Regulatory mechanism of Ca2+/calmodulin-dependent protein kinase kinase

Ca(2+)/calmodulin-dependent protein kinase kinase (CaM-KK) is a novel member of the CaM kinase family, which specifically phosphorylates and activates CaM kinase I and IV. In this study, we characterized the CaM-binding peptide of alphaCaM-KK (residues 438-463), which suppressed the activity of constitutively active CaM-KK (84-434) in the absence of Ca(2+)/CaM but competitively with ATP. Truncation and site-directed mutagenesis of the CaM-binding region in CaM-KK reveal that Ile(441) is essential for autoinhibition of CaM-KK. Furthermore, CaM-KK chimera mutants containing the CaM-binding sequence of either myosin light chain kinases or CaM kinase II located C-terminal of Leu(440), exhibited enhanced Ca(2+)/CaM-independent activity (60% of total activity). Although the CaM-binding domains of myosin light chain kinases and CaM kinase II bind to the N- and C-terminal domains of CaM in the opposite orientation to CaM-KK (Osawa, M., Tokumitsu, H., Swindells, M. B., Kurihara, H., Orita, M., Shibanuma, T., Furuya, T., and Ikura, M. (1999) Nat. Struct. Biol. 6, 819-824), the chimeric CaM-KKs containing Ile(441) remained Ca(2+)/CaM-dependent. This result demonstrates that the orientation of the CaM binding is not critical for relief of CaM-KK autoinhibition. However, the requirement of Ile(441) for autoinhibition, which is located at the -3 position from the N-terminal anchoring residue (Trp(444)) to CaM, accounts for the opposite orientation of CaM binding of CaM-KK compared with other CaM kinases.     

Mechanism of the generation of autonomous activity of Ca2+/calmodulin-dependent protein kinase IV

Ca2+/calmodulin-dependent protein kinase IV (CaM-KIV) is phosphorylated at Thr196 by Ca2+/calmodulin-dependent protein kinase kinase (CaM-KK), resulting in induction of both autonomous activity and a high level of Ca2+/CaM-dependent activity. We have shown that the kinetics of Thr196 phosphorylation of CaM-KIV by CaM-KK is well correlated with the generation of its autonomous activity, although Thr177 phosphorylation of CaM-KI does not induce its autonomous activity. The activities of CaM-KI chimera mutants fused with C-terminal regions (residues 296-469 and 296-350) of CaM-KIV are completely dependent on Ca2+/CaM, which is also the case for CaM-KI. Unlike wild-type CaM-KI, however, phosphorylation of Thr177 in the chimera mutants by CaM-KK resulted in generation of significant autonomous activities, indicating that the phosphorylation of Thr in the activation loop is sufficient to partially release the autoinhibitory region of CaM-KIV from the catalytic core. Indeed, the CaM-KIV peptide (residues 304-325) containing minimum autoinhibitory sequences (residues 314-321) suppressed the activity of non-phosphorylated CaM-KIV with an IC50 of approximately 50 microm, and this suppression was competitive with respect to the peptide substrate; however, the CaM-KIV peptide was not capable of inhibiting Thr196-phosphorylated CaM-KIV. Taken together, these results indicated that the Thr196 phosphorylation of CaM-KIV by CaM-KK reduced the interaction of the catalytic core with the autoinhibitory region, resulting in generation of the autonomous activity.     

Generation of autonomous activity of Ca(2+)/calmodulin-dependent protein kinase kinase β by autophosphorylation

Ca(2+)/calmodulin-dependent protein kinase kinases (CaMKKs) phosphorylate and activate specific downstream protein kinases, including CaMKI, CaMKIV, and 5'-AMP-activated protein kinase, which mediates a variety of Ca(2+) signaling cascades. CaMKKs have been shown to undergo autophosphorylation, although their role in enzymatic regulation remains unclear. Here, we found that CaMKKα and β isoforms expressed in nonstimulated transfected COS-7 cells, as well as recombinant CaMKKs expressed in and purified from Escherichia coli, were phosphorylated at Thr residues. Introduction of a kinase-dead mutation completely impaired the Thr phosphorylation of these recombinant CaMKK isoforms. In addition, wild-type recombinant CaMKKs were unable to transphosphorylate the kinase-dead mutants, suggesting that CaMKK isoforms undergo Ca(2+)/CaM-independent autophosphorylation in an intramolecular manner. Liquid chromatography-tandem mass spectrometry analysis identified Thr(482) in the autoinhibitory domain as one of the autophosphorylation sites in CaMKKβ, but phosphorylation of the equivalent Thr residue (Thr(446)) in the α isoform was not observed. Unlike CaMKKα that has high Ca(2+)/CaM-dependent activity, wild-type CaMKKβ displays enhanced autonomous activity (Ca(2+)/CaM-independent activity, 71% of total activity). This activity was significantly reduced (to 37%) by substitution of Thr(482) with a nonphosphorylatable Ala, without significant changes in Ca(2+)/CaM binding. In addition, a CaMKKα mutant containing the CaMKKβ regulatory domain was shown to be partially phosphorylated at Thr(446), resulting in a modest elevation of its autonomous activity. The combined results indicate that, in contrast to the α isoform, CaMKKβ exhibited increased autonomous activity, which was caused, at least in part, by autophosphorylation at Thr(482), resulting in partial disruption of the autoinhibitory mechanism.     

Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent protein kinases

Endothelial nitric-oxide synthase (NOS-III) is defined as being strictly dependent on Ca(2+)/calmodulin (CaM) for activity, although NO release from endothelial cells has been reported to also occur at intracellular free Ca(2+) levels that are substimulatory for the purified enzyme. We demonstrate here that NOS-III, but neither NOS-I nor -II, is rapidly and strongly activated and phosphorylated on both Ser and Thr in the presence of cGMP-dependent protein kinase II (cGK II) and the catalytic subunit of cAMP-dependent protein kinase (cAK) in vitro. Phosphopeptide analysis by mass spectrometry identified Ser(1177), as well as Ser(633) which is situated in a recently defined CaM autoinhibitory domain within the flavin-binding region of human NOS-III. Phosphoamino acid analysis identified a putative phosphorylation site at Thr(495) in the CaM-binding domain. Importantly, both cAK and cGK phosphorylation of NOS-III in vitro caused a highly reproducible partial (10-20%) NOS-III activation which was independent of Ca(2+)/CaM, and as much as a 4-fold increase in V(max) in the presence of Ca(2+)/CaM. cAK stimulation in intact endothelial cells also increased both Ca(2+/)CaM-independent and -dependent activation of NOS-III. These data collectively provide new evidence for cAK and cGK stimulation of both Ca(2+)/CaM-independent and -dependent NOS-III activity, and suggest possible cross-talk between the NO and prostaglandin I(2) pathways and a positive feedback mechanism for NO/cGMP signaling.     

Mechanisms for regulation of calmodulin kinase IIalpha by Ca(2+)/calmodulin and autophosphorylation of threonine 286

A mechanism that relates calmodulin (CaM) binding to enzyme activation remains to be established within the context of full-length calmodulin kinase IIalpha (CaM KIIalpha). Previous studies using peptides and/or truncated enzymes have shown that L299 of CaM KIIalpha represents an "anchor" for Ca(2+)/CaM binding and that F293 is required for autoinhibition. We have substituted each of these residues with a W in full-length CaM KIIalpha and measured the W fluorescence to evaluate the location of these side chains in the absence and presence of Ca(2+)/CaM. Fluorescence emission of the L299W mutant indicates that L299 is solvent accessible in the absence of Ca(2+)/CaM but becomes internalized in the presence of Ca(2+)/CaM. On the other hand, examination of F293W indicates that Ca(2+)/CaM binding promotes enzyme activation by transferring F293 from an internal location in the inactive enzyme to a more solvent accessible position in the active enzyme. In addition, F293 interacts with Ca(2+)/CaM as a consequence of autophosphorylation at T286, thus providing a mechanism for CaM trapping. Whereas in the absence of autophosphorylation the exposure of F293 is reversed by dissociation of CaM leading to enzyme autoinhibition, after autophosphorylation of T286, F293 is retained in an exposed position due to dissociation of CaM, consistent with the retention of autonomous activity. Proline mutants were introduced at positions between T286 and F293 to explore the basis of CaM-independent, autonomous activity. The observation that an L290P mutant displayed a high level of activity independent of Ca(2+)/CaM or phosphorylation of T286 indicates that a change in the conformation of the polypeptide main chain at L290 might contribute to the mechanism for generating autophosphorylation-dependent autonomous activity.     

Substrate recognition by Ca2+/Calmodulin-dependent protein kinase kinase. Role of the arg-pro-rich insert domain.

Mammalian Ca2+/CaM-dependent protein kinase kinase (CaM-KK) has been identified and cloned as an activator for two kinases, CaM kinase I (CaM-KI) and CaM kinase IV (CaM-KIV), and a recent report (Yano, S., Tokumitsu, H., and Soderling, T. R. (1998) Nature 396, 584-587) demonstrates that CaM-KK can also activate and phosphorylate protein kinase B (PKB). In this study, we identify a CaM-KK from Caenorhabditis elegans, and comparison of its sequence with the mammalian CaM-KK alpha and beta shows a unique Arg-Pro (RP)-rich insert in their catalytic domains relative to other protein kinases. Deletion of the RP-domain resulted in complete loss of CaM-KIV activation activity and physical interaction of CaM-KK with glutathione S-transferase-CaM-KIV (T196A). However, CaM-KK autophosphorylation and phosphorylation of a synthetic peptide substrate were normal in the RP-domain mutant. Site-directed mutagenesis of three conserved Arg in the RP- domain of CaM-KK confirmed that these positive charges are important for CaM-KIV activation. The RP- domain deletion mutant also failed to fully activate and phosphorylate CaM-KI, but this mutant was indistinguishable from wild-type CaM-KK for the phosphorylation and activation of PKB. These results indicate that the RP-domain in CaM-KK is critical for recognition of downstream CaM-kinases but not for its catalytic activity (i.e. autophosphorylation) and PKB activation.     

A single amino acid difference between alpha and beta Ca2+/calmodulin-dependent protein kinase kinase dictates sensitivity to the specific inhibitor,STO-609

We recently developed STO-609, a selective inhibitor of Ca(2+)/calmodulin-dependent protein kinase kinase (CaM-KK), and we demonstrated that CaM-KK beta is more sensitive to STO-609 than the CaM-KK alpha isoform (Tokumitsu H., Inuzuka H., Ishikawa Y., Ikeda M., Saji I., and Kobayashi R. (2002) J. Biol. Chem. 277, 15813-15818). By using catalytic chimera and point mutants of both isoforms, we demonstrated that Val(269) in CaM-KK beta/Leu(233) in CaM-KK alpha confers a distinct sensitivity ( approximately 10-fold) to STO-609 on CaM-KK isoforms. Various mutations of Val(269) in CaM-KK beta indicate that substitution by hydrophobic residues with bulky side chains significantly decreases drug sensitivity and that the V269F mutant is the most effective drug-resistant enzyme ( approximately 80-fold higher IC(50) value). These findings are consistent with a result obtained with a full-length mutant expressed in COS-7 cells. Furthermore, suppression of CaM-KK-mediated CaM-KIV activation in transfected HeLa cells by STO-609 treatment was completely abolished by the co-expression of the CaM-KK beta V269F mutant. Based on the results that the distinct sensitivity of CaM-KK isoforms to STO-609 is because of a single amino acid substitution (Val/Leu) in the ATP-binding pocket, we have generated an STO-609-resistant CaM-KK mutant, which might be useful for validating the pharmacological effects and specificity of STO-609 in vivo.     

Calmodulin kinase II chimeras used to investigate the structural requirements for smooth muscle myosin light chain kinase autoinhibition and calmodulin-dependent activation

Segments of the autoregulatory domain of MK, a catalytically active fragment of the monomeric smooth muscle myosin light chain kinase (smMLCK) (residues 472-972), were replaced with their counterparts from a homologous but multimeric enzyme, calmodulin-dependent protein kinase II (CaM KII). Chimeric proteins in which both the autoregulatory and oligomerization domains of CaM KII (residues 281-478) were substituted for residues 781-972 of smMLCK, MK(CK281-478), or only the autoregulatory domain of CaM KII (residues 281-315) was exchanged for residues 781-813 of smMLCK, MK(CK281-315), exhibited significant enzymatic activity in the absence of Ca(2+)/CaM. In contrast, both MK and a chimeric protein in which the C-terminal half of the autoregulatory domain of smMLCK was replaced with CaM KII residues 301-315, MK(CK301-315), were inactive in the absence of Ca(2+)/CaM. These results indicate that the sequence of the N-terminal half of the autoregulatory domain of smMLCK is important for complete autoinhibition of its enzymatic activity. All proteins bound to Ca(2+)/CaM, and the chimeric proteins MK(CK281-478) and MK(CK281-315) were activated by Ca(2+)/CaM with activation constants (K(CaM)) and maximal enzymatic activities comparable to those of the wild-type MK enzyme. This demonstrates that the entire autoregulatory domain of CaM KII can replace that of smMLCK in its ability to promote efficient CaM-dependent activation of the smMLCK enzyme. However, the inability of the chimeric protein MK(CK301-315) to be activated by Ca(2+)/CaM suggests that replacement of only the C-terminal half of the autoregulatory domain of smMLCK, while still retaining the ability to bind Ca(2+)/CaM, also substitutes residues that prevent activation of the enzyme by Ca(2+)/CaM.     

Regulatory domain of calcium/calmodulin-dependent protein kinase II. Mechanism of inhibition and regulation by phosphorylation

Regulatory mechanisms of rat brain Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) were probed using a synthetic peptide (CaMK-(281-309] corresponding to residues 281-309 (alpha-subunit) which contained the calmodulin (CaM)-binding and inhibitory domains and also the initial autophosphorylation site (Thr286). Kinetic analyses indicated that inhibition of a completely Ca2+/CaM-independent form of CaM-kinase II by CaMK-(281-309) was noncompetitive with respect to peptide substrate (syntide-2) but was competitive with respect to ATP. Interaction of CaMK-(281-309) with the ATP-binding site was independently confirmed since inactivation of proteolyzed CaM-kinase II by phenylglyoxal (t1/2 = 7 min) was blocked by ATP analog plus Mg2+ or by CaMK-(281-309). In the presence of Ca2+/CaM, CaMK-(281-309) no longer protected against phenylglyoxal inactivation, consistent with our previous observations (Colbran, R.J., Fong, Y.-L., Schworer, C.M., and Soderling, T.R. (1988) J. Biol. Chem. 263, 18145-18151) that binding of Ca2+/CaM to CaMK-(281-309) 1) blocks its inhibitory property, and 2) enhances its phosphorylation at Thr 286. The present study also showed that phosphorylation of CaMK-(281-309) decreased its inhibitory potency at least 10-fold without affecting its Ca2+/CaM-binding ability. Thus, CaM-kinase II is inactive in the absence of Ca2+/CaM because an inhibitory domain within residues 281-309 interacts with the catalytic domain and blocks ATP binding. Autophosphorylation of Thr286 results in a Ca2+/CaM-independent form of the kinase by disrupting the inhibitory interaction with the catalytic domain.     

Characterization of a novel calcium/calmodulin-dependent protein kinase from tobacco

A cDNA encoding a calcium (Ca2+)/calmodulin (CaM)-dependent protein kinase (CaMK) from tobacco (Nicotiana tabacum), NtCaMK1, was isolated by protein-protein interaction-based screening of a cDNA expression library using 35S-labeled CaM as a probe. The genomic sequence is about 24.6 kb, with 21 exons, and the full-length cDNA is 4.8 kb, with an open reading frame for NtCaMK1 consisting of 1,415 amino acid residues. NtCaMK1 has all 11 subdomains of a kinase catalytic domain, lacks EF hands for Ca2+-binding, and is structurally similar to other CaMKs in mammal systems. Biochemical analyses have identified NtCaMK1 as a Ca2+/CaMK since NtCaMK1 phosphorylated itself and histone IIIs as substrate only in the presence of Ca2+/CaM with a Km of 44.5 microm and a Vmax of 416.2 nm min(-1) mg(-1). Kinetic analysis showed that the kinase not previously autophosphorylated had a Km for the synthetic peptide syntide-2 of 22.1 microm and a Vmax of 644.1 nm min(-1) mg(-1) when assayed in the presence of Ca2+/CaM. Once the autophosphorylation of NtCaMK1 was initiated, the phosphorylated form displayed Ca2+/CaM-independent behavior, as many other CaMKs do. Analysis of the CaM-binding domain (CaMBD) in NtCaMK1 with truncated and site-directed mutated forms defined a stretch of 20 amino acid residues at positions 913 to 932 as the CaMBD with high CaM affinity (Kd = 5 nm). This CaMBD was classified as a 1-8-14 motif. The activation of NtCaMK1 was differentially regulated by three tobacco CaM isoforms (NtCaM1, NtCaM3, and NtCaM13). While NtCaM1 and NtCaM13 activated NtCaMK1 effectively, NtCaM3 did not activate the kinase.     

Regulation of Ca2+/calmodulin kinase II by a small C-terminal domain phosphatase

We present here the identification and characterization of an SCP3 (small C-terminal domain phosphatase-3) homologue in smooth muscle and show, for the first time, that it dephosphorylates CaMKII [Ca(2+)/CaM (calmodulin)-dependent protein kinase II]. SCP3 is a PP2C (protein phosphatase 2C)-type phosphatase that is primarily expressed in vascular smooth muscle tissues and specifically binds to the association domain of the CaMKIIgamma G-2 variant. The dephosphorylation is site-specific, excluding the Thr(287) associated with Ca(2+)/CaM-independent activation of the kinase. As a result, the autonomous activity of CaMKIIgamma G-2 is not affected by the phosphatase activity of SCP3. SCP3 co-localizes with CaMKIIgamma G-2 on cytoskeletal filaments, but is excluded from the nucleus in differentiated vascular smooth muscle cells. Upon depolarization-induced Ca(2+) influx, CaMKIIgamma G-2 is activated and dissociates from SCP3. Subsequently, CaMKIIgamma G-2 is targeted to cortical adhesion plaques. We show here that SCP3 regulates phosphorylation sites in the catalytic domain, but not those involved in regulation of kinase activation. This selective dephosphorylation by SCP3 creates a constitutively active kinase that can then be differentially regulated by other phosphorylation-dependent regulatory mechanisms.     

Functional assembly of fragments from bisected smooth muscle myosin light chain kinase

The C-terminal regulatory segment of smooth muscle myosin light chain kinase folds back on its catalytic core to inhibit kinase activity. This regulatory segment consists of autoinhibitory residues linking the catalytic core to the calmodulin-binding sequence and perhaps additional C-terminal residues including an immunoglobulin-like motif. However, mutational and biochemical analyses showed no specific involvement of residues C-terminal to the calmodulin-binding sequence. To obtain additional insights on the proposed mechanisms for autoinhibition and Ca(2+)/calmodulin activation of the kinase, the polypeptide backbone chain of myosin light chain kinase was cleaved by genetic means to produce N- and C-terminal protein fragments. The N-terminal fragment containing the catalytic core was catalytically inactive when expressed alone. Co-expression of the N-terminal fragment with the C-terminal fragment containing the regulatory segment restored kinase activity. Deletion of the autoinhibitory linker residues without or with the calmodulin-binding sequence prevented restoration of kinase activity. In the presence or absence of Ca(2+)/calmodulin, regulatory segment binding occurred through the linker region connecting the catalytic core to the calmodulin-binding sequence. Collectively, these results indicate that residues C-terminal to the calmodulin-binding sequence (including the immunoglobulin-like motif) are not functional components of the regulatory segment. Furthermore, the principal autoinhibitory motif is contained in the sequence linking the catalytic core of myosin light chain kinase to the calmodulin-binding sequence.     

Isolation and characterization of a novel calcium/calmodulin-dependent protein kinase, AtCK, from arabidopsis

Protein phosphorylation is one of the major mechanisms by which eukaryotic cells transduce extracellular signals into intracellular responses. Calcium/calmodulin (Ca(2+)/CaM)-dependent protein phosphorylation has been implicated in various cellular processes, yet little is known about Ca(2+)/CaM-dependent protein kinases (CaMKs) in plants. From an Arabidopsis expression library screen using a horseradish peroxidase-conjugated soybean calmodulin isoform (SCaM-1) as a probe, we isolated a full-length cDNA clone that encodes AtCK (Arabidopsis thaliana calcium/calmodulin-dependent protein kinase). The predicted structure of AtCK contains a serine/threonine protein kinase catalytic domain followed by a putative calmodulin-binding domain and a putative Ca(2+)-binding domain. Recombinant AtCK was expressed in E. coli and bound to calmodulin in a Ca(2+)-dependent manner. The ability of CaM to bind to AtCK was confirmed by gel mobility shift and competition assays. AtCK exhibited its highest levels of autophosphorylation in the presence of 3 mM Mn(2+). The phosphorylation of myelin basic protein (MBP) by AtCK was enhanced when AtCK was under the control of calcium-bound CaM, as previously observed for other Ca(2+)/CaM-dependent protein kinases. In contrast to maize and tobacco CCaMKs (calcium and Ca(2+)/CaM-dependent protein kinase), increasing the concentration of calmodulin to more than 3 microgram suppressed the phosphorylation activity of AtCK. Taken together our results indicate that AtCK is a novel Arabidopsis Ca(2+)/CaM-dependent protein kinase which is presumably involved in CaM-mediated signaling.     

The pro-apoptotic function of death-associated protein kinase is controlled by a unique inhibitory autophosphorylation-based mechanism

Death-associated protein kinase is a calcium/calmodulin serine/threonine kinase, which positively mediates programmed cell death in a variety of systems. Here we addressed its mode of regulation and identified a mechanism that restrains its apoptotic function in growing cells and enables its activation during cell death. It involves autophosphorylation of Ser(308) within the calmodulin (CaM)-regulatory domain, which occurs at basal state, in the absence of Ca(2+)/CaM, and is inversely correlated with substrate phosphorylation. This type of phosphorylation takes place in growing cells and is strongly reduced upon their exposure to the apoptotic stimulus of C(6)-ceramide. The substitution of Ser(308) to alanine, which mimics the ceramide-induced dephosphorylation at this site, increases Ca(2+)/CaM-independent substrate phosphorylation as well as binding and overall sensitivity of the kinase to CaM. At the cellular level, it strongly enhances the death-promoting activity of the kinase. Conversely, mutation to aspartic acid reduces the binding of the protein to CaM and abrogates almost completely the death-promoting function of the protein. These results are consistent with a molecular model in which phosphorylation on Ser(308) stabilizes a locked conformation of the CaM-regulatory domain within the catalytic cleft and simultaneously also interferes with CaM binding. We propose that this unique mechanism of auto-inhibition evolved to impose a locking device, which keeps death-associated protein kinase silent in healthy cells and ensures its activation only in response to apoptotic signals.     

STO-609, a specific inhibitor of the Ca(2+)/calmodulin-dependent protein kinase kinase

STO-609, a selective inhibitor of Ca(2+)/calmodulin-dependent protein kinase kinase (CaM-KK) was synthesized, and its inhibitory properties were investigated both in vitro and in vivo. STO-609 inhibits the activities of recombinant CaM-KK alpha and CaM-KK beta isoforms, with K(i) values of 80 and 15 ng/ml, respectively, and also inhibits their autophosphorylation activities. Comparison of the inhibitory potency of the compound against various protein kinases revealed that STO-609 is highly selective for CaM-KK without any significant effect on the downstream CaM kinases (CaM-KI and -IV), and the IC(50) value of the compound against CaM-KII is approximately 10 microg/ml. STO-609 inhibits constitutively active CaM-KK alpha (glutathione S-transferase (GST)-CaM-KK-(84-434)) as well as the wild-type enzyme. Kinetic analysis indicates that the compound is a competitive inhibitor of ATP. In transfected HeLa cells, STO-609 suppresses the Ca(2+)-induced activation of CaM-KIV in a dose-dependent manner. In agreement with this observation, the inhibitor significantly reduces the endogenous activity of CaM-KK in SH-SY5Y neuroblastoma cells at a concentration of 1 microg/ml (approximately 80% inhibitory rate). Taken together, these results indicate that STO-609 is a selective and cell-permeable inhibitor of CaM-KK and that it may be a useful tool for evaluating the physiological significance of the CaM-KK-mediated pathway in vivo as well as in vitro.     

Alternative splicing modulates the frequency-dependent response of CaMKII to Ca(2+) oscillations

Ca(2+) oscillations are required in various signal trans duction pathways, and contain information both in their amplitude and frequency. Remarkably, the Ca(2+)/calmodulin(CaM)-dependent protein kinase II (CaMKII) can decode such frequencies. A Ca(2+)/CaM-stimulated autophosphorylation leads to Ca(2+)/CaM-independent (autonomous) activity of the kinase that outlasts the initial stimulation. This autonomous activity increases exponentially with the frequency of Ca(2+) oscillations. Here we show that three beta-CaMKII splice variants (beta(M), beta and beta(e)') have very similar specific activity and maximal autonomy. However, their autonomy generated by Ca(2+) oscillations differs significantly. A mechanistic basis was found in alterations of the CaM activation constant and of the initial rate of autophosphorylation. Structurally, the splice variants differ only in a variable 'linker' region between the kinase and association domains. Therefore, we propose that differences in relative positioning of kinase domains within multimeric holoenzymes are responsible for the observed effects. Notably, the beta-CaMKII splice variants are differentially expressed, even among individual hippocampal neurons. Taken together, our results suggest that alternative splicing provides cells with a mechanism to modulate their sensitivity to Ca(2+) oscillations.     

Structural insights into enzyme regulation for inositol 1,4,5-trisphosphate 3-kinase B

D-Myoinositol 1,4,5-trisphophate 3-kinases (IP(3)-3Ks) play important roles in metazoan cellular signaling. It has been demonstrated that mice without a functional version of IP(3)-3K isoform B are deficient in peripheral T-cells, indicating that IP(3)-3KB is essential to the developing immune system. The recent apo IP(3)-3KA structure exhibited a helix at the catalytic domain N-terminus exhibited a helix at the N-terminus of the catalytic domain, with a tryptophan indole moiety mimicking the binding mode of the substrate ATP purine ring, suggesting a mechanism of autoinhibition. Here we present the structure of the complete catalytic domain of IP(3)-3KB, including the CaM binding domain in complex with Mg(2+) and ATP. The crystal structure reveals a homodimeric arrangement of IP(3)-3KB catalytic domains, mediated via an intermolecular antiparallel beta-sheet formed from part of the CaM binding region. Residues from the putative autoinhibitory helix are rearranged into a loop configuration, with extensive interactions with the bound ATP. Mutagenesis of residues from this region reveals that substitution of the putative autoinhibitory tryptophan generates a hyperactive enzyme which retains Ca(2+)/CaM sensitivity. The IP(3)-3KB structure suggests a mechanism of enzyme activation, and raises the possibility that an interaction between IP(3)-3KB molecules may occur as part of the catalytic or regulatory cycle.