nature neuroscience • volume 3 no 9 • september 2000 881 A broad range of pharmacological, biochemical and genetic stud- ies establish that the serine/threonine kinase CaMKII is a key reg- ulator of long-term potentiation as well as of other forms of neuronal plasticity. CaMKII was initially discovered as one of the most abundant neuronal serine/threonine kinases with an activ- ity that is induced by binding calcium–calmodulin (Ca 2+ –CaM) 1 . The kinase is enriched at the postsynaptic density (PSD), a cytoskeletal structure beneath the postsynaptic membrane that contains many structural and signaling proteins. In adult mam- malian central neurons, the predominantly expressed CaMKIIs are the α and β isoforms 1,2 , with lower expression of the γ and δ isoforms. The α isoform of CaMKII is a multimeric enzyme that con- sists of approximately 12 subunits per holoenzyme 3 . After acti- vation by Ca 2+ –CaM, CaMKII undergoes a characteristic trans-subunit autophosphorylation on Thr286. This autophos- phorylation requires CaM binding to two neighboring subunits and renders the kinase partially Ca 2+ –CaM independent 3 . Autophosphorylation also leads to a several-hundredfold increase in Ca 2+ –CaM binding affinity 4 . This partial activity is sustained until Thr286 is dephosphorylated, presumably by phosphatase 1 action. In addition to phosphorylating itself, CaMKII has a wide spectrum of substrates in vitro, including AMPA receptors and NMDA receptors, which are key compo- nents of the postsynaptic membrane. The broad interest in the neuronal role of CaMKII stems in part from the activation- induced autonomous activity of the kinase. This property of the kinase led to the suggestion that autophosphorylation may function as a biochemical ‘memory’ process that can prolong a sufficiently strong but brief calcium signal into a long-lasting change in CaMKII activity. The first direct evidence for a role of CaMKII in synaptic plas- ticity came from electrophysiological studies in hippocampal neurons in which CaMKII was blocked using peptide blockers and pharmacological agents 5 . This role was confirmed by the introduction of constitutively active CaMKII into neurons by purified protein or viral transfection 6,7 . Furthermore, CaMKIIα- deficient mice show impaired hippocampal LTP and behavioral defects in spatial learning and memory 8,9 . A different mouse model, in which a constitutively active CaMKII is expressed in hippocampal and other neurons, shows a shift in the frequency dependence of long-term potentiation and defects in spatial learning 10-12 . A mutant mouse in which the endogenous CaMKII can be activated but not autophosphorylated showed that the autophosphorylation site at Thr286 is necessary for hippocampal LTP and spatial learning 13,14 . Taken together, these experiments strongly supported the functional importance of CaMKIIα and its autophosphorylation in hippocampal LTP and in spatial learn- ing and memory. As biochemical studies show that CaMKII can phosphorylate AMPA and NMDA glutamate receptor subtypes 3 and as signifi- cant amounts of CaMKII can exist at PSDs 1 , it is proposed that CaMKII may exert its function at synapses by regulating postsy- naptic AMPA and/or NMDA receptors. CaMKII also translocates from the cytosol to this postsynaptic region in response to stim- uli that activate NMDA receptors 15 , and biochemical studies sug- gest that this translocation is mediated by the binding of CaMKII to postsynaptic density-localized NMDA receptors 16–18 . Here we used confocal imaging of green fluorescent protein (GFP)–CaMKII in cultured hippocampal neurons to identify the key signaling steps in the translocation cycle of CaMKII. Partic- ularly, we were interested in whether trapping of CaMKII at its postsynaptic sites (target trapping) and facilitation of the translo- cation of CaMKII by previous activity (translocation priming) are potential prolongation mechanisms that could generate altered biochemical ‘memory’ states of the synapse. articles Molecular memory by reversible translocation of calcium/ calmodulin- dependent protein kinase II K. Shen 1 , M. N. Teruel 1,2 , J. H. Connor 3 , S. Shenolikar 3 and T. Meyer 1,2 1 Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA 2 Department of Molecular Pharmacology, 269 Campus Drive, Rm #3215, Stanford University Medical School, Stanford, California 94305, USA 3 Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA Correspondence should be addressed to T.M. (tobias1@stanford.edu) Synaptic plasticity is thought to be a key process for learning, memory and other cognitive functions of the nervous system. The initial events of plasticity require the conversion of brief electrical signals into alterations of the biochemical properties of synapses that last for much longer than the initial stimuli. Here we show that a regulator of synaptic plasticity, calcium/ calmodulin-dependent protein kinase II α (CaM KII), sequentially translocates to postsynaptic sites, undergoes autophosphorylation and gets trapped for several minutes until its dissociation is induced by secondary autophosphoryla- tion and phosphatase 1 action. Once dissociated, CaM KII shows facilitated translocation for several minutes. This suggests that trapping of CaM KII by its targets and priming of CaM KII translocation may function as biochemical memory mechanisms that change the signaling capacity of synapses. © 2000 Nature America Inc. • http://neurosci.nature.com © 2000 Nature America Inc. • http://neurosci.nature.com