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[This wiki should grow to a annotated list of known interactions of CaMKII, constituting the basis for a detailed kappa model of the system] 2. Agents
Ca2+/calmodulin-dependent protein kinase II (CaMKII)
CaMKII is one of the most abundant proteins in neurons making 1-2% of the total protein concentration. CaMKII forms heterooligomers of 12 subunits and arrange in a hexameric doubledoughnut structure. There are now > 30 isoforms of CaMKII known, comprised of alternately spliced variants of alpha, beta, delta. Primary isoforms are alpha and beta. Depending on the location in the brain the ratio between the two forms can range from 3:1 = alpha:beta (forebrain mouse) to 1:4 (cerebellum). BetaCaMKII differs from the alpha form in the variable region for F-actin targeting. It was reported that alpha and beta isoforms have sharply different affinities for CaM (Half-max autophos level is achieved at 130nM CaM for alphaCaMKII and at 15nM CaM for betaCaMKII). Heterooligomers comprised of two beta subunits translocate to the postsynaptic density (PSD) because of the F-actin targeting of betaCaMKII. At its site Thr286 CaMKII can be phosed by a particular binding event with CaM (detailed below). When phosed on T286 CaMKII can trap bound CaM by a several hundred fold increase in binding affinity. CaMKII (also alphaCaMKII) translocates to the PSD in response to NMDA receptor activation. It is likely to bind to the NR2B subunit of the NMDA receptor. At the PSD active CaMKII actives AMPA receptors that in turn causes an increase of conductance. But there are more the 30 known targets of CaMKII (Fink and Meyer 2002, PMID:12049936). Note that the F-actin targeting beta isoforms can not be the reason for the translocation to the PSD, as actin bound betaCaMKII are released upon Ca2+ stimulation. At the PSD, CaMKII in addition to NMDA binds to structural scaffold proteins such PSD95, actin, densin-180 and SAP97. PP1 also interacts with PSD95 and only dephos CaMKII if it is bound in the PSD. Conversely another phosphatase PP2A is able to dephos soluble CaMKII at Thr286 but not when it is bound in the PSD. Zhabotinsky explains the bistability of CaMKII through the differential access of PP1 and PP2A on CaMKII. One hyphothesis is that NR2B binding of CaMKII is phos level dependent, such that active CaMKII get recruited to the PSD (Fink and Meyer 2002, PMID:12049936). The time CaMKII remains in the PSD depends on whether it is autophosed or not. alanine and aspartic acide mutants for T286 show shorter and longer remain time in the PSD, respectively. A ten-fold prolongation was observed for the aspartic acid mutant (Shen and Meyer 2000, PMID: 10966618). Thus "target trapping" at the PSD is depending on the T286 phos state. This trapping state is terminated by the action of the PP1 on T286. It also seems that phos of secondary autophos sites T305 and T306 are required for the dissociation from the PSD. Futhermore a "translocation priming" was observed, i.e., the dissociated CaMKII still have some active (phosed T286) subunits are thus more inclined to retranslocate to the PSD at a subsquent Ca+ stimulation. As translocation does not require phosed T286, the reason for this priming might be the higher affinity of autophosed for CaM and binding of CaM seems to be the requirement for translocation. One of the anchor proteins in the PSD for CaMKII is the subunit NR2B of NMDA receptor. Recently based on the work of (Shen and Meyer 2000, PMID: 10966618) two qualitatively different binding of CaMKII to NR2B were reported (Bayer and De Koninck 2006, PMID: 16436603). The first binding is reversible and involves the substrate binding site and is dependent on Ca2+/CaM binding at CaMKII, while the second binding event is persistent, involves the T286 binding site and enables a Ca2+/CaM independent CaMKII activity. The autonomous activitiy of CaMKII when in the PSD is attributed to the NR2B-bound state rather than CaM-bound state. Dissocation from the PSD occurs if Thr286 dephos through PP1 and (possibly) in addition the inhibtory autophos sites T305 and T306 get phosed. NR2B bound state supresses the activation of these two sites. In a phos deficient Thr286A mutant CaMKII dissociates quickly from the PSD, while consistently active Thr286D mutant did not dissociate. www.ncbi.nlm.nih.gov/pubmed/
Calmodulin (CaM)
Its an important Ca2+ effector protein that can bind four Ca2+, two on its N-terminal lobe and two on this C-terminal lobe. It has many downstream target, e.g. CaMKII and PP2B. The affinity of CaM for Ca2+ can be modulated by its binding to a target. In (Shifman and Kennedy 2006, PMID:) they show that the coorperativity of CaM-Ca2+ binding is strongly increase if CaM is bound to CaMKII. The affinity of CaM for Ca2+ can change by at least one order of magnitude. Protein phosphatase 1 (PP1) Four major potein phosphatase are able to specifically remove phosphate groups from serine (Ser) and threonine (Thr) residues, ie PP1, PP2A and PP2B and PP2C. PP1 has be shown to be involved in multiple cellular processes such as cell division, muscle contraction, translation, transcription and apotosis. Various interaction proteins of PP1 leads to compartmentalize different isoforms of PP1. The Beta isoform is predominantly found in the cell soma, whereas the gamma1 form is found in large quantities in the soma, the dendrites and presynaptic boutons where it colocalizes with abundant protein such as CaMKII. The specifity and activity of PP1 is regulated by its interactions proteins. Most of them have inhibitory action on PP1. Another level of regulation is present through the phos level of these interacting proteins. For example cAmp PKA -> I-1 -> PP1 (see below in Events). G-substrate and dopamine and cAMP-regulated phosphoprotein Mr3200 (DARPP-32) inhibit PP1 (they show sequence overlap with I-1).The localization of PP1 at the PSD is believed to be caused by binding to spinophillin. PP1 acts directly on the GluR1 subunit of AMPA receptors by dephos of their site Ser-845, responsible for the basal receptor currents. The second phos site Ser-831 get phosed by active CaMKII. PP1 (and PP2B) are implicated in the endocytotic internalization of AMPAR during the LTD. (I guess thats translocation from PSD to cytosol) as a prerequist for it is the dephos of Ser-845. Disruption of PP1 binding to PSD target proteins, blocks the induction of LTD, but has no effect on the basal synaptic current mediated by AMPAR and NMDAR. Thus PP1 does not regulate the basal current but is involved in the activity-dependent regulation. Pharmacological agents inhibiting PP1, showed to prevent depotentiation, i.e. reversal of LTP. Calcineurin (PP2B) Inhibitor protein 1 (I-1) Protein kinase A (PKA)
3. Events
Binding of Ca2+ to CaM The affinities for N and C terminal binding sites for Ca2+ are different. The C-terminal intriniscally has a 6-fold higher affinity for Ca2+ than the N-terminal sites (Shifman and Kennedy 2006, PMID: ). However the on-rate for Ca2+ binding to the N-terminal is 10 fold faster. In the absence of Ca2+ CaM assumes a closed conformation not able to interaction with most targets. Binding of Ca2+ causes a conformational change, where the hydrophopic residues are exposed.
Translocation of CaMKII to the PSD Activiation of CaMKII through CaM binding
Phosphatase activity of PP1 on CaMKII Phosphatase cascade PP2B-I1-PP1 4. Perturbations experiments
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