Characterization of a soluble Mr-30,000 catalytic fragment of the neuronal calmodulin-dependent protein kinase II

High level expression and preparation of autonomous Ca2+/calmodulin-dependent protein kinase II in Escherichia coli

The chymotryptic fragment of Ca2+/calmodulin-dependent protein kinase II (30K-CaMKII) is a constitutively active enzyme that phosphorylates a variety of protein substrates in vitro. Although 30K-CaMKII is an often used and powerful tool for protein phosphorylation, the efficient production of catalytically active 30K-CaMKII in Escherichia coli has not yet been successfully realized, probably due to its toxicity in host cells. In this study, we found that a high-level expression of 30K-CaMKII as an insoluble form was attained when the N-terminal 43 amino acid residues of Xenopus CaMKI were fused to the N-terminal end of 30K-CaMKII (CX-30K-CaMKII). The inactive CX-30K-CaMKII thus expressed in E. coli was reactivated by simple denaturation/renaturation processes and purified on a Ni2+-chelating column. The renatured CX-30K-CaMKII exhibited specific activity similar to that of rat brain CaMKII, and phosphorylated various proteins such as histones, myosin light chain, myelin basic protein, and synapsin I, as in case of 30K-CaMKII or purified CaMKII. Thus, CX-30K-CaMKII, an autonomous CaMKII, can be obtained with a simple procedure using E. coli expression system.

Structure-function of the multifunctional Ca2+/calmodulin-dependent protein kinase II

Ca2+/calmodulin (CaM)-dependent protein kinase (CaMKII) is a ubiquitous mediator of Ca2+-linked signalling that phosphorylates a wide range of substrates to co-ordinate and regulate Ca2+-mediated alterations in cellular function. The transmission of information by the kinase from extracellular stimuli and the intracellular Ca2+ rise is not passive. Rather, its multimeric structure and autoregulation enable this enzyme to participate actively in the sensitivity, timing and location of its action. CaMKII can: (i) be activated in a Ca2+-spike frequency-dependent manner; (ii) become independent of its initial Ca2+/CaM activators; and (iii) undergo a 'molecular switch-like' behaviour, which is crucial for certain forms of learning and memory. CaMKII is derived from a family of four homologous but distinct genes, with over 30 alternatively spliced isoforms described at present. These isoforms possess diverse developmental and anatomical expression patterns, as well as subcellular localization. Six independent catalytic/autoregulatory domains are connected by a narrow stalk-like appendage to each hexameric ring within the dodecameric structure. Ca2+/CaM binding activates the enzyme by disinhibiting the autoregulatory domain; this process initiates an intra-holoenzyme autophosphorylation reaction that induces complex changes in the enzyme's sensitivity to Ca2+/CaM, including the generation of Ca2+/CaM-independent (autonomous) activity and marked increase in affinity for CaM. The role of CaMKII in Ca2+ signal transduction is shaped by its autoregulation, isoenzymic type and subcellular localization. The molecular determinants and mechanisms producing these processes are discussed as they relate to the structure-function of this multifunctional protein kinase.

A recombinant form of the catalytic subunit of phosphorylase kinase that is soluble, monomeric, and includes key C-terminal residues

Residues 302-326 of the catalytic (gamma) subunit of phosphorylase kinase (PhK) may comprise an autoinhibitory, pseudosubstrate domain that binds calmodulin. To study this, the cDNA corresponding to rabbit muscle PhKgamma was expressed using Escherichia coli. This yielded two stable, high-activity PhKgamma forms (35 and 42 kDa by SDS-PAGE) that were smaller than an authentic sample of rabbit muscle PhKgamma (45 kDa by SDS-PAGE). Each recombinant form was purified to homogeneity. The N-terminal sequence of the larger, 42-kDa form (pk42) matched that of the rabbit muscle enzyme. This suggested that pk42 consisted of PhKgamma residues 1-362, including the putative calmodulin-binding, autoinhibitory domain. Kinetic parameters obtained for pk42 were like those previously reported for the intact gamma subunit. This implied that the lack of 25 PhKgamma C-terminal residues did not affect phosphorylase kinase activity, but greatly improved enzyme stability. An additional 60 residues were removed from the C-terminus of pk42 using the protease m-calpain. This increased the kinase activity 1.5-fold. Consistent with this, the activity of a mutant PhKgamma that consisted of residues 1-300, denoted gamma1-300, was like that of the m-calpain-treated enzyme. Therefore, although the effect was small, some influence by the C-terminus of pk42 was noted. Moreover, when pk42 was incubated with ATP alone, a C-terminal threonine residue became phosphorylated. Although the influence of this autophosphorylation cannot be inferred from this data, it was evidence that the C-terminus accessed the enzyme's active site. Taken together, these data imply that pk42 will be useful to study phosphorylase kinase structure/activity relationships.

Identification of domains essential for the assembly of calcium/calmodulin-dependent protein kinase II holoenzymes

Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), as isolated from brain, is a multimeric complex composed predominantly of two subunits, alpha and beta, products of unique genes. Little is known about how subunit composition influences holoenzyme structure or how the domain(s) of each subunit interact to form holoenzymes. We show here that holoenzymes composed of only alpha or only beta subunits exhibit different biophysical properties. The S values of alpha and beta are 17.2 and 14.5 S while the Stokes's radii are 85 and 111 A, respectively, indicating their structures are different. C-terminal truncations of the alpha subunit show that amino acids 382-478 are necessary for holoenzyme formation and that amino acids 427-478 contribute to holoenzyme stability. Additionally, the C-terminal domains of both the alpha subunit, alpha315-478, and beta subunit, beta314-542, formed oligomers indicating the sufficiency of the C-terminal domain for multimer formation. Using the yeast two-hybrid system we show, in vivo, that full-length subunits, alpha1-478 and beta1-542, interact with themselves or each other interchangeably. Additionally, the C-terminal domains of the alpha subunit, alpha315-478 and beta subunit, beta314-542 associated with themselves in a manner indistinguishable from their association with full-length alpha or beta subunits. Further studies revealed that the C-terminal domains of the alpha and beta subunits contain information necessary for interaction with beta but not alpha. These data are summarized into a model describing the assembly of CaM kinase II holoenzymes.

UV irradiation-induced apoptosis leads to activation of a 36-kDa myelin basic protein kinase in HL-60 cells

UV irradiation induces apoptosis (or programmed cell death) in HL-60 promyelocytic leukemia cells within 3 h. UV-induced apoptosis is accompanied by activation of a 36-kDa myelin basic protein kinase (p36 MBP kinase). This kinase is also activated by okadaic acid and retinoic acid-induced apoptosis. Irrespective of the inducing agent, p36 MBP kinase activation is restricted to the subpopulation of cells actually undergoing apoptosis. Activation of p36 MBP kinase occurs in enucleated cytoplasts, indicating no requirement for a nucleus or fragmented DNA in signaling. We also demonstrate the activation of p36 kinase in tumor necrosis factor-alpha- and serum starvation-induced cell death using the human prostatic tumor cell line LNCap and NIH 3T3 fibroblasts, respectively. We postulate that p36 MBP kinase is a common component in diverse signaling pathways leading to apoptosis.

PITSLRE protein kinase activity is associated with apoptosis

Minimal ectopic expression of a 58-kDa protein kinase (PITSLRE beta 1), distantly related to members of the cdc2 gene family, induces telophase delay, abnormal chromosome segregation, and decreased growth rates in Chinese hamster ovary cells. Here we show that this decrease in cell growth rate is due to apoptosis. Apoptosis is also induced by ectopic expression of an amino-terminal deletion mutant containing the catalytic and C-terminal domains of PITSLRE beta 1 but not by other mutants lacking histone H1 kinase activity or by other members of the cdc2 gene family. However, unlike the wild-type PITSLRE beta 1 over-expressors, ectopic expression of the N-terminal PITSLRE beta 1 mutant does not result in telophase delay or abnormal chromosome segregation. These results suggested that the function of this protein kinase could be linked to apoptotic signaling. To test this hypothesis, we examined levels of PITSLRE mRNA, steady-state protein, and enzyme activity in human T cells undergoing apoptosis after activation with the anti-Fas monoclonal antibody (MAb). All were substantially elevated shortly after Fas MAb treatment. In addition to new transcription and translation, proteolysis contributed to the increased steady-state levels of a novel 50-kDa PITSLRE protein, as suggested by the diminution of larger PITSLRE isoforms observed in the same cells. Indeed, treatment of the Fas-activated T cells with a serine protease inhibitor prevented apoptotic death and led to the accumulation of larger, less active PITSLRE kinase isoforms but not the enzymatically active 50-kDa PITSLRE isoform. Finally, induction of apoptosis by glucocorticoids in the same cell line, as well as by Fas MAb treatment of another T-cell line, led to a similar induction of 50-kDa PITSLRE protein levels over time. These findings suggest that (i) PITSLRE kinase(s) may lie within apoptotic signaling pathway(s), (ii) serine protease activation may be an early event in Fas-activated apoptosis of human T cells, and (iii) some PITSLRE kinase isoforms may be targets of apoptotic proteases.

New isoforms of Ca2+/calmodulin-dependent protein kinase II in smooth muscle

Four novel isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) were found in rat aorta smooth muscle. Two of them were related to gamma-isoform of brain CaM kinase II (gamma-a). Differences in the primary structure of these isoforms were located in the variable region. One of them (gamma-b) contained 23 unique amino acid residues, whereas the other (gamma-c) did not contain this sequence. Both isoforms lacked the two segments (Val-316 to Gln-337 and Lys-353 to Leu-362) present in gamma-a. The DNA sequence of these gamma-isoforms except the variable region was exactly the same, suggesting that they are produced by alternative splicing. Another two isoforms were related to the delta-isoform of brain CaM kinase II (delta-a). delta-b contained a unique 11-residue sequence in the variable region whereas delta-c did not. As found for gamma-isoforms, the sequence analysis suggested that the three delta-isoforms are also produced by alternative splicing. Analysis of RNA by reverse transcription PCR confirmed the existence of specific messages for gamma-b, delta-a and delta-b. The variety of isoforms of CaM kinase II suggest that each isoform may play a specialized role in cell regulation.

Identification and characterization of delta B-CaM kinase and delta C-CaM kinase from rat heart, two new multifunctional Ca2+/calmodulin-dependent protein kinase isoforms

We have identified, expressed and characterized two new isoforms of the multifunctional Ca2+/calmodulin-dependent kinase (CaM kinase) cloned from rat heart. Both isoforms are variants of the neuronal delta-CaM kinase (termed delta A-CaM kinase), and are designated as delta B-CaM kinase and delta C-CaM kinase. The new isoforms differ from delta A-CaM kinase in its isoform-specific insert region, between nucleotides 984 to 1087 of the delta A-CaM kinase cDNA. Replacing these 102 nucleotides, a sequence of 33 nucleotides which code for 11 amino acids (KRKSSSSQMM) are introduced in delta B-CaM kinase. The delta C-CaM kinase lacks all 102 nucleotides and the corresponding 34 amino acids which they encode. The predicted molecular masses of the delta B- and delta C-CaM kinase isoforms are 57,697 Da and 56,446 Da, respectively. Recombinant delta-CaM kinases purified from transfected COS-7 cells were found to associate into a larger holoenzyme estimated to contain 8 to 10 subunits. The relative subunit molecular masses on SDS-polyacrylamide gel electrophoresis are 59 kDa, 54 kDa and 52 kDa for delta A-, delta B- and delta C-CaM kinase, respectively. All three isoforms showed a strict dependence on Ca2+/calmodulin for activity and exhibited the Ca(2+)-dependent autophosphorylation and resultant conversion to Ca(2+)-independent kinase activity, characteristic features of multifunctional CaM kinase. Phosphopeptide analysis after partial CNBr digestion suggests that delta B-CaM kinase is the predominant soluble CaM kinase species purified from rat heart.

Regulation of type-II calmodulin kinase: functional implications

Calmodulin-kinase II (CaM kinase) is a calcium/calmodulin-dependent protein kinase which is highly enriched in the nervous system and mediates many of calcium's actions. Regulation of CaM kinase activity plays an important role in modulating synaptic transmission, synaptic plasticity and in neuropathology. Primary regulation of CaM kinase occurs via changes in intracellular calcium concentrations. Increased calcium stimulates protein kinase activity and induces autophosphorylation. Autophosphorylation of CaM kinase at specific sites results in altered activity and responsiveness to subsequent changes in calcium concentrations. Intracellular translocation of CaM kinase also appears to result from autophosphorylation. These mechanisms of regulation play an important role in synaptic plasticity (e.g., Aplysia ganglia), status epilepticus and cerebral ischemia. Long-lasting alterations in the expression of CaM kinase have been demonstrated in the kindling model of epilepsy and in monocular deprivation and therefore modulation of gene expression, in addition to autophosphorylation and translocation, appears to be another important mechanism of regulating CaM kinase activity.

Multifunctional Ca2+/calmodulin-dependent protein kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is a prominent mediator of neurotransmitters which elevate Ca2+. It coordinates cellular responses to external stimuli by phosphorylating proteins involved in neurotransmitter synthesis, neurotransmitter release, carbohydrate metabolism, ion flux and neuronal plasticity. Structure/function studies of CaM kinase have provided insights into how it decodes Ca2+ signals. The kinase is kept relatively inactive in its basal state by the presence of an autoinhibitory domain. Binding of Ca2+/calmodulin eliminates this inhibitory constraint and allows the kinase to phosphorylate its substrates, as well as itself. This autophosphorylation significantly slows dissociation of calmodulin, thereby trapping calmodulin even when Ca2+ levels are subthreshold. The kinase may respond particularly well to multiple Ca2+ spikes since trapping may enable a spike frequency-dependent recruitment of calmodulin with each successive Ca2+ spike leading to increased activation of the kinase. Once calmodulin dissociates, CaM kinase remains partially active until it is dephosphorylated, providing for an additional period in which its response to brief Ca2+ transients is potentiated.

Regulation and role of brain calcium/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase II (CaMKII) exhibits a broad substrate specificity and regulates diverse responses to physiological changes of intracellular Ca2+ concentrations. Five isozymic subunits of the highly abundant brain kinase are encoded by four distinct genes. Expression of each gene is tightly regulated in a cell-specific and developmental manner. CaMKII immunoreactivity is broadly distributed within neurons but is discretely associated with a number of subcellular structures. The unique regulatory properties of CaMKII have attracted a lot of attention. Ca2+/calmodulin-dependent autophosphorylation of a specific threonine residue (alpha-Thr286) within the autoinhibitory domain generates partially Ca(2+)-independent CaMKII activity. Phosphorylation of this threonine in CaMKII is modulated by changes in intracellular Ca2+ concentrations in a variety of cells, and may prolong physiological responses to transient increases in Ca2+. Additional residues within the calmodulin-binding domain are autophosphorylated in the presence of Ca2+ chelators and block activation by Ca2+/calmodulin. This Ca(2+)-independent autophosphorylation is very rapid following prior Ca2+/calmodulin-dependent autophosphorylation at alpha-Thr286 and generates constitutively active, Ca2+/calmodulin-insensitive CaMKII activity. Ca(2+)-independent autophosphorylation of CaMKII also occurs at a slower rate when alpha-Thr286 is not autophosphorylated and results in inactivation of CaMKII. Thus, Ca(2+)-independent autophosphorylation of CaMKII generates a form of the kinase that is refractory to activation by Ca2+/calmodulin. CaMKII phosphorylates a wide range of neuronal proteins in vitro, presumably reflecting its involvement in the regulation of diverse functions such as postsynaptic responses (e.g. long-term potentiation), neurotransmitter synthesis and exocytosis, cytoskeletal interactions and gene transcription. Recent evidence indicates that the levels of CaMKII are altered in pathological states such as Alzheimer's disease and also following ischemia.

Decoding calcium signals by multifunctional CaM kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is one of the three major protein kinases coordinating cellular responses to hormones and neurotransmitters. It mediates the action of Ca2+ on neurotransmitter synthesis and release, on carbohydrate metabolism and on the cytoskeleton. CaM kinase has structural/functional properties that facilitate its response to distinctive attributes of Ca2+ signals which often involve transient increases that span a narrow concentration range and increases that are pulsatile rather than persistent. The kinase responds to the narrow working range of Ca2+ signals by the use of calmodulin as the Ca2+ sensor. It is activated by the binding of calmodulin to an autoinhibitory domain that keeps the kinase inactive in the basal state. The transient nature of the signal is accommodated by autophosphorylation of this autoinhibitory domain which allows the kinase to remain partially active after calmodulin dissociates and thereby switches it to a Ca(2+)-independent species. The pulsatile nature of Ca2+ signals may also be decoded by CaM kinase. Autophosphorylation traps calmodulin on autophosphorylated subunits by greatly reducing its off-rate. At high frequency of stimulation, calmodulin would remain trapped during the brief interval between Ca2+ oscillations and each successive rise in Ca2+ would recruit more calmodulin. This may enable a stimulus frequency dependent activation of CaM kinase.

Molecular analysis of neuronal physiology by gene transfer into neurons with herpes simplex virus vectors

A genetic analysis of mammalian neuronal physiology might now be possible due to the development of defective herpes simplex virus vectors, which allow gene transfer directly into mature neurons, in culture or in the adult brain. Genetically altered proteins that play critical roles in neuronal physiology, including those responsible for the generation of action potentials, synthesis and release of neurotransmitters, and signal transduction enzymes, can now be stably expressed in neurons. The effect of such altered proteins on neuronal physiology can therefore be examined, using the tools of modern neuroscience. Genetic manipulation is biochemically specific and stable, and can be targeted both to a particular cell type and to a particular subregion of the cell to yield insights into the molecular basis for specific brain functions.

Concerted regulation of protein phosphorylation and dephosphorylation by calmodulin

The multiple functions of calmodulin in brain bring to light an apparent paradox in the mechanism of action of this multifunctional regulatory protein: How can the simultaneous calmodulin stimulation of enzymes with opposing functions, such as cyclic nucleotide phosphodiesterases and adenylate cyclase, which are responsible for the degradation and synthesis of cAMP, respectively, be physiologically significant? The same question applies to the simultaneous activation of protein kinases (in particular calmodulin kinase II) and a protein phosphatase (calcineurin). One could propose that the protein kinase(s) and the phosphatase may be located in different cells or in different cellular compartments, and are therefore not antagonizing each other. The same result could be achieved if the specific substrates of these enzymes have different cellular localizations. This does not seem to be the case. In many areas of the brain the two enzymes and their substrates coexist in the same cell. For example, the hippocampus is rich in calmodulin kinase II, calcineurin and substrates for the two enzymes. A more general scheme is presented here, based on different mechanisms of the calmodulin regulation of the two classes of enzyme, which helps to solve this apparent inconsistency in the mechanism of action of calmodulin.

Expression of a catalytically active polypeptide of calmodulin-dependent protein kinase II alpha subunit in Escherichia coli

Two cDNAs, one containing the entire coding region of alpha subunit of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) and the other containing only its protein kinase domain, were separately ligated into the bacterial expression vector pET3a and expressed in Escherichia coli. The activity of the recombinant alpha subunit protein was dependent on Ca2+/calmodulin, whereas the activity of the recombinant protein containing only the protein kinase domain (recombinant alpha-I protein) was absolutely independent of Ca2+/calmodulin. These proteins showed similar enzymatic properties to brain CaM kinase II with some minor differences. These results directly demonstrated that the protein kinase domain alone without the rest of the subunit was sufficient to exhibit its activity.

Multifunctional Ca2+/calmodulin-dependent protein kinase made Ca2+ independent for functional studies

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) that is transiently expressed in COS-7 cells is essentially inactive when assayed without Ca2+. Physiological activation of the kinase occurs by binding of Ca2+/calmodulin near a putative autoinhibitory subdomain that contains the sequence His282-Arg-Gln-Glu-Thr286. We have markedly increased the Ca2(+)-independent activity of CaM kinase by altering the charge of this sequence by site-directed mutagenesis. The mutant containing Asp282-Gly-Glu-Glu-Thr286 is 67% Ca2+ independent. We also mimicked the effect of autophosphorylation at Thr286 by the mutant containing His282-Arg-Gln-Glu-Asp286, which is 36% Ca2+ independent. In addition to delineating the autoinhibitory domain by use of mutations that disable it, these constructs are of immediate practical value for simulating CaM kinase action in vivo without elevating Ca2+. To this end, we show that nuclear microinjection of cDNA of a constitutive mutant, but not of the wild-type kinase, initiates maturation of Xenopus oocytes.

Regulation of Ca2+/calmodulin-dependent protein kinase II by brain gangliosides

Purified rat brain Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is stimulated by brain gangliosides to a level of about 30% the activity obtained in the presence of Ca2+/calmodulin (CaM). Of the various gangliosides tested, GT1b was the most potent, giving half-maximal activation at 25 microM. Gangliosides GD1a and GM1 also gave activation, but asialo-GM1 was without effect. Activation was rapid and did not require calcium. The same gangliosides also stimulated the autophosphorylation of CaM-kinase II on serine residues, but did not produce the Ca2+-independent form of the kinase. Ganglioside stimulation of CaM-kinase II was also present in rat brain synaptic membrane fractions. Higher concentrations (125-250 microM) of GT1b, GD1a, and GM1 also inhibited CaM-kinase II activity. This inhibition appears to be substrate-directed, as the extent of inhibition is very dependent on the substrate used. The molecular mechanism of the stimulatory effect of gangliosides was further investigated using a synthetic peptide (CaMK 281-309), which contains the CaM-binding, inhibitory, and autophosphorylation domains of CaM-kinase II. Using purified brain CaM-kinase II in which these regulatory domains were removed by limited proteolysis. CaMK 281-309 strongly inhibited kinase activity (IC50 = 0.2 microM). GT1b completely reversed this inhibition, but did not stimulate phosphorylation of the peptide on threonine-286. These results demonstrate that GT1b can partially mimic the effects of Ca2+/CaM on native CaM-kinase II and on peptide CaMK 281-309.

Calcium/calmodulin-dependent protein kinase II

There is a great deal known about the in vitro properties of CaM kinase II, both in terms of its substrate specificity and its regulation by calmodulin and autophosphorylation. Much of this characterization is based on experiments performed with the rat brain isozyme of CaM kinase II, although in the aspects examined to date isozymes of the kinase from other tissues appear to behave in a broadly similar manner in vitro. However, relatively little is known about the functions of the kinase in vivo. The proteins phosphorylated by the kinase (with the probable exception of synapsin I and tyrosine hydroxylase) and the role of kinase autophosphorylation in vivo remain largely unknown. Investigation of the physiological role of the kinase in brain and other tissues will be a particularly exciting area for future work. The current knowledge of the in vitro properties and the availability of cDNA clones will hopefully expedite this research.

Properties of a microtubule-associated cofactor-independent protein kinase from pig brain

A protein kinase activity was identified in pig brain that co-purified with microtubules through repeated cycles of temperature-dependent assembly and disassembly. The microtubule-associated protein kinase (MTAK) phosphorylated histone H1; this activity was not stimulated by cyclic nucleotides. Ca2+ plus calmodulin, phospholipids or polyamines. MTAK did not phosphorylate synthetic peptides which are substrates for cyclic AMP-dependent protein kinase, cyclic GMP-dependent protein kinase. Ca2+/calmodulin-dependent protein kinase II, protein kinase C or casein kinase II. MTAK activity was inhibited by trifluoperazine [IC50 (median inhibitory concn.) = 600 microM] in a Ca2+-independent fashion. Ca2+ alone was inhibitory [IC50 = 4 mM). MTAK was not inhibited by heparin, a potent inhibitor of casein kinase II, nor a synthetic peptide inhibitor of cyclic AMP-dependent protein kinase. MTAK demonstrated a broad pH maximum (7.5-8.5) and an apparent Km for ATP of 45 microM. Mg2+ was required for enzyme activity and could not be replaced by Mn2+. MTAK phosphorylated serine and threonine residues on histone H1. MTAK is a unique cofactor-independent protein kinase that binds to microtubule structures.

Calmodulin-binding proteins as calpain substrates

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Expression of a multifunctional Ca2+/calmodulin-dependent protein kinase and mutational analysis of its autoregulation

Autophosphorylation of multifunctional Ca2+/calmodulin-dependent protein kinase converts it from a Ca2(+)-dependent to a Ca2(+)-independent or autonomous kinase, a process that may underlie some long-term enhancement of transient Ca2+ signals. We demonstrate that the neuronal alpha subunit clone expressed in COS-7 cells (alpha-CaM kinase) is sufficient to encode the regulatory phenomena characteristic of the multisubunit kinase isolated from brain. Activity of alpha-CaM kinase is highly dependent on Ca2+/calmodulin. It is converted by autophosphorylation to an enzyme capable of Ca2(+)-independent (autonomous) substrate phosphorylation and autophosphorylation. Using site-directed mutagenesis, we separately eliminate five putative autophosphorylation sites within the regulatory domain and directly examine their individual roles. Ca2+/calmodulin-dependent kinase activity is fully retained by each mutant, but Thr286 is unique among the sites in being indispensable for generation of an autonomous kinase.

Calcium/calmodulin-dependent protein kinase II

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Affinity labeling of the ATP-binding site of type II calmodulin-dependent protein kinase by 5'-p-fluorosulfonylbenzoyl adenosine

Modification of the type II calmodulin-dependent protein kinase by 5'-p-fluorosulfonylbenzoyl adenosine (FSBA) resulted in a time-dependent inactivation of the enzyme. The reaction followed pseudo-first-order kinetics and showed a nonlinear dependence on reagent concentration. The rate of inactivation was sensitive to Mg2+- and calmodulin-induced conformational changes on the enzyme. However, the enhancing effects of these ligands were not additive; indeed, the kinetic parameters of the Mg2+-stimulated inactivation reaction with FSBA (Kinact = 2.4 mM; kappa max = 0.12 min-1) were almost unaffected by the simultaneous addition of calmodulin (Kinact = 1.5 mM; kappa max = 0.086 min-1). Protection from inactivation by FSBA was provided by Mg2+-ADP which is consistent with modification of the catalytic site. An analysis of the protective effect of Mg2+-ADP in the absence (Kd = 590 microM) and presence (Kd = 68 microM) of calmodulin demonstrated that binding of the modulator protein to the enzyme increases the affinity of the protein kinase for nucleotides. Modification by FSBA resulted in labeling of both Tyr and Lys residues but only labeling of Lys was decreased by Mg2+-ADP which is consistent with the hypothesis that a conserved Lys residue is important in nucleotide binding to the protein kinase. However, the kinetic results of the inactivation reaction suggest that this Lys is not involved in mediating the calmodulin-promoted increase in the affinity of the enzyme for Mg2+-nucleotide complexes.

A monoclonal antibody against brain calmodulin-dependent protein kinase type II detects putative conformational changes induced by Ca2+-calmodulin

A mouse monoclonal IgG1 antibody has been generated against the soluble form of the calmodulin-dependent protein kinase type II. This antibody recognizes both the soluble and cytoskeletal forms of the enzyme, requiring Ca2+ (EC50 = 20 microM) for the interaction. Other divalent cations such as Zn2+, Mn2+, Cd2+, Co2+, and Ni2+ will substitute for Ca2+, while Mg2+ and Ba2+ will not. The antibody reacts with both the alpha- and beta-subunits on Western blots in a similar Ca2+-dependent fashion but with a lower sensitivity. The affinity of the antibody for the kinase is 0.13 nM determined by displacement of 125I Bolton-Hunter-labeled kinase with unlabeled enzyme. A variety of other proteins including tubulin do not compete for antibody binding. The Mr 30,000 catalytic fragment obtained by proteolysis of either the soluble or the cytoskeletal form of the kinase fails to react with the antibody. Calmodulin and antibody reciprocally potentiate each other's interaction with the enzyme. This is illustrated both by direct binding studies and by a decrease of the Kmapp for calmodulin and an increase in the Vmax for the autophosphorylation reaction of the enzyme. The antibody thus appears to recognize and stabilize a conformation of the kinase which favors calmodulin binding although it does not itself activate the kinase in the absence of calmodulin. Since the Mr 30,000 catalytic fragment of the kinase is not immunoreactive, either the antibody combining site of the kinase must be present in the noncatalytic portion of the protein along with the calmodulin binding site or proteolysis interferes with the putative Ca2+-dependent conformational change.(ABSTRACT TRUNCATED AT 250 WORDS)