Prokaryotic expression cloning of a novel human tyrosine kinase

Fibroblast polarization is a matrix-rigidity-dependent process controlled by focal adhesion mechanosensing

Cell elongation and polarization are basic morphogenetic responses to extracellular matrix adhesion. We demonstrate here that human cultured fibroblasts readily polarize when plated on rigid, but not on compliant, substrates. On rigid surfaces, large and uniformly oriented focal adhesions are formed, whereas cells plated on compliant substrates form numerous small and radially oriented adhesions. Live-cell monitoring showed that focal adhesion alignment precedes the overall elongation of the cell, indicating that focal adhesion orientation may direct cell polarization. siRNA-mediated knockdown of 85 human protein tyrosine kinases (PTKs) induced distinct alterations in the cell polarization response, as well as diverse changes in cell traction force generation and focal adhesion formation. Remarkably, changes in rigidity-dependent traction force development, or focal adhesion mechanosensing, were consistently accompanied by abnormalities in the cell polarization response. We propose that the different stages of cell polarization are regulated by multiple, PTK-dependent molecular checkpoints that jointly control cell contractility and focal-adhesion-mediated mechanosensing.

Actin-depolymerizing factor homology domain: a conserved fold performing diverse roles in cytoskeletal dynamics

Actin filaments form contractile and protrusive structures that play central roles in many processes such as cell migration, morphogenesis, endocytosis, and cytokinesis. During these processes, the dynamics of the actin filaments are precisely regulated by a large array of actin-binding proteins. The actin-depolymerizing factor homology (ADF-H) domain is a structurally conserved protein motif, which promotes cytoskeletal dynamics by interacting with monomeric and/or filamentous actin, and with the Arp2/3 complex. Despite their structural homology, the five classes of ADF-H domain proteins display distinct biochemical activities and cellular roles, only parts of which are currently understood. ADF/cofilin promotes disassembly of aged actin filaments, whereas twinfilin inhibits actin filament assembly via sequestering actin monomers and interacting with filament barbed ends. GMF does not interact with actin, but instead binds Arp2/3 complex and promotes dissociation of Arp2/3-mediated filament branches. Abp1 and drebrin are multidomain proteins that interact with actin filaments and regulate the activities of other proteins during various actin-dependent processes. The exact function of coactosin is currently incompletely understood. In this review article, we discuss the biochemical functions, cellular roles, and regulation of the five groups of ADF-H domain proteins.

Genomic implications of H(2)O (2) for cell proliferation and growth of Caco-2 cells

Evidence indicates that oxidative stress inhibits cell proliferation in several cell systems. To determine whether the proliferation of Caco-2 cells is inhibited by oxidative stress and to identify any novel key regulatory factors involved in protecting or damaging the intestine from oxidative stress, Caco-2 cells were treated with an oxidizing agent and analyzed by transcriptomic oligonucleotide microarrays. Results indicated that expression of genes involved in cell proliferation and growth, including genes involved in lipid synthesis, cell cycle progression and cell division, angiogenesis, RNA processing and translation, cAMP metabolism, cytoskeleton and cell to cell adhesion, receptor tyrosine kinases, and intracellular and extracellular signaling, were repressed. If an oxidant-induced inhibition in cell proliferation is involved in the pathogenesis of intestinal disease, information gained could help explain the mechanisms contributing to the causes and consequences of intestinal disease and could aid in the elucidation of mechanisms by which intestinal cells protect against oxidative stress.

Specific peptide ligand for Grb7 signal transduction protein and pancreatic cancer metastasis

BACKGROUND

Pancreatic cancer is one of the most aggressive malignancies, with high rates of invasion and metastasis and with generally poor prognosis. We previously found that metastasis was strongly associated with the expression of growth factor receptor-bound protein 7 (Grb7), which contains a Src homology 2 (SH2) domain. In this study, we evaluated Grb7 protein as a molecular target of therapy for metastatic pancreatic cancer.

METHODS

Grb7 protein expression was measured by immunohistochemistry in 36 human pancreatic cancer specimens and adjacent normal pancreatic tissue. We synthesized a nonphosphorylated peptide inhibitor that binds specifically to the SH2 domain of Grb7. Intracellular signaling was assessed by immunoprecipitation and immunoblot assays in cultured human pancreatic cancer cells. Cell migration was measured with a modified Boyden chamber method. Peritoneal metastasis of the pancreatic cancer cells was measured with a mouse model. All statistical tests were two-sided.

RESULTS

We found that 22 (61%) of 36 pancreatic cancer specimens had higher levels of Grb7 protein than their corresponding normal pancreatic tissue specimens. Grb7 expression was statistically significantly different between specimens from patients without lymph node metastasis (stage N0; two of the 10 patients) and patients with lymph node metastasis (stages N1 + N2; 20 of the 26 patients) (P = .006). The Grb7 peptide inhibitor selectively blocked the interaction between Grb7 and focal adhesion kinase and blocked the phosphorylation of Grb7 protein. In vivo Grb7 peptide inhibitor statistically significantly attenuated cell migration (for control peptide, 87.5 cells migrated, 95% confidence interval [CI] = 82.6 to 92.4 cells; for Grb7 peptide, 5.7 cells migrated, 95% CI = 2.3 to 9.0 cells; P < .001) and peritoneal metastasis of the pancreatic cancer cells in a mouse model, as assessed by the number of nodules (control = 72.6 nodules, 95% CI = 55.8 to 89.4 nodules; and for Grb7 peptide = 3.2 nodules, 95% CI = 1.6 to 4.8 nodules; P < .001, t test) and their weight (control = 4.13 g, 95% CI = 3.40 to 4.86 g; Grb7 peptide = 0.19 g, 95% CI = 0.06 to 0.32 g; P < .001, t test).

CONCLUSIONS

The Grb7 peptide inhibitor appears to be a promising molecularly targeted therapeutic agent against metastatic pancreatic cancer.

PhosphoregDB: the tissue and sub-cellular distribution of mammalian protein kinases and phosphatases

Background

Protein kinases and protein phosphatases are the fundamental components of phosphorylation dependent protein regulatory systems. We have created a database for the protein kinase-like and phosphatase-like loci of mouse that integrates protein sequence, interaction, classification and pathway information with the results of a systematic screen of their sub-cellular localization and tissue specific expression data mined from the GNF tissue atlas of mouse.

Results

The database lets users query where a specific kinase or phosphatase is expressed at both the tissue and sub-cellular levels. Similarly the interface allows the user to query by tissue, pathway or sub-cellular localization, to reveal which components are co-expressed or co-localized. A review of their expression reveals 30% of these components are detected in all tissues tested while 70% show some level of tissue restriction. Hierarchical clustering of the expression data reveals that expression of these genes can be used to separate the samples into tissues of related lineage, including 3 larger clusters of nervous tissue, developing embryo and cells of the immune system. By overlaying the expression, sub-cellular localization and classification data we examine correlations between class, specificity and tissue restriction and show that tyrosine kinases are more generally expressed in fewer tissues than serine/threonine kinases.

Conclusion

Together these data demonstrate that cell type specific systems exist to regulate protein phosphorylation and that for accurate modelling and for determination of enzyme substrate relationships the co-location of components needs to be considered.

The RIO kinases: an atypical protein kinase family required for ribosome biogenesis and cell cycle progression

Atypical protein kinases (aPKs) include proteins known to be involved in the phosphorylation-mediated regulation of a wide variety of cellular processes, as well as some for which the function is, as yet, unknown. At present, 13 families of aPKs have been identified in the human genome. This review briefly summarizes their known properties, but concentrates in particular on the RIO family of aPKs. Representatives of this family are present in organisms varying from archaea to humans. All these organisms contain at least two RIO proteins, Rio1 and Rio2, but a third Rio3 group is present in multicellular eukaryotes. Crystal structures of A. fulgidus Rio1 and Rio2 have shown that whereas the overall fold of these enzymes resembles typical protein kinases, some of the kinase structural domains, particularly those involved in peptide substrate binding, are not present. The mode of binding of nucleotides also differs from other kinases. While the enzymatic activity of Rio1 and Rio2 has been demonstrated and both have been shown to be essential in S. cerevisiae and required for proper cell cycle progression and chromosome maintenance, the biological substrates of RIO proteins still remain to be identified.

Identification of novel non-phosphorylated ligands, which bind selectively to the SH2 domain of Grb7

Grb7 is an adapter-type signaling protein, which is recruited via its SH2 domain to a variety of receptor tyrosine kinases (RTKs), including ErbB2 and ErbB3. It is overexpressed in breast, esophageal, and gastric cancers, and may contribute to the invasive potential of cancer cells. Molecular interactions involving Grb7 therefore provide attractive targets for therapeutic intervention. We have utilized phage display random peptide libraries as a source of small peptide ligands to the SH2 domain of Grb7. Screening these libraries against purified Grb7 SH2 resulted in the identification of Grb7-binding peptide phage clones that contained a non-phosphorylated Tyr-X-Asn (YXN) motif. The tyrosine-phosphorylated form of this motif is characteristic of Grb7 SH2 domain binding sites identified in RTKs and other signaling proteins such as Shc. Peptides that are non-phosphorylated have greater potential in the development of therapeutics because of the instability of a phosphate group in vivo. Using a biased library approach with this conserved YXN motif, we identified seven different peptide phage clones, which bind specifically to the SH2 domain of Grb7. These peptides did not bind to the SH2 domain of Grb2 (which also selects for Asn at pY(+2)) or Grb14, a closely related family member. The cyclic structure of the peptides was required to bind to the Grb7 SH2 domain. Importantly, the synthetic Grb7-binding peptide G7-18 in cell lysates was able to specifically inhibit the association of Grb7 with the ErbB family of RTKs, in particular ErbB3, in a dose-dependent manner. These peptides will be useful in the development of targeted molecular therapeutics for cancers overexpressing Grb7 and in the development of Grb7-specific inhibitors to gain a complete understanding of the physiological role of Grb7.

Twinfilin, a molecular mailman for actin monomers

Twinfilin is a ubiquitous actin-monomer-binding protein that is composed of two ADF-homology domains. It forms a 1:1 complex with ADP-actin-monomers,inhibits nucleotide exchange on actin monomers and prevents assembly of the monomer into filaments. The two ADF-H domains in twinfilin probably have 3D structures similar to those of the ADF/cofilin proteins and overlapping actin-binding sites. Twinfilin also interacts with PtdIns(4,5)P2, which inhibits its actin-monomer-sequestering activity in vitro. Mutations in the twinfilin gene result in defects in the bipolar budding pattern in S. cerevisiae and in a rough eye phenotype and aberrant bristle morphology in Drosophila melanogaster. These phenotypes are caused by the uncontrolled polymerization of actin filaments in the absence of twinfilin. Studies on budding yeast suggest that twinfilin contributes to actin filament turnover by localizing actin monomers, in their `inactive'ADP-form, to the sites of rapid filament assembly. This is mediated through direct interactions between twinfilin and capping protein. Therefore,twinfilin might serve as a link between rapid actin filament depolymerization and assembly in cells.

Effects of HIV-1 Nef on cellular gene expression profiles

The early human immunodeficiency virus (HIV) accessory protein Nef makes an important contribution to virulence, but the mechanisms by which Nef influences pathogenesis remain unclear. Many well-studied effects of Nef, like CD4 and class I MHC downregulation, occur posttranslationally. However, Nef has the potential to affect gene expression by interfering with cell signaling pathways and by virtue of structural features such as the Pro-X-X-Pro motif, which may interact with src homology region-3 domains of src-like kinases. We used a cDNA microarray screening strategy to identify cellular genes whose steady state transcriptional levels may be affected by Nef. We generated HeLa cell lines expressing wild-type or mutant HIV-1 nef protein sequences. Using cDNA microarray technology, we compared the patterns of cellular gene expression in the various cell lines to the pattern in non-Nef-expressing HeLa cells. By matching the patterns of cellular gene expression in HeLa cell lines expressing various Nefs with that of parental HeLa cells, we identified several cellular genes whose expression was modulated differentially by Nef and its mutants. We confirmed the differential expression of selected genes by RNA filter blotting. Genes expressed at higher levels included proteases, transcription factors, protein kinases, nuclear import/export proteins, adaptor molecules and cyclins, some of which have previously been implicated as being important for HIV replication and pathogenesis. The results indicate that Nef expression can alter the expression of cellular genes and suggest that this alteration in cellular gene expression may serve to optimize the cell to support the subsequent stages of viral replication.

Cathepsin B-like cysteine proteases and Caenorhabditis elegans homologues dominate gene products expressed in adult Haemonchus contortus intestine

Proteins expressed by nematode intestinal cells are potential targets for parasite control by immune or chemical based strategies. To expand our knowledge on nematode intestinal proteins, expressed sequence tags were generated for 131 cDNA clones from the intestine of adult female Haemonchus contortus. An estimated 55 distinct protein genes or gene families were identified. Predicted proteins represented diverse functions. Several predicted polypeptides were related to H. contortus proteins implicated in inducing protective immunity against challenge infections of this parasite. The dominant intestinal transcripts were represented by cathepsin B-like cysteine protease genes (cbl) (17% of protein coding expressed sequence tags (ESTs) analyzed). An estimated 11 previously undescribed cbl genes were identified, doubling the recognized members of this gene family. Multiple C-type lectin sequences were identified. Other notable sequences included a predicted Y-box binding protein, serine/threonine kinases and a cyclin E-like sequence. Predicted protein homologues were found in Caenorhabditis elegans for all but one H. contortus sequence (99%), while fewer homologues from other parasitic nematodes were found. Many of the proteases, lipase and C-type lectin homologues in C. elegans had apparent signal peptides, suggesting that they are secreted. Several gene products had no obvious similarity outside the phylum Nematoda. The ESTs identified intestinal genes with potential application to immune control, understanding of basic intestinal regulatory processes and refinement of nematode genomic resources.

COS cells expression cloning of tyrosine-phosphorylated proteins by immunocytochemistry

Tyrosine phosphorylation is an important post-translational modification of proteins, essential in many aspects of the cell economy, particularly in signal transduction pathways. Despite the importance of protein tyrosine phosphorylation, the approaches available for molecular cloning remain limited. We have developed a COS cell-based eukaryotic expression cloning procedure for phosphotyrosine-containing proteins by immunocytochemistry of cell monolayers. The approach takes advantage of the low basal levels of tyrosine phosphorylated, robust transient expression, availability of specific antibodies against tyrosine-phosphorylated residues, and rescue of episomal DNA after immunocytochemistry. The technique is validated by cloning the rat proto-oncogene c-fgr in its tyrosine-phosphorylated form out of a rat kidney cDNA library containing over 10(6) primary recombinants. This technique set the grounds for expression cloning of tyrosine-phosphorylated proteins in eukaryotic cells, and it is anticipated that further modifications and refinements will allow the identification of protein tyrosine phosphatase substrates.

HsRec2/Rad51L1, a protein influencing cell cycle progression, has protein kinase activity

Human Rec2/Rad51L1 is a member of the Rad51 family of proteins. Although recombinase activity, typical of this family, could not be established, its overexpression in mammalian cells has been shown to cause a delay in G1. Moreover, since hsRec2/Rad51L1 has been found to be induced by both ionizing and UV irradiation, it is likely that hsRec2/Rad51L1 is elevated following any DNA damage and causes a G1 delay to allow time for DNA repair to occur. Limited homology with catalytic domains X and XI of protein kinase A suggested that kemptide, an artificial substrate containing one phosphorylatable residue, a serine, might serve as a substrate for hsRec2/Rad51L1. Here, we report that hsRec2/Rad51L1 can phosphorylate kemptide, as well as myelin basic protein, p53, cyclin E, and cdk2, but not a peptide substrate containing tyrosine only. The finding that hsRec2/Rad51L1 exhibits protein kinase activity is a first step toward identifying a mechanism whereby this protein affects the cell cycle.

Cloning, expression and characterization of an A6-related protein

By interaction cloning (yeast two-hybrid system) using the catalytic domain of protein kinase Czeta (PKCzeta) as bait, we cloned a human full-length cDNA with 62% nucleotide homology to the A6 protein recently cloned and characterized by Beeler et al. [Beeler, J.F., LaRochelle, W.J., Chedid, M., Tronick, S.R. & Aaronson, S. A. (1994) Mol. Cell. Biol. 14, 982-988]. The deduced amino acid sequence (349 amino acids) of the A6-related protein (A6rp) contained potential actin-binding sites and ATP-binding sites. We also cloned the murine homolog of A6rp. Human A6rp was expressed in an in-vitro transcriptional/translational system with an apparent molecular mass of 40 kDa and as a glutathione S-transferase (GST) fusion protein in bacteria. A polyclonal anti-(A6rp) was raised in rabbits and used for the identification of A6rp by immunoblotting. A6rp was found to be expressed at the mRNA and the protein levels in all cells and tissues investigated. GST-A6rp was phosphorylated by PKCzeta but not significantly by other PKC isoenzymes. Moreover, it was phosphorylated by casein kinase 2 and most effectively by the tyrosine kinase Src. In contrast to GST-A6rp, GST-A6 was also phosphorylated by PKC isoforms other than PKCzeta and strongly by CK2, but just weakly by Src. In contrast to the results of Beeler et al. on beta-galactosidase-A6, we were unable to demonstrate autokinase activity or tyrosine phosphorylation of either GST-A6 or GST-A6rp. In accordance with the potential ATP-binding sites, both proteins were able to bind ATP.

Potent inhibitory ligands of the GRB2 SH2 domain from recombinant peptide libraries

We cloned and expressed the SH2 domain of human GRB2 as glutathione S-transferase and maltose binding protein fusion proteins. We screened three phagemid-based fd pVIII-protein phage display libraries against SH2 domain fusion proteins. Sequence analysis of the peptide extensions yielded a variety of related peptides. By examining the ability of the phage clones to bind other SH2 domains, we demonstrated that the phage were specific for the SH2 domain of GRB2. Based on the sequence motif identified in the "random" library screening experiment, we also built and screened a phage display library based on a Tyr-X-Asn motif (X5-Tyr-X-Asn-X8). To examine the affinity of the phage derived peptides for GRB2, we set up a radioligand competition binding assay based on immobilized GRB2 and radiolabelled autophosphorylated EGFR ICD as the radioligand. Results obtained with peptide competitors derived from the phage sequences demonstrated that nonphosphotyrosine-containing peptides identified with the phage display technology had an affinity for the receptor similar to tyrosine-phosphorylated peptides derived from the EGFR natural substrate. Interestingly, when the phage display peptides were then phosphorylated on tyrosine, their affinity for GRB2 increased dramatically. We also demonstrated the ability of the peptides to block the binding of the GRB2 SH2 domain to EGFR in a mammalian cell-based binding assay.

Characterization of a novel type of serine/threonine kinase that specifically phosphorylates the human goodpasture antigen

Goodpasture disease is an autoimmune disorder that occurs naturally only in humans. Also exclusive to humans is the phosphorylation process that targets the unique N-terminal region of the Goodpasture antigen. Here we report the molecular cloning of GPBP (Goodpasture antigen-binding protein), a previously unknown 624-residue polypeptide. Although the predicted sequence does not meet the conventional structural requirements for a protein kinase, its recombinant counterpart specifically binds to and phosphorylates the exclusive N-terminal region of the human Goodpasture antigen in vitro. This novel kinase is widely expressed in human tissues but shows preferential expression in the histological structures that are targets of common autoimmune responses. The work presented in this report highlights a novel gene to be explored in human autoimmunity.

Identification and characterization of mRNAs differentially expressed in thyroid cells stimulated by a mitogenic treatment

The aim of our work is to identify new genes and proteins involved in the control of the proliferation of thyroid cells as putative protooncogenes and antioncogenes. Several strategies are discussed. A first study has allowed to identify three new genes. Further search will use the differential display and gene arrays methodology. The role of the identified proteins coded by the genes is studied in vitro by the search of partner proteins by the double hybrid method and in vivo by mice gene knockout technology.

Walleye retroviruses associated with skin tumors and hyperplasias encode cyclin D homologs

Walleye dermal sarcoma (WDS) and walleye epidermal hyperplasia (WEH) are skin diseases of walleye fish that appear and regress on a seasonal basis. We report here that the complex retroviruses etiologically associated with WDS (WDS virus [WDSV]) and WEH (WEH viruses 1 and 2 [WEHV1 and WEHV2, respectively]) encode D-type cyclin homologs. The retroviral cyclins (rv-cyclins) are distantly related to one another and to known cyclins and are not closely related to any walleye cellular gene based on low-stringency Southern blotting. Since aberrant expression of D-type cyclins occurs in many human tumors, we suggest that expression of the rv-cyclins may contribute to the development of WDS or WEH. In support of this hypothesis, we show that rv-cyclin transcripts are made in developing WDS and WEH and that the rv-cyclin of WDSV induces cell cycle progression in yeast (Saccharomyces cerevisiae). WEHV1, WEHV2, and WDSV are the first examples of retroviruses that encode cyclin homologs. WEH and WDS and their associated retroviruses represent a novel paradigm of retroviral tumor induction and, importantly, tumor regression.

Regulation of the cortical actin cytoskeleton in budding yeast by twinfilin, a ubiquitous actin monomer-sequestering protein

Here we describe the identification of a novel 37-kD actin monomer binding protein in budding yeast. This protein, which we named twinfilin, is composed of two cofilin-like regions. In our sequence database searches we also identified human, mouse, and Caenorhabditis elegans homologues of yeast twinfilin, suggesting that twinfilins form an evolutionarily conserved family of actin-binding proteins. Purified recombinant twinfilin prevents actin filament assembly by forming a 1:1 complex with actin monomers, and inhibits the nucleotide exchange reaction of actin monomers. Despite the sequence homology with the actin filament depolymerizing cofilin/actin-depolymerizing factor (ADF) proteins, our data suggests that twinfilin does not induce actin filament depolymerization. In yeast cells, a green fluorescent protein (GFP)-twinfilin fusion protein localizes primarily to cytoplasm, but also to cortical actin patches. Overexpression of the twinfilin gene (TWF1) results in depolarization of the cortical actin patches. A twf1 null mutation appears to result in increased assembly of cortical actin structures and is synthetically lethal with the yeast cofilin mutant cof1-22, shown previously to cause pronounced reduction in turnover of cortical actin filaments. Taken together, these results demonstrate that twinfilin is a novel, highly conserved actin monomer-sequestering protein involved in regulation of the cortical actin cytoskeleton.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

Characterization of a phosphoprotein whose mRNA is regulated by the mitogenic pathways in dog thyroid cells

We have isolated cDNA clones encoding the dog and human forms of a novel protein whose function is still unknown. Sequence analysis indicates that dog clone c5fw protein contains 343 amino acid residues. several potential phosphorylation sites. and two of the 12 conserved subdomains (VIII and IX) that fold into a common catalytic core structure of the large family of protein kinases. Human clone c5fw shares 95% amino acid identity with its dog counterpart. We have also isolated another human-related clone c5fw sharing 70% amino acid identity with the dog sequence. We transiently expressed c-myc epitope-tagged clone c5fw protein in COS-7 cells and infected thyrocytes in primary culture with a recombinant adenovirus containing clone c5fw cDNA (adenovirus c5fw). In both experiments, a 46-kDa protein was detected and subsequently more extensively characterized. By two-dimensional gel electrophoresis and V8 protease digestion, we showed that this overexpressed protein is phosphorylated on different sites. Moreover, cells stimulated with thyrotropin or epidermal growth factor, thyrotropin and fetal calf serum increased the level of clone c5fw protein produced after infection by adenovirus containing clone c5fw. The disappearance of this 46-kDa protein after 1 h of puromycin treatment indicates that it is a labile protein. Immunofluorescence and subcellular fractionation analysis have revealed that c-myc-tagged clone c5fw was insoluble and localized mainly in the cytoplasm, in the form of granules.

Cloning and characterization of the mouse homolog of the human A6 gene

The mouse homolog of a novel human protein tyrosine kinase encoding gene, A6, was cloned and characterized. The human A6 cDNA is unique in that its gene product exhibited in vitro kinase activity but its predicted amino acid (aa) sequence revealed no consensus motifs commonly found within the kinase domain of protein kinase family members. Here, we isolated a mouse A6 cDNA clone from a murine myeloid progenitor 32D cell library using a 1.1 kb cDNA probe containing the entire human A6 open reading frame (ORF). Determination of the mouse A6 cDNA nucleotide (nt) sequence revealed an ORF of 1050 nt encoding a protein of 350 aa and a molecular mass of 40,201 Da. The mouse and human A6 gene products shared 93% identity. In vitro translation, as well as immunoprecipitation of 32D cell lysates confirmed expression of mouse A6 as a 40 kDa protein. Northern blot analysis of total RNA from mouse cell lines derived from diverse tissues including NIH 3T3 fibroblasts, L cell fibroblasts, C2C12 myoblasts, M1 myeloblasts, BALB/MK cells, 70Z/3 preB lymphocytes, and p388D1 monocytes demonstrated widespread A6 mRNA expression. A6 mRNA was also ubiquitously expressed at varying levels in all tissues examined. The identification of a potential actin/phosphoinositide binding domain and consensus phosphorylation sites, coupled with A6's expression in a variety of cell types suggest that the A6 gene product may play a role in basic cellular processes.

The protein kinases of budding yeast: six score and more

The completion of the budding yeast genome sequencing project has made it possible to determine not only the total number of genes, but also the exact number of genes of a particular type 1-3. As a consequence, we now know exactly how many protein kinases are encoded by the yeast genome, a number of considerable interest because of the importance of protein phosphorylation in the control of so many cellular processes.

Identification and characterization of novel genes modulated in the thyroid of dogs treated with methimazole and propylthiouracil

Induction of cell proliferation by mitogen or growth factor stimulation leads to the specific stimulation or repression of a large number of genes. To better understand differentiated epithelial cell growth regulation, we have initiated a study to identify genes which are regulated by the thyrotropin-dependent mitogenic pathway in dog thyroid cells. A thyroid cDNA library was prepared from a methimazole and propylthiouracil-treated dog and differentially screened with probes derived from control or stimulated thyroids. Among 19 clones isolated, 6 encode known proteins (inwardly rectifying potassium channel, nucleosome assembly protein, ribosomal protein L7, elongation factor 1alpha, non-muscle myosin light chain, and heat shock protein 90beta). The 13 others correspond to proteins whose function is unknown. Among them, 5 correspond to mRNAs whose expression was modulated by mitogenic stimulation of thyrocytes in primary culture. A preliminary characterization of two of these cDNAs is reported: clone 5, which might represent a novel, atypical protein kinase, and clone 3, which contains ankyrin-like repeats, suggesting that it might interact with other proteins.

A novel type of protein kinase phosphorylates actin in the actin-fragmin complex

Actin-fragmin kinase (AFK) from Physarum polycephalum specifically phosphorylates actin in the EGTA-resistant 1:1 actin-fragmin complex. The cDNA deduced amino acid sequence reveals two major domains of approximately 35 kDa each that are separated by a hinge-like proline/serine-rich segment of 50 residues. Whereas the N-terminal domain does not show any significant similarity to protein sequences from databases, there are six complete kelch repeats in the protein that comprise almost the entire C-terminal half of the molecule. To prove the intrinsic phosphorylation activity of AFK, full-length or partial cDNA fragments were expressed both in a reticulocyte lysate and in Escherichia coli. In both expression systems, we obtained specific actin phosphorylation and located the catalytic domain in the N-terminal half. Interestingly, this region did not contain any of the known protein kinase consensus sequences. The only known sequence motif present that could have been involved in nucleotide binding was a nearly perfect phosphate binding loop (P-loop). However, introduction of two different point mutations into this putative P-loop sequence did not alter the catalytic activity of the kinase, which indicates an as yet unknown mechanism for phosphate transfer. Our data suggest that AFK belongs to a new class of protein kinases and that this actin phosphorylation might be the first example of a widely distributed novel type of regulation of the actin cytoskeleton in non-muscle cells.

Structural analysis of myosin heavy chain kinase A from Dictyostelium. Evidence for a highly divergent protein kinase domain, an amino-terminal coiled-coil domain, and a domain homologous to the beta-subunit of heterotrimeric G proteins

We report here the cloning and characterization of the gene encoding the 130-kDa myosin heavy chain kinase (MHCK A) from the amoeba Dictyostelium. Previous studies have shown that purified MHCK A phosphorylates threonines in the carboxyl-terminal tail portion of the Dictyostelium myosin II heavy chain and that phosphorylation of these sites is critical in regulating the assembly and disassembly of myosin II filaments in vitro and in vivo. Biochemical analysis of MHCK A, together with analysis of the primary sequence, suggests that the amino-terminal approximately 500 amino acids form an alpha-helical coiled-coil domain and that residues from position approximately 860 to the carboxyl terminus (residue 1146) form a domain with significant similarity to the beta-subunit of heterotrimeric G proteins. No part of the MHCK A sequence displays significant similarity to the catalytic domain of conventional eukaryotic protein kinases. However, both native and recombinant MHCK A displayed autophosphorylation activity following renaturation from SDS gels, and MHCK A expressed in Escherichia coli phosphorylated purified Dictyostelium myosin, confirming that MHCK A is a bona fide protein kinase. Cross-linking studies demonstrated that native MHCK A is a multimer, consistent with the presence of an amino-terminal coiled-coil domain. Southern blot analysis indicates that MHCK A is encoded by a single gene that has no detectable introns.

Protein kinases and phosphatases that act on histidine, lysine, or arginine residues in eukaryotic proteins: a possible regulator of the mitogen-activated protein kinase cascade

Phosphohistidine goes undetected in conventional studies of protein phosphorylation, although it may account for 6% of total protein phosphorylation in eukaryotes. Procedures for studying protein N- kinases are described. Genes whose products are putative protein histidine kinases occur in a yeast and a plant. In rat liver plasma membranes, activation of the small G-protein, Ras, causes protein histidine phosphorylation. Cellular phosphatases dephosphorylate phosphohistidine. One eukaryotic protein histidine kinase has been purified, and specific proteins phosphorylated on histidine have been observed. There is a protein arginine kinase in mouse and protein lysine kinases in rat. Protein phosphohistidine may regulate the mitogen-activated protein kinase cascade.