Oligomerization states of the association domain and the holoenyzme of Ca2+/CaM kinase II

Ca2+/calmodulin activated protein kinase II (CaMKII) is an oligomeric protein kinase with a unique holoenyzme architecture. The subunits of CaMKII are bound together into the holoenzyme by the association domain, a C-terminal region of approximately 140 residues in the CaMKII polypeptide. Single particle analyses of electron micrographs have suggested previously that the holoenyzme forms a dodecamer that contains two stacked 6-fold symmetric rings. In contrast, a recent crystal structure of the isolated association domain of mouse CaMKIIalpha has revealed a tetradecameric assembly with two stacked 7-fold symmetric rings. In this study, we have determined the crystal structure of the Caenorhabditis elegans CaMKII association domain and it too forms a tetradecamer. We also show by electron microscopy that in its fully assembled form the CaMKII holoenzyme is a dodecamer but without the kinase domains, either from expression of the isolated association domain in bacteria or following their removal by proteolysis, the association domains form a tetradecamer. We speculate that the holoenzyme is held in its 6-fold symmetric state by the interactions of the N-terminal approximately 1-335 residues and that the removal of this region allows the association domain to convert into a more stable 7-fold symmetric form.     

The eag potassium channel binds and locally activates calcium/calmodulin-dependent protein kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the regulation of neuronal excitability in many systems. Recent studies suggest that local regulation of membrane potential can have important computational consequences for neuronal function. In Drosophila, CaMKII regulates the eag potassium channel, but if and how this regulation was spatially restricted was unknown. Using coimmunoprecipitation from head extracts and in vitro binding assays, we show that CaMKII and Eag form a stable complex and that association with Eag activates CaMKII independently of CaM and autophosphorylation. Ca(2+)/CaM is necessary to initiate binding of CaMKII to Eag but not to sustain association because binding persists when CaM is removed. The Eag CaMKII-binding domain has homology to the CaMKII autoregulatory region, and the constitutively active CaMKII mutant, T287D, binds Eag Ca(2+)-independently in vitro and in vivo. These results favor a model in which the CaMKII-binding domain of Eag displaces the CaMKII autoinhibitory region. Displacement results in autophosphorylation-independent activation of CaMKII which persists even when Ca(2+) levels have gone down. Activity-dependent binding to this potassium channel substrate allows CaMKII to remain locally active even when Ca(2+) levels have dropped, providing a novel mechanism by which CaMKII can regulate excitability locally over long time scales.     

A mechanism for tunable autoinhibition in the structure of a human Ca2+/calmodulin- dependent kinase II holoenzyme

Calcium/calmodulin-dependent kinase II (CaMKII) forms a highly conserved dodecameric assembly that is sensitive to the frequency of calcium pulse trains. Neither the structure of the dodecameric assembly nor how it regulates CaMKII are known. We present the crystal structure of an autoinhibited full-length human CaMKII holoenzyme, revealing an unexpected compact arrangement of kinase domains docked against a central hub, with the calmodulin-binding sites completely inaccessible. We show that this compact docking is important for the autoinhibition of the kinase domains and for setting the calcium response of the holoenzyme. Comparison of CaMKII isoforms, which differ in the length of the linker between the kinase domain and the hub, demonstrates that these interactions can be strengthened or weakened by changes in linker length. This equilibrium between autoinhibited states provides a simple mechanism for tuning the calcium response without changes in either the hub or the kinase domains.     

Structure of the autoinhibited kinase domain of CaMKII and SAXS analysis of the holoenzyme

Ca2+/calmodulin-dependent protein kinase-II (CaMKII) is unique among protein kinases for its dodecameric assembly and its complex response to Ca2+. The crystal structure of the autoinhibited kinase domain of CaMKII, determined at 1.8 A resolution, reveals an unexpected dimeric organization in which the calmodulin-responsive regulatory segments form a coiled-coil strut that blocks peptide and ATP binding to the otherwise intrinsically active kinase domains. A threonine residue in the regulatory segment, which when phosphorylated renders CaMKII calmodulin independent, is held apart from the catalytic sites by the organization of the dimer. This ensures a strict Ca2+ dependence for initial activation. The structure of the kinase dimer, when combined with small-angle X-ray scattering data for the holoenzyme, suggests that inactive CaMKII forms tightly packed autoinhibited assemblies that convert upon activation into clusters of loosely tethered and independent kinase domains.     

Variation in assembly stoichiometry in non‐metazoan homologs of the hub domain of Ca2+/calmodulin‐dependent protein kinase II

The multi‐subunit Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) holoenzyme plays a critical role in animal learning and memory. The kinase domain of CaMKII is connected by a flexible linker to a C‐terminal hub domain that assembles into a 12‐ or 14‐subunit scaffold that displays the kinase domains around it. Studies on CaMKII suggest that the stoichiometry and dynamic assembly/disassembly of hub oligomers may be important for CaMKII regulation. Although CaMKII is a metazoan protein, genes encoding predicted CaMKII‐like hub domains, without associated kinase domains, are found in the genomes of some green plants and bacteria. We show that the hub domains encoded by three related green algae, Chlamydomonas reinhardtii, Volvox carteri f. nagarensis, and Gonium pectoral, assemble into 16‐, 18‐, and 20‐subunit oligomers, as assayed by native protein mass spectrometry. These are the largest known CaMKII hub domain assemblies. A crystal structure of the hub domain from C. reinhardtii reveals an 18‐subunit organization. We identified four intra‐subunit hydrogen bonds in the core of the fold that are present in the Chlamydomonas hub domain, but not in metazoan hubs. When six point mutations designed to recapitulate these hydrogen bonds were introduced into the human CaMKII‐α hub domain, the mutant protein formed assemblies with 14 and 16 subunits, instead of the normal 12‐ and 14‐subunit assemblies. Our results show that the stoichiometric balance of CaMKII hub assemblies can be shifted readily by small changes in sequence.     

Site-specific phosphorylation of neurofilament-L is mediated by calcium/calmodulin-dependent protein kinase II in the apical dendrites during long-term potentiation

Neurofilament-L (NF-L), one subunit of the neuronal intermediate filaments, is a major element of neuronal cytoskeletons. The dynamics of NF-L are regulated by phosphorylation of its head domain. The phosphorylation sites of the NF-L head domain by protein kinase A, protein kinase C, and Rho-associated kinase have been previously identified, and those by calcium/calmodulin-dependent protein kinase II (CaMKII) were identified in this study. A series of site- and phosphorylation state-specific antibodies against NF-L was prepared to investigate NF-L phosphorylation in neuronal systems. Long-term potentiation (LTP) is a cellular model of neuronal plasticity that is thought to involve the phosphorylation of various proteins. NF-L is considered a possible substrate for phosphorylation. During LTP stimulation of mouse hippocampal slices, the series of antibodies demonstrated the increase in the phosphorylation level of Ser(57) in NF-L and the visualization of the localized distribution of Ser(57) phosphorylation in a subpopulation of apical dendrites of the pyramidal neurons. Furthermore, Ser(57) phosphorylation during LTP is suggested to be mediated by CaMKII. Here we show that NF-L is phosphorylated by CaMKII in a subpopulation of apical dendrites during LTP, indicating that Ser(57) is a novel phosphorylation site of NF-L in vivo related to the neuronal signal transduction.     

Association of calcium/calmodulin-dependent kinase II with developmentally regulated splice variants of the postsynaptic density protein densin-180

In a continuing search for proteins that target calcium/calmodulin-dependent protein kinase II (CaMKII) to postsynaptic density (PSD) substrates important in synaptic plasticity, we showed that the PSD protein densin-180 binds CaMKII. Four putative splice variants (A-D) of the cytosolic tail of densin-180 are shown to be differentially expressed during brain development. Densin-180 splicing affects CaMKII phosphorylation of specific serine residues. Variants A, B, and D, but not C, bind CaMKII stoichiometrically and with high affinity, mediated by a differentially spliced domain. Densin-180 differs from the previously identified CaMKII-binding protein NR2B in that binding does not strictly require CaMKII autophosphorylation. Binding of densin-180 and NR2B to CaMKII is noncompetitive, indicating different interaction sites on CaMKII. Expression of the membrane-targeted CaMKII-binding domain of densin-180 confers membrane localization to coexpressed CaMKII without requiring calcium mobilization, suggesting that densin-180 plays a role in the constitutive association of CaMKII with PSDs.     

Hyaluronan-CD44 interaction with leukemia-associated RhoGEF and epidermal growth factor receptor promotes Rho/Ras co-activation, phospholipase C epsilon-Ca2+ signaling, and cytoskeleton modification in head and neck squamous cell carcinoma cells

In this study we have examined the interaction of CD44 (a major hyaluronan (HA) receptor) with a RhoA-specific guanine nucleotide exchange factor (leukemia-associated RhoGEF (LARG)) in human head and neck squamous carcinoma cells (HNSCC-HSC-3 cell line). Immunoprecipitation and immunoblot analyses indicate that CD44 and the LARG protein are expressed in HSC-3 cells and that these two proteins are physically associated as a complex. HA-CD44 binding induces LARG-specific RhoA signaling and phospholipase C epsilon (PLC epsilon) activity. In particular, the activation of RhoA-PLC epsilon by HA stimulates inositol 1,4,5-triphosphate production, intracellular Ca2+ mobilization, and the up-regulation of Ca2+/calmodulin-dependent kinase II (CaMKII), leading to phosphorylation of the cytoskeletal protein, filamin. The phosphorylation of filamin reduces its interaction with filamentous actin, promoting tumor cell migration. The CD44-LARG complex also interacts with the EGF receptor (EGFR). Most importantly, the binding of HA to the CD44-LARG-EGFR complex activates the EGFR receptor kinase, which in turn promotes Ras-mediated stimulation of a downstream kinase cascade including the Raf-1 and ERK pathways leading to HNSCC cell growth. Using a recombinant fragment of LARG (the LARG-PDZ domain) and a binding assay, we have determined that the LARG-PDZ domain serves as a direct linker between CD44 and EGFR. Transfection of the HSC-3 cells with LARG-PDZcDNA significantly reduces LARG association with CD44 and EGFR. Overexpression of the LARG-PDZ domain also functions as a dominant-negative mutant (similar to the PLC/Ca2+-calmodulin-dependent kinase II (CaMKII) and EGFR/MAPK inhibitor effects) to block HA/CD44-mediated signaling events (e.g. EGFR kinase activation, Ras/RhoA co-activation, Raf-ERK signaling, PLC epsilon-mediated inositol 1,4,5-triphosphate production, intracellular Ca2+ mobilization, CaMKII activity, filamin phosphorylation, and filamin-actin binding) and to abrogate tumor cell growth/migration. Taken together, our findings suggest that CD44 interaction with LARG and EGFR plays a pivotal role in Rho/Ras co-activation, PLC epsilon-Ca2+ signaling, and Raf/ERK up-regulation required for CaMKII-mediated cytoskeleton function and in head and neck squamous cell carcinoma progression.     

Mechanism and regulation of calcium/calmodulin-dependent protein kinase II targeting to the NR2B subunit of the N-methyl-D-aspartate receptor

Calcium influx through the N-methyl-d-aspartate (NMDA)-type glutamate receptor and activation of calcium/calmodulin-dependent kinase II (CaMKII) are critical events in certain forms of synaptic plasticity. We have previously shown that autophosphorylation of CaMKII induces high-affinity binding to the NR2B subunit of the NMDA receptor (Strack, S., and Colbran, R. J. (1998) J. Biol. Chem. 273, 20689-20692). Here, we show that residues 1290-1309 in the cytosolic tail of NR2B are critical for CaMKII binding and identify by site-directed mutagenesis several key residues (Lys(1292), Leu(1298), Arg(1299), Arg(1300), Gln(1301), and Ser(1303)). Phosphorylation of NR2B at Ser(1303) by CaMKII inhibits binding and promotes slow dissociation of preformed CaMKII.NR2B complexes. Peptide competition studies imply a role for the CaMKII catalytic domain, but not the substrate-binding pocket, in the association with NR2B. However, analysis of monomeric CaMKII mutants indicates that the holoenzyme structure may also be important for stable association with NR2B. Residues 1260-1316 of NR2B are sufficient to direct the subcellular localization of CaMKII in intact cells and to confer dynamic regulation by calcium influx. Furthermore, mutation of residues in the CaMKII-binding domain in full-length NR2B bidirectionally modulates colocalization with CaMKII after NMDA receptor activation, suggesting a dynamic model for the translocation of CaMKII to postsynaptic targets.     

Substrate-selective and calcium-independent activation of CaMKII by α-actinin

Protein-protein interactions are thought to modulate the efficiency and specificity of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) signaling in specific subcellular compartments. Here we show that the F-actin-binding protein α-actinin targets CaMKIIα to F-actin in cells by binding to the CaMKII regulatory domain, mimicking CaM. The interaction with α-actinin is blocked by CaMKII autophosphorylation at Thr-306, but not by autophosphorylation at Thr-305, whereas autophosphorylation at either site blocks Ca(2+)/CaM binding. The binding of α-actinin to CaMKII is Ca(2+)-independent and activates the phosphorylation of a subset of substrates in vitro. In intact cells, α-actinin selectively stabilizes CaMKII association with GluN2B-containing glutamate receptors and enhances phosphorylation of Ser-1303 in GluN2B, but inhibits CaMKII phosphorylation of Ser-831 in glutamate receptor GluA1 subunits by competing for activation by Ca(2+)/CaM. These data show that Ca(2+)-independent binding of α-actinin to CaMKII differentially modulates the phosphorylation of physiological targets that play key roles in long-term synaptic plasticity.     

Differential association of postsynaptic signaling protein complexes in striatum and hippocampus

Distinct physiological stimuli are required for bidirectional synaptic plasticity in striatum and hippocampus, but differences in the underlying signaling mechanisms are poorly understood. We have begun to compare levels and interactions of key excitatory synaptic proteins in whole extracts and subcellular fractions isolated from micro‐dissected striatum and hippocampus. Levels of multiple glutamate receptor subunits, calcium/calmodulin‐dependent protein kinase II (CaMKII), a highly abundant serine/threonine kinase, and spinophilin, a F‐actin and protein phosphatase 1 (PP1) binding protein, were significantly lower in striatal extracts, as well as in synaptic and/or extrasynaptic fractions, compared with similar hippocampal extracts/fractions. However, CaMKII interactions with spinophilin were more robust in striatum compared with hippocampus, and this enhanced association was restricted to the extrasynaptic fraction. NMDAR GluN2B subunits associate with both spinophilin and CaMKII, but spinophilin‐GluN2B complexes were enriched in extrasynaptic fractions whereas CaMKII‐GluN2B complexes were enriched in synaptic fractions. Notably, the association of GluN2B with both CaMKII and spinophilin was more robust in striatal extrasynaptic fractions compared with hippocampal extrasynaptic fractions. Selective differences in the assembly of synaptic and extrasynaptic signaling complexes may contribute to differential physiological regulation of excitatory transmission in striatum and hippocampus.     

The last twenty residues in the head domain of mouse lamin A contain important structural elements for formation of head-to-tail polymers in vitro

Nuclear lamins are a type of intermediate filament (IF) proteins. They have a characteristic tripartite domain structure with a alpha-helical rod domain flanked by non-alpha-helical N-terminal head and C-terminal tail domains. While the head domain has been shown to be important for the formation of head-to-tail polymers that are critical assembly intermediates for lamin IFs, essential structural elements in this domain have remained obscure. As a first step to remedy this, a series of mouse lamin A mutants in which the head domain (30 amino acid residues) was deleted stepwise from the N-terminus at intervals of 10 residues were bacterially expressed. The assembly properties in vitro of the purified recombinant proteins were explored by electron microscopy. We observed that while a lamin A mutant lacking N-terminal 10 residues formed head-to-tail polymers, a mutant lacking N-terminal 20 residues or the whole head domain (30 residues) showed significantly decreased potency to form head-to-tail polymers. These results suggest that the last 20 residues (from Arg-11 to Gln-30) of the head domain of mouse lamin A contain essential structures for the formation of head-to-tail polymers. The last 20 residues of the head domain include several conserved residues between A- and B-type lamins and also the phosphorylation site for cdc2 kinase, which affects lamin IF organization in vivo and in vitro. Our results provide clues to the molecular mechanism by which the head domain plays a crucial role in lamin polymerization.     

Characterization of CaMKIIα holoenzyme stability

Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) is a Ser/Thr kinase necessary for long‐term memory formation and other Ca²⁺‐dependent signaling cascades such as fertilization. Here, we investigated the stability of CaMKIIα using a combination of differential scanning calorimetry (DSC), X‐ray crystallography, and mass photometry (MP). The kinase domain has a low thermal stability (apparent T_m = 36°C), which is slightly stabilized by ATP/MgCl₂ binding (apparent T_m = 40°C) and significantly stabilized by regulatory segment binding (apparent T_m = 60°C). We crystallized the kinase domain of CaMKII bound to p‐coumaric acid in the active site. This structure reveals solvent‐exposed hydrophobic residues in the substrate‐binding pocket, which are normally buried in the autoinhibited structure when the regulatory segment is present. This likely accounts for the large stabilization that we observe in DSC measurements comparing the kinase alone with the kinase plus regulatory segment. The hub domain alone is extremely stable (apparent T_m ~ 90°C), and the holoenzyme structure has multiple unfolding transitions ranging from ~60°C to 100°C. Using MP, we compared a CaMKIIα holoenzyme with different variable linker regions and determined that the dissociation of both these holoenzymes occurs at a higher concentration (is less stable) compared with the hub domain alone. We conclude that within the context of the holoenzyme structure, the kinase domain is stabilized, whereas the hub domain is destabilized. These data support a model where domains within the holoenzyme interact.     

Characterization of a central Ca2+/calmodulin-dependent protein kinase IIalpha/beta binding domain in densin that selectively modulates glutamate receptor subunit phosphorylation

The densin C-terminal domain can target Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) in cells. Although the C-terminal domain selectively binds CaMKIIα in vitro, full-length densin associates with CaMKIIα or CaMKIIβ in brain extracts and in transfected HEK293 cells. This interaction requires a second central CaMKII binding site, the densin-IN domain, and an "open" activated CaMKII conformation caused by Ca(2+)/calmodulin binding, autophosphorylation at Thr-286/287, or mutation of Thr-286/287 to Asp. Mutations in the densin-IN domain (L815E) or in the CaMKIIα/β catalytic domain (I205/206K) disrupt the interaction. The amino acid sequence of the densin-IN domain is similar to the CaMKII inhibitor protein, CaMKIIN, and a CaMKIIN peptide competitively blocks CaMKII binding to densin. CaMKII is inhibited by both CaMKIIN and the densin-IN domain, but the inhibition by densin is substrate-selective. Phosphorylation of a model peptide substrate, syntide-2, or of Ser-831 in AMPA receptor GluA1 subunits is fully inhibited by densin. However, CaMKII phosphorylation of Ser-1303 in NMDA receptor GluN2B subunits is not effectively inhibited by densin in vitro or in intact cells. Thus, densin can target multiple CaMKII isoforms to differentially modulate phosphorylation of physiologically relevant downstream targets.     

Calcium/Calmodulin-Dependent Protein Kinase II Is Associated with NR2A/B Subunits of NMDA Receptor in Postsynaptic Densities

Abstract: NMDA receptors and Ca²⁺/calmodulin-dependent kinase II (CaMKII) have been reported to be highly concentrated in the postsynaptic density (PSD). Although the possibility that CaMKII in PSD might be associated with specific proteins has been put forward, the protein or proteins determining the targeting of the kinase in PSD have not yet been identified. Here we report that CaMKII binds to NR2A and NR2B subunits of NMDA receptors in PSD isolated from cortex and hippocampus. The association of NMDA receptor subunits and CaMKII was assessed by immunoprecipitating PSD proteins with antibodies specific for NR2A/B and CaMKII: CaMKII coprecipitated with NR2A/B and NR1 but not with other glutamate ionotropic receptor subunits, such as GluR1 and GluR2-3. A direct association between CaMKII and NR2A/B subunits was further confirmed by overlay experiments using either ³²P-autophosphorylated CaMKII or ³²P-NR2A/B and by evaluating the formation of a CaMKII-NR2A/B complex by means of the cross-linker disuccimidyl suberate. These data demonstrate an association between the NMDA receptor complex and CaMKII in the postsynaptic compartment, suggesting that this colocalization may be relevant for synaptic plasticity.     

Ca2+-independent Activation of Ca2+/Calmodulin-dependent Protein Kinase II Bound to the C-terminal Domain of CaV2.1 Calcium Channels

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) forms a major component of the postsynaptic density where its functions in synaptic plasticity are well established, but its presynaptic actions are poorly defined. Here we show that CaMKII binds directly to the C-terminal domain of Ca_V2.1 channels. Binding is enhanced by autophosphorylation, and the kinase-channel signaling complex persists after dephosphorylation and removal of the Ca²⁺/CaM stimulus. Autophosphorylated CaMKII can bind the Ca_V2.1 channel and synapsin-1 simultaneously. CaMKII binding to Ca_V2.1 channels induces Ca²⁺-independent activity of the kinase, which phosphorylates the enzyme itself as well as the neuronal substrate synapsin-1. Facilitation and inactivation of Ca_V2.1 channels by binding of Ca²⁺/CaM mediates short term synaptic plasticity in transfected superior cervical ganglion neurons, and these regulatory effects are prevented by a competing peptide and the endogenous brain inhibitor CaMKIIN, which blocks binding of CaMKII to Ca_V2.1 channels. These results define the functional properties of a signaling complex of CaMKII and Ca_V2.1 channels in which both binding partners are persistently activated by their association, and they further suggest that this complex is important in presynaptic terminals in regulating protein phosphorylation and short term synaptic plasticity.     

Protein kinase C promotes N-methyl-D-aspartate (NMDA) receptor trafficking by indirectly triggering calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation

Regulation of neuronal NMDA receptor (NMDAR) is critical in synaptic transmission and plasticity. Protein kinase C (PKC) promotes NMDAR trafficking to the cell surface via interaction with NMDAR-associated proteins (NAPs). Little is known, however, about the NAPs that are critical to PKC-induced NMDAR trafficking. Here, we showed that calcium/calmodulin-dependent protein kinase II (CaMKII) could be a NAP that mediates the potentiation of NMDAR trafficking by PKC. PKC activation promoted the level of autophosphorylated CaMKII and increased association with NMDARs, accompanied by functional NMDAR insertion, at postsynaptic sites. This potentiation, along with PKC-induced long term potentiation of the AMPA receptor-mediated response, was abolished by CaMKII antagonist or by disturbing the interaction between CaMKII and NR2A or NR2B. Further mutual occlusion experiments demonstrated that PKC and CaMKII share a common signaling pathway in the potentiation of NMDAR trafficking and long-term potentiation (LTP) induction. Our results revealed that PKC promotes NMDA receptor trafficking and induces synaptic plasticity through indirectly triggering CaMKII autophosphorylation and subsequent increased association with NMDARs.     

AlphaCaMKII binding to the C-terminal tail of NMDA receptor subunit NR2A and its modulation by autophosphorylation.

Ca2+/calmodulin-dependent protein kinase II (CaMKII), a multifunctional, widely distributed enzyme, is enriched in post-synaptic densities (PSDs). Here, we demonstrate that CaMKII binds to a discrete C-terminal region of the NR2A subunit of NMDA receptors and promotes the phosphorylation of a Ser residue of this NMDA receptor subunit. Glutathione S-transferase (GST)-NR2A(1349-1464) binds native CaMKII from solubilised hippocampal PSDs in 'pull-out' and overlay experiments and this binding is competed by recombinant alphaCaMKII(1-315). The longer GST-NR2A(1244-1464), although containing the CaMKII phosphosite Ser-1289, binds the kinase with a lower efficacy. CaMKII association to NR2A(1349-1464) is positively modulated by kinase autophosphorylation in the presence of Ca2+/calmodulin. These data provide direct evidence for a mechanism modulating the synaptic strength.