk may be a part of a multiprotein complex containing GAP.

Genetic analysis of tyrosine kinase function in B cell development

B lymphopoiesis is regulated by multiple signals from stromal cell contact, soluble cytokines, antigen, and T helper cells. In vitro and biochemical experiments have implicated tyrosine kinases as key components of many of these signaling pathways. Genetic analysis of the role of these tyrosine kinases has been facilitated by recent advance in transgenic and gene targeting technology as well as by the identification of the genetic basis of several human and murine immune deficiencies. This review discusses the effect of gain and loss of function mutations of selected tyrosine kinases and their regulators and substrates on B cell development and function.

Detection of a physical and functional interaction between Csk and Lck which involves the SH2 domain of Csk and is mediated by autophosphorylation of Lck on tyrosine 394

The COOH-terminal Src kinase (Csk) is responsible for the phosphorylation of the conserved, negative regulatory, carboxyl-terminal tyrosine of most of the Src family protein tyrosine kinases. Up to now, no stable binding of Csk to Src kinases has been detected. We therefore decided to analyze this interaction using two systems which allow detection of transient interaction. We produced and purified recombinant proteins in the glutathione S-transferase prokaryotic expression system. First, using real-time biospecific interaction analysis (BIAcore(TM)), we detected in vitro a specific interaction between Csk and one of its substrates Lck, a lymphocyte-specific member of the Src family. This interaction requires the autophosphorylation of Lck on tyrosine 394 (the phosphorylation of which is correlated with an increase of the kinase activity) and involves a functional Csk SH2 domain. Second, using the yeast two-hybrid system, we confirmed in vivo the physical interaction between Csk and Lck. Furthermore, in vitro we showed that autophosphorylation of Lck on tyrosine 394 enhances the phosphorylation of Lck by Csk on the negative regulatory site, tyrosine 505, suggesting that activated Lck serves preferentially as substrate for Csk. These findings might explain the mechanism(s) by which Csk interacts with most of Src kinases to down-regulate their kinase activity.

Cooperation of tyrosine kinases p72syk and p53/56lyn regulates calcium mobilization in chicken B cell oxidant stress signaling

A chicken B cell line DT40 and its syk-negative or lyn-negative mutants were used to investigate the roles of protein-tyrosine kinases in oxidant stress signaling. The data presented here for wild-type cells demonstrate that hydrogen peroxide stimulates p53/56lyn-dependent tyrosine phosphorylation and activation of p72syk, and induces a rapid and prolonged elevation of intracellular calcium, which consists of calcium release from intracellular stores and influx from the extracellular space. Hydrogen-peroxide-triggered calcium mobilization was impaired in both syk-negative and lyn-negative cells, which was mainly due to the loss of calcium release from intracellular stores. Further studies indicated that inositol trisphosphate production was also abolished in both syk-negative and lyn-negative cells, which is consistent with the loss of calcium release. Taken together, these observations suggest that the defect of p72syk or p53/56lyn was responsible for the abnormality of calcium mobilization in both lyn-negative and syk-negative cells, and that both p72syk and p53/56lyn might regulate calcium mobilization through the phosphatidylinositol pathway in B cell oxidant stress signaling.

Nuclear signaling by endothelin-1 requires Src protein-tyrosine kinases

In response to changes in vascular homeostasis, endothelial cells secrete endothelin-1 (ET-1), which in turn regulates gene expression and phenotype in underlying vascular cells. We characterized a nuclear signaling cascade in which Src protein-tyrosine kinases link the ET-1 receptor to induction of c-fos transcription. A dominant negative SrcK- kinase mutant blocked ET-1-stimulated c-fos transcription. Expression of the COOH-terminal Src kinase (Csk), which represses Src kinases, also blocked induction of c-fos transcription by ET-1. Activation of the c-fos promoter by ET-1 required both the CArG DNA sequence of the c-fos serum response element and the Ca2+/cAMP response element. In contrast, Src-induced c-fos transcription required only the CArG cis-element, demonstrating a divergence in signals regulating c-fos transcription. Thus, Src kinases contribute to a nuclear signaling cascade linking an ET-1 receptor to the CArG element of the c-fos serum response element. A Src-based pathway might play a more general role to propagate ET-1 nuclear signals that regulate cell growth and development. In addition, these results point to a widening role for nonreceptor protein-tyrosine kinases in propagating signals from G protein-coupled receptors.

Targeted disruption of H2B-V encoding a particular H2B histone variant causes changes in protein patterns on two-dimensional polyacrylamide gel electrophoresis in the DT40 chicken B cell line

The chicken H2B gene family comprises eight members (H2B-I to H2B-VIII), which are all located in two major histone gene clusters. All of them have been shown to encode four different protein variants (classes I to IV). In the DT40 chicken B cell line, the H2B-V gene, encoding the class III H2B variant, constituted about 10% of the total intracellular mRNA from all the H2B genes. To study the nature of this particular variant in vivo, we generated heterozygous (H2B-V, +/-) and homozygous (H2B-V, -/-) DT40 mutants by targeted integration. The remaining H2B genes were shown to be expressed more in these mutants than in the wild-type cell lines. The growth rate of DT40 cells was unchanged in the absence of the H2B-V gene. Two-dimensional polyacrylamide gel electrophoresis showed that the protein patterns were, on the whole, similar between the wild-type and homozygous cell lines. However, within this constant background, some cellular proteins disappeared or decreased quantitatively in the homozygous mutants, and several other proteins increased or newly appeared. These results suggest that the class III H2B variant participates negatively or positively in regulation of the expression of particular genes that encode the proteins that vary in DT40 cells. This type of regulation is possibly mediated through alterations in nucleosome structure over the restricted regions involving the putative genes of the DT40 genome.

Transmembrane signaling by antigen receptors of B and T lymphocytes

The specificity of immune responses depends upon the activation of only those lymphocytes that recognize the introduced antigen. In recent years, a great deal has been learned about the structure of lymphocyte receptors for antigens and about their signal transduction mechanism. These receptors activate intracellular protein tyrosine kinases of at least two families, the Src family and the Syk/ZAP-70 family. Recent studies have given us considerable insight into the interactions of these two types of kinases and how they mediate antigen receptor signaling.