Highly enriched in brain tissue and present throughout the body, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is central to the coordination and execution of Ca(2+) signal transduction. The substrates phosphorylated by CaMKII are implicated in homeostatic regulation of the cell, as well as in activity-dependent changes in neuronal function that appear to underlie complex cognitive and behavioral responses, including learning and memory. The architecture of CaMKII holoenzymes is unique in nature. The kinase functional domains (12 per holoenzyme) are attached by stalklike appendages to a gear-shaped core, grouped into two clusters of six. Each subunit contains a catalytic, an autoregulatory, and an association domain. Ca(2+)/calmodulin (CaM) binding disinhibits the autoregulatory domain, allowing autophosphorylation and complex changes in the enzyme's sensitivity to Ca(2+)/CaM, including the generation of Ca(2+)/CaM-independent activity, CaM trapping, and CaM capping. These processes confer a type of molecular memory to the autoregulation and activity of CaMKII. Its function is intimately shaped by its multimeric structure, autoregulation, isozymic type, and subcellular localization; these features and processes are discussed as they relate to known and potential cellular functions of this multifunctional protein kinase.

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.

Chronic pain hypersensitivity depends on N-type voltage-gated calcium channels (CaV2.2). However, the use of CaV2.2 blockers in pain therapeutics is limited by side effects that result from inhibited physiological functions of these channels. Here we report suppression of both inflammatory and neuropathic hypersensitivity by inhibiting the binding of the axonal collapsin response mediator protein 2 (CRMP-2) to CaV2.2, thus reducing channel function. A 15-amino acid peptide of CRMP-2 fused to the transduction domain of HIV TAT protein (TAT-CBD3) decreases neurotransmitter release from nociceptive dorsal root ganglion neurons, reduces meningeal blood flow, reduces nocifensive behavior induced by subcutaneous formalin injection or following corneal capsaicin application, and reverses neuropathic hypersensitivity produced by the antiretroviral drug 2’,3’-dideoxycytidine. TAT-CBD3 was mildly anxiolytic but innocuous on sensorimotor and cognitive functions and despair. By preventing CRMP-2-mediated enhancement of CaV2.2 function, TAT-CBD3 alleviates inflammatory and neuropathic hypersensitivity, an approach that may prove useful in managing clinical pain.

concentrations, likely attained in quiescent cells. Two stretches of amino acids were found to support the tethering and to contain putative CaM-binding sequences close to or overlapping residues previously shown to affect CDI and Ca 2؉ -independent inactivation. Synthetic peptides containing these sequences displayed differences in CaM-binding properties, both in affinity and Ca 2؉ dependence, leading us to propose a novel mechanism for CDI. In contrast to a traditional disinhibitory scenario, we suggest that apoCaM is tethered at two sites and signals actively to slow inactivation. When the C-terminal lobe of CaM binds to the nearby CaM effector sequence (IQ motif), the braking effect is relieved, and CDI is accelerated.The voltage-gated L-type Ca 2ϩ channel is unique among ion channels in displaying two gating properties that are regulated by the ion that permeates the channel, calcium-dependent inactivation (CDI) 1 and calcium-dependent facilitation (CDF). These feedback mechanisms are of critical importance for regulation of the electromechanical activity of the heart and other essential physiological processes. CDI helps determine the length of the cardiac action potential plateau (1) and CDF contributes to the positive force-frequency relationship of the cardiac contraction (2).Several lines of evidence from recent work suggest that the calcium sensor mediating both of these processes may be the calcium-binding protein calmodulin (CaM). We (3) and others (4, 5) have shown that there is a Ca 2ϩ -dependent CaM-binding sequence ("IQ motif") in the cytoplasmic C-terminal tail of the pore-forming ␣ 1C subunit of the channel, within a region previously shown to confer Ca 2ϩ sensitivity (6). We have also shown that those mutations within the IQ motif that render the channel subunit unable to bind CaM also disrupt CDI (3, 7), suggesting that the IQ motif serves as the effector region for CDI. Furthermore, we (3) and others (4) have shown CDI can be blocked in a dominant negative fashion by those CaM mutants that lack Ca 2ϩ binding in their C-terminal EF-hand domains.Several important questions remain unanswered about how Ca 2ϩ and CaM might regulate L-type Ca 2ϩ channel inactivation. The first question concerns how CaM may be tethered to the L-type channel (8). There are multiple reasons for thinking that there must be a binding site that tethers the Ca 2ϩ sensor in the channel's resting state, keeping it poised for signaling as soon as Ca 2ϩ entry begins. Without tethering it would be difficult to explain the rapid development of CDI, beginning within milliseconds after L-type channel opening is initiated by depolarization (5). Tethering would also explain the dominant negative inhibitory action of mutant CaM molecules inasmuch as their binding to the tethering site would preclude binding of wild-type CaM (3, 4, 7). Recent studies have identified sequences in a cytoplasmic domain of the ␣ 1C subunit that display significant affinity for CaM even at low Ca 2ϩ concentrations (9 -11), but these do not appear to bin...

Ca2+-dependent facilitation (CDF) of voltage-gated calcium current is a powerful mechanism for up-regulation of Ca2+ influx during repeated membrane depolarization. CDF of L-type Ca2+ channels (Cav1.2) contributes to the positive force–frequency effect in the heart and is believed to involve the activation of Ca2+/calmodulin-dependent kinase II (CaMKII). How CaMKII is activated and what its substrates are have not yet been determined. We show that the pore-forming subunit α1C (Cavα1.2) is a CaMKII substrate and that CaMKII interaction with the COOH terminus of α1C is essential for CDF of L-type channels. Ca2+ influx triggers distinct features of CaMKII targeting and activity. After Ca2+-induced targeting to α1C, CaMKII becomes tightly tethered to the channel, even after calcium returns to normal levels. In contrast, activity of the tethered CaMKII remains fully Ca2+/CaM dependent, explaining its ability to operate as a calcium spike frequency detector. These findings clarify the molecular basis of CDF and demonstrate a novel enzymatic mechanism by which ion channel gating can be modulated by activity.