Extracellular signal-regulated kinase 7 (ERK7), a novel ERK with a C-terminal domain that regulates its activity, its cellular localization, and cell growth

Mkp1 of Pneumocystis carinii associates with the yeast transcription factor Rlm1 via a mechanism independent of the activation state

The mitogen-activated protein (MAP) kinase Mkp1 of the fungal pathogen Pneumocystis carinii is a functional MAP kinase that complements the loss of Slt2p, the MAP kinase component of the cell integrity pathway of Saccharomyces cerevisiae, and is activated within P. carinii in response to oxidative stress. Mkp1 displays an unusual feature in that it contains a phosphorylation motif repeat (TEYMTEY) within the activation loop not present in any other fungal MAPK identified to date. Mutagenesis of the T186,Y188 phosphorylation motif within the activation domain of Mkp1 results in the loss of detectable kinase activity but still retains partial complementation function. In addition to the ability of Mkp1 to restore partial activity to the cell integrity pathway in the absence of phosphorylatable residues within the activation loop, the association of Mkp1 with a substrate of Slt2p, the transcription factor Rlm1p, can also occur in the absence of MAP kinase activation. The results of this study suggest that the presence of phosphorylatable residues within the activation loop of Mkp1 is not absolutely required for functional (complementation) activity or for the association of Mkp1 with the transcription factor Rlm1p. In contrast, the catalytic lysine of the ATP-binding domain of Mkp1 is necessary for both complementation function and interaction with Rlm1p.

Identification of variants and dual promoters of murine serine/threonine kinase KKIAMRE

KKIAMRE is a serine/threonine protein kinase whose transcripts increase in the deep cerebellar nuclei of the rabbit after eyeblink conditioning, a model of associative learning and memory. We here characterized the expression, isoforms, and promoters of murine KKIAMRE gene. The expression of KKIAMRE was detected, by in situ hybridization and immunohistochemistry, in neurons in various brain regions including deep cerebellar nuclei. The gene spans approximately 40 kb and consists of 15 exons. Analysis of cDNA clones revealed multiple variants, having diversity in the putative carboxy-terminal regulatory domain, generated by alternative splicing and intraexonal termination. Furthermore, they had alternative 5' noncoding sequences. Primer extension, RNase protection, and transient expression assays revealed that two alternative promoters linked to distinct noncoding exons direct the expression of KKIAMRE. The gene was mapped on chromosomes 5 and 4 in mouse and human, respectively.

cRel-TD kinase: a serine/threonine kinase binding in vivo and in vitro c-Rel and phosphorylating its transactivation domain

The activity of transcription factors is often modulated by signal responsive protein kinases. Rel/NF-kappaB transcription factors are regulated by IkappaB inhibitors, the phosphorylation of which causes ubiquitination and degradation, resulting in nuclear translocation of NF-kappaB and activation of target genes. Here we report pulldown and immunoprecipitation experiments showing that a mammalian 66 kDa protein kinase binds murine c-Rel, both in vitro and in vivo. This kinase appears to have at least two binding sites on c-Rel, a proline-directed serine/ threonine substrate specificity similar to MAP kinases and to specifically phosphorylate the C-terminal domain of murine c-Rel at an ERK consensus site.

Eotaxin induces degranulation and chemotaxis of eosinophils through the activation of ERK2 and p38 mitogen-activated protein kinases

Eotaxin and other CC chemokines acting via CC chemokine receptor-3 (CCR3) are believed to play an integral role in the development of eosinophilic inflammation in asthma and allergic inflammatory diseases. However, little is known about the intracellular events following agonist binding to CCR3 and the relationship of these events to the functional response of the cell. The objectives of this study were to investigate CCR3-mediated activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase-2 (ERK2), p38, and c-jun N-terminal kinase (JNK) in eosinophils and to assess the requirement for MAP kinases in eotaxin-induced eosinophil cationic protein (ECP) release and chemotaxis. MAP kinase activation was studied in eotaxin-stimulated eosinophils (more than 97% purity) by Western blotting and immune-complex kinase assays. ECP release was measured by radioimmunoassay. Chemotaxis was assessed using Boyden microchambers. Eotaxin (10−11 to 10−7 mol/L) induced concentration-dependent phosphorylation of ERK2 and p38. Phosphorylation was detectable after 30 seconds, peaked at about 1 minute, and returned to baseline after 2 to 5 minutes. Phosphorylation of JNK above baseline could not be detected. The kinase activity of ERK2 and p38 paralleled phosphorylation. PD980 59, an inhibitor of the ERK2-activating enzyme MEK (MAP ERK kinase), blocked phosphorylation of ERK2 in a concentration-dependent manner. The functional relevance of ERK2 and p38 was studied using PD98 059 and the p38 inhibitor SB202 190. PD98 059 and SB202 190 both caused inhibition of eotaxin-induced ECP release and chemotaxis. We conclude that eotaxin induces a rapid concentration-dependent activation of ERK2 and p38 in eosinophils and that the activation of these MAP kinases is required for eotaxin-stimulated degranulation and directed locomotion.

Cloning and characterization of mouse extracellular-signal-regulated protein kinase 3 as a unique gene product of 100 kDa

MAP (mitogen-activated protein) kinases are a family of serine/threonine kinases that have a pivotal role in signal transduction. Here we report the cloning and characterization of a mouse homologue of extracellular-signal-regulated protein kinase (ERK)3. The mouse Erk3 cDNA encodes a predicted protein of 720 residues, which displays 94% identity with human ERK3. Transcription and translation of this cDNA in vitro generates a 100 kDa protein similar to the human gene product ERK3. Immunoblot analysis with an antibody raised against a unique sequence of ERK3 also recognizes a 100 kDa protein in mouse tissues. A single transcript of Erk3 was detected in every adult mouse tissue examined, with the highest expression being found in the brain. Interestingly, expression of Erk3 mRNA is acutely regulated during mouse development, with a peak of expression observed at embryonic day 11. The mouse Erk3 gene was mapped to a single locus on central mouse chromosome 9, adjacent to the dilute mutation locus and in a region syntenic to human chromosome 15q21. Finally, we provide several lines of evidence to support the existence of a unique Erk3 gene product of 100 kDa in mammalian cells.

Distantly related cousins of MAP kinase: biochemical properties and possible physiological functions

MAP kinases have been established to be key regulators of cellular signal transduction systems and are conserved from baker's yeast to human beings. Until now, three major types of mammalian MAP kinases (ERK, p38, and JNK/SAPK) have been reported and extensively studied. Advancement of genomic research as well as homology cloning techniques has revealed that there are several other protein kinase families that are structurally modestly related to those conventional MAP kinases. Indeed, most of them possess the TXY motif characteristic to MAP kinases in their activation loop, and can be regarded as members of the MAP kinase superfamily, yet some of them show closest overall similarity to Cdks. These kinases, all of mammalian origin, include MAK, MRK, MOK, p42KKIALRE, p56KKIAMRE, NLK, DYRK/Mnb, and Prp4. Although most of their physiological roles remain unknown, recent progress starts shedding some light on their functions.

Ca2+-induced p38/SAPK signalling inhibited by the immunosuppressant cyclosporin A in human peripheral blood mononuclear cells

To understand the effects of the immunosuppressant cyclosporin A (CsA) on Ca2+-mediated intracellular signalling pathways in human peripheral blood mononuclear cells (PBMCs), we investigated its effects on the activity profiles of mitogen-activated protein kinase (MAPK) cascades. PBMCs, or subpopulations thereof, were simultaneously stimulated with a phorbol ester and the calcium ionophore ionomycin, in the presence or absence of therapeutic concentrations of CsA. In these primary human cells, CsA significantly inhibited PMA/ionomycin-mediated and ionomycin-mediated activation of the MAPK kinase MKK6, as well as its downstream kinases SAPK2a (p38alpha) and MAPKAP-K2. PMA/ionomycin treatment also mediated activation of SAPK1 (JNKs) which was inhibited by CsA. Treatment with ionomycin alone also resulted in CsA-sensitive activation of SAPK1. With regard to transcription factors targeted by the Ca2+-induced MAPK signalling network, we found CsA to inhibit the ionomycin-mediated phosphorylation of ATF2 at Thr71. We identified the heterodimeric transcription factor ATF2/CREB as constitutively binding to the essential cAMP response element (CRE) site within the Ca2+-regulated DNA polymerase beta promoter and contributing to the activation of this promoter. Our data implicate ATF2 phosphorylation status as a nuclear sensor within PBMCs that monitors converging intracellular Ca2+-signalling pathways.

The role of protein phosphatases in the regulation of mitogen and stress-activated protein kinases

It is now established that a family of dual-specificity protein phosphatases are able to interact with mitogen and stress-activated protein kinases in a highly specific manner to differentially regulate these enzymes in mammalian cells. A role for these proteins in negative feedback regulation of MAP kinase activity is also supported by genetic and biochemical studies in yeasts and Drosophila. More recently it has become clear that other classes of protein phosphatase also play key roles in the regulated dephosphorylation of MAP kinases, including tyrosine-specific protein phosphatases and serine/threonine protein phosphatases. It is likely that a complex balance between upstream activators and these different classes of MAP kinase specific phosphatase are responsible for determining, at least in part, the magnitude and duration of MAP kinase activation and hence the physiological outcome of signalling.

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

Molecular cloning and characterization of a novel member of the MAP kinase superfamily

BACKGROUND

Members of the MAP kinase superfamily play important roles in a wide variety of signal transduction pathways, and several members have been identified. However, the diversity and complexity of cellular responses in mammalian systems may imply existence of hitherto unidentified members of the MAP kinase superfamily.

RESULTS

We report the molecular cloning and characterization of a novel member of the MAP kinase superfamily. We isolated full-length mouse and human cDNAs that encode complete open reading frames of a novel protein kinase, termed MOK. MOK consists of 419 (human) and 420 (mouse) amino acids, with a calculated molecular weight of 48kDa. MOK contains all of the protein serine/threonine kinase consensus motifs and shows a modest similarity to members of the MAP kinase superfamily and MAK and MAK-related kinase (MRK). In addition, MOK possesses a Thr-Glu-Tyr (TEY) motif in the activation loop domain, as do classical MAP kinases. MOK is widely expressed in normal tissues and organs and localizes to the cytoplasm. MOK is able to phosphorylate several known MAP kinase substrates and to undergo autophosphorylation. A mutation in the TEY motif to AEF abolished the kinase activity of MOK, and the treatment of cells with a phosphatase inhibitor, okadaic acid, enhanced the kinase activity of MOK, suggesting the existence of an upstream kinase. Phorbol ester TPA was found to stimulate the kinase activity of MOK, whereas serum stimulation, osmotic shock, or anisomycin treatment did not significantly activate MOK.

CONCLUSION

These results indicate that MOK is distantly related to members of known subfamilies of the MAP kinase superfamily and can therefore be classified as a novel member.