Expression of a neuronal Ca2+/calmodulin-dependent protein kinase, CaM kinase-Gr, in rat thymus

Attenuated β-adrenergic response in calcium/calmodulin-dependent protein kinase IV-knockout mice

In the present study, we examined the importance of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) in the regulation of cardiac function using genetically modified CaMKIV-null mice. RT-PCR analysis revealed decreased expression of voltage-dependent calcium channels in the cardiac myocytes of CaMKIV-null mice compared with wild-type mice. CaMKIV-null mice showed shortened QT time on electrocardiograms. Pharmacological analysis revealed decreased responsiveness to the β-adrenergic blocker propranolol in CaMKIV-null mice, whereas the plasma norepinephrine level was not affected. CaMKIV-null mice showed decreased baroreflex on electrocardiograms. Heart rate variability analysis showed unstable R-R intervals, a decreased low frequency power/high frequency power (LF/HF) ratio, and increased standard deviation of the normal to normal R-R intervals (SDNN) in CaMKIV-null mice, suggesting decreased responsiveness to β-adrenergic stimulation in CaMKIV-null mice. Atrial contraction analysis and cardiac action potential recording showed a decreased response to the β-adrenoceptor agonist isoproterenol in CaMKIV-null mice. Furthermore, fluorescence imaging in a CRE-hrGFP assay revealed a decreased response to isoproterenol in CaMKIV-null cardiac myocytes. Taken together, our data strongly suggest a significant effect of CaMKIV gene ablation on cardiac β-adrenergic signal transduction.

Persulfide Signaling in Stress-Initiated Calmodulin Kinase Response

Significance: Calcium ion (Ca²⁺)/calmodulin (CaM)-dependent protein kinases (CaMKs) are activated by phosphorylation of a crucial threonine residue either by itself (CaMKII) or by upstream kinases, CaMK kinases (CaMKKs) (CaMKI and CaMKIV). CaMKs, present in most mammalian tissues, can phosphorylate many downstream targets, thereby regulating numerous cellular functions. Recent Advances: Aside from canonical post-translational modifications, cysteine-based redox switches in CaMKs affect their enzyme activities. In addition to reactive oxygen species (ROS) and reactive nitrogen species (RNS), reactive sulfur species (RSS) are also recognized as key signaling molecules, regulating protein function through polysulfidation, formation of polysulfides [-S-(S)_n-H] on their reactive cysteine residues. To comprehend the biological significance of RSS signaling-related CaMK regulation, here we introduce a novel concept defining CaMKs as RSS targets in stress responses. The stress responses include an irreversible electrophile attack for CaMKI, inflammation for CaMKII, and endoplasmic reticulum stress for CaMKIV. Critical Issues: Development of various human diseases is associated with increased ROS, RNS, and RSS generation. Therefore, depending on specific pathophysiology, RSS could have very particular effects on CaMK functions. Future Directions: How multiple sources and mutual reactions of ROS, RNS, and RSS are coordinated is obscure. Elucidating the mechanisms through applications of enzymology, chemical biology, and mass spectrometry enables to uncover the complexities of redox regulation of CaMK cascades.

Redox regulation of Ca2+/calmodulin-dependent protein kinase IV via oxidation of its active-site cysteine residue

We have recently reported that Ca²⁺/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) is inactivated by reactive sulfur species via polysulfidation of the active-site Cys residue. Here, we show that hydrogen peroxide (H₂O₂) limit CaMKIV activity at the same active-site Cys residue through oxidation and downstream signaling in cells. CaMKIV is phosphorylated at Thr¹⁹⁶ by its upstream CaMK kinase (CaMKK), which induces its full activity. In vitro incubation of CaMKIV with H₂O₂ resulted in reversible inhibition of CaMKK-induced phospho-Thr¹⁹⁶ and the consequent inactivation of CaMKIV. In contrast, mutated CaMKIV (C198V) was refractory to the H₂O₂-induced enzyme inhibition. In transfected cells expressing CaMKIV, Ca²⁺ ionophore-induced CaMKIV phosphorylation at Thr¹⁹⁶ was decreased upon treatment with H₂O₂, whereas cells expressing mutant CaMKIV (C198V) were resistant to H₂O₂ treatment. Modification of free thiol with N-ethylmaleimide revealed that Cys¹⁹⁸ in CaMKIV is a target for S-oxidation. Additionally, the Ca²⁺ influx-induced phospho-Thr¹⁹⁶ of endogenous CaMKIV was also inhibited upon treatment with H₂O₂ in Jurkat T-lymphocytes and cerebellar granule cells. Phosphorylation of cyclic AMP response element-binding protein (CREB) at Ser¹³³, which is downstream of CaMKIV, was also decreased upon treatment with H₂O₂. Thus, our results indicate that oxidation stress regulates cellular function by decreasing the activity of CaMKIV through Cys¹⁹⁸ oxidation.

Reactive sulfur species inactivate Ca2+/calmodulin-dependent protein kinase IV via S-polysulfidation of its active-site cysteine residue

Reactive sulfur species (RSS) modulate protein functions via S-polysulfidation of reactive Cys residues. Here, we report that Ca²⁺/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) was reversibly inactivated by RSS via polysulfidation of the active-site Cys residue. CaMKIV is phosphorylated at Thr¹⁹⁶ by its upstream CaMK kinase (CaMKK), resulting in the induction of its full activity. In vitro incubation of CaMKIV with the exogenous RSS donors Na₂S _n (n = 2-4) resulted in dose-dependent inhibition of the CaMKK-induced phospho-Thr¹⁹⁶ and consequent inactivation of the enzyme activity. Conversely, mutated CaMKIV (C198V) was refractory to the Na₂S _n -induced enzyme inhibition. A biotin-polyethylene glycol-conjugated maleimide capture assay revealed that Cys¹⁹⁸ in CaMKIV represents a target for S-polysulfidation. Furthermore, phosho-Thr¹⁹⁶ and CaMKIV activity were inhibited by incubation with cysteine hydropersulfide, a newly identified RSS that is generated from cystine by cystathionine-γ-lyase. In transfected cells expressing CaMKIV, ionomycin-induced CaMKIV phosphorylation at Thr¹⁹⁶ was decreased upon treatment with either Na₂S₄ or the endoplasmic reticulum (ER) stress inducer thapsigargin, whereas cells expressing mutant CaMKIV (C198V) were resistant to this treatment. In addition, the ionomycin-induced phospho-Thr¹⁹⁶ of endogenous CaMKIV was also inhibited by treatment either with Na₂S₄ or thapsigargin in Jurkat T lymphocytes. Taken together, these data define a novel signaling function for intracellular RSS in inhibiting CaMKIV activity via S-polysulfidation of its Cys¹⁹⁸ during the response to ER stress.

Nicotine reward and affective nicotine withdrawal signs are attenuated in calcium/calmodulin-dependent protein kinase IV knockout mice

The influx of Ca(2+) through calcium-permeable nicotinic acetylcholine receptors (nAChRs) leads to activation of various downstream processes that may be relevant to nicotine-mediated behaviors. The calcium activated protein, calcium/calmodulin-dependent protein kinase IV (CaMKIV) phosphorylates the downstream transcription factor cyclic AMP response element binding protein (CREB), which mediates nicotine responses; however the role of CaMKIV in nicotine dependence is unknown. Given the proposed role of CaMKIV in CREB activation, we hypothesized that CaMKIV might be a crucial molecular component in the development of nicotine dependence. Using male CaMKIV genetically modified mice, we found that nicotine reward is attenuated in CaMKIV knockout (-/-) mice, but cocaine reward is enhanced in these mice. CaMKIV protein levels were also increased in the nucleus accumbens of C57Bl/6 mice after nicotine reward. In a nicotine withdrawal assessment, anxiety-related behavior, but not somatic signs or the hyperalgesia response are attenuated in CaMKIV -/- mice. To complement our animal studies, we also conducted a human genetic association analysis and found that variants in the CaMKIV gene are associated with a protective effect against nicotine dependence. Taken together, our results support an important role for CaMKIV in nicotine reward, and suggest that CaMKIV has opposing roles in nicotine and cocaine reward. Further, CaMKIV mediates affective, but not physical nicotine withdrawal signs, and has a protective effect against nicotine dependence in human genetic association studies. These findings further indicate the importance of calcium-dependent mechanisms in mediating behaviors associated with drugs of abuse.

Calcium/calmodulin and cAMP/protein kinase-A pathways regulate sperm motility in the stallion

In spite of the importance of sperm motility to fertility in the stallion, little is known about the signaling pathways that regulate motility in this species. In other mammals, calcium/calmodulin signaling and the cyclic AMP/protein kinase-A pathway are involved in sperm motility regulation. We hypothesized that these pathways also were involved in the regulation of sperm motility in the stallion. Using immunoblotting, calmodulin and the calmodulin-dependent protein kinase II β were shown to be present in stallion sperm and with indirect immunofluorescence calmodulin was localized to the acrosome and flagellar principal piece. Additionally, inhibition of either calmodulin or protein kinase-A significantly reduced sperm motility without affecting viability. Following inhibition of calmodulin, motility was not restored with agonists of the cyclic AMP/protein kinase-A pathway. These data suggest that calcium/calmodulin and cyclic AMP/protein kinase-A pathways are involved in the regulation of stallion sperm motility. The failure of cyclic AMP/protein kinase-A agonists to restore motility of calmodulin inhibited sperm suggests that both pathways may be required to support normal motility.

Regulation of calcium/calmodulin-dependent kinase IV by O-GlcNAc modification

Similar to phosphorylation, GlcNAcylation (the addition of O-GlcNAc to Ser(Thr) residues on polypeptides) is an abundant, dynamic, and inducible post-translational modification. GlcNAcylated proteins are crucial in regulating virtually all cellular processes, including signaling, cell cycle, and transcription. Here we show that calcium/calmodulin-dependent kinase IV (CaMKIV) is highly GlcNAcylated in vivo. In addition, we show that upon activation of HEK293 cells, hemagglutinin-tagged CaMKIV GlcNAcylation rapidly decreases, in a manner directly opposing its phosphorylation at Thr-200. Correspondingly, there is an increase in CaMKIV interaction with O-GlcNAcase during CaMKIV activation. Furthermore, we identify at least five sites of GlcNAcylation on CaMKIV. Using site-directed mutagenesis, we determine that the GlcNAcylation sites located in the active site of CaMKIV can modulate its phosphorylation at Thr-200 and its activity toward cAMP-response element-binding transcription factor. Our results strongly indicate that the O-GlcNAc modification participates in the regulation of CaMKIV activation and function, possibly coordinating nutritional signals with the immune and nervous systems. This is the first example of an O-GlcNAc/phosphate cycle involving O-GlcNAc transferase/kinase cross-talk.

Post-translational excision of the carboxyl-terminal segment of CaM kinase phosphatase N and its cytosolic occurrence in the brain

Ca2+/Calmodulin-dependent protein kinase (CaM kinase) phosphatase, occurring in the cytoplasm of all tissues, dephosphorylates and thereby deactivates multifunctional CaM kinases, such as CaM kinases I, II and IV. In contrast, CaM kinase phosphatase N has been reported to occur almost exclusively in the brain and to be localized in the nucleus in the transfected COS-7 cells, as examined immunocytochemically with antibodies against the carboxyl-terminal segment of the enzyme, indicating its involvement in the deactivation of CaM kinase IV. Here, we show that the majority of the naturally occurring CaM kinase phosphatase N in the brain exists not in the intact form of the enzyme (83.4 kDa) but in a form (61.1 kDa) in which the carboxyl-terminal segment containing nuclear localization signals is deleted, and that it is present mostly in the cytoplasm but a little in the nucleus throughout the central nervous system, although occurring mostly in the nucleus in some large neurons. Strong immunostaining of the enzyme was also observed at postsynaptic density. These findings suggest that CaM kinase phosphatase N is involved in the regulation of not only CaM kinase IV but also CaM kinases II and I.

A proposed mechanism to explain the stimulatory effect of melatonin on antioxidative enzymes

Melatonin, the main secretory product of the pineal gland, is known to collaborate against oxidative stress within cells, but its mechanism of action in terms of stimulating antioxidant enzymes remains unclear. Herein, we propose that melatonin modulates antioxidant enzyme activities via its interaction with calmodulin, which in turn inhibits downstream processes that lead to the inactivation of nuclear RORalpha melatonin receptor. Eventually, this nuclear transcription factor downregulates NF-kappaB-induced antioxidant enzyme expression. Therefore, the increment in antioxidant enzyme activities induced by melatonin involves the inhibition of the RORalpha pathway. Thus, in addition to its direct free radical scavenging activities, melatonin has important actions in oxidative defense by stimulating enzymes which metabolize free radicals and radical products to innocuous metabolites.

Calmodulin-Dependent Kinase IV Stimulates Vitamin D Receptor-Mediated Transcription

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] promotes intestinal absorption of calcium primarily by binding to the vitamin D receptor (VDR) and regulating gene expression. 1,25-(OH)2D3 also exerts rapid actions at the cell membrane that include increasing intracellular calcium levels and activating protein kinase cascades. To explore potential cross talk between calcium signaling elicited by the nongenomic actions of 1,25-(OH)2D3 and the genomic pathway mediated by VDR, we examined the effects of activated Ca2+/calmodulin-dependent kinases (CaMKs) on 1,25-(OH)2D3/VDR-mediated transcription. Expression of a constitutively active form of CaMKIV dramatically stimulated 1,25-(OH)2D3-activated reporter gene expression in COS-7, HeLa, and ROS17/2.8 cell lines. Metabolic labeling studies indicated that CaMKIV increased VDR phosphorylation levels. In addition, CaMKIV increased the independent transcription activity of the VDR coactivator SRC (steroid receptor coactivator) 1, and promoted ligand-dependent interaction between VDR and SRC coactivator proteins in mammalian two-hybrid studies. The functional consequences of this multifaceted mechanism of CaMKIV action were revealed by reporter gene studies, which showed that CaMKIV and select SRC coactivators synergistically enhanced VDR-mediated transcription. These studies support a model in which CaMKIV signaling stimulates VDR-mediated transcription by increasing phosphorylation levels of VDR and enhancing autonomous SRC activity, resulting in higher 1,25-(OH)2D3-dependent interaction between VDR and SRC coactivators.

Systemic lupus erythematosus serum IgG increases CREM binding to the IL-2 promoter and suppresses IL-2 production through CaMKIV

Systemic lupus erythematosus (SLE) T cells express high levels of cAMP response element modulator (CREM) that binds to the IL-2 promoter and represses the transcription of the IL-2 gene. This study was designed to identify pathways that lead to increased binding of CREM to the IL-2 promoter in SLE T cells. Ca(2+)/calmodulin-dependent kinase IV (CaMKIV) was found to be increased in the nucleus of SLE T cells and to be involved in the overexpression of CREM and its binding to the IL-2 promoter. Treatment of normal T cells with SLE serum resulted in increased expression of CREM protein, increased binding of CREM to the IL-2 promoter, and decreased IL-2 promoter activity and IL-2 production. This process was abolished when a dominant inactive form of CaMKIV was expressed in normal T cells. The effect of SLE serum resided within the IgG fraction and was specifically attributed to anti-TCR/CD3 autoantibodies. This study identifies CaMKIV as being responsible for the increased expression of CREM and the decreased production of IL-2 in SLE T cells and demonstrates that anti-TCR/CD3 antibodies present in SLE sera can account for the increased expression of CREM and the suppression of IL-2 production.

The autonomous activity of calcium/calmodulin-dependent protein kinase IV is required for its role in transcription

Calcium/calmodulin-dependent kinase IV (CaMKIV) is a multifunctional serine/threonine kinase that is positively regulated by two main events. The first is the binding of calcium/calmodulin (Ca(2+)/CaM), which relieves intramolecular autoinhibition of the enzyme and leads to basal kinase activity. The second is activation by the upstream kinase, Ca(2+)/calmodulin-dependent kinase kinase. Phosphorylation of Ca(2+)/CaM-bound CaMKIV on its activation loop threonine (residue Thr(200) in human CaMKIV) by Ca(2+)/calmodulin-dependent kinase kinase leads to increased CaMKIV kinase activity. It has also been repeatedly noted that activation of CaMKIV is accompanied by the generation of Ca(2+)/CaM-independent or autonomous activity, although the significance of this event has been unclear. Here we demonstrate the importance of autonomous activity to CaMKIV biological function. We show that phosphorylation of CaMKIV on Thr(200) leads to the generation of a fully Ca(2+)/CaM-independent enzyme. By analyzing the behavior of wild-type and mutant CaMKIV proteins in biochemical experiments and cellular transcriptional assays, we demonstrate that CaMKIV autonomous activity is necessary and sufficient for CaMKIV-mediated transcription. The ability of wild-type CaMKIV to drive cAMP response element-binding protein-mediated transcription is strictly dependent upon an initiating Ca(2+) stimulus, which leads to kinase activation and development of autonomous activity in cells. Mutant CaMKIV proteins that are incapable of developing autonomous activity within a cellular context fail to drive transcription, whereas certain CaMKIV mutants that possess constitutive autonomous activity drive transcription in the absence of a Ca(2+) stimulus and independent of Ca(2+)/CaM binding or Thr(200) phosphorylation.

Regulation and function of the calcium/calmodulin-dependent protein kinase IV/protein serine/threonine phosphatase 2A signaling complex

Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is a member of the broad substrate specificity class of Ca(2+)/calmodulin (CaM)-dependent protein kinases and functions as a potent stimulator of Ca(2+)-dependent gene expression. Activation of CaMKIV is a transient, tightly regulated event requiring both Ca(2+)/CaM binding and phosphorylation of the kinase on T200 by an upstream CaMK kinase (CaMKK). Previously, CaMKIV was shown to stably associate with protein serine/threonine phosphatase 2A (PP2A), which was proposed to play a role in negatively regulating the kinase. Here we report that the Ca(2+)/CaM binding-autoinhibitory domain of CaMKIV is required for association of the kinase with PP2A and that binding of PP2A and Ca(2+)/CaM appears to be mutually exclusive. We demonstrate that inhibition of the CaMKIV/PP2A association in cells results in enhanced CaMKIV-mediated gene transcription that is independent of Ca(2+)/CaM. The enhanced transcriptional activity correlates with the elevated level of phospho-T200 that accumulates when CaMKIV is prevented from interacting with PP2A. Collectively, these data suggest a molecular basis for the sequential activation and inactivation of CaMKIV. First, in response to an increase in intracellular Ca(2+), CaMKIV binds Ca(2+)/CaM and becomes phosphorylated on T200 by CaMKK. These events result in the generation of autonomous activity required for CaMKIV-mediated transcriptional regulation. The CaMKIV-associated PP2A then dephosphorylates CaMKIV T200, thereby terminating autonomous activity and CaMKIV-mediated gene transcription.

Ca(2+)/calmodulin-dependent protein kinase IV expression in epithelial ovarian cancer

Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is a multifunctional protein kinase expressed abundantly in the central nervous system. Because changes in intracellular Ca(2+) concentrations affect progression through the mitotic cell cycle, enhanced expression of CaMKIV has been reported in small cell lung carcinoma and hepatocellular carcinoma. To elucidate the involvement of CaMKIV in epithelial ovarian carcinogenesis, we analyzed serial frozen sections for CaMKIV protein expression in 26 patients with ovarian epithelial carcinoma and ten patients with benign cystadenoma of the ovary by fluorescent immunohistochemistry. We analyzed the relationship between the percentages of CaMKIV-stained cells and the patient's characteristics, including histological classification, clinical stage, histological grade, and clinical outcome. In the benign ovarian cystadenoma, CaMKIV was detected in none of the cases examined. Most of the CaMKIV proteins were found in the nucleus of epithelial ovarian cancer tissue. CaMKIV expression was significantly associated with clinical stage (P<0.01), histological grade (P<0.01), and clinical outcome (P<0.01). Survival data were available for all patients, and univariate Cox regression analysis showed that CaMKIV expression was significantly associated with poor prognosis (P<0.05). Our results demonstrate that CaMKIV expression in epithelial ovarian cancer correlates with the malignant potential of this tumor.

Defective signaling in a subpopulation of CD4(+) T cells in the absence of Ca(2+)/calmodulin-dependent protein kinase IV

Ca(2+)/calmodulin-dependent protein kinase IV-deficient (CaMKIV(-/-)) mice have been used to investigate the role of this enzyme in CD4(+) T cells. We identify a functional defect in a subpopulation of CD4(+) T cells, characterized by a cell surface marker profile usually found on memory phenotype CD4(+) T cells. Upon T-cell receptor engagement, the mutant cells produce diminished levels of interleukin-2 (IL-2), IL-4, and gamma interferon protein and mRNA. The defect is secondary to an inability to phosphorylate CREB and to induce CREB-dependent immediate-early genes, including c-jun, fosB, fra2, and junB, which are required for cytokine gene induction. In contrast, stimulated naive CD4(+) T cells from CaMKIV(-/-) mice show normal CREB phosphorylation, induction of immediate-early genes, and cytokine production. Thus, in addition to defining an important signaling role for CaMKIV in a subpopulation of T cells, we identify differential signaling requirements for cytokine production between naive T cells and T cells that express cell surface markers characteristic of the memory phenotype.

Requirement for Ca2+/calmodulin-dependent kinase type IV/Gr in setting the thymocyte selection threshold

The outcome of thymocyte selection is influenced by the nature of Ca2+ signals transduced by the TCR. Robust Ca2+ responses characterize high-affinity, negatively selecting peptide/TCR interactions, while modest responses typify lower-affinity, positively selecting interactions. To elucidate mechanisms by which thymocytes decode distinct Ca2+ signals, we examined selection events in mice lacking Ca2+/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr), which is enriched in thymocytes. CaMKIV/Gr-deficient thymocytes exhibited impaired positive selection and defective Ca2+-dependent gene transcription. Significantly, CaMKIV/Gr deficiency raised the selection threshold of peptide/TCR interactions such that a peptide that normally induced weak negative selection instead promoted positive selection. These results demonstrate an important role for CaMKIV/Gr in sensitizing thymocytes to selection by low-affinity peptides.

Inhibition of neuronal nitric-oxide synthase by calcium/ calmodulin-dependent protein kinase IIalpha through Ser847 phosphorylation in NG108-15 neuronal cells

We have previously demonstrated that phosphorylation of neuronal nitric-oxide synthase (nNOS) at Ser(847) by Ca(2+)/calmodulin-dependent protein kinases (CaM kinases) attenuates the catalytic activity of the enzyme in vitro (Hayashi Y., Nishio M., Naito Y., Yokokura H., Nimura Y., Hidaka H., and Watanabe Y. (1999) J. Biol. Chem. 274, 20597-20602). In the present study we determined that CaM kinase IIalpha (CaM-K IIalpha) can directly phosphorylate nNOS on Ser(847), leading to a reduction of nNOS activity in cells. The phosphorylation abilities of purified CaM kinase Ialpha (CaM-K Ialpha), CaM-K IIalpha, and CaM-kinase IV (CaM-K IV) on Ser(847) were analyzed using the synthetic peptide nNOS-(836-859) (Glu-Glu-Arg-Lys-Ser-Tyr-Lys-Val-Arg-Phe-Asn-Ser-Val-Ser-Ser-Tyr-Ser- Asp-Ser-Arg-Lys-Ser-Ser-Gly) from nNOS as substrate. The relative V(max)/K(m) ratios of CaM kinases for nNOS-(836-859) were found to be as follows: CaM-K IIalpha, 100; CaM-K Ialpha, 54.5; CaM-K IV, 9.1. Co-transfection of constitutively active CaM-K IIalpha1-274 but not inactive CaM-K IIalpha1-274, generated by mutation of Lys(42) to Ala, with nNOS into NG108-15 cells, resulted in increased Ser(847) phosphorylation in the presence of okadaic acid, an inhibitor of protein phosphatase (PP)1 and PP2A, with a concomitant inhibition of NOS enzyme activity. In addition, this latter decrease could be reversed by treatment with exogenous PP2A. Cells expressing mutant nNOS (S847A) proved resistant to phosphorylation and a decrease of NOS activity. Thus, our results indicate that Ca(2+) triggers cross-talk signal transduction between CaM kinase and NO and CaM-K IIalpha phosphorylating nNOS on Ser(847), which in turn decreases the gaseous second messenger NO in neuronal cells.

Activation of orphan receptor-mediated transcription by Ca(2+)/calmodulin-dependent protein kinase IV

Retinoid-related receptor alpha (RORalpha) is an orphan nuclear receptor that constitutively activates transcription from its cognate response element. We show that RORalpha is Ca(2+ )responsive, and a Ca(2+)/calmodulin-independent form of Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) potentiates RORalpha-dependent transcription 20- to 30-fold. Other orphan receptors including RORalpha2, RORgamma and COUP-TFI are also potentiated by CaMKIV. Transcriptional activation by CaMKIV is orphan receptor selective and does not occur with either the thyroid hormone or estrogen receptor. CaMKIV does not phosphorylate RORalpha or its ligand-binding domain (LBD) in vitro, although the LBD is essential for transactivation. Therefore, the RORalpha LBD was used in the mammalian two-hybrid assay to identify a single class of small peptide molecules containing LXXLL motifs that interacted with greater affinity in the presence of CaMKIV. This class of peptides antagonized activation of orphan receptor-mediated transcription by CaMKIV. These studies demonstrate a pivotal role for CaMKIV in the regulation of orphan receptor-mediated transcription.

Age-related decline in activation of calcium/calmodulin-dependent phosphatase calcineurin and kinase CaMK-IV in rat T cells

We have previously shown that the DNA binding activity of the transcription factor NFAT which plays a predominant role in IL-2 transcription decreases with age. Because the transactivation (dephosphorylation and nuclear translocation) of the NFAT-c (cytoplasmic component of the NFAT complex) is mediated by the calcium/calmodulin-dependent phosphatase, calcineurin (CaN), and because Ca2+/calmodulin-dependent kinases (CaMK-II and IV/Gr) have been shown to play a critical role in calcium signaling in T cells, it was of interest to determine what effect aging has on the activation and the levels of these calcium regulating enzymes. The induction of calcineurin phosphatase activity, and CaMK-II and IV/Gr activities, were studied in splenic T cells isolated from Fischer 344 rats at 6, 15, and 24 months of age. In addition, the changes in the protein levels of these enzymes were measured by Western blot. The calcineurin phosphatase activity and CaMK-II and IV kinase activities were at a maximum after the cells were incubated with anti-CD3 antibody for 5-10 minutes. The induction of calcineurin activity by anti-CD3 and by calcium ionophore (A23187) declined 65 and 55%, respectively, between 6 and 24 months of age. The induction of CaMK-IV activity, but not CaMK-II activity by anti-CD3, was significantly less (by 54%) in T cells from old rats compared to T cells from young rats. The decline in the activation of these enzymes with age was not associated with changes in their corresponding protein levels. These results demonstrate that alterations in calcineurin phosphatase activity and CaMK-IV activity may contribute to the well-documented age-related decline in T cell function.

The Ca-calmodulin-dependent protein kinase cascade

The Ca2+-calmodulin-dependent protein kinase (CaM kinase) cascade includes three kinases: CaM-kinase kinase (CaMKK); and the CaM kinases CaMKI and CaMKIV, which are phosphorylated and activated by CaMKK. Members of this cascade respond to elevation of intracellular Ca2+ levels and are particularly abundant in brain and in T cells. CaMKK and CaMKIV localize both to the nucleus and to the cytoplasm, whereas CaMKI is only cytosolic. Nuclear CaMKIV regulates transcription through phosphorylation of several transcription factors, including CREB. In the cytoplasm, there is extensive cross-talk between CaMKK, CaMKIV and other signaling cascades, including those that involve the cAMP-dependent kinase (PKA), MAP kinases and protein kinase B (PKB; also known as Akt). Activation of PKB by CaMKK appears to be important in protection of neurons from programmed cell death during development.

Calmodulin-dependent protein kinase IV: regulation of function and expression

Calmodulin-dependent protein kinase IV (CaMKIV) is a key mediator of Ca2+-induced gene expression. This serine/threonine kinase is itself activated by a calmodulin kinase kinase. In the present contribution the gene structure, regulation of activity, the role in Ca2+-dependent gene expression, and the hormonal induction and controlled expression of CaMKIV during tissue development are reviewed.

Components of a calmodulin-dependent protein kinase cascade. Molecular cloning, functional characterization and cellular localization of Ca2+/calmodulin-dependent protein kinase kinase beta

Ca2+/calmodulin-dependent protein kinases I and IV (CaMKI and CaMKIV, respectively) require phosphorylation on an equivalent single Thr in the activation loop of subdomain VIII for maximal activity. Two distinct CaMKI/IV kinases, CaMKKalpha and CaMKKbeta, were purified from rat brain and partially sequenced (Edelman, A. M., Mitchelhill, K., Selbert, M. A., Anderson, K. A., Hook, S. S., Stapleton, D., Goldstein, E. G., Means, A. R., and Kemp, B. E. (1996) J. Biol. Chem. 271, 10806-10810). We report here the cloning and sequencing of cDNAs for human and rat CaMKKbeta, tissue and regional brain localization of CaMKKbeta protein, and mRNA and functional characterization of recombinant CaMKKbeta in vitro and in Jurkat T cells. The sequences of human and rat CaMKKbeta demonstrate 65% identity and 80% similarity with CaMKKalpha and 30-40% identity with CaMKI and CaMKIV themselves. CaMKKbeta is broadly distributed among rat tissues with highest levels in CaMKIV-expressing tissues such as brain, thymus, spleen, and testis. In brain, CaMKKbeta tracks more closely with CaMKIV than does CaMKKalpha. Bacterially expressed CaMKKbeta undergoes intramolecular autophosphorylation, is regulated by Ca2+/CaM, and phosphorylates CaMKI and CaMKIV on Thr177 and Thr200, respectively. CaMKKbeta activates both CaMKI and CaMKIV when coexpressed in Jurkat T cells as judged by phosphorylated cAMP response element-binding protein-dependent reporter gene expression. CaMKKbeta activity is enhanced by elevation of intracellular Ca2+, although substantial activity is observed at the resting Ca2+ concentration. The strict Ca2+ requirement of CaMKIV-dependent phosphorylation of cAMP response element-binding protein, is therefore controlled at the level of CaMKIV rather than CaMKK.

Calcium/calmodulin-dependent protein kinase IV is cleaved by caspase-3 and calpain in SH-SY5Y human neuroblastoma cells undergoing apoptosis

We have previously demonstrated cleavage of alpha-spectrin by caspase-3 and calpain during apoptosis in SH-SY5Y neuroblastoma cells (Nath, R., Raser, K. J., Stafford, D., Hajimohammadreza, I., Posner, A., Allen, H., Talanian, R. V., Yuen, P., Gilbertsen, R. B., and Wang, K. K. (1996) Biochem. J. 319, 683-690). We demonstrate here that calcium/calmodulin-dependent protein kinase IV (CaMK IV) is cleaved during apoptosis by caspase-3 and calpain. We challenged SH-SY5Y cells with the pro-apoptotic agent thapsigargin. Western blot analysis revealed major CaMK IV breakdown products of 40, 38, and 33 kDa. Digestion of control SH-SY5Y lysate with purified caspase-3 produced a 38-kDa CaMK IV fragment; digestion with purified calpain produced a major fragment of 40 kDa. Pretreatment with carbobenzoxy-Asp-CH2OC(O)-2,6-dichlorobenzene or Z-Val-Ala-Asp-fluoromethylketone was able to block the caspase-3-mediated production of the 38-kDa fragment both in situ and in vitro. Calpain inhibitor II similarly blocked formation of the calpain-mediated 40-kDa fragment both in situ and in vitro. Digestion of recombinant CaMK IV by other caspase family members revealed that only caspase-3 produces a fragmentation pattern consistent to that seen in situ. The major caspase-3 and calpain cleavage sites are respectively identified as PAPD176*A and CG201*A, both within the CaMK IV catalytic domain. Furthermore, calmodulin-stimulated protein kinase activity decreases within 6 h in thapsigargin-treated SH-SY5Y. The loss of activity precedes cell death.

The role of Ca2+/calmodulin-dependent protein kinases within the nucleus

Stimulation of cells by Ca(2+)-linked signaling agents increases Ca2+ levels within both the cell cytosol and nucleus. The multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) family, consisting of CaM kinases I, II and IV, have all been detected within the nucleus and each may serve as a mediator of nuclear Ca2+ signals. Certain isoforms of the large multimeric CaM kinase II are targeted to the nucleus as a result of an alternatively spliced nuclear localization signal. By contrast, CaM kinases I and IV are monomeric and likely gain nuclear access by passive diffusion through nuclear pores. These kinases have activation properties which may allow them to discriminate between Ca2+ signals which differ in their spike frequency, amplitude and duration. In addition, these kinases have the ability to control gene expression through the phosphorylation of key regulatory sites on nuclear transcription factors. CaM kinases may thus serve to decode Ca2+ signals to the nucleus in order to produce a multitude of cellular responses including control of cell cycle, apoptosis and synaptic efficacy.

Reticalmin: a novel calcium/calmodulin-dependent protein kinase IV-like protein in rat retina

Western blot analysis of 100,000 g supernatant of rat retina using a polyclonal anti-Ca2+/ calmodulin-dependent protein kinase IV (CaM-kinase IV) antibody revealed an immunoreactive mass of 35 kDa, termed reticalmin. Lower amount of a isoform of CaM-kinase IV was also expressed in rat retina. Reticalmin did not react with anti-CaM-kinase IV C-terminal peptide antibody which recognized alpha and beta isoforms of CaM-kinase IV and calspermin. Immunohistochemically reticalmin was shown to be localized mainly in the outer segment of photo-receptor cells, and in dendrites of inner plexiform layers and may be in nuclei of ganglion cells and some inner nuclear layer cells.

Defective survival and activation of thymocytes in transgenic mice expressing a catalytically inactive form of Ca2+/calmodulin-dependent protein kinase IV

We have generated transgenic mice that express a catalytically inactive form of Ca2+/calmodulin-dependent protein kinase IV (CaMKIV) specifically in thymic T cells. The presence of this protein results in a markedly reduced thymic cellularity, although the distribution of the remaining cells is normal based on evaluation of the CD4 and CD8 cell surface antigens that are used to gauge T cell development. Isolated thymic T cells from the transgenic mice also show a dramatically decreased survival rate when evaluated in culture under conditions that do not favor activation. When challenged with an activating stimulus such as alpha-CD3 or a combination of phorbol ester plus ionophore, the cells are severely compromised in their ability to produce the cytokine interleukin-2 (IL-2). Reduction of IL-2 production is secondary to the inability to phosphorylate the cAMP response element binding protein, CREB, and induce expression of the immediate early genes such as Fos B that are required to transactivate the IL-2 promoter. Because transgene expression was regulated by the proximal promoter of the murine lck gene and this promoter is inactivated in T cells that exit the thymus, the mutant hCaMKIV is not present in peripheral T cells. Consequently, T lymphocytes present in the spleen can be activated normally in response to either stimulus mentioned above, demonstrating that the effects of the inactive CaMKIV on activation are reversible. Our results suggest that CaMKIV may represent a physiologically relevant CREB kinase in T cells and that the enzyme is also required to ensure normal expansion of T cells in the thymus. Whereas the pathway responsible for this latter role is yet to be elucidated, it is unlikely to include CREB phosphorylation.

Regulation of phospholipid biosynthesis by Ca(2+)-calmodulin-dependent protein kinase inhibitors

Inhibitors of Ca(2+)-calmodulin (CaM)-dependent protein kinases strongly modify phospholipid metabolism. Two compounds, KN62 and KT5926 recognized as blockers of Ca(2+)-CaM-dependent protein kinase II, induced a specific increase in phosphatidylserine (PtdSer) synthesis without noticeable changes in phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) biosynthesis. The increase of PtdSer synthesis was dependent on the presence of Ca2+ in the incubation medium and was impaired in cells whose Ca2+ stores were depleted by pretreatment with CD3 mAb, thapsigargin or EGTA. The mechanism of the stimulation of PtdSer synthesis by these two compounds seems to involve an accumulation of Ca2+ into the endoplasmic reticulum, possibly due to an increased activity of the endoplasmic reticulum Ca(2+)-ATPase. By contrast, ML-7 and ML-9, two inhibitors of the myosin light chain kinase (MLCK), another Ca(2+)-CaM-dependent kinase, were both capable of increasing PtdSer synthesis and decreasing PtdCho and PtdEtn synthesis, reproducing the effect previously described with CaM-antagonists. The increase of PtdSer caused by ML-7 and ML-9 was Ca(2+)-dependent while the inhibition of PtdCho and PtdEtn synthesis was not. The use of these four protein kinase inhibitors thus suggests the possible existence of two CaM-dependent pathways that differentially regulates phospholipid metabolism in T cells.

Calmodulin kinase IV: expression and function during rat brain development

The expression of calmodulin kinase IV (CaMKIV) can be induced by the thyroid hormone T3 in a time- and concentration-dependent manner at a very early stage of brain differentiation using a fetal rat telencephalon primary cell culture system which can grow and differentiate under chemically defined conditions (Krebs et al. (1996) J. Biol. Chem. 271, 11055-11058). After the induction of CaMKIV by T3 we examined the influence of prolonged absence of T3 from the culture medium on the expression of CaMKIV. We could demonstrate that after the T3-dependent induction of CaMKIV, omission of the hormone, even for 8 days, from the medium did not downregulate the expression of CaMKIV indicating that different regulatory mechanisms became important for the expression of the enzyme. We further showed that CaMKIV could be involved in the Ca(2+) -dependent expression of the immediate early gene c-fos, probably via phosphorylation of the transcription factor CREB. Convergence of signal transduction pathways on this transcription factor by using different protein kinases may explain the importance of CREB for the regulation of different cellular processes.

A unique phosphorylation-dependent mechanism for the activation of Ca2+/calmodulin-dependent protein kinase type IV/GR

The activity of the Ca2+/calmodulin-dependent protein kinase IV/Gr (CaMKIV/Gr) is shown to be strictly regulated by phosphorylation of three residues both in vitro and in response to antigen receptor-mediated signaling in lymphocytes. One residue, Thr-200, is indispensable for enhancement of Ca2+/calmodulin-dependent basal activity by CaMKIV/Gr kinase. This event requires Ca2+/calmodulin in the full-length CaMKIV/Gr but is Ca2+/calmodulin-independent when a truncated version of CaMKIV/Gr is used as a substrate (DeltaCaMKIV/Gr1-317 (Delta1-317)). The other two residues, Ser12 and Ser13, are apparently autophosphorylated by the Ca2+/calmodulin-bound CaMKIV/Gr. Phosphorylation of neither Ser12-Ser13 nor Thr312 (the residue in a homologous position to Thr286 of CaMKIIalpha influences the development of Ca2+/calmodulin-independent activity or any other property of CaMKIV/Gr examined. Similarly, removal of the NH2-terminal 20 amino acids has no effect on the activation or function of CaMKIV/Gr. However, mutation of both Ser12 and Ser13 residues to Ala in Delta1-317 completely abrogates activity, while individual substitutions have no effect. These results indicate that the NH2-terminal Ser cluster mediates a novel type of intrasteric inhibition and suggest that three events are required for CaMKIV/Gr activation: 1) Ca2+/calmodulin binding; 2) phosphorylation of the Ca2+/calmodulin-bound enzyme on Thr200 by a Ca2+/calmodulin-dependent protein kinase kinase; and 3) autophosphorylation of Ser12-Ser13. This three-step requirement is unique among the multifunctional Ca2+/calmodulin-dependent kinases.

Induction of calmodulin kinase IV by the thyroid hormone during the development of rat brain

This communication reports the specific induction of calmodulin kinase IV by the thyroid hormone 3,3',5-triiodo-L-thyronine (T3) in a time- and concentration-dependent manner at a very early stage of brain differentiation using a fetal rat telencephalon primary cell culture system, which can grow and differentiate under chemically defined conditions. The induction of the enzyme that can be observed both on the mRNA and on the protein level is T3-specific, i.e. it cannot be induced by retinoic acid or reverse T3, and can be inhibited on both the transcriptional and the translational level by adding to the culture medium actinomycin D or cycloheximide, respectively. The earliest detection of calmodulin kinase IV in the fetal brain tissue of the rat is at days E16/E17, both on the mRNA as well as on the protein level. This is the first report in which a second messenger-dependent kinase involved in the control of cell regulatory processes is itself controlled by a primary messenger, the thyroid hormone.

Multiple Ca(2+)-calmodulin-dependent protein kinase kinases from rat brain. Purification, regulation by Ca(2+)-calmodulin, and partial amino acid sequence

We have purified to near homogeneity from rat brain two Ca(2+)-calmodulin-dependent protein kinase I (CaM kinase I) activating kinases, termed here CaM kinase I kinase-alpha and CaM kinase I kinase-beta (CaMKIK alpha and CaMKIK beta, respectively). Both CaMKIK alpha and CaMKIK beta are also capable of activating CaM kinase IV. Activation of CaM kinase I and CaM kinase IV occurs via phosphorylation of an equivalent Thr residue within the "activation loop" region of both kinases, Thr-177 and Thr-196, respectively. The activities of CaMKIK alpha and CaMKIK beta are themselves strongly stimulated by the presence of Ca(2+)-CaM, and both appear to be capable of Ca(2+)-CaM-dependent autophosphorylation. Automated microsequence analysis of the purified enzymes established that CaMKIK alpha and -beta are the products of distinct genes. In addition to rat, homologous nucleic acids corresponding to these CaM kinase kinases are present in humans and the nematode, Caenorhabditis elegans. CaMKIK alpha and CaMKIK beta are thus representatives of a family of enzymes, which may function as key intermediaries in Ca(2+)-CaM-driven signal transduction cascades in a wide variety of eukaryotic organisms.

Inactivation of calmodulin-dependent protein kinase IV by autophosphorylation of serine 332 within the putative calmodulin-binding domain

When brain calmodulin-dependent protein kinase IV is incubated with calmodulin-dependent protein kinase IV kinase under the phosphorylation conditions in the presence of Ca2+/calmodulin, rapid initial incorporation of 1 mol of phosphate into 1 mol of the enzyme by the action of the kinase kinase occurs, resulting in marked activation of the enzyme, and the subsequent incorporation of more than 3 mol of phosphate by autophosphorylation occurs, resulting in no significant change in the activity (Okuno, S., Kitani, T., and Fujisawa, H. (1994) J. Biochem. (Tokyo) 116, 923-930; Okuno, S., Kitani, T., and Fujisawa, H. (1995) J. Biochem. (Tokyo) 117, 686-690). After the maximal phosphorylation, the continued incubation in the presence of excess EGTA resulted in additional autophosphorylation of the enzyme, leading to a complete loss of the Ca2+/calmodulin-dependent activity, while causing no significant change in the Ca2+/calmodulin-independent activity. The amino acid sequence analysis revealed that the autophosphorylation after removal of Ca2+ occurred on Ser332, Ser333, Ser337, and Ser341. Analysis by site-directed mutagenesis clearly showed that the autophosphorylation site responsible for the inactivation is Ser332. Thus, calmodulin-dependent protein kinase IV activated by the kinase kinase may lose its Ca2+/calmodulin-dependent activity by autophosphorylation on Ser332 located within the putative calmodulin-binding domain in the absence of Ca2+.

Requirements for calcium and calmodulin in the calmodulin kinase activation cascade

We have previously purified and cloned rat brain Ca2+/calmodulin-dependent protein kinase kinase (CaM-KK), and the 68-kDa recombinant CaM-KK activates in vitro both CaM-kinase IV (CaM-K IV) and CaM-K I (Tokumitsu, H., Enslen, H., and Soderling, T. R. (1995) J. Biol. Chem. 270, 19320-19324). In the present study we have determined that activation of CaM-K IV through phosphorylation of Thr196 by CaM-KK is triggered by elevated intracellular Ca2+ in intact cells and requires binding of Ca2+/CaM to both enzymes. An expressed fragment of CaM-K IV (CaM-K IV178-246), which contains the activating phosphorylation site (Thr196) but not the autoinhibitory domain or the CaM-binding domain, still required Ca2+/CaM for phosphorylation by wild-type CaM-KK. A truncated mutant of CaM-KK (CaM-KK1-434) phosphorylated CaM-K IV178-246 in a Ca2+/CaM-independent manner, but this constitutively active CaM-KK1 434 required Ca2+/CaM for phosphorylation and activation of wild-type CaM-K IV. These results demonstrate that binding of Ca2+/CaM to both CaM-K IV and CaM-KK is required for the CaM-kinase cascade. Both CaM-KK and CaM-K IV appear to have similar Ca2+/CaM requirements with EC50 values of approximately 100 nM. Studies using co-expression of CaM-K IV with CaM-KK in COS-7 cells demonstrated that CaM-KK rapidly activated both total and Ca2+/CaM-independent activities of wild-type CaM-K IV, but not the Thr196 --> Ala mutant, upon ionomycin stimulation.

Activation of Ca2+/calmodulin-dependent protein kinase (CaM-kinase) IV by CaM-kinase kinase in Jurkat T lymphocytes

Ca2+/calmodulin-dependent protein kinase IV (CaM-kinase IV), a member of the CaM-kinase family involved in transcriptional regulation, is stimulated by Ca2+/CaM but also requires phosphorylation by a CaM-kinase kinase for full activation. In this study we investigated the physiological role of a CaM-kinase cascade in Jurkat T human lymphocytes through antigen receptor (CD3) signaling. Total and Ca(2+)-independent CaM-kinase IV activities were increased 8-14-fold by anti-CD3 antibody. This CD3-mediated activation involved phosphorylation since the immunoprecipitated CaM-kinase IV from stimulated Jurkat cells could be subsequently inactivated in vitro by protein phosphatase 2A. CaM-kinase IV immunoprecipitated from unstimulated Jurkat cells or CD3-negative mutant Jurkat cells could be activated in vitro 10-40-fold by CaM-kinase kinase purified from rat brain or thymus, whereas CaM-kinase IV from CD3-stimulated wild-type Jurkat cells was only activated to 2-3-fold by exogenous CaM-kinase kinase. CaM-kinase IV activation was triggered by Ca2+ acting through calmodulin since activation could also be elicited by ionomycin treatment, and CD3-mediated activation was blocked by the calmodulin antagonist calmidazolium. These data are consistent with a CaM-kinase cascade in which CaM-kinase IV is activated by a CaM-kinase kinase cascade triggered by elevated intracellular calcium in Jurkat cells.

Characterization of a Ca2+/calmodulin-dependent protein kinase cascade. Molecular cloning and expression of calcium/calmodulin-dependent protein kinase kinase

Recent studies have demonstrated that Ca2+/calmodulin-dependent protein kinase IV (CaM-kinase IV) can mediate Ca(2+)-dependent regulation of gene expression through the phosphorylation of transcriptional activating proteins. We have previously identified and purified a 68-kDa rat brain CaM-kinase kinase that phosphorylates and increases total and Ca(2+)-independent activities of CaM-kinase IV (Tokumitsu, H., Brickey, D. A., Gold, J., Hidaka, H., Sikela, J., and Soderling, T. R. (1994) J. Biol. Chem. 269, 28640-28647). Using a partial amino acid sequence of the purified brain kinase, a CaM-kinase kinase cDNA was cloned from a rat brain cDNA library. Northern blot analysis showed that CaM-kinase kinase mRNA (3.4 kilobases) was expressed in rat brain, thymus, and spleen. Sequence analyses revealed that the cDNA encoded a 505-amino acid protein, which contained consensus protein kinase motifs and was 30-40% homologous with members of the CaM-kinase family. Expression of the cDNA in COS-7 cells yielded an apparent 68-kDa CaM-binding protein, which catalyzed in vitro activation in the presence of Mg2+/ATP and Ca2+/ CaM of CaM-kinases I and IV but not of CaM-kinase II. Co-expression of CaM-kinase kinase with CaM-kinase IV gave a 14-fold enhancement of cAMP-response element-binding protein-dependent gene expression compared with CaM-kinase IV alone. These results are consistent with the hypothesis that CaM-kinases I and IV are regulated through a unique signal transduction cascade involving CaM-kinase kinase.

Phosphorylation and activation of Ca(2+)-calmodulin-dependent protein kinase IV by Ca(2+)-calmodulin-dependent protein kinase Ia kinase. Phosphorylation of threonine 196 is essential for activation

Purified pig brain Ca(2+)-calmodulin (CaM)-dependent protein kinase Ia kinase (Lee, J. C., and Edelman, A. M. (1994) J. Biol. Chem. 269, 2158-2164) enhances, by up to 24-fold, the activity of recombinant CaM kinase IV in a reaction also requiring Ca(2+)-CaM and MgATP. The addition of brain extract, although capable of activating CaM kinase IV by itself, provides no further activation beyond that induced by purified CaM kinase Ia kinase, consistent with the lack of a requirement of additional components for activation. Activation is accompanied by the development of significant (38%) Ca(2+)-CaM-independent CaM kinase IV activity. In parallel fashion to its activation, CaM kinase IV is phosphorylated in a CaM kinase Ia kinase-, Ca(2+)-CaM-, and MgATP-dependent manner. Phosphorylation occurs on multiple serine and threonine residues with a Ser-P:Thr-P ratio of approximately 3:1. The identical requirements for phosphorylation and activation and a linear relationship between extent of phosphorylation of CaM kinase IV and its activation state indicate that CaM kinase IV activation is induced by its phosphorylation. Replacement of Thr-196 of CaM kinase IV with a nonphosphorylatable alanine by site-directed mutagenesis abolishes both the phosphorylation and activation of CaM kinase IV, demonstrating that Thr-196 phosphorylation is essential for activation.

Serine 16 of oncoprotein 18 is a major cytosolic target for the Ca2+/calmodulin-dependent kinase-Gr

Oncoprotein 18 (Op18) is a cytosolic protein that was initially identified due to its up-regulated expression in acute leukemia and its complex pattern of phosphorylation in response to diverse extracellular signals. We have previously identified in vivo phosphorylation sites and some of the protein kinase systems involved. Two distinct proline-directed kinase families phosphorylate Ser25 and Ser38 of Op18 with overlapping but distinct site preference. These two kinase families, mitogen-activated protein (MAP) kinases and cyclin-dependent cdc2 kinases, are involved in receptor-regulated and cell-cycle-regulated phosphorylation events, respectively. During analysis of Op18 phosphorylation in the Jurkat T-cell line, we also found that Ser16 of Op18 is phosphorylated in response to a Ca2+ signal generated by T-cell receptor stimulation or the Ca2+ ionophore ionomycin. As suggested by a previous study, T-cell-receptor-induced phosphorylation events may be mediated by the Ca2+/CaM-dependent protein kinase type Gr (CaM kinase-Gr). The present study shows that activation of this protein kinase correlates with phosphorylation of Ser16 of Op18, and in vitro experiments reveal efficient and selective phosphorylation of this residue. The CaM kinase-Gr is only expressed in certain lymphoid cell lines, and the present study shows that ionomycin-induced phosphorylation of Op18 Ser16 is restricted to cells expressing this protein kinase. Finally, CaM kinase-Gr-dependent in vitro phosphorylation of a crude cellular extract reveals a striking preference of this protein kinase for Op18 compared to other cellular substrates. In conclusion, the results suggest that Ser16 of Op18 is a major cytosolic target for activated CaM kinase-Gr.

The cDNA sequence and characterization of the Ca2+/calmodulin-dependent protein kinase-Gr from human brain and thymus

We have isolated and sequenced cDNAs encoding Ca2+/calmodulin-dependent protein kinase type Gr (CaM-K-Gr, also called CaM-K-IV) from human brain and thymus. The sequence of the protein coding region of the cDNA is identical in both brain and thymus, although Northern hybridization analysis shows variation of the mRNA transcripts in these tissues. The sequence predicts a protein of M(r) 51,897 that is 83.7% identical and shows 89.2% similarity with the rat homologue. The deduced human CaM-K-Gr is identical to the rat and mouse proteins in the portion of the enzyme involved in ATP binding, the catalytic domain and Ca2+/calmodulin-binding domain; however, the N terminus of the human kinase, which may comprise a second regulatory domain [McDonald et al., J. Biol. Chem. 268 (1993) 10054-10059], contains a 4-amino-acid (aa) insertion relative to the rodent enzymes. Additionally, the C-terminal association domain shows only 45.2 and 41.6% identity with the rat and mouse proteins, respectively, suggesting that this domain is not constrained by stringent structural and functional requirements. Based on the predicted aa sequence of the human kinase, we produced polyclonal antisera against a C-terminal peptide that recognizes two forms of CaM-K-Gr in human T-cell lymphoma and neuroblastoma cell lines. The human antiserum cross-reacts with the rat and mouse proteins and immunoprecipitates the active kinase.

A Ca2+/calmodulin-dependent protein kinase, CaM kinase-Gr, expressed after transformation of primary human B lymphocytes by Epstein-Barr virus (EBV) is induced by the EBV oncogene LMP1

CaM kinase-Gr is a multifunctional Ca2+/calmodulin-dependent protein kinase which is enriched in neurons and T lymphocytes. The kinase is absent from primary human B lymphocytes but is expressed in Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines, suggesting that expression of the kinase can be upregulated by an EBV gene product(s). We investigated the basis of CaM kinase-Gr expression in EBV-transformed cells and the mechanisms that regulate its activity therein by using an EBV-negative Burkitt lymphoma cell line, BJAB, and BJAB cells converted to expression of individual EBV proteins by single-gene transfer. CaM kinase-Gr expression was upregulated in BJAB cells by EBV latent-infection membrane protein 1 (LMP1) but not by LMP2A or by nuclear proteins EBNA1, EBNA2, EBNA3A, and EBNA3C. In LMP1-converted BJAB cells, the kinase was functional and was dramatically activated upon cross-linking of surface immunoglobulin M. Overlapping cDNA clones that encode human CaM kinase-Gr were sequenced, revealing 81% amino acid identity between the rat and human proteins. Transfection of BJAB cells with an expression construct for the human enzyme resulted in a functional kinase which was shown by epitope tagging to localize primarily to cytoplasmic and perinuclear structures. Induction of CaM kinase-Gr expression by LMP1 provides the first example of a Ca2+/calmodulin-dependent protein kinase upregulated by a viral protein. In view of the key role played by LMP1 in B-lymphocyte immortalization by EBV, these findings implicate CaM kinase-Gr as a potential mediator of B-lymphocyte growth transformation.

Identification of alternate 5' untranslated regions in the gene encoding Ca2+/calmodulin-dependent kinase-G1

Alternate 5' untranslated regions (UTR) of the Ca2+/calmodulin-dependent kinase-Gr cDNA were identified by sequencing of anchored reverse transcription-polymerase chain reaction (RT-PCR) products. Three alternate 5' ends of the mRNA were spliced at the same point to a common 5' UTR. A 3-kb EcoRI-BamHI genomic DNA restriction fragment containing the alternate UTR and 5' flanking sequence was characterized by sequence analysis and identification of several potential transcriptional control elements.

Cloning and sequencing of a gene encoding the beta polypeptide of Ca2+/calmodulin-dependent protein kinase IV and its expression confined to the mature cerebellar granule cells

A cDNA encoding the beta polypeptide of Ca2+/calmodulin-dependent protein kinase IV (CaM kinase IV) was isolated and sequenced from a rat cerebellar cDNA library. By in situ hybridization histochemistry, we demonstrated the differential gene expression for alpha and beta polypeptides of CaM kinase IV in mature and developing rat brains using oligonucleotide probes specific for each polypeptide.