Ca(2+)-calmodulin-dependent protein kinases Ia and Ib from rat brain. II. Enzymatic characteristics and regulation of activities by phosphorylation and dephosphorylation

The Phosphatase-Resistant Isoform of CaMKI, Ca²⁺/Calmodulin-Dependent Protein Kinase Iδ (CaMKIδ), Remains in Its "Primed" Form without Ca²⁺ Stimulation

Ca²⁺/calmodulin-dependent protein kinase I (CaMKI) is known to play pivotal roles in Ca²⁺ signaling pathways. Four isoforms of CaMKI (α, β, γ, and δ) have been reported so far. CaMKI is activated through phosphorylation by the upstream kinase, CaMK kinase (CaMKK), and phosphorylates downstream targets. When CaMKI was transiently expressed in 293T cells, CaMKIα was not phosphorylated at all under low-Ca²⁺ conditions in the cells. In contrast, we found that CaMKIδ was significantly phosphorylated and activated to phosphorylate cAMP response element-binding protein (CREB) under the same conditions. Herein, we report that the sustained activation of CaMKIδ is ascribed to its phosphatase resistance resulting from the structure of its N-terminal region. First, we examined whether CaMKIδ is more readily phosphorylated by CaMKK than CaMKIα, but no significant difference was observed. Next, to compare the phosphatase resistance between CaMKIα and CaMKIδ, we assessed the dephosphorylation of the phosphorylated CaMKIs by CaMK phosphatase (CaMKP/PPM1F). Surprisingly, CaMKIδ was hardly dephosphorylated by CaMKP, whereas CaMKIα was significantly dephosphorylated under the same conditions. To date, there have been no detailed reports concerning dephosphorylation of CaMKI. Through extensive analysis of CaMKP-catalyzed dephosphorylation of various chimeric and point mutants of CaMKIδ and CaMKIα, we identified the amino acid residues responsible for the phosphatase resistance of CaMKIδ (Pro-57, Lys-62, Ser-66, Ile-68, and Arg-76). These results also indicate that the phosphatase resistance of CaMKI is largely affected by only several amino acids in its N-terminal region. The phosphatase-resistant CaMKI isoform may play a physiological role under low-Ca²⁺ conditions in the cells.

Identification of an endogenous substrate of zebrafish doublecortin-like protein kinase using a highly active truncation mutant

Doublecortin-like protein kinase (DCLK), a Ser/Thr protein kinase predominantly expressed in brain and eyes, is believed to play crucial roles in neuronal functions. However, the regulatory mechanisms for DCLK activation and its physiological targets are still unknown. In the present study, we found that a deletion mutant consisting of the catalytic domain of zebrafish DCLK, zDCLK(377-677), exhibited the highest activity among various mutants. Since fully active zDCLK(377-677) showed essentially the same substrate specificity as wild-type zDCLK, we used it to search for physiological substrates of zDCLK. When a zebrafish brain extract was resolved by isoelectric focusing and then phosphorylated by zDCLK(377-677), a highly basic protein with a molecular mass of approximately 90 kDa was detected. This protein was identified as synapsin II by mass spectrometric analysis. Synapsin II was found to interact with the catalytic domain of zDCLK and was phosphorylated at Ser-9 and Ser-58. When synaptosomes were isolated from zebrafish brain, both synapsin II and zDCLK were found to coexist in this preparation. Furthermore, synapsin II in the synaptosomes was efficiently phosphorylated by zDCLK. These results suggest that zDCLK mediates its neuronal functions through phosphorylation of physiological substrates such as synapsin II.

Pregnancy-upregulated nonubiquitous calmodulin kinase induces ligand-independent EGFR degradation

We describe here an important function of the novel calmodulin kinase I isoform, pregnancy-upregulated nonubiquitous calmodulin kinase (Pnck). Pnck (also known as CaM kinase Ibeta(2)) was previously shown to be differentially overexpressed in a subset of human primary breast cancers, compared with benign mammary epithelial tissue. In addition, during late pregnancy, Pnck mRNA was shown to be strongly upregulated in epithelial cells of the mouse mammary gland exhibiting decreased proliferation and terminal differentiation. Pnck mRNA is also significantly upregulated in confluent and serum-starved cells, compared with actively growing proliferating cells (Gardner HP, Seung HI, Reynolds C, Chodosh LA. Cancer Res 60: 5571-5577, 2000). Despite these suggestive data, the true physiological role(s) of, or the signaling mechanism(s) regulated by Pnck, remain unknown. We now report that epidermal growth factor receptor (EGFR) levels are significantly downregulated in a ligand-independent manner in human embryonic kidney-293 (HEK-293) cells overexpressing Pnck. MAP kinase activation was strongly inhibited by EGFR downregulation in the Pnck-overexpressing cells. The EGFR downregulation was not the result of reduced transcription of the EGFR gene but from protea-lysosomal degradation of EGFR protein. Knockdown of endogenous Pnck mRNA levels by small interfering RNA transfection in human breast cancer cells resulted in upregulation of unliganded EGFR, consistent with the effects observed in the overexpression model of Pnck-mediated ligand-independent EGFR downregulation. Pnck thus emerges as a new component of the poorly understood mechanism of ligand-independent EGFR degradation, and it may represent an attractive therapeutic target in EGFR-regulated oncogenesis.

Negative regulation of multifunctional Ca2+/calmodulin-dependent protein kinases: physiological and pharmacological significance of protein phosphatases

Multifunctional Ca2+/calmodulin-dependent protein kinases (CaMKs) play pivotal roles in intracellular Ca2+ signaling pathways. There is growing evidence that CaMKs are involved in the pathogenic mechanisms underlying various human diseases. In this review, we begin by briefly summarizing our knowledge of the involvement of CaMKs in the pathogenesis of various diseases suggested to be caused by the dysfunction/dysregulation or aberrant expression of CaMKs. It is widely known that the activities of CaMKs are strictly regulated by protein phosphorylation/dephosphorylation of specific phosphorylation sites. Since phosphorylation status is balanced by protein kinases and protein phosphatases, the mechanism of dephosphorylation/deactivation of CaMKs, corresponding to their 'switching off', is extremely important, as is the mechanism of phosphorylation/activation corresponding to their 'switching on'. Therefore, we focus on the regulation of multifunctional CaMKs by protein phosphatases. We summarize the current understanding of negative regulation of CaMKs by protein phosphatases. We also discuss the biochemical properties and physiological significance of a protein phosphatase that we designated as Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKP), and those of its homologue CaMKP-N. Pharmacological applications of CaMKP inhibitors are also discussed. These compounds may be useful not only for exploring the physiological functions of CaMKP/CaMKP-N, but also as novel chemotherapies for various diseases.

Involvement of Ca2+/calmodulin-dependent protein kinases in mycelial growth of the basidiomycetous mushroom, Coprinus cinereus

Although multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaM-kinases) are widely distributed in animal cells, the occurrence of CaM-kinases in the basidiomycetous mushroom has not previously been documented. When the extracts from various developmental stages from mycelia to the mature fruiting body of Coprinus cinereus were analyzed by Western blotting using Multi-PK antibodies, which had been generated to detect a wide variety of protein serine/threonine kinases (Ser/Thr kinases), a variety of stage-specific Ser/Thr kinases was detected. Calmodulin (CaM) overlay assay using digoxigenin-labeled CaM detected protein bands of 65 kDa, 58 kDa, 46 kDa, 42 kDa, and 38 kDa only in the presence of CaCl(2), suggesting that these bands were CaM-binding proteins. When the CaM-binding fraction was prepared from mycelial extract of C. cinereus by CaM-Sepharose and analyzed with Multi-PK antibodies, two major immunoreactive bands corresponding to 65 kDa and 46 kDa were detected. CaM-binding fraction, thus obtained, exhibited Ca(2+)/CaM-dependent protein kinase activity toward protein substrates such as histones. These CaM-kinases were found to be highly expressed in the actively growing mycelia, but not in the resting mycelial cells. Mycelial growth was enhanced by the addition of CaCl(2) in the culture media, but inhibited by the addition of EGTA or trifluoperazine, a potent CaM inhibitor. This suggested that CaM-dependent enzymes including CaM-kinases play crucial roles in mycelial growth of basidiomycete C. cinereus.

Expression, characterization, and gene knockdown of zebrafish doublecortin-like protein kinase

Doublecortin-like protein kinase (DCLK) is a protein Ser/Thr kinase expressed in brain and believed to play crucial roles in neuronal development. To investigate the biological significance of DCLK, we isolated cDNA clones for zebrafish DCLK (zDCLK) and found that there were five splice variants of the kinase. In this study, the catalytic properties of a major isoform of zDCLK, which we designated as zDCLK1, and of an N-terminal truncated mutant retaining the kinase domain were examined by expressing them in Escherichia coli. Mutational analysis of recombinant zDCLK suggested that the kinase was activated not only by phosphorylation at Thr-576 in the activation loop but also by autophosphorylation at the other site(s) in the catalytic domain. zDCLK significantly phosphorylated protein substrates such as myelin basic protein, histones, and synapsin I. Subcellular localization of zDCLK and its N-terminal deletion mutant implicated that microtubule-association of zDCLK is mediated through N-terminal doublecortin like domain of this enzyme. Western blotting analysis and whole mount in situ hybridization revealed that zDCLK was highly expressed in brain and eyes after 24-h post fertilization. Gene knockdown of zDCLK using morpholino-based antisense oligonucleotides induced significant increase of apoptotic cells in the central nervous systems and resulted in the increase of the morphologically abnormal embryos in a dose-dependent manner. These results suggest that zDCLK may play crucial roles in the central nervous systems during the early stage of embryogenesis.

Protein phosphatases that regulate multifunctional Ca2+/calmodulin-dependent protein kinases: from biochemistry to pharmacology

Multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaMKs) play pivotal roles in Ca(2+) signaling pathways, such as the regulation of the neuronal functions of learning, memory, and neuronal cell death. The activities of the kinases are strictly regulated by protein phosphorylation/dephosphorylation. Although the activation mechanisms for multifunctional CaMKs through phosphorylation, which correspond to "switch on," have been extensively studied, the negative regulatory mechanisms through dephosphorylation, which correspond to "switch off," have not. In this review, we focused on the regulation of multifunctional CaMKs by the protein phosphatases responsible. We first summarized the current understanding of negative regulation of CaMKs by known protein phosphatases and their physiological significance. We then discussed newly developed methods for detection of protein phosphatases involved in the regulation of CaMKs. We also summarized the biochemical properties of a novel protein phosphatase, which we isolated with the new methods and designated as CaMK phosphatase (CaMKP), and its homologue. Pharmacological implications for neuronal functions including memory and neuronal cell death are discussed from the viewpoint that regulation of protein kinase activity can be elucidated by focusing on protein phosphatases involved in its "switch off" mechanism.

Functional differences between two DCLK splice variants

Recently, we have cloned two splice variants of the doublecortin-like kinase (DCLK) gene, called DCLK-short-A and -B, both of which encode calcium/calmodulin-dependent protein kinase (CaMK)-like proteins with different C-terminal ends. Using in situ hybridization, we have found that both are highly expressed in limbic structures of the brain and that their expression differs in a number of brain areas. DCLK-short-A is relatively more strongly expressed than DCLK-short-B in the subependymal zone. The DCLK-short-B variant shows stronger expression in the cortex, the ventromedial and dorsomedial hypothalamic nuclei, the arcuate nucleus, the zona incerta and the subincertal nucleus. Also, within the hippocampus, the relative distribution of these two splice variants differs. DCLK-short-B expression compared to DCLK-short-A is highest in the CA1 area. The expression of the A variant is highest in the CA3/CA4 area. Additionally, DCLK-short-B is expressed at a higher level than DCLK-short-A in the substantia nigra and the mammillary nucleus. Both DCLK-short-A and -B were located in the cytoplasm, however DCLK-short-B was also found specifically in growth cone like structures and near the nucleus. Both DCLK-short proteins phosphorylate autocamtide and syntide, two highly specific CaMK substrates. Finally, removal of the C-terminal end of DCLK-short leads to a 10-fold increase of kinase activity, indicating that the different C-termini represent auto-inhibitory domains. Our results indicate that DCLK-short-A and -B control different neuronal processes that overlap with those controlled by CaMKs.

Phosphorylation screening identifies translational initiation factor 4GII as an intracellular target of Ca(2+)/calmodulin-dependent protein kinase I

CaMKI is a Ca2+/calmodulin-dependent protein kinase that is widely expressed in eukaryotic cells and tissues but for which few, if any, physiological substrates are known. We screened a human lung cDNA expression library for potential CaMKI substrates by solid phase in situ phosphorylation ("phosphorylation screening"). Multiple overlapping partial length cDNAs encoding three proteins were detected. Two of these proteins are known: 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase and eukaryotic translation initiation factor (eIF) 4GII. To determine whether CaMKI substrates identified by phosphorylation screening represent authentic physiological targets, we examined the potential for [Ca2+]i- and CaMKI-dependent phosphorylation of eIF4GII in vitro and in vivo. Endogenous eIF4GII immunoprecipitated from HEK293T cells was phosphorylated by CaMKI, in vitro as was a recombinant fragment of eIF4GII encompassing the central and C-terminal regions. The latter phosphorylation occurred with favorable kinetics (Km = 1 microm; kcat = 1.8 s-1) at a single site, Ser1156, located in a segment of eIF4GII aligning with the phosphoregion of eIF4GI. Phosphopeptide mapping and back phosphorylation experiments revealed [Ca2+]i-dependent, CaMKI site-specific, eIF4GII phosphorylation in vivo. This phosphorylation was blocked by kinase-negative CaMKI consistent with a requirement for endogenous CaMKI for in vivo eIF4GII phosphorylation. We conclude that phosphorylation screening is an effective method for searching for intracellular targets of CaMKI and may have identified a new role of Ca2+ signaling to the translation apparatus.

Activation of Ca2+/calmodulin-dependent protein kinase I in cultured rat hippocampal neurons

We have focused on activation mechanisms of calcium/calmodulin-dependent protein kinase (CaM) kinase I in the hippocampal neurons and compared them with that of CaM kinase IV. Increased activation of CaM kinase I occurred by stimulation with glutamate and depolarization in cultured rat hippocampal neurons. Similar to CaM kinases II and IV, CaM kinase I was essentially activated by stimulation with the NMDA receptor. Although both CaM kinases I and IV seem to be activated by CaM kinase kinase, the activation of CaM kinase I was persistent during stimulation with glutamate in contrast to a transient activation of CaM kinase IV. In addition, CaM kinase I was activated in a lower concentration of glutamate than that of CaM kinase IV. Depolarization-induced activation of CaM kinase I was also evident in the cultured neurons and was largely blocked by nifedipine. In the experiment with 32P-labeled cells, phosphorylation of CaM kinase I was stimulated by glutamate treatment and depolarization. The glutamate- and depolarization-induced phosphorylation was inhibited by the NMDA receptor antagonist and nifedipine, respectively. These results suggest that, although CaM kinases I and IV are activated by the NMDA receptor and depolarization stimulation, these kinase activities are differently regulated in the hippocampal neurons.

Expression of Ca2+/calmodulin-dependent protein kinase (CaMK) Ibeta2 in developing rat CNS

We observed the onset time and distribution pattern of beta2 isoform of Ca2+/calmodulin-dependent protein kinase I (CaMKIbeta2) in the CNS of the rat during the embryonic period until birth using an immunohistochemical method. The expression of CaMKIbeta2 started at embryological day 10 when the three primary brain vesicles and neural tube are generated from the neural plate. During the embryonic period, highly immunoreactive products were ubiquitously detected in neurons in the CNS, although neurons in the caudate-putamen and globus pallidus were faintly immunostained or immunonegative. High expression of CaMKIbeta2 persisted in the olfactory bulb, lymbic system, neocortex, septal nuclei, amygdala complex, some hypothalamic nuclei, pontine nuclei, Purkinje cells and granule cells in the cerebellar cortex through the developing period. At the subcellular level, CaMKIbeta2 was strongly expressed in nuclei of neurons but faintly in their cytoplasm, suggesting that this protein has an important role in the nuclear signaling pathway. This study demonstrates that expression of CaMKIbeta2 begins at the earliest developmental stage of the rat CNS and persists through the developing period.

Calcium binding is required for calmodulin function in Aspergillus nidulans

To explore the structural basis for the essential role of calmodulin (CaM) in Aspergillus nidulans, we have compared the biochemical and in vivo properties of A. nidulans CaM (AnCaM) with those of heterologous CaMs. Neither Saccharomyces cerevisiae CaM (ScCaM) nor a Ca2+ binding mutant of A. nidulans CaM (1234) interacts appreciably with A. nidulans CaM binding proteins by an overlay assay or activates two essential CaMKs, CMKA and CMKB. In contrast, although vertebrate CaM (VCaM) binds a spectrum of proteins similar to that for AnCaM, it is unable to fully activate CMKA and CMKB, displaying a higher K(CaM) and reduced Vmax for both enzymes. In correlation with the biochemical analysis, neither ScCaM nor 1234 can support A. nidulans growth in the absence of the endogenous protein, whereas VCaM only partially complements the absence of wild-type CaM. Analysis of VCaM and AnCaM chimeras demonstrates that amino acid variations in both N- and C-terminal domains contribute to the inability of VCaM to activate CMKB, but differences in the N terminus are largely responsible for the reduced activity towards CMKA. In vivo, the chimeric molecules support growth equivalently, but only to levels intermediate between those of VCaM and AnCaM, suggesting that the reduced ability to activate the CaMKs is not solely responsible for the inability of VCaM to complement the absence of the wild-type protein. Thus, not only is Ca2+ binding required for CaM function in A. nidulans, but the essential in vivo functions of A. nidulans CaM are uniquely sensitive to the subtle amino acid variations present in vertebrate CaM.

3-phosphoinositide-dependent protein kinase 1, an Akt1 kinase, is involved in dephosphorylation of Thr-308 of Akt1 in Chinese hamster ovary cells

To investigate the role of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in the Akt1 phosphorylation state, wild-type (wt) PDK1 and its kinase dead (kd) mutant were expressed using an adenovirus gene transduction system in Chinese hamster ovary cells stably expressing insulin receptor. Immunoblotting using anti-phosphorylated Akt1 antibody revealed Thr-308 already to be maximally phosphorylated at 1 min but completely dephosphorylated at 5 min, with insulin stimulation, whereas insulin-induced Akt1 activation was maintained even after dephosphorylation of Thr-308. Overexpression of wt-PDK1 further increased insulin-stimulated phosphorylation of Thr-308, also followed by rapid dephosphorylation. The insulin-stimulated Akt1 activity was also enhanced by wt-PDK1 expression but was maintained even at 15 min. Thus, phosphorylation of Thr-308 is not essential for maintaining the Akt1 activity once it has been achieved. Interestingly, the insulin-stimulated phosphorylation state of Thr-308 was maintained even at 15 min in cells expressing kd-PDK1, suggesting that kd-PDK1 has a dominant negative effect on dephosphorylation of Thr-308 of Akt1. Calyculin A, an inhibitor of PP1 and PP2A, also prolonged the insulin-stimulated phosphorylation state of Thr-308. In addition, in vitro experiments revealed PP2A, but not PP1, to dephosphorylate completely Thr-308 of Akt1. These findings suggest that a novel pathway involving dephosphorylation of Akt1 at Thr-308 by a phosphatase, possibly PP2A, originally, identified as is regulated downstream from PDK1, an Akt1 kinase.

The expression of Ca2+/calmodulin-dependent protein kinase I in rat retina is regulated by light stimulation

Ca2+/calmodulin-dependent protein kinase I (CaM-kinase I) in rat retina was analyzed by immunohistochemical analysis, Western blot analysis and kinase activity assay. Western blot analysis revealed two immunoreactive bands similar to those detected in the brain. Developmental studies revealed that CaM-kinase I expression increased in accordance with postnatal development. Expression of CaM-kinase I in the retinas of rats raised in the complete darkness markedly decreased. CaM-kinase I activity assay supported these findings. Synapsin I was shown to be a possible intrinsic substrate of CaM-kinase I in rat retina. These results elucidated that CaM-kinase I is expressed in the retina and may play an important role in the retinal functions and that the expression of CaM-kinase I is regulated by light stimulation.

Peptide specificity determinants at P-7 and P-6 enhance the catalytic efficiency of Ca2+/calmodulin-dependent protein kinase I in the absence of activation loop phosphorylation

Phosphorylation of Ca2+/calmodulin-dependent protein kinase I (CaM KI) at Thr-177 by recombinant rat Ca2+/calmodulin-dependent kinase kinase B (CaM KKB) modulates the kinetics of synapsin-(4-13) peptide phosphorylation by reducing the Km 44-fold and decreasing the KCaM 4-fold. There is also a slight decrease in Km for ATP and increase in enzyme Vmax. A synthetic peptide substrate from the yeast transcription factor, ADR1-(222-234)G233 is a 15-fold better substrate for the Thr-177 dephospho-form of CaM KI than synapsin-(4-13). The Thr-177 dephospho-enzyme has a Km and Vmax for ADR1-(222-234)G233 similar to the values with synapsin-(4-13) using the Thr-177 phosphorylated enzyme. Likewise, with ADR1-(222-234)G233 as substrate, phosphorylation of Thr-177 or substitution of T177A had very little effect on the kinetic values. Using chimeric peptides between synapsin-(4-13) and ADR1-(222-234)G233 we found that N-terminal basic residues at P-7 and P-6 positions were sufficient to allow efficient phosphorylation by the Thr-177 dephospho-form of CaM KI. Phosphorylation of Thr-177 expands the substrate specificity of CaM KI and is not merely an "on-off" switch for kinase activity.

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.

Regulation of protein kinase cascades by protein phosphatase 2A

Many protein kinases themselves are regulated by reversible phosphorylation. Upon cell stimulation, specific kinases are transiently phosphorylated and activated. Several of these protein kinases are substrates for protein phosphatase 2A (PP2A), and PP2A appears to be the major kinase phosphatase in eukaryotic cells that downregulates activated protein kinases. This idea is substantiated by the observation that some viral proteins and naturally occurring toxins target PP2A and modulate its activity. There is increasing evidence that PP2A activity is regulated by extracellular signals and during the cell cycle. Thus, PP2A is likely to play an important role in determining the activation kinetics of protein kinase cascades.

Inhibition of the Ca2+/calmodulin-dependent protein kinase I cascade by cAMP-dependent protein kinase

Several recent studies have shown that Ca2+/calmodulin-dependent protein kinase I (CaMKI) is phosphorylated and activated by a protein kinase (CaMKK) that is itself subject to regulation by Ca2+/calmodulin. In the present study, we demonstrate that this enzyme cascade is regulated by cAMP-mediated activation of cAMP-dependent protein kinase (PKA). In vitro, CaMKK is phosphorylated by PKA and this is associated with inhibition of enzyme activity. The major site of phosphorylation is threonine 108, although additional sites are phosphorylated with lower efficiency. In vitro, CaMKK is also phosphorylated by CaMKI at the same sites as PKA, suggesting that this regulatory phosphorylation might play a role as a negative-feedback mechanism. In intact PC12 cells, activation of PKA with forskolin resulted in a rapid inhibition of both CaMKK and CaMKI activity. In hippocampal slices CaMKK was phosphorylated under basal conditions, and activation of PKA led to an increase in phosphorylation. Two-dimensional phosphopeptide mapping indicated that activation of PKA led to increased phosphorylation of multiple sites including threonine 108. These results indicate that in vitro and in intact cells the CaMKK/CaMKI cascade is subject to inhibition by PKA-mediated phosphorylation of CaMKK. The phosphorylation and inhibition of CaMKK by PKA is likely to be involved in modulating the balance between cAMP- and Ca2+-dependent signal transduction pathways.

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.

Characterization of the mechanism of regulation of Ca2+/ calmodulin-dependent protein kinase I by calmodulin and by Ca2+/calmodulin-dependent protein kinase kinase

Ca2+/calmodulin-dependent protein kinase I (CaMKI) is maintained in an autoinhibited state by the interaction of a COOH-terminal helix-loop-helix (Ile286-Met316) regulatory domain with the catalytic core. Activation of the enzyme by calmodulin (CaM) also allows CaMKI to be phosphorylated and activated by a second enzyme, CaMK kinase (CaMKK). To more thoroughly characterize the regulation of CaMKI by CaM and its interrelationship with phosphorylation by CaMKK, we have carried out a detailed structure-function analysis using recombinant wild-type (WT) and mutant forms of CaMKI and CaMKK. CaMKI-WT, in the absence of CaM, or CaMKI-299 and CaMKI-298 were autoinhibited and could not be phosphorylated by CaMKK-433 (a truncated constitutively active form of CaMKK). Removal of Phe298 (CaMK-297) generated a constitutively active form of CaMKI that was also phosphorylated by CaMKK-433. CaMKI-WT was essentially inactive in the absence of CaM (K0.5 for activation by CaM approximately 30 nM). Mutation of Ile294 and Phe298 to alanine (CaMKI-2A) resulted in measurable basal enzyme activity. Additional mutation of Ile286 and Val290 to alanine (CaMKI-4A) increased this basal activity. Mutation of Trp303 (CaMKI-W303S) resulted in a large increase in the K0.5 for CaM ( approximately 100 microM), supporting a role for this residue as an initial target for CaM. Mutation of Phe307 (CaMKI-F307A) resulted in increased basal enzyme activity, supporting a role for this residue in autoinhibition of CaMKI. Together these studies demonstrate the critical role of specific amino acids in the autoinhibition of CaMKI and also in its activation by CaM and phosphorylation by CaMKK.

Unique inhibitory action of the synthetic compound 2-[N-(2-aminoethyl)-N-(5-isoquinolinesulfonyl)] amino-N-(4-chlorocinnamyl)-N-methylbenzylamine (CKA-1306) against calcium/calmodulin-dependent protein kinase I

A newly synthesized compound, 2-[N-(2-aminoethyl)-N-(5-isoquinolinesulfonyl)]amino-N-(4-chlorocinnamyl )-N-methylbenzylamine (CKA-1306), was found to inhibit cyclic AMP-dependent protein kinase (PKA) and Ca2+/calmodulin-dependent protein kinase I (CaMK I) with IC50 values of 1.6+/-0.14 and 2.5+/-0.16 microM, respectively. In contrast, the established PKA inhibitors H-8 and H-89 inhibited CaMK I with relatively high IC50 values of >100 and 24.4+/-3.2 microM, respectively. An additional inhibitor, KN-62, against Ca2+/calmodulin-dependent protein kinase II (CaMK II) did not inhibit either PKA or CaMK I at the concentrations tested. In our library of many isoquinolinesulfonamide derivatives, only CKA-1306 inhibited CaMK I to a satisfactory degree, suggesting a unique mode of action. Indeed, the inhibition of CaMK I by CKA-1306 was competitive in every respect to Mg2+/ATP, peptide substrate (syntide-2), and Ca2+/calmodulin. This phenomenon may be understood from the context of the recently determined structure of the enzyme in its autoinhibited state. Such kinetic analysis was also extended to cases using a phosphorylated and activated enzyme at Thr177 or a constitutively active, COOH-terminal truncated mutant at Gln293. CKA-1306 still competed with Mg2+/ATP for the two enzymes, but it no longer achieved any competitive advantage over syntide-2. These results may reflect some differences in the active conformation of CaMK I. However, the compound should be constant in its recognition of an Mg2+/ATP-binding site of the enzyme. Though CKA-1306 is not specific to CaMK I, the compound will be useful in studying the enzyme further under limited conditions.

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.

Isoform-specific activation and structural diversity of calmodulin kinase I

We earlier confirmed that there are isoforms of Ca2+/calmodulin (CaM)-dependent protein kinase I (CaM kinase I) (CaM kinase Ibeta1 and Igamma) beside CaM kinase Ialpha by cDNA cloning (Yokokura, H., Terada, O., Naito, Y., and Hidaka, H. (1997) Biochim. Biophys. Acta 1338, 8-12). Here, we demonstrate the existence of an isoform-specific activation mechanism of CaM kinase I and alternative splicing specifically regulating CaM kinase I (CaM kinase Ibeta2) in the central nervous system. To cast light on isoform structure-enzyme activity relationships, CaM kinase Ibeta1, Ibeta2, and Ialpha were expressed separately using a baculovirus/Sf9 cell expression system. The novel CaM kinase Ibeta2 isoform demonstrated similar catalytic activity to those of CaM kinase Ibeta1 and Ialpha. Interestingly, CaM kinase Ibeta1 and Ibeta2 both can activate CaM kinase Ialpha activity via phosphorylation at Thr177. Reverse transcribed-polymerase chain reaction analysis showed that CaM kinase Ibeta2 is dominant in the cerebrum and cerebellum, whereas CaM kinase Ibeta1 is present in peripheral tissues such as liver, heart, lung, kidney, spleen, and testis. CaM kinase Ibeta2 was also detected with an anti-CaM kinase Ibeta2 antibody in PC12 cells. The results indicate that alternative splicing is a means for tissue-specific expression of CaM kinase Ibeta. Thus the Thr177 residue of CaM kinase Ialpha is phosphorylated by not only CaM kinase kinase but also CaM kinase Ibeta for activation of the enzyme.

Characterization of substrate phosphorylation and use of calmodulin mutants to address implications from the enzyme crystal structure of calmodulin-dependent protein kinase I

Calcium/calmodulin (CaM) directly activates CaM-dependent protein kinase I (CaMKI) by binding to the enzyme and indirectly promotes the phosphorylation and synergistic activation of CaMKI by an exogenous kinase. We have evaluated the initial CaM-dependent activation of the unphosphorylated form of CaMKI. The kinetics of bacterially expressed human CaMKI show that the peptide syntide-2 is a relatively poor substrate, whereas the synapsin site-1 peptide is 17-fold more specific. The peptide ADR1G is 400-fold more specific than syntide-2, and its catalytic rate is among the highest reported for a kinase peptide substrate. To understand how CaM activates CaMKI, we have characterized the activation of the enzyme by CaM mutants with substitutions at hydrophobic residues. The point mutant M124Q located in the C-terminal domain of CaM produced a 57-fold increase in the CaM activation constant for CaMKI and suggests the involvement of methionine 124 in an important hydrophobic interaction with tryptophan 303 of CaMKI. Substituting two, three, and five hydrophobic residues in the N-terminal domain of CaM increased the CaM activation constant for CaMKI by 10-190-fold and lowered the maximal enzyme activity by more than 80%. Two of these N-terminal mutants of CaM do not affect the Km for peptide substrate but instead produce a 5-10-fold higher Km for ATP. This result demonstrates the critical role of the N-terminal domain of CaM in regulating the access of ATP to CaMKI.

Activation of a calcium-calmodulin-dependent protein kinase I cascade in PC12 cells

It has been observed that the activity of Ca2+-calmodulin (CaM)-dependent protein kinase I is enhanced up to 50-fold by its phosphorylation in vitro by a distinct CaM kinase I kinase (Lee, J. C., and Edelman, A. M. (1994) J. Biol. Chem. 269, 2158-2164). It has, however, been unclear whether this event represents an acute form of cellular regulation. We demonstrate here the phosphorylation and activation of CaM kinase I in PC12 pheochromocytoma cells in response to elevation of intracellular Ca2+. Treatment of PC12 cells with the Ca2+-ionophore, ionomycin, or with a depolarizing concentration of KCl, led to rapid, biphasic phosphorylation of CaM kinase I and to increases in CaM kinase I activity of 5.1- and 7. 3-fold, respectively. Depolarization-induced activation of CaM kinase I was reduced by approximately 80% by blockade of Ca2+ influx through L-type voltage-dependent Ca2+ channels and completely abolished by removal of extracellular Ca2+. The ability of PC12 cell CaM kinase I to be phosphorylated and activated by purified CaM kinase I kinase in vitro was markedly reduced by prior depolarization of the cells, consistent with intracellular phosphorylation and activation of CaM kinase I by CaM kinase I kinase. These results demonstrate the existence in PC12 cells of a CaM kinase I cascade, the function of which may be to sensitize cells to signal-induced elevations of intracellular Ca2+.

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.

5'-AMP activates the AMP-activated protein kinase cascade, and Ca2+/calmodulin activates the calmodulin-dependent protein kinase I cascade, via three independent mechanisms

AMP-activated protein kinase (AMPK) and Ca2+/calmodulin (CaM)-dependent protein kinase I (CaMKI) are protein kinases that are regulated both by allosteric activation (AMP and Ca2+/CaM, respectively) and by phosphorylation by upstream protein kinases (AMPK kinase (AMPKK) and CaMKI kinase (CaMKIK), respectively). We now report that AMPKK can activate CaMKI and that, conversely, CaMKIK can activate AMPK. CaMKIK is 68-fold more effective at activating CaMKI than AMPK, while AMPKK is 17-fold more effective at activating AMPK than CaMKI. Our results suggest that CaMKIK and AMPKK are distinct enzymes dedicated to their respective kinase targets but with some overlap in their substrate specificities. The availability of alternative substrates for AMPKK and CaMKIK allowed the unequivocal demonstration that AMP and Ca2+/calmodulin promote the activation of AMPK and Ca2+/calmodulin promote the activation of AMPK and CaMKI, respectively, via three independent mechanisms: 1) direct activation of AMPK and CaMKI, 2) activation of AMPKK and CaMKIK, and 3) by binding to AMPK and CaMKI, inducing exposure of their phosphorylation sites. Since AMP and Ca2+/calmodulin each has a triple effect in its respective system, in vivo, the two systems would be expected to be exquisitely sensitive to changes in concentration of their respective activating ligands.

The regulatory region of calcium/calmodulin-dependent protein kinase I contains closely associated autoinhibitory and calmodulin-binding domains

The mechanism for the regulation of Ca2+/calmodulin-dependent protein kinase I (CaM kinase I) was investigated using a series of COOH-terminal truncated mutants. These mutants were expressed in bacteria as fusion proteins with glutathione S-transferase and purified by affinity chromatography using glutathione Sepharose 4B. A mutant (residues 1-332) showed complete Ca2+/CaM-dependent activity. Truncation mutants (residues 1-321, 1-314, and 1-309) exhibited decreasing affinities for Ca2+/CaM and also exhibited decreasing Ca2+/CaM-dependent activities. Truncation mutants (residues 1-305 or 1-299) were unable to bind Ca2+/CaM and were inactive. In contrast, truncation mutants (residues 1-293 or 1-277) were constitutively active at a slightly higher level (2-fold) than fully active CaM kinase I. These results indicate the location of the Ca2+/CaM-binding domain on CaM kinase I (residues 294-321) and predict the existence of an autoinhibitory domain near, or overlapping, the Ca2+/CaM-binding domain. These conclusions were supported by studies which showed that a synthetic peptide (CaM kinase I (294-321)) corresponding to residues 294-321 of CaM kinase I inhibited the fully active kinase in a manner that was competitive with Ca2+/CaM and also inhibited the constitutively active mutant (residues 1-293) in a manner that was competitive with Syntide-2, a peptide substrate, (Ki = 1.2 microM) but was non-competitive with ATP. Thus, these results suggest that CaM kinase I is regulated through an intrasteric mechanism common to other members of the family of Ca2+/CaM-dependent protein kinases.

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.

Immunochemical localization of calcium/calmodulin-dependent protein kinase I

Ca2+/calmodulin-dependent protein kinase I (CaM kinase I) was originally identified in rat brain based on its ability to phosphorylate site 1 of synapsin I. Recently a cDNA for the rat brain enzyme has been cloned and the primary structure elucidated [Picciotto et al. (1993), J. Biol. Chem., 268:26512-26521]. The rat cDNA encoded a protein of 374 amino acids with a calculated M(r) of 41,636. Antibodies have now been raised against the recombinant kinase expressed in E. coli as a glutathione-S-transferase fusion protein. Immunoblot analysis of rat cortex lysates revealed two major immunoreactive bands of approximately M(r) 38,000 and 42,000. Minor immunoreactive species of slightly lower M(r) were also detected. Two distinct CaM kinase I activities were partially purified from rat brain and shown to correspond to the two major immunoreactive species. A variety of immunoreactive species of M(r) 35-43,000 were detected in "brain" tissue from cow, zebra finch, goldfish, Xenopus, lamprey, and Drosophila. In rat brain, immunocytochemistry revealed strong staining in cortex, hippocampus, amygdala, hypothalamus, brain stem, and choroid plexus. The labelling was mainly observed in neuropil but clusters of intensely labelled neuronal cell bodies were also detected all along the neuraxis. Neuronal nuclei and glial cells did not appear to be stained. Subcellular fractionation studies confirmed the cytosolic localization of the kinase in the brain. In various rat non-neuronal tissues and in a number of cell lines, immunoreactive species of approximately M(r) 38,000 and approximately 42,000 were detected at lower levels than that detected in brain. The M(r) 38,000 and 42,000 species were also found in different ratios and at different levels in the non-neuronal tissues. These results support a role for CaM kinase I in the regulation of multiple neuronal processes. Furthermore, the widespread cell and tissue distribution suggests that CaM kinase I may function as a ubiquitous multi-functional protein kinase. Finally, the multiple immunoreactive species may represent isoforms of CaM kinase I.

Similar substrate recognition motifs for mammalian AMP-activated protein kinase, higher plant HMG-CoA reductase kinase-A, yeast SNF1, and mammalian calmodulin-dependent protein kinase I

We have analysed phosphorylation of the synthetic peptide AMARAASAAALARRR, and 23 variants by mammalian, higher plant and yeast members of the SNF1 protein kinase subfamily (AMP-activated protein kinase (AMPK), HMG-CoA reductase kinase (HRK-A), and SNF1 itself), and by mammalian calmodulin-dependent protein kinase I (CaMKI). These four kinases recognize motifs which are very similar, although distinguishable. Our studies define the following recognition motifs: AMPK: phi (X beta)XXS/TXXX phi; HRK-A: phi (X,beta)XXSXXX phi; Snf1: phi XRXXSXXX phi; CaMKI: phi XRXXS/TXXX phi; where phi is a hydrophobic residue (M, V, L, I or F) and beta is a basic residue (R, K or H).

Characterization of a rat cDNA clone encoding calcium/calmodulin-dependent protein kinase I

Two cDNA clones encoding calcium/calmodulin-dependent (CaM) protein kinase I were isolated. In contrast to the previously reported CaM kinase I cDNA, which encodes a protein with a mass of 37 kDa, the clones identified in this study encode a protein (10-1/CaM kinase I) with a predicted mass of 42 kDa; the size of 10-1/CaM kinase I was verified by hybrid-selected translation.

The multifunctional Ca2+/calmodulin-dependent protein kinases

Ca2+ mediates the effect of many hormones, neurotransmitters and growth factors on contractility and motility, carbohydrate metabolism, cell cycle, gene expression and neuronal plasticity. Multifunctional Ca2+/calmodulin-dependent (CaM) kinase, CaM kinase Ia, CaM kinase Ib and CaM kinase IV are four of the kinases that mediate Ca(2+)-signaling pathways. Recent studies have clarified our understanding of their structure, regulation and function.