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spinqualitylabelsall models Calcium-bound calmodulin Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Calmodulin (CaM) for Calcium-Modulated protein is an important protein that intervenes in a wide range of activities.

CaM is expressed in many cell types and can have different subcellular locations, including the cytoplasm, within organelles, or associated with the plasma or organelle membranes. Indeed, it is a small (16.7 kDa = 148 aa) and highly conserved protein that is necessary in all eukaryotic cells because it represents an essential calcium sensor with troponin C its isoform.

Calcium is an important mineral salt which represents 2% of body weight and 99% in teeth and bones. Calcium levels are finely regulated in the organism because it ediates and controls many biochemical processes, inflammation, metabolism, apoptosis, smooth muscle contraction, intracellular movement, short-term and long-term memory, and the immune response. Calcium ions play a major role in the metabolism and physiology of eukaryotes cells, that's why calmodulin is important for the organism.

Biological functionBiological function

Calmodulin is a receptor protein owing a wide ranging effects. About 30 enzymes and structural proteins have been identified that bind calmodulin with high affinity. The Ca²⁺- calmodulin complex can regulate the intracellular concentration of some second messengers such as cyclic AMP and inositoltriphosphate (IP₃)

StructureStructure

spinqualitylabelsall models Structure of calmodulin (PDB entry 3cln) Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Calmodulin [1] Calmodulin contains four Ca2+ binding sites and the binding of calcium induces a conformational change in calmodulin that can cause the activation of key enzymes such as kinases or phosphatases proteins (especially phosphorylase kinases) which are not necessarily themselves Ca2+-sensitive and allows a large diversity of cellular response. The calmodulin structure has been determined by NMR. This method reveals that calmodulin is a long molecule which looks like a dumbbell because it contains two globular domains (the and the ) linked by a . Each lobe contains a pair of motifs (called EF-hand or calmodulin-fold) that can bind two Ca2+ ions. However those lobes do not have the same properties because the C-lobe has higher Ca2+ affinity than the N-lobe. The two EF-hands are located in the vicinity of each other. Those neighboring sited are very likely to structurally influence each other upon Ca2+ binding to one of them. A superfamily of proteins utilizes the configuration of helix-loop-helix to bind Ca2+ but calmodulin is the only member that subserves so many functions. The affinity of the individual Ca2+ ion binding sites are in the range 10−5-10−6 mol.L−1 and adjacent sites bind Ca2+ with positive cooperativity, so that attachment of the first Ca2+ ion enhances the affinity of its neighbour. This has the effect of making the protein sensitive to small changes in the concentration of Ca2+ within the signaling range. Ca2+-calmodulin itself has no intrinsic catalytic activity. Its action depends on its close association with a target enzyme. The C-terminal domain solution structure is similar while the EF hands of the N-terminal domain are considerably less open. The backbone flexibility within calmodulin is key to its ability to bind a wide range of targets . Up to four calcium ions are bound by calmodulin via its four EF hand motifs. EF hands supply an electronegative environment for ion coordination. After calcium binding, hydrophobic methyl groups from methionine residues become exposed on the protein via conformational change. This presents hydrophobic surfaces, which can in turn bind to Basic Amphiphilic Helices (BAA helices) on the target protein. These helices contain complementary hydrophobic regions. The flexibility of Calmodulin's hinged region allows the molecule to "wrap around" its target. This property allows it to tightly bind to a wide range of different target proteins. .

• Three-dimensional structure of apocalmodulin

In the absence of bound Ca²⁺, the helices of calmodulin pack so that their hydrophobic side chains are not exposed. In this form it is unable to interact with its targets.The C-terminal lobe of each CaM adopts a semi-open conformation that grips the first part of the IQ motif (IQxxxR), whereas the N-terminal lobe adopts a closed conformation that interacts more weakly with the second part of the motif (GxxxR). .

Apocalmodulin (ApoCaM) binds to other proteins and has other specific effects. It is essential role for life. The ApoCaM-binding proteins are a diverse group of at least 15 proteins including enzymes, actin-binding proteins, as well as cytoskeletal and other membrane proteins, including receptors and ion channels. .

• Ca²⁺-bound calmodulin

Calmodulin uses a linear sequence of 12 amino acids to bind Ca²⁺. Binding of Ca²⁺ to the four sites induces a large conformational change causing the terminal helices to expose hydrophobic surfaces and also a long central α-helical segment. Ca²⁺-bound calmodulin binds to its targets with high affinity (K_D≈10⁻⁹ mol.L⁻¹).

• Calmodulin bound to a target peptide

spinqualitylabelsall models Calmodulin bound to a target peptide Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

To form the bound state, the central residues of the link region unwind form their α-helical arrangement to form a hinge that allows the molecule to bend and wrap itself around the target. The N-terminal and C-terminal regions approach each other and by their hydrophobic surfaces bind to it, rather like two hands holding a rope. This encourages the target sequence to adopt an α-helical arrangement so that it occupies the center of a hydrophobic tunnel. The consequence of this interaction is a conformational change in the target, a state that persists only as long as the Ca²⁺ concentration remains high .

When the Ca²⁺ concentration falls, calcium dissociates and calmodulin is quickly released, inactivating the target. However, at least one important target protein is an exception to this rule. This is CaM-kinase II which can retain its active state after it has been activated by calmodulin.

CaMII kinaseCaMII kinase

Calmodulin plays an important role through kinase enzymes such as calcium/calmodulin-dependent kinase II (CaMKII) that is a multifunctional serine/threonine kinase found in many tissues. Activation of CaMKII contributes to synaptic plasticity and regulation of excitory synaptic transmission. The regulatory domain of CaMKII contains an autophosphorylation site, which is essential for its calcium-dependent activation .

CaM kinases have a catalytic N-terminal domain, a regulator domain and a domain of association. Many CaM form an holoenzyme with a dodecameric structure(the catalytics domains are found one's way out side) that permit to phosphorylate the residues between the sub-unit. Without Ca2+/calmodulin, the catalytic domain is self-inhibited by the regulator domain, which contains one sequence of type pseudo-substrate. Many CaM kinases give an homo-oligomeres or hetero-oligomeres. When there is an activation by the Ca2+/calmodulin complex, the actif CaM kinases are autophosphorylated one by one at the level of the threonine 286 residues.

Family membersFamily members

• Calmodulin 1 (CALM1)

Synonyms: CALM, CAM, CAM1

• Calmodulin 2 (CALM2)

Synonyms: CAM2, CAMB

• Calmodulin 3 (CALM3)

Synonyms: CALML2, CAM3, CAMC, CAMIII

• Calmodulin-like 1 (CALML1)
• Calmodulin-like 3 (CALML3)
• Calmodulin-like 4 (CALML4)
• Calmodulin-like 5 (CALML5)
• Calmodulin-like 6 (CALML6)

Mutation of Calmodulin gene and consequencesMutation of Calmodulin gene and consequences

Calmodulin is an essential protein that is encoded in vertebrates by tree different genes (CaMI, CaMII and CaMIII) whereas it is encoded by a single gene in yeast and fungi (CMDI). Consequently the disruption of CMDI in yeast and fungi is lethal. However studies on Saccaromyces cerevisae had shown that vertebrate calmodulin can rescue yeast cells whose calmodulin has been deleted. That experience indicates that vertebrate calmodulin can functionally substitute for yeast calmodulin because Saccaromyces cerevisae calmodulin amino acids sequence is 60% identical to vertebrate Calmodulin.

Calmodulin in laboratoriesCalmodulin in laboratories

Because of widespread interest in calmodulin research many useful reagents are now available commercially. Purified calmodulin, anti-calmodulin, calmodulin-sepharose or -agarose and several calmodulin deficient target enzymes are available from major biochemical suppliers. For laboratories that plan extensive calmodulin research it may be more economically feasible to produce at least some of their reagents by various protocols such as calmodulin bioassay or calmodulin radioimmunoassay.

External ResourcesExternal Resources

• Protein Data Bank file on 2CLP
• Biochimie générale 10ième édition. Jacques-Henri Weil 2005
• Signal Transduction Gomperts.Kramer.Tatham Seconde edition

Proteopedia page contributors and editorProteopedia page contributors and editor

Tonazzini Saphia, Planchenault Charlène