ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

c-Src tyrosine kinase binds the beta 2-adrenergic receptor via phospho-Tyr-350, phosphorylates G-protein-linked receptor kinase 2, and mediates agonist-induced receptor desensitization

The nonreceptor tyrosine kinase Src has been implicated in the switching of signaling of beta2-adrenergic receptors from adenylylcyclase coupling to the mitogen-activated protein kinase pathway. In the current work, we demonstrate that Src plays an active role in the agonist-induced desensitization of beta2-adrenergic receptors. Both the expression of dominant-negative Src and treatment with the 4-amine-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) inhibitor of Src kinase activity blocks agonist-induced desensitization. Agonist triggers tyrosine phosphorylation of the beta2-adrenergic receptor and recruitment and activation of Src. Because phosphorylation of the Tyr-350 residue of the beta2-adrenergic receptor creates a conditional, canonical SH2-binding site on the receptor, we examined the effect of the Y350F mutation on Src phosphorylation, Src recruitment, and desensitization. Mutant beta2-adrenergic receptors with a Tyr-to-Phe substitution at Tyr-350 do not display agonist-induced desensitization, Src recruitment, or Src activation. Downstream of binding to the receptor, Src phosphorylates and activates G-protein-linked receptor kinase 2 (GRK2), a response obligate for agonist-induced desensitization. Constitutively active Src increases GRK phosphorylation, whereas either expression of dominant-negative Src or treatment with the PP2 inhibitor abolishes tyrosine phosphorylation of GRK and desensitization. Thus, in addition to its role in signal switching to the mitogen-activated protein kinase pathway, Src recruitment to the beta2-adrenergic receptor and activation are obligate for normal agonist-induced desensitization.

ARG tyrosine kinase activity is inhibited by STI571

The tyrosine kinase inhibitor STI571 inhibits BCR/ABL and induces hematologic remission in most patients with chronic myeloid leukemia. In addition to BCR/ABL, STI571 also inhibits v-Abl, TEL/ABL, the native platelet-derived growth factor (PDGF)beta receptor, and c-KIT, but it does not inhibit SRC family kinases, c-FMS, FLT3, the epidermal growth factor receptor, or multiple other tyrosine kinases. ARG is a widely expressed tyrosine kinase that shares substantial sequence identity with c-ABL in the kinase domain and cooperates with ABL to regulate neurulation in the developing mouse embryo. As described here, ARG has recently been implicated in the pathogenesis of leukemia as a fusion partner of TEL. A TEL/ARG fusion was constructed to determine whether ARG can be inhibited by STI571. When expressed in the factor-dependent murine hematopoietic cell line Ba/F3, the TEL/ARG protein was heavily phosphorylated on tyrosine, increased tyrosine phosphorylation of multiple cellular proteins, and induced factor-independent proliferation. The effects of STI571 on Ba/F3 cells transformed with BCR/ABL, TEL/ABL, TEL/PDGFbetaR, or TEL/ARG were then compared. STI571 inhibited tyrosine phosphorylation and cell growth of Ba/F3 cells expressing BCR/ABL, TEL/ABL, TEL/PDGFbetaR, and TEL/ARG with an IC(50) of approximately 0.5 microM in each case, but it had no effect on untransformed Ba/F3 cells growing in IL-3 or on Ba/F3 cells transformed by TEL/JAK2. Culture of TEL/ARG-transfected Ba/F3 cells with IL-3 completely prevented STI571-induced apoptosis in these cells, similar to what has been observed with BCR/ABL- or TEL/ABL-transformed cells. These results indicate that ARG is a target of the small molecule, tyrosine kinase inhibitor STI571.

Recurrent pneumonia with mild hypogammaglobulinemia diagnosed as X-linked agammaglobulinemia in adults

BACKGROUND

X-linked agammaglobulinemia (XLA) is a humoral immunodeficiency caused by disruption of the Bruton's tyrosine kinase (BTK) gene. Typical XLA patients suffer recurrent and severe bacterial infections in childhood.

METHODS

Flow cytometric analysis of the peripheral monocytes using the anti-BTK antibody was used to characterize a 27 year old male patient with mild hypogammaglobulinemia (IgG, 635 mg/dl; IgM, 11 mg/dl; IgA, <5 mg/dl). He had suffered from frequent pneumonia since age 25 but had no history of frequent infections in his childhood or in adolescence. Sequencing of the BTK cDNA obtained from an Epstein-Barr virus-transformed B lymphoblastoid cell line derived from the bone marrow of the patient was performed to confirm a genetic defect.

RESULTS

Flow cytometric analysis of cytoplasmic BTK protein in peripheral monocytes indicated that the patient presents a rare case of adult-onset XLA and that his mother is an XLA carrier. Sequencing of the BTK gene revealed a deletion of AG in the codon for Glu605 (AGT), resulting in an aberrant stop codon that truncates the BTK protein in its kinase domain.

CONCLUSIONS

This case suggests that some XLA cases may remain undiagnosed because they only show mild hypogammaglobulinemia and they lack repeated infections in childhood. Flow cytometric analysis is a powerful method to screen these patients.

Dynamic coupling between the SH2 and SH3 domains of c-Src and Hck underlies their inactivation by C-terminal tyrosine phosphorylation

The effect of C-terminal tyrosine phosphorylation on molecular motions in the Src kinases Hck and c-Src is investigated by molecular dynamics simulations. The SH2 and SH3 domains of the inactive kinases are seen to be tightly coupled by the connector between them, impeding activation. Dephosphorylation of the tail reduces the coupling between the SH2 and SH3 domains in the simulations, as does replacement of connector residues with glycine. A mutational analysis of c-Src expressed in Schizosaccharomyces pombe demonstrates that replacement of residues in the SH2-SH3 connector with glycine activates c-Src. The SH2-SH3 connector appears to be an inducible "snap lock" that clamps the SH2 and SH3 domains upon tail phosphorylation, but which allows flexibility when the tail is released.

A mutant Drosophila insulin receptor homolog that extends life-span and impairs neuroendocrine function

The Drosophila melanogaster gene insulin-like receptor (InR) is homologous to mammalian insulin receptors as well as to Caenorhabditis elegans daf-2, a signal transducer regulating worm dauer formation and adult longevity. We describe a heteroallelic, hypomorphic genotype of mutant InR, which yields dwarf females with up to an 85% extension of adult longevity and dwarf males with reduced late age-specific mortality. Treatment of the long-lived InR dwarfs with a juvenile hormone analog restores life expectancy toward that of wild-type controls. We conclude that juvenile hormone deficiency, which results from InR signal pathway mutation, is sufficient to extend life-span, and that in flies, insulin-like ligands nonautonomously mediate aging through retardation of growth or activation of specific endocrine tissue.

Extension of life-span by loss of CHICO, a Drosophila insulin receptor substrate protein

The Drosophila melanogaster gene chico encodes an insulin receptor substrate that functions in an insulin/insulin-like growth factor (IGF) signaling pathway. In the nematode Caenorhabditis elegans, insulin/IGF signaling regulates adult longevity. We found that mutation of chico extends fruit fly median life-span by up to 48% in homozygotes and 36% in heterozygotes. Extension of life-span was not a result of impaired oogenesis in chico females, nor was it consistently correlated with increased stress resistance. The dwarf phenotype of chico homozygotes was also unnecessary for extension of life-span. The role of insulin/IGF signaling in regulating animal aging is therefore evolutionarily conserved.

Autonomous maturation of alpha/beta T lineage cells in the absence of COOH-terminal Src kinase (Csk)

The deletion of COOH-terminal Src kinase (Csk), a negative regulator of Src family protein tyrosine kinases (PTKs), in immature thymocytes results in the development of alpha/beta T lineage cells in T cell receptor (TCR) beta-deficient or recombination activating gene (rag)-1-deficient mice. The function of Csk as a repressor of Lck and Fyn activity suggests activation of these PTKs is solely responsible for the phenotype observed in csk-deficient T lineage cells. We provide genetic evidence for this notion as alpha/beta T cell development is blocked in lck(-/)-fyn(-/)- csk-deficient mice. It remains unclear whether activation of Lck and Fyn in the absence of Csk uncouples alpha/beta T cell development entirely from engagement of surface-expressed receptors. We show that in mice expressing the alpha/beta TCR on csk-deficient thymocytes, positive selection is biased towards the CD4 lineage and does not require the presence of major histocompatibility complex (MHC) class I and II. Furthermore, the introduction of an MHC class I-restricted transgenic TCR into a csk-deficient background results in the development of mainly CD4 T cells carrying the transgenic TCR both in selecting and nonselecting MHC background. Thus, TCR-MHC interactions have no impact on positive selection and commitment to the CD4 lineage in the absence of Csk. However, TCR-mediated negative selection of csk-deficient, TCR transgenic cells is normal. These data suggest a differential involvement of the Csk-mediated regulation of Src family PTKs in positive and negative selection of developing thymocytes.

Requirement of tyrosine-phosphorylated Vav for morphological differentiation of all-trans-retinoic acid-treated HL-60 cells

Our previous data demonstrated that cellular and nuclear tyrosine-phosphorylated Vav associate with phosphoinositide 3-kinase during all-trans-retinoic acid-dependent granulocytic differentiation of HL-60 cells. In this study, aimed to analyze the mechanism by which Vav is recruited and activated, we report that the Src homology 2 domain of Vav interacts with tyrosine-phosphorylated proteins in a differentiation-dependent manner. Two adaptor proteins, Cbl and SLP-76, were identified, showing a discrete distribution inside the cells, with Cbl absent from the nuclei and SLP-76 particularly abundant in the nuclear compartment. Of note, Vav interacts with the tyrosine kinase Syk, which is also present in the nuclear compartment and may phosphorylate Vav in vitro when cells differentiate. Inhibition of Syk activity by piceatannol prevents both in vitro and in vivo Vav tyrosine phosphorylation, its association with the regulatory subunit of phosphoinositide 3-kinase, and the nuclear modifications typically observed during granulocytic differentiation of this cell line. These findings suggest that tyrosine-phosphorylated Vav and its association with phosphoinositide 3-kinase play a crucial role in all-trans-retinoic acid-induced reorganization of the nucleoskeleton, which is responsible for the changes in nuclear morphology observed during granulocytic differentiation of HL-60 cells.

A contiguous deletion syndrome of X-linked agammaglobulinemia and sensorineural deafness

Hearing loss in patients with X-linked agammaglobulinemia is often attributed to recurrent infections. However, recent genetic studies suggest a different etiology in some patients. We present three unrelated patients, 6, 9, and 14 years of age, with large deletions of the terminal portion of the Bruton tyrosine kinase (Btk) gene extending 4.2-19 kb beyond the 3' end of the gene. The DNA immediately downstream of the 3' end of Btk contains the deafness-dystonia protein gene (DDP). Mutations in this gene have recently been shown to underlie the Mohr-Tranebjaerg syndrome, which is characterized by sensorineural deafness, dystonia, and mental deficiency. Besides the immunodeficiency, our patients exhibited progressive sensorineural deafness. The clue to an associated hearing problem was delayed development of speech in one patient and post-lingual deafness noticed between the age of 3-4 years in the other two. These patients have not yet exhibited significant associated neurologic deficits.

Evidence of a possible epigenetic inactivation mechanism operating on a region of mouse chromosome 19 in γ-radiation-induced thymic lymphomas

Loss of heterozygosity (LOH) analysis, performed in 68 gamma-radiation-induced primary thymic lymphomas of F1 hybrid mice, provided evidence of significant LOH on chromosome 19 in a region defined by the D19Mit106 (22 cM) and D19Mit100 (27 cM) markers (Thymic Lymphoma Suppressor Region 8, TLSR8). Cd95 and Pten, two genes mapped at this region, were inactivated in a vast majority of these tumors (85.3% for Cd95 and 61.8% for Pten). Moreover, altered expression of Cd95 and Pten occurred concomitantly in 34 of 68 (50%) thymic lymphomas suggesting a coordinated mechanism of inactivation of these genes. Surprisingly, we also found that Jak2, a proto-oncogene located between Cd95 and Pten, was simultaneously inactivated in a significant fraction of the tumors analysed (24 of 34, 70.6%). Taken together these findings and the lack of mutations in the coding sequences of the mentioned genes clearly suggest a possible regional epigenetic inactivation mechanism on mouse chromosome 19 operating during the development of these tumors.

Involvement of 2p23 in pulmonary inflammatory pseudotumors

Pulmonary inflammatory pseudotumors (IP) are rare mesenchymal proliferations that have a polymorphic histology and an unpredictable biologic behavior. The histologic spectrum of IP has led to uncertainty as to whether this tumor has a reactive or neoplastic pathogenesis. Reports of extrapulmonary IP have identified clonal chromosomal aberrations involving 2p23 in the region of the ALK gene. Using fluorescence in situ hybridization with a probe flanking the ALK gene at 2p23 and immunostaining for the ALK gene product, we studied formalin-fixed, paraffin-embedded tissues of pulmonary IP and found a subset (33%) with 2p23 aberrations. We suggest that chromosomal rearrangements and ALK immunostaining may be helpful in the diagnosis of a group of pulmonary IP and should be investigated as a potential tool for predicting their future biologic behavior. An association with anaplastic large-cell lymphoma was also observed. HUM PATHOL 32:428-433.

Bruton's tyrosine kinase is required for signaling the CD79b-mediated pro-B to pre-B cell transition

Formation of the pre-BCR complex is a critical check point during B cell development and induces the transition of pro-B to pre-B cells. CD79b (Igbeta) is a signaling component in the pre-BCR complex, since differentiation to the pre-B phenotype is induced by cross-linking the CD79b expressed on developmentally arrested pro-B cells from recombination-activating gene (RAG)-2-deficient mice. Bruton's tyrosine kinase (BTK) plays important roles in B cell development. However, its molecular mechanisms in early B cell development are not fully understood. To examine whether BTK functions in CD79b-mediated signaling for the pro-B/pre-B transition, we utilized RAG2/BTK double-knockout (DKO) mice. Pro-B cells from RAG2/BTK-DKO mice did not differentiate into pre-B cells following CD79b cross-linking, although tyrosine phosphorylation of cellular proteins including Erk1/2 and phospholipase C-gamma2 was induced in the same manner as RAG2-KO mice. BTK is phosphorylated after cross-linking of CD79b on RAG2-deficient pro-B cells. These findings suggest that BTK-dependent pathways downstream of CD79b are critical for the pro-B/pre-B transition and BTK-independent signaling pathways are also activated via the pre-BCR complex.

PKC-dependent activation of FAK and src induces tyrosine phosphorylation of Cas and formation of Cas-Crk complexes

SH-SY5Y neuroblastoma cells are a well-characterized model for studying the induction of neuronal differentiation. TPA treatment of these cells induces cytoskeletal rearrangements that ultimately result in neurite extension. However, the signaling pathways that precede these changes are poorly understood. Other investigators have shown that TPA treatment of SH-SY5Y cells results in increased tyrosine phosphorylation of cytoskeletal-associated proteins, including the adapter protein Cas. In this report, we examine the events upstream and downstream of Cas phosphorylation. We show that TPA treatment induces the PKC-dependent association of tyrosine-phosphorylated Cas with Crk. The activity of two protein tyrosine kinases, Src and FAK, was shown to be necessary and sufficient for TPA-induced Cas phosphorylation. We propose that the PKC-dependent phosphorylation of Cas by Src and FAK promotes the establishment of Cas-Crk complexes and that these interactions may play an important role in regulating the actin cytoskeleton during neuronal differentiation.

Chronic cocaine-mediated changes in non-human primate nucleus accumbens gene expression

Chronic cocaine use elicits changes in the pattern of gene expression within reinforcement-related, dopaminergic regions. cDNA hybridization arrays were used to illuminate cocaine-regulated genes in the nucleus accumbens (NAcc) of non-human primates (Macaca fascicularis; cynomolgus macaque), treated daily with escalating doses of cocaine over one year. Changes seen in mRNA levels by hybridization array analysis were confirmed at the level of protein (via specific immunoblots). Significantly up-regulated genes included: protein kinase A alpha catalytic subunit (PKA(calpha)); cell adhesion tyrosine kinase beta (PYK2); mitogen activated protein kinase kinase 1 (MEK1); and beta-catenin. While some of these changes exist in previously described cocaine-responsive models, others are novel to any model of cocaine use. All of these adaptive responses coexist within a signaling scheme that could account for known inductions of genes(e.g. fos and jun proteins, and cyclic AMP response element binding protein) previously shown to be relevant to cocaine's behavioral actions. The complete data set from this experiment has been posted to the newly created Drug and Alcohol Abuse Array Data Consortium (http://www.arraydata.org) for mining by the general research community.

Residues Y429 and Y463 of the human CD5 are targeted by protein tyrosine kinases

The human CD5 lymphocyte cell surface co-receptor modulates activation and differentiation responses mediated by the antigen-specific receptor of T and B cells. CD5 is phosphorylated following lymphocyte activation; however, the exact sites and kinases involved are yet to be determined. Jurkat T cell transfectants expressing tyrosine-mutated CD5 molecules have been used to show that residues Y429 and Y463 are targeted in vivo by protein tyrosine kinases following cell stimulation with anti-CD3 mAb or pervanadate. This is in agreement with data from direct in vitro kinase assays using purified recombinant Lck and Fyn protein tyrosine kinases. The analysis of Lck- and CD3-deficient Jurkat cells shows that tyrosine phosphorylation of CD5 requires Lck activity. We propose that T cell activation mediates CD5 tyrosine phosphorylation at residues Y429 and Y463 mainly through the activation of Lck.

The spindle checkpoint of the yeast Saccharomyces cerevisiae requires kinetochore function and maps to the CBF3 domain

We have measured the activity of the spindle checkpoint in null mutants lacking kinetochore activity in the yeast Saccharomyces cerevisiae. We constructed deletion mutants for nonessential genes by one-step gene replacements. We constructed heterozygous deletions of one copy of essential genes in diploid cells and purified spores containing the deletion allele. In addition, we made gene fusions for three essential genes to target the encoded proteins for proteolysis (degron alleles). We determined that Ndc10p, Ctf13p, and Cep3p are required for checkpoint activity. In contrast, cells lacking Cbf1p, Ctf19p, Mcm21p, Slk19p, Cse4p, Mif2p, Mck1p, and Kar3p are checkpoint proficient. We conclude that the kinetochore plays a critical role in checkpoint signaling in S. cerevisiae. Spindle checkpoint activity maps to a discreet domain within the kinetochore and depends on the CBF3 protein complex.

p38 mitogen-activated protein kinase-dependent activation of protein phosphatases 1 and 2A inhibits MEK1 and MEK2 activity and collagenase 1 (MMP-1) gene expression

Degradation of collagenous extracellular matrix by collagenase 1 (also known as matrix metalloproteinase 1 [MMP-1]) plays a role in the pathogenesis of various destructive disorders, such as rheumatoid arthritis, chronic ulcers, and tumor invasion and metastasis. Here, we have investigated the role of distinct mitogen-activated protein kinase (MAPK) pathways in the regulation of MMP-1 gene expression. The activation of the extracellular signal-regulated kinase 1 (ERK1)/ERK2 (designated ERK1,2) pathway by oncogenic Ras, constitutively active Raf-1, or phorbol ester resulted in potent stimulation of MMP-1 promoter activity and mRNA expression. In contrast, activation of stress-activated c-Jun N-terminal kinase and p38 pathways by expression of constitutively active mutants of Rac, transforming growth factor beta-activated kinase 1 (TAK1), MAPK kinase 3 (MKK3), or MKK6 or by treatment with arsenite or anisomycin did not alone markedly enhance MMP-1 promoter activity. Constitutively active MKK6 augmented Raf-1-mediated activation of the MMP-1 promoter, whereas active mutants of TAK1 and MKK3b potently inhibited the stimulatory effect of Raf-1. Activation of p38 MAPK by arsenite also potently abrogated stimulation of MMP-1 gene expression by constitutively active Ras and Raf-1 and by phorbol ester. Specific activation of p38alpha by adenovirus-delivered constitutively active MKK3b resulted in potent inhibition of the activity of ERK1,2 and its upstream activator MEK1,2. Furthermore, arsenite prevented phorbol ester-induced phosphorylation of ERK1,2 kinase-MEK1,2, and this effect was dependent on p38-mediated activation of protein phosphatase 1 (PP1) and PP2A. These results provide evidence that activation of signaling cascade MKK3-MKK3b-->p38alpha blocks the ERK1,2 pathway at the level of MEK1,2 via PP1-PP2A and inhibits the activation of MMP-1 gene expression.

A case of X-linked agammaglobulinemia diagnosed in adulthood

X-linked agammaglobulinemia (XLA), caused by mutations in Bruton's tyrosine kinase (BTK), typically presents in early childhood. We report here the case of a male diagnosed at age 23 years with hypogammaglobulinemia, originally classified as common variable immunodeficiency (CVID). On further analysis at age 40, flow cytometric analysis of lymphocytes showed only 0.1% B cells and Western blot analysis showed a deficiency of BTK protein in peripheral blood mononuclear cells, indicating the patient has XLA. BTK cDNA and genomic DNA analysis revealed a splice site mutation at the 3' end of intron 13. Multiple abnormally spliced mRNA species were identified, one of which was predicted to produce a protein with a 24-amino-acid insertion between the SH2 and kinase domains. In vitro kinase assay of this product showed weak kinase activity, perhaps resulting in milder than usual disease. XLA can present in adult males, and sporadic cases may be misdiagnosed as CVID.

A homozygous missense mutation in the tyrosine E kinase domain of the RET proto-oncogene in an infant with total intestinal aganglionosis

Germline mutations of the RET proto-oncogene (RET), its ligand glial cell-derived neurotrophic factor (GDNF), and neurturin (NTN) gene have been reported in patients with Hirschsprung's disease. A targeted mutation in the tyrosine kinase domain of RET produced total intestinal aganglionosis and renal agenesis in homozygous transgenic mice. Here we describe a homozygous mutation of the human gene for the RET tyrosine kinase domain that was present in a male neonate with total intestinal aganglionosis. Gut wall biopsy specimens from the stomach to the anorectum showed no ganglion cells. No urinary tract abnormalities were detected. Genomic DNAs were isolated from peripheral blood lymphocytes of the infant and his parents. DNA sequences of all the RET/GDNF/NTN coding regions were determined using a direct DyeDeoxy Terminator Cycle method. A homozygous missense mutation (CGG-to-TGG) at RET codon 969 was identified in this patient, which resulted in an amino acid change from arginine to tryptophan. No germline RET/GDNF/NTN mutations were found in his parents. In this case, the homozygous RET mutation seemed to cause a critical alteration of the Ret tyrosine kinase activity, which resulted in total intestinal aganglionosis but not renal agenesis. Discrepancies in phenotypic expression between humans and mice suggest differing threshold values for RET signal transduction in species or organs.

A role for the Pkc1p/Mpk1p kinase cascade in the morphogenesis checkpoint

In many cells the timing of entry into mitosis is controlled by the balance between the activity of inhibitory Wee1-related kinases (Swe1p in budding yeast) and the opposing effect of Cdc25-related phosphatases (Mih1p in budding yeast) that act on the cyclin-dependent kinase Cdc2 (Cdc28p in budding yeast). Wee1 and Cdc25 are key elements in the G2 arrest mediated by diverse checkpoint controls. In budding yeast, a 'morphogenesis checkpoint' that involves Swe1p and Mih1p delays mitotic activation of Cdc28p. Many environmental stresses (such as shifts in temperature or osmolarity) provoke transient depolarization of the actin cytoskeleton, during which bud construction is delayed while cells adapt to environmental conditions. During this delay, the morphogenesis checkpoint halts the cell cycle in G2 phase until actin can repolarize and complete bud construction, thus preventing the generation of binucleate cells. A similar G2 delay can be triggered by mutations or drugs that specifically impair actin organization, indicating that it is probably actin disorganization itself, rather than specific environmental stresses, that triggers the delay. The G2 delay involves stabilization of Swe1p in response to various actin perturbations, although this alone is insufficient to produce a long G2 delay.

Depletion of focal adhesion kinase by antisense depresses contractile activation of smooth muscle

Focal adhesion kinase (FAK) undergoes tyrosine phosphorylation in response to the contractile stimulation of tracheal smooth muscle. We hypothesized that FAK may play an important role in signaling pathways that regulate smooth muscle contraction. FAK antisense or FAK sense was introduced into muscle strips by reversible permeabilization, and strips were incubated with antisense or sense for 7 days. Antisense decreased FAK expression compared with that in untreated and sense-treated tissues, but it did not affect the expression of vinculin or myosin light chain kinase. Increases in force, intracellular free Ca2+ and myosin light chain phosphorylation in response to stimulation with ACh or KCl were depressed in FAK-depleted tissues, but FAK depletion did not affect the activation of permeabilized tracheal muscle strips with Ca2+. The tyrosine phosphorylation of paxillin, a substrate for FAK, was also significantly reduced in FAK-depleted strips. We conclude that FAK is a necessary component of the signaling pathways that regulate smooth muscle contraction and that FAK plays a role in regulating intracellular free Ca2+ and myosin light chain phosphorylation.

Crystallographic and solution studies of an activation loop mutant of the insulin receptor tyrosine kinase: insights into kinase mechanism

The tyrosine kinase domain of the insulin receptor is subject to autoinhibition in the unphosphorylated basal state via steric interactions involving the activation loop. A mutation in the activation loop designed to relieve autoinhibition, Asp-1161 --> Ala, substantially increases the ability of the unphosphorylated kinase to bind ATP. The crystal structure of this mutant in complex with an ATP analog has been determined at 2.4-A resolution. The structure shows that the active site is unobstructed, but the end of the activation loop is disordered and therefore the binding site for peptide substrates is not fully formed. In addition, Phe-1151 of the protein kinase-conserved DFG motif, at the beginning of the activation loop, hinders closure of the catalytic cleft and proper positioning of alpha-helix C for catalysis. These results, together with viscometric kinetic measurements, suggest that peptide substrate binding induces a reconfiguration of the unphosphorylated activation loop prior to the catalytic step. The crystallographic and solution studies provide new insights into the mechanism by which the activation loop controls phosphoryl transfer as catalyzed by the insulin receptor.

Evaluation of the tyrosine kinase domain of the Met proto-oncogene in sporadic ovarian carcinomas*

Most of the ovarian cancers originate from the ovarian surface epithelium derived from the coelomic mesothelium. The Met proto-oncogene encodes a transmembrane tyrosine kinase receptor (Met) that has the capacity to regulate cell proliferation and differentation and it is activated by hepatocyte growth factor. Trisomy of chromosome 7 and Met protein overexpression have been were observed in ovarian carcinomas, the papillary renal cancers and other solid tumors. Frequent mutations of Met proto-oncogene have been found in hereditary papillary renal cancer (HPRC) and most of the mutations are located in the tyrosine kinase domain. The aim of this study to perform a mutation analysis of exons 17 19 of Met proto-oncogene in epithelial ovarian tumors (EOTs). We have examined 24 tumor samples from patients, operated with EOTs. Mutation was detected in exon 18 in only one sample of 24 EOTs. Our results indicate that mutations located in the Met proto-oncogene is not a common event in EOT. It is not clear whether the mutation plays a role in the tumorigenesis or progression of EOT or not.