Target enzyme recognition by calmodulin: 2.4 A structure of a calmodulin-peptide complex

Atomic resolution experimental phase information reveals extensive disorder and bound 2-methyl-2,4-pentanediol in Ca(2+)-calmodulin

Calmodulin (CaM) is the primary calcium signaling protein in eukaryotes and has been extensively studied using various biophysical techniques. Prior crystal structures have noted the presence of ambiguous electron density in both hydrophobic binding pockets of Ca(2+)-CaM, but no assignment of these features has been made. In addition, Ca(2+)-CaM samples many conformational substates in the crystal and accurately modeling the full range of this functionally important disorder is challenging. In order to characterize these features in a minimally biased manner, a 1.0 Å resolution single-wavelength anomalous diffraction data set was measured for selenomethionine-substituted Ca(2+)-CaM. Density-modified electron-density maps enabled the accurate assignment of Ca(2+)-CaM main-chain and side-chain disorder. These experimental maps also substantiate complex disorder models that were automatically built using low-contour features of model-phased electron density. Furthermore, experimental electron-density maps reveal that 2-methyl-2,4-pentanediol (MPD) is present in the C-terminal domain, mediates a lattice contact between N-terminal domains and may occupy the N-terminal binding pocket. The majority of the crystal structures of target-free Ca(2+)-CaM have been derived from crystals grown using MPD as a precipitant, and thus MPD is likely to be bound in functionally critical regions of Ca(2+)-CaM in most of these structures. The adventitious binding of MPD helps to explain differences between the Ca(2+)-CaM crystal and solution structures and is likely to favor more open conformations of the EF-hands in the crystal.

Application of Circular Dichroism Spectroscopy to the Analysis of the Interaction Between the Estrogen Receptor Alpha and Coactivators: The Case of Calmodulin

The estrogen receptor α ligand-binding domain (ERα-LBD) binds the natural hormone 17β-estradiol (E2) to induce transcription and cell proliferation. This process occurs with the contribution of protein and peptide partners (also called coactivators) that can modulate the structure of ERα, and therefore its specificity of action. As with most transcription factors, ERα exhibits a high content of α helix, making it difficult to routinely run spectroscopic studies capable of deciphering the secondary structure of the different partners under binding conditions. Ca(2+)-calmodulin, a protein also highly structured in α-helix, is a key coactivator for ERα activity. Here, we show how circular dichroism can be used to study the interaction of ERα with Ca(2+)-calmodulin. Our approach allows the determination not only of the conformational changes induced upon complex formation but also the dissociation constant (K d) of this interaction.

Human adenosine A2A receptor binds calmodulin with high affinity in a calcium-dependent manner

Understanding how ligands bind to G-protein-coupled receptors and how binding changes receptor structure to affect signaling is critical for developing a complete picture of the signal transduction process. The adenosine A2A receptor (A2AR) is a particularly interesting example, as it has an exceptionally long intracellular carboxyl terminus, which is predicted to be mainly disordered. Experimental data on the structure of the A2AR C-terminus is lacking, because published structures of A2AR do not include the C-terminus. Calmodulin has been reported to bind to the A2AR C-terminus, with a possible binding site on helix 8, next to the membrane. The biological meaning of the interaction as well as its calcium dependence, thermodynamic parameters, and organization of the proteins in the complex are unclear. Here, we characterized the structure of the A2AR C-terminus and the A2AR C-terminus-calmodulin complex using different biophysical methods, including native gel and analytical gel filtration, isothermal titration calorimetry, NMR spectroscopy, and small-angle X-ray scattering. We found that the C-terminus is disordered and flexible, and it binds with high affinity (Kd = 98 nM) to calmodulin without major conformational changes in the domain. Calmodulin binds to helix 8 of the A2AR in a calcium-dependent manner that can displace binding of A2AR to lipid vesicles. We also predicted and classified putative calmodulin-binding sites in a larger group of G-protein-coupled receptors.

Identification and Characterization of the Interaction Site between cFLIPL and Calmodulin

Overexpression of the cellular FLICE-like inhibitory protein (cFLIP) has been reported in a number of tumor types. As an inactive procaspase-8 homologue, cFLIP is recruited to the intracellular assembly known as the Death Inducing Signaling Complex (DISC) where it inhibits apoptosis, leading to cancer cell proliferation. Here we characterize the molecular details of the interaction between cFLIP_L and calmodulin, a ubiquitous calcium sensing protein. By expressing the individual domains of cFLIP_L, we demonstrate that the interaction with calmodulin is mediated by the N-terminal death effector domain (DED1) of cFLIP_L. Additionally, we mapped the interaction to a specific region of the C-terminus of DED1, referred to as DED1 R4. By designing DED1/DED2 chimeric constructs in which the homologous R4 regions of the two domains were swapped, calmodulin binding properties were transferred to DED2 and removed from DED1. Furthermore, we show that the isolated DED1 R4 peptide binds to calmodulin and solve the structure of the peptide-protein complex using NMR and computational refinement. Finally, we demonstrate an interaction between cFLIP_L and calmodulin in cancer cell lysates. In summary, our data implicate calmodulin as a potential player in DISC-mediated apoptosis and provide evidence for a specific interaction with the DED1 of cFLIP_L.

Calmodulin and STIM proteins: Two major calcium sensors in the cytoplasm and endoplasmic reticulum

The calcium (Ca(2+)) ion is a universal signalling messenger which plays vital physiological roles in all eukaryotes. To decode highly regulated intracellular Ca(2+) signals, cells have evolved a number of sensor proteins that are ideally adapted to respond to a specific range of Ca(2+) levels. Among many such proteins, calmodulin (CaM) is a multi-functional cytoplasmic Ca(2+) sensor with a remarkable ability to interact with and regulate a plethora of structurally diverse target proteins. CaM achieves this 'multi-talented' functionality through two EF-hand domains, each with an independent capacity to bind targets, and an adaptable flexible linker. By contrast, stromal interaction molecule-1 and -2 (STIMs) have evolved for a specific role in endoplasmic reticulum (ER) Ca(2+) sensing using EF-hand machinery analogous to CaM; however, whereas CaM structurally adjusts to dissimilar binding partners, STIMs use the EF-hand machinery to self-regulate the stability of the Ca(2+) sensing domain. The molecular mechanisms underlying the Ca(2+)-dependent signal transduction by CaM and STIMs have revealed a remarkable repertoire of actions and underscore the flexibility of nature in molecular evolution and adaption to discrete Ca(2+) levels. Recent genomic sequencing efforts have uncovered a number of disease-associated mutations in both CaM and STIM1. This article aims to highlight the most recent key structural and functional findings in the CaM and STIM fields, and discusses how these two Ca(2+) sensor proteins execute their biological functions.

Practical Aspects of Paramagnetic Relaxation Enhancement in Biological Macromolecules

In this brief review, we summarize various aspects of NMR paramagnetic relaxation enhancement (PRE). We discuss the types of spin labels used in NMR studies, describe the relevant theory used to accurately calculate PREs from coordinates, including how to take into account the fact that paramagnetic labels tend to be highly mobile and sample a wide range of conformational space, and outline methods to refine structures or ensembles of structures directly against PRE data using simulated annealing. Finally, we show how the PRE can be used to detect, characterize, and visualize sparsely populated states of proteins and their complexes that are invisible to all other biophysical techniques.

Characterization and identification of calmodulin and calmodulin binding proteins in hemocyte of the black tiger shrimp (Penaeus monodon)

Calmodulin (CaM), a ubiquitous intracellular calcium (Ca(2+)) sensor in all eukaryotic cells, is one of the well-known signaling proteins. Previously, CaM gene has shown a high transcriptional level in hemocyte of the pathogen infected shrimp, suggesting that shrimp CaM does not only regulate Ca(2+) metabolism, but is also involved in immune response cascade. In the present study, the CaM gene of shrimp Penaeus monodon was identified and the recombinant P.monodon CaM (rPmCaM) was produced and biochemically characterized. The identification of CaM-binding proteins was also performed. The PmCaM cDNA consisted of an open reading frame of 447 bp encoding for 149 amino acid residues with a calculated mass of 16,810 Da and an isoelectric point of 4.09. Tissue distribution showed that the PmCaM transcript was expressed in all examined tissues. The results of gel mobility shift assay, circular dichroism spectroscopy and fluorescence spectroscopy all confirmed that the conformational changes of the rPmCaM were observed after the calcium binding. According to the gene silencing of PmCaM transcript levels, the shrimp's susceptibility to pathogenic Vibrio harveyi infection increased in comparison with that of the control groups. Protein pull-down assay and LC-MS/MS analysis were performed to identify rPmCaM-binding proteins involved in shrimp immune responses and transglutaminase, elongation factor 1-alpha, elongation factor 2 and actin were found. However, by computational analysis, only the first three proteins contained CaM-binding domain. These findings suggested that PmCaM may play an important role in regulation of shrimp immune system.

The cytosolic tail of the tumor marker protein Trop2--a structural switch triggered by phosphorylation

Trop2 is a transmembrane signaling glycoprotein upregulated in stem and carcinoma cells. Proliferation-enhancing signaling involves regulated intramembrane proteolytic release of a short cytoplasmic fragment, which is later engaged in a cytosolic signaling complex. We propose that Trop2 function is modulated by phosphorylation of a specific serine residue within this cytosolic region (Ser303), and by proximity effects exerted on the cytosolic tail by Trop2 dimerization. Structural characterization of both the transmembrane (Trop2TM) and cytosolic regions (Trop2IC) support this hypothesis, and shows that the central region of Trop2IC forms an α-helix. Comparison of NMR structures of non-phosphorylated and phosphorylated forms suggest that phosphorylation of Trop2IC triggers salt bridge reshuffling, resulting in significant conformational changes including ordering of the C-terminal tail. In addition, we demonstrate that the cytosolic regions of two Trop2 subunits can be brought into close proximity via transmembrane part dimerization. Finally, we show that Ser303-phosphorylation significantly affects the structure and accessibility of functionally important regions of the cytosolic tail. These observed structural features of Trop2 at the membrane-cytosol interface could be important for regulation of Trop2 signaling activity.

Parvulin 17-catalyzed Tubulin Polymerization Is Regulated by Calmodulin in a Calcium-dependent Manner

Recently we have shown that the peptidyl-prolyl cis/trans isomerase parvulin 17 (Par17) interacts with tubulin in a GTP-dependent manner, thereby promoting the formation of microtubules. Microtubule assembly is regulated by Ca(2+)-loaded calmodulin (Ca(2+)/CaM) both in the intact cell and under in vitro conditions via direct interaction with microtubule-associated proteins. Here we provide the first evidence that Ca(2+)/CaM interacts also with Par17 in a physiologically relevant way, thus preventing Par17-promoted microtubule assembly. In contrast, parvulin 14 (Par14), which lacks only the first 25 N-terminal residues of the Par17 sequence, does not interact with Ca(2+)/CaM, indicating that this interaction is exclusive for Par17. Pulldown experiments and chemical shift perturbation analysis with (15)N-labeled Par17 furthermore confirmed that calmodulin (CaM) interacts in a Ca(2+)-dependent manner with the Par17 N terminus. The reverse experiment with (15)N-labeled Ca(2+)/CaM demonstrated that the N-terminal Par17 segment binds to both CaM lobes simultaneously, indicating that Ca(2+)/CaM undergoes a conformational change to form a binding channel between its two lobes, apparently similar to the structure of the CaM-smMLCK(796-815) complex. In vitro tubulin polymerization assays furthermore showed that Ca(2+)/CaM completely suppresses Par17-promoted microtubule assembly. The results imply that Ca(2+)/CaM binding to the N-terminal segment of Par17 causes steric hindrance of the Par17 active site, thus interfering with the Par17/tubulin interaction. This Ca(2+)/CaM-mediated control of Par17-assisted microtubule assembly may provide a mechanism that couples Ca(2+) signaling with microtubule function.

The exchanged EF-hands in calmodulin and troponin C chimeras impair the Ca²⁺-induced hydrophobicity and alter the interaction with Orai1: a spectroscopic, thermodynamic and kinetic study

Background

Calmodulin (CaM) plays an important role in Ca²⁺-dependent signal transduction. Ca²⁺ binding to CaM triggers a conformational change, forming a hydrophobic patch that is important for target protein recognition. CaM regulates a Ca²⁺-dependent inactivation process in store-operated Ca²⁺ entry, by interacting Orai1. To understand the relationship between Ca²⁺-induced hydrophobicity and CaM/Orai interaction, chimera proteins constructed by exchanging EF-hands of CaM with those of Troponin C (TnC) are used as an informative probe to better understand the functionality of each EF-hand.

Results

ANS was used to assess the context of the induced hydrophobic surface on CaM and chimeras upon Ca²⁺ binding. The exchanged EF-hands from TnC to CaM resulted in reduced hydrophobicity compared with wild-type CaM. ANS lifetime measurements indicated that there are two types of ANS molecules with rather distinct fluorescence lifetimes, each specifically corresponding to one lobe of CaM or chimeras. Thermodynamic studies indicated the interaction between CaM and a 24-residue peptide corresponding to the CaM-binding domain of Orail1 (Orai-CMBD) is a 1:2 CaM/Orai-CMBD binding, in which each peptide binding yields a similar enthalpy change (ΔH = −5.02 ± 0.13 kcal/mol) and binding affinity (K_a = 8.92 ± 1.03 × 10⁵ M⁻¹). With the exchanged EF1 and EF2, the resulting chimeras noted as CaM(1TnC) and CaM(2TnC), displayed a two sequential binding mode with a one-order weaker binding affinity and lower ΔH than that of CaM, while CaM(3TnC) and CaM(4TnC) had similar binding thermodynamics as CaM. The dissociation rate constant for CaM/Orai-CMBD was determined to be 1.41 ± 0.08 s⁻¹ by rapid kinetics. Stern-Volmer plots of Orai-CMBD Trp76 indicated that the residue is located in a very hydrophobic environment but becomes more solvent accessible when EF1 and EF2 were exchanged.

Conclusions

Using ANS dye to assess induced hydrophobicity showed that exchanging EFs for all Ca²⁺-bound chimeras impaired ANS fluorescence and/or binding affinity, consistent with general concepts about the inadequacy of hydrophobic exposure for chimeras. However, such ANS responses exhibited no correlation with the ability to interact with Orai-CMBD. Here, the model of 1:2 binding stoichiometry of CaM/Orai-CMBD established in solution supports the already published crystal structure.

NMR studies of dynamic biomolecular conformational ensembles

Multidimensional heteronuclear NMR approaches can provide nearly complete sequential signal assignments of isotopically enriched biomolecules. The availability of assignments together with measurements of spin relaxation rates, residual spin interactions, J-couplings and chemical shifts provides information at atomic resolution about internal dynamics on timescales ranging from ps to ms, both in solution and in the solid state. However, due to the complexity of biomolecules, it is not possible to extract a unique atomic-resolution description of biomolecular motions even from extensive NMR data when many conformations are sampled on multiple timescales. For this reason, powerful computational approaches are increasingly applied to large NMR data sets to elucidate conformational ensembles sampled by biomolecules. In the past decade, considerable attention has been directed at an important class of biomolecules that function by binding to a wide variety of target molecules. Questions of current interest are: "Does the free biomolecule sample a conformational ensemble that encompasses the conformations found when it binds to various targets; and if so, on what time scale is the ensemble sampled?" This article reviews recent efforts to answer these questions, with a focus on comparing ensembles obtained for the same biomolecules by different investigators. A detailed comparison of results obtained is provided for three biomolecules: ubiquitin, calmodulin and the HIV-1 trans-activation response RNA.

Visualizing transient dark states by NMR spectroscopy

Myriad biological processes proceed through states that defy characterization by conventional atomic-resolution structural biological methods. The invisibility of these 'dark' states can arise from their transient nature, low equilibrium population, large molecular weight, and/or heterogeneity. Although they are invisible, these dark states underlie a range of processes, acting as encounter complexes between proteins and as intermediates in protein folding and aggregation. New methods have made these states accessible to high-resolution analysis by nuclear magnetic resonance (NMR) spectroscopy, as long as the dark state is in dynamic equilibrium with an NMR-visible species. These methods - paramagnetic NMR, relaxation dispersion, saturation transfer, lifetime line broadening, and hydrogen exchange - allow the exploration of otherwise invisible states in exchange with a visible species over a range of timescales, each taking advantage of some unique property of the dark state to amplify its effect on a particular NMR observable. In this review, we introduce these methods and explore two specific techniques - paramagnetic relaxation enhancement and dark state exchange saturation transfer - in greater detail.

Protein intrinsic disorder in Arabidopsis NAC transcription factors: transcriptional activation by ANAC013 and ANAC046 and their interactions with RCD1

The regulatory domains of NAC [no apical meristem, ATAF (Arabidopsis transcription activation factor), cup-shaped cotyledon] transcription factors (TFs) are mostly disordered. The single molecular recognition feature (MoRF) in ANAC046 (Arabidopsis NAC domain containing protein 46) is a functional hot spot mediating interactions with RCD1 (radical-induced cell death 1), a stress-associated hub which exploits disorder and different mechanisms for interactions.

Molecular modeling of peptides

This article presents a review of the field of molecular modeling of peptides. The main focus is on atomistic modeling with molecular mechanics potentials. The description of peptide conformations and solvation through potentials is discussed. Several important computer simulation methods are briefly introduced, including molecular dynamics, accelerated sampling approaches such as replica-exchange and metadynamics, free energy simulations and kinetic network models like Milestoning. Examples of recent applications for predictions of structure, kinetics, and interactions of peptides with complex environments are described. The reliability of current simulation methods is analyzed by comparison of computational predictions obtained using different models with each other and with experimental data. A brief discussion of coarse-grained modeling and future directions is also presented.

Structural analysis of calmodulin binding by nNOS inhibitory amphibian peptides

Calmodulin (CaM) is a ubiquitous protein in nature and plays a regulatory role in numerous biological processes, including the upregulation of nitric oxide (NO) synthesis in vivo. Several peptides that prevent NO production by interacting with CaM have been isolated in the cutaneous secretions of Australian amphibians, and are thought to serve as a defense mechanism against predators. In this work, we probe the mechanism by which three of these peptides, namely, caerin 1.8, dahlein 5.6, and a synthetic modification of citropin 1.1, interact with CaM to inhibit NO signaling. Isothermal titration calorimetry was used to determine thermodynamic parameters of the binding interactions and revealed that all the peptides bind to CaM in a similar fashion, with the peptide encapsulated between the two lobes of CaM. Ion mobility-mass spectrometry was used to investigate the changes in collision cross section that occur as a result of complexation, providing additional evidence for this binding mode. Finally, nuclear magnetic resonance spectroscopy was used to track chemical shift changes upon binding. The results obtained confirm that these complexes adopt canonical collapsed structures and demonstrate the strength of the interaction between the peptides and CaM. An understanding of these molecular recognition events provides insights into the underlying mechanism of the amphibian host-defense system.

Structural plasticity of calmodulin on the surface of CaF2 nanoparticles preserves its biological function

Nanoparticles are increasingly used in biomedical applications and are especially attractive as biocompatible and biodegradable protein delivery systems. Herein, the interaction between biocompatible 25 nm CaF2 nanoparticles and the ubiquitous calcium sensor calmodulin has been investigated in order to assess the potential of these particles to serve as suitable surface protein carriers. Calmodulin is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells by changing its conformation in a calcium-dependent manner. Isothermal titration calorimetry and circular dichroism studies have shown that the interaction between calmodulin and CaF2 nanoparticles occurs with physiologically relevant affinity and that the binding process is fully reversible, occurring without significant alterations in protein secondary and tertiary structures. Experiments performed with a mutant form of calmodulin having an impaired Ca(2+)-binding ability in the C-terminal lobe suggest that the EF-hand Ca(2+)-binding motifs are directly involved in the binding of calmodulin to the CaF2 matrix. The residual capability of nanoparticle-bound calmodulin to function as a calcium sensor protein, binding to and altering the activity of a target protein, was successfully probed by biochemical assays. Even if efficiently carried by CaF2 nanoparticles, calmodulin may dissociate, thus retaining the ability to bind the peptide encompassing the putative C-terminal calmodulin-binding domain of glutamate decarboxylase and activate the enzyme. We conclude that the high flexibility and structural plasticity of calmodulin are responsible for the preservation of its function when bound in high amounts to a nanoparticle surface.

Calcium-induced conformational changes of the regulatory domain of human mitochondrial aspartate/glutamate carriers

The transport activity of human mitochondrial aspartate/glutamate carriers is central to the malate-aspartate shuttle, urea cycle, gluconeogenesis and myelin synthesis. They have a unique three-domain structure, comprising a calcium-regulated N-terminal domain with eight EF-hands, a mitochondrial carrier domain, and a C-terminal domain. Here we present the calcium-bound and calcium-free structures of the N- and C-terminal domains, elucidating the mechanism of calcium regulation. Unexpectedly, EF-hands 4-8 are involved in dimerization of the carrier and form a static unit, whereas EF-hands 1-3 form a calcium-responsive mobile unit. On calcium binding, an amphipathic helix of the C-terminal domain binds to the N-terminal domain, opening a vestibule. In the absence of calcium, the mobile unit closes the vestibule. Opening and closing of the vestibule might regulate access of substrates to the carrier domain, which is involved in their transport. These structures provide a framework for understanding cases of the mitochondrial disease citrin deficiency.

Evaluation of the calmodulin-SOX9 interaction by "magnetic fishing" coupled to mass spectrometry

Disruption of calmodulin (CaM)-based protein interactions has been touted as a potential means for modulating several disease pathways. Among these is SOX9, which is a DNA binding protein that is involved in chrondrocyte differentiation and regulation of the hormones that control sexual development. In this work, we employed a "magnetic fishing"/mass spectrometry assay in conjunction with intrinsic fluorescence to examine the interaction of CaM with the CaM-binding domain of SOX9 (SOX-CAL), and to assess the modulation of this interaction by known anti-CaM compounds. Our data show that there is a high affinity interaction between CaM and SOX-CAL (27±9 nM), and that SOX-CAL bound to the same location as the well-known CaM antagonist melittin; unexpectedly, we also found that addition of CaM-binding small molecules initially produced increased SOX-CAL binding, indicative of binding to both the well-known high-affinity CaM binding site and a second, lower-affinity binding site.

A new calmodulin-binding motif for inositol 1,4,5-trisphosphate 3-kinase regulation

Inositol 1,4,5-trisphosphate 3-kinase regulation by Ca2+/calmodulin involves multiple protein–protein interactions, which form a highly hydrophobic interface and defines a new calmodulin-binding motif. The structural data support that calmodulin binds to an autoinhibitory segment facilitating the kinase activity.

Structural analyses of Ca²⁺/CaM interaction with NaV channel C-termini reveal mechanisms of calcium-dependent regulation

Ca²⁺ regulates voltage-gated Na⁺ (Na_V) channels and perturbed Ca²⁺ regulation of Na_V function is associated with epilepsy syndromes, autism, and cardiac arrhythmias. Understanding the disease mechanisms, however, has been hindered by a lack of structural information and competing models for how Ca²⁺ affects Na_V channel function. Here, we report the crystal structures of two ternary complexes of a human Na_V cytosolic C-terminal domain (CTD), a fibroblast growth factor homologous factor, and Ca²⁺/calmodulin (Ca²⁺/CaM). These structures rule out direct binding of Ca²⁺ to the Na_V CTD, and uncover new contacts between CaM and the Na_V CTD. Probing these new contacts with biochemical and functional experiments allows us to propose a mechanism by which Ca²⁺ could regulate Na_V channels. Further, our model provides hints towards understanding the molecular basis of the neurologic disorders and cardiac arrhythmias caused by Na_V channel mutations.

Seeing the invisible by paramagnetic and diamagnetic NMR

Sparsely populated transient states of proteins and their complexes play an important role in many biological processes including protein-protein and protein-DNA recognition, allostery, conformational selection, induced fit and self-assembly. These states are difficult to study as their low population and transient nature makes them effectively invisible to conventional structural and biophysical techniques. In the present article, I summarize recent NMR developments in our laboratory, including the use of paramagnetic relaxation enhancement, lifetime line broadening and dark-state exchange saturation transfer spectroscopy, that have permitted such sparsely populated states to be detected, characterized and, in some instances, visualized. I illustrate the application of these methods to the elucidation of mechanisms whereby transcription factors locate their specific target sites within an overwhelming sea of non-specific DNA, to the characterization of encounter complexes in protein-protein recognition, to large-scale interdomain motions involved in ligand binding, and to the interaction of monomeric amyloid β-peptide with the surface of amyloid protofibrils and the internal cavity surface of the chaperonin GroEL.

Stoichiometry of the calcineurin regulatory domain-calmodulin complex

Calcineurin is an essential serine/threonine phosphatase that plays vital roles in neuronal development and function, heart growth, and immune system activation. Calcineurin is unique in that it is the only phosphatase known to be activated by calmodulin in response to increasing intracellular calcium concentrations. Calcium-loaded calmodulin binds to the regulatory domain of calcineurin, resulting in a conformational change that removes an autoinhibitory domain from the active site of the phosphatase. We have determined a 1.95 Å crystal structure of calmodulin bound to a peptide corresponding to its binding region from calcineurin. In contrast to previous structures of this complex, our structure has a stoichiometry of 1:1 and has the canonical collapsed, wraparound conformation observed for many calmodulin-substrate complexes. In addition, we have used size-exclusion chromatography and time-resolved fluorescence to probe the stoichiometry of binding of calmodulin to a construct corresponding to almost the entire regulatory domain from calcineurin, again finding a 1:1 complex. Taken in sum, our data strongly suggest that a single calmodulin protein is necessary and sufficient to bind to and activate each calcineurin enzyme.

Specificity and affinity quantification of flexible recognition from underlying energy landscape topography

Flexibility in biomolecular recognition is essential and critical for many cellular activities. Flexible recognition often leads to moderate affinity but high specificity, in contradiction with the conventional wisdom that high affinity and high specificity are coupled. Furthermore, quantitative understanding of the role of flexibility in biomolecular recognition is still challenging. Here, we meet the challenge by quantifying the intrinsic biomolecular recognition energy landscapes with and without flexibility through the underlying density of states. We quantified the thermodynamic intrinsic specificity by the topography of the intrinsic binding energy landscape and the kinetic specificity by association rate. We found that the thermodynamic and kinetic specificity are strongly correlated. Furthermore, we found that flexibility decreases binding affinity on one hand, but increases binding specificity on the other hand, and the decreasing or increasing proportion of affinity and specificity are strongly correlated with the degree of flexibility. This shows more (less) flexibility leads to weaker (stronger) coupling between affinity and specificity. Our work provides a theoretical foundation and quantitative explanation of the previous qualitative studies on the relationship among flexibility, affinity and specificity. In addition, we found that the folding energy landscapes are more funneled with binding, indicating that binding helps folding during the recognition. Finally, we demonstrated that the whole binding-folding energy landscapes can be integrated by the rigid binding and isolated folding energy landscapes under weak flexibility. Our results provide a novel way to quantify the affinity and specificity in flexible biomolecular recognition.

Flexibility in biomolecular recognition is crucial for the function. Flexibility often leads to moderate binding affinity but high binding specificity, challenging the conventional wisdom that high specificity is guaranteed by high affinity. Currently, understanding of the relationship between affinity and specificity in flexible biomolecular recognition is still obscure, even in a qualitative way. By exploring the intrinsic biomolecular recognition energy landscapes, we provided a novel way to quantify the thermodynamic intrinsic specificity by energy landscape topography and kinetic specificity by association rate. We show quantitatively that flexibility decreases binding affinity while increases binding specificity, and the relative changes in affinity and specificity are strongly correlated with the degree of flexibility. Our results show that more (less) flexibility leads to weaker (stronger) coupling between affinity and specificity. Importantly, we demonstrated that flexibility modulates affinity and specificity through the underlying energy landscape. Our study establishes the quantitative relationship among flexibility, affinity and specificity, bridging the gap between theory and experiments.

Computational prediction of hinge axes in proteins

BACKGROUND

A protein's function is determined by the wide range of motions exhibited by its 3D structure. However, current experimental techniques are not able to reliably provide the level of detail required for elucidating the exact mechanisms of protein motion essential for effective drug screening and design. Computational tools are instrumental in the study of the underlying structure-function relationship. We focus on a special type of proteins called "hinge proteins" which exhibit a motion that can be interpreted as a rotation of one domain relative to another.

RESULTS

This work proposes a computational approach that uses the geometric structure of a single conformation to predict the feasible motions of the protein and is founded in recent work from rigidity theory, an area of mathematics that studies flexibility properties of general structures. Given a single conformational state, our analysis predicts a relative axis of motion between two specified domains. We analyze a dataset of 19 structures known to exhibit this hinge-like behavior. For 15, the predicted axis is consistent with a motion to a second, known conformation. We present a detailed case study for three proteins whose dynamics have been well-studied in the literature: calmodulin, the LAO binding protein and the Bence-Jones protein.

CONCLUSIONS

Our results show that incorporating rigidity-theoretic analyses can lead to effective computational methods for understanding hinge motions in macromolecules. This initial investigation is the first step towards a new tool for probing the structure-dynamics relationship in proteins.

The many structural faces of calmodulin: a multitasking molecular jackknife

Calmodulin (CaM) is a highly conserved protein and a crucial calcium sensor in eukaryotes. CaM is a regulator of hundreds of diverse target proteins. A wealth of studies has been carried out on the structure of CaM, both in the unliganded form and in complexes with target proteins and peptides. The outcome of these studies points toward a high propensity to attain various conformational states, depending on the binding partner. The purpose of this review is to provide examples of different conformations of CaM trapped in the crystal state. In addition, comparisons are made to corresponding studies in solution. The different CaM conformations in crystal structures are also compared based on the positions of the metal ions bound to their EF hands, in terms of distances, angles, and pseudo-torsion angles. Possible caveats and artifacts in CaM crystal structures are discussed, as well as the possibilities of trapping biologically relevant CaM conformations in the crystal state.

Structural and thermodynamic characterization of the recognition of the S100-binding peptides TRTK12 and p53 by calmodulin

Calmodulin (CaM) is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells in a calcium-dependent manner. CaM has been proposed to be functionally distinct from the S100 proteins, a related family of eukaryotic calcium-binding proteins. Previously, it was demonstrated that peptides derived from the actin-capping protein, TRTK12, and the tumor-suppressor protein, p53, interact with multiple members of the S100 proteins. To test the specificity of these peptides, they were screened using isothermal titration calorimetry against 16 members of the human S100 protein family, as well as CaM, which served as a negative control. Interestingly, both the TRTK12 and p53 peptides were found to interact with CaM. These interactions were further confirmed by both fluorescence and nuclear magnetic resonance spectroscopies. These peptides have distinct sequences from the known CaM target sequences. The TRTK12 peptide was found to independently interact with both CaM domains and bind with a stoichiometry of 2:1 and dissociations constants Kd,C-term  = 2 ± 1 µM and Kd,N-term  = 14 ± 1 µM. In contrast, the p53 peptide was found to interact only with the C-terminal domain of CaM, Kd,C-term = 2 ± 1 µM, 25°C. Using NMR spectroscopy, the locations of the peptide binding sites were mapped onto the structure of CaM. The binding sites for both peptides were found to overlap with the binding interface for previously identified targets on both domains of CaM. This study demonstrates the plasticity of CaM in target binding and may suggest a possible overlap in target specificity between CaM and the S100 proteins.

Distinguishing unfolding and functional conformational transitions of calmodulin using ultraviolet resonance Raman spectroscopy

Calmodulin (CaM) is a ubiquitous moderator protein for calcium signaling in all eukaryotic cells. This small calcium-binding protein exhibits a broad range of structural transitions, including domain opening and folding-unfolding, that allow it to recognize a wide variety of binding partners in vivo. While the static structures of CaM associated with its various binding activities are fairly well-known, it has been challenging to examine the dynamics of transition between these structures in real-time, due to a lack of suitable spectroscopic probes of CaM structure. In this article, we examine the potential of ultraviolet resonance Raman (UVRR) spectroscopy for clarifying the nature of structural transitions in CaM. We find that the UVRR spectral change (with 229 nm excitation) due to thermal unfolding of CaM is qualitatively different from that associated with opening of the C-terminal domain in response to Ca(2+) binding. This spectral difference is entirely due to differences in tertiary contacts at the interdomain tyrosine residue Tyr138, toward which other spectroscopic methods are not sensitive. We conclude that UVRR is ideally suited to identifying the different types of structural transitions in CaM and other proteins with conformation-sensitive tyrosine residues, opening a path to time-resolved studies of CaM dynamics using Raman spectroscopy.

Structural insights into the mechanism of calmodulin binding to death receptors

The death receptors Fas, p75NTRand DR6 are key components of extrinsically activated apoptosis. Characterization of how they interact with the adaptors is crucial in order to unravel the signalling mechanisms. However, the exact conformation that their intracellular death domain adopts upon binding downstream partners remains unclear. One model suggests that it adopts a typical compact fold, whilst a second model proposed an open conformation. Calmodulin (CaM), a major calcium sensor, has previously been reported to be one of the Fas adaptors that modulate apoptosis. This work reports that CaM also binds directly to the death domains of p75NTRand DR6, indicating that it serves as a common modulator of the death receptors. Two crystal structures of CaM in complexes with the corresponding binding regions of Fas and p75NTRare also reported. Interestingly, the precise CaM-binding sites were mapped to different regions: helix 1 in Fas and helix 5 in p75NTRand DR6. A novel 1–11 motif for CaM binding was observed in p75NTR. Modelling the complexes of CaM with full-length receptors reveals that the opening of the death domains would be essential in order to expose their binding sites for CaM. These results may facilitate understanding of the diverse functional repertoire of death receptors and CaM and provide further insights necessary for the design of potential therapeutic peptide agents.

Kinase activity and calmodulin binding are essential for growth signaling by the phytosulfokine receptor PSKR1

The cell growth-promoting peptide phytosulfokine (PSK) is perceived by leucine-rich repeat (LRR) receptor kinases. To elucidate PSK receptor function we analyzed PSKR1 kinase activity and binding to Ca(2+) sensors and evaluated the contribution of these activities to growth control in planta. Ectopically expressed PSKR1 was capable of auto- and transphosphorylation. Replacement of a conserved lysine within the ATP-binding region by a glutamate resulted in the inhibition of auto- and transphosphorylation kinase activities. Expression of the kinase-inactive PSKR1(K762E) receptor in the pskr null background did not restore root or shoot growth. Instead, the mutant phenotype was enhanced suggesting that the inactive receptor protein exerts growth-inhibitory activity. Bioinformatic analysis predicted a putative calmodulin (CaM)-binding site within PSKR1 kinase subdomain VIa. Bimolecular fluorescence complementation analysis demonstrated that PSKR1 binds to all isoforms of CaM, more weakly to the CaM-like protein CML8 but apparently not to CML9. Mutation of a conserved tryptophan (W831S) within the predicted CaM-binding site strongly reduced CaM binding. Expression of PSKR1(W831S) in the pskr null background resulted in growth inhibition that was similar to that of the kinase-inactive receptor. We conclude that PSK signaling requires Ca(2+) /CaM binding and kinase activity of PSKR1 in planta. We further propose that the inactivated kinase interferes with other growth-promoting signaling pathway(s).

Pivoting between calmodulin lobes triggered by calcium in the Kv7.2/calmodulin complex

Kv7.2 (KCNQ2) is the principal molecular component of the slow voltage gated M-channel, which strongly influences neuronal excitability. Calmodulin (CaM) binds to two intracellular C-terminal segments of Kv7.2 channels, helices A and B, and it is required for exit from the endoplasmic reticulum. However, the molecular mechanisms by which CaM controls channel trafficking are currently unknown. Here we used two complementary approaches to explore the molecular events underlying the association between CaM and Kv7.2 and their regulation by Ca²⁺. First, we performed a fluorometric assay using dansylated calmodulin (D-CaM) to characterize the interaction of its individual lobes to the Kv7.2 CaM binding site (Q2AB). Second, we explored the association of Q2AB with CaM by NMR spectroscopy, using ¹⁵N-labeled CaM as a reporter. The combined data highlight the interdependency of the N- and C-lobes of CaM in the interaction with Q2AB, suggesting that when CaM binds Ca²⁺ the binding interface pivots between the N-lobe whose interactions are dominated by helix B and the C-lobe where the predominant interaction is with helix A. In addition, Ca²⁺ makes CaM binding to Q2AB more difficult and, reciprocally, the channel weakens the association of CaM with Ca²⁺.

Crystallographic snapshots of initial steps in the collapse of the calmodulin central helix

Calmodulin is one of the most well characterized proteins and a widely used model system for calcium binding and large-scale protein conformational changes. Its long central helix is usually cut in half when a target peptide is bound. Here, two new crystal structures of calmodulin are presented, in which conformations possibly representing the first steps of calmodulin conformational collapse have been trapped. The central helix in the two structures is bent in the middle, causing a significant movement of the N- and C-terminal lobes with respect to one another. In both of the bent structures, a nearby polar side chain is inserted into the helical groove, disrupting backbone hydrogen bonding. The structures give an insight into the details of the factors that may be involved in the distortion of the central helix upon ligand peptide binding.

A self-sequestered calmodulin-like Ca²⁺ sensor of mitochondrial SCaMC carrier and its implication to Ca²⁺-dependent ATP-Mg/P(i) transport

The mitochondrial carriers play essential roles in energy metabolism. The short Ca²⁺-binding mitochondrial carrier (SCaMC) transports ATP-Mg in exchange for Pi and is important for activities that depend on adenine nucleotides. SCaMC adopts, in addition to the transmembrane domain (TMD) that transports solutes, an extramembrane N-terminal domain (NTD) that regulates solute transport in a Ca²⁺-dependent manner. Crystal structure of the Ca²⁺-bound NTD reveals a compact architecture in which the functional EF hands are sequestered by an endogenous helical segment. Nuclear magnetic resonance (NMR) relaxation rates indicated that removal of Ca²⁺ from NTD results in a major conformational switch from the rigid and compact Ca²⁺-bound state to the dynamic and loose apo state. Finally, we showed using surface plasmon resonance and NMR titration experiments that free apo NTDs could specifically interact with liposome-incorporated TMD, but that Ca²⁺ binding drastically weakened the interaction. Our results together provide a molecular explanation for Ca²⁺-dependent ATP-Mg flux in mitochondria.

A fluorogenic peptide probe developed by in vitro selection using tRNA carrying a fluorogenic amino acid

A peptide that binds and emits fluorescence in response to conformational change in a target protein was developed by in vitro selection using tRNA carrying a fluorogenic amino acid. This technology could prove to be useful for the development of separation-free immunoassays and bio-imaging analyses.

Asymmetry of calmodulin revealed by peptide binding

The binding of amphiphilic peptides to calmodulin has been studied using fluorescence energy transfer techniques. Calmodulin has no tryptophan residues but possesses two tyrosines (at positions 99 and 138) in the C-terminal half of the protein. The peptides have a single tryptophan which serves as energy acceptor for the protein tyrosine fluorescence. For the binding of mastoparan or peptide Baa17, with a tryptophan at position 3, the observed quenching of the tyrosine fluorescence of over a factor of 2 corresponds to an average tyrosine-trytophan distance of less than 14 Å. These results indicate that the peptides binds preferentially with the tryptophan in the C-terminal half of the protein.

Microscopic insights into the NMR relaxation-based protein conformational entropy meter

Conformational entropy is a potentially important thermodynamic parameter contributing to protein function. Quantitative measures of conformational entropy are necessary for an understanding of its role but have been difficult to obtain. An empirical method that utilizes changes in conformational dynamics as a proxy for changes in conformational entropy has recently been introduced. Here we probe the microscopic origins of the link between conformational dynamics and conformational entropy using molecular dynamics simulations. Simulation of seven proteins gave an excellent correlation with measures of side-chain motion derived from NMR relaxation. The simulations show that the motion of methyl-bearing side chains are sufficiently coupled to that of other side chains to serve as excellent reporters of the overall side-chain conformational entropy. These results tend to validate the use of experimentally accessible measures of methyl motion--the NMR-derived generalized order parameters--as a proxy from which to derive changes in protein conformational entropy.

Transient disorder: Calcineurin as an example

How intrinsically disordered proteins and regions evade degradation by cellular machinery evolved to recognize unfolded and misfolded chains remains a vexing question. One potential means by which this can occur is the disorder is transient in nature. That is, the disorder exists just long enough for it to be bound by a partner biomolecule and fold. A review of 30 y of studies of calmodulin’s activation of calcineurin suggests that the regulatory domain of this vital phosphatase is a transiently disordered region. During activation, the regulatory domain progresses from a folded state, to disordered, followed by folding upon being bound by calmodulin. The transient disordered state of this domain is part of a critical intermediate state that facilitates the rapid binding of calmodulin. Building upon “fly-casting” as a means of facilitating partner binding, the mechanism by which calcineurin undergoes activation and subsequent deactivation could be considered “catch and release.”

Interplay between conformational selection and induced fit in multidomain protein-ligand binding probed by paramagnetic relaxation enhancement

The binding of ligands and substrates to proteins has been extensively studied for many years and can be described, in its simplest form, by two limiting mechanisms: conformational selection and induced fit. Conformational selection involves the binding of ligand to a pre-existing sparsely-populated conformation of the free protein that is the same as that in the final protein-ligand complex. In the case of induced fit, the ligand binds to the major conformation of the free protein and only subsequent to binding undergoes a conformational change to the final protein-ligand complex. While these two mechanisms can be dissected and distinguished by transient kinetic measurements, direct direction, characterization and visualization of transient, sparsely-populated states of proteins are experimentally challenging. Unless trapped, sparsely-populated states are generally invisible to conventional structural and biophysical techniques, including crystallography and most NMR measurements. In this review we summarize some recent developments in the use of paramagnetic relaxation enhancement to directly study sparsely-populated states of proteins and illustrate the application of this approach to two proteins, maltose binding protein and calmodulin, both of which undergo large rigid body conformational rearrangements upon ligand binding from an open apo state to a closed ligand-bound holo state. We show that the apo state ensemble comprises a small population of partially-closed configurations that are similar but not identical to that of the holo state. These results highlight the complementarity and interplay of induced fit and conformational selection and suggest that the existence of partially-closed states in the absence of ligand facilitates the transition to the closed ligand-bound state.

Reconstruction of Calmodulin Single-Molecule FRET States, Dye-Interactions, and CaMKII Peptide Binding by MultiNest and Classic Maximum Entropy

We analyze single molecule FRET burst measurements using Bayesian nested sampling. The MultiNest algorithm produces accurate FRET efficiency distributions from single-molecule data. FRET efficiency distributions recovered by MultiNest and classic maximum entropy are compared for simulated data and for calmodulin labeled at residues 44 and 117. MultiNest compares favorably with maximum entropy analysis for simulated data, judged by the Bayesian evidence. FRET efficiency distributions recovered for calmodulin labeled with two different FRET dye pairs depended on the dye pair and changed upon Ca2+ binding. We also looked at the FRET efficiency distributions of calmodulin bound to the calcium/calmodulin dependent protein kinase II (CaMKII) binding domain. For both dye pairs, the FRET efficiency distribution collapsed to a single peak in the case of calmodulin bound to the CaMKII peptide. These measurements strongly suggest that consideration of dye-protein interactions is crucial in forming an accurate picture of protein conformations from FRET data.

Structural and functional diversity in the activity and regulation of DAPK-related protein kinases

Within the large group of calcium/calmodulin-dependent protein kinases (CAMKs) of the human kinome, there is a distinct branch of highly related kinases that includes three families: death-associated protein-related kinases, myosin light-chain-related kinases and triple functional domain protein-related kinases. In this review, we refer to these collectively as DMT kinases. There are several functional features that span the three families, such as a broad involvement in apoptotic processes, cytoskeletal association and cellular plasticity. Other CAMKs contain a highly conserved HRD motif, which is a prerequisite for kinase regulation through activation-loop phosphorylation, but in all 16 members of the DMT branch, this is replaced by an HF/LD motif. This DMT kinase signature motif substitutes phosphorylation-dependent active-site interactions with a local hydrophobic core that maintains an active kinase conformation. Only about half of the DMT kinases have an additional autoregulatory domain, C-terminal to the kinase domain that binds calcium/calmodulin in order to regulate kinase activity. Protein substrates have been identified for some of the DMT kinases, but little is known about the mechanism of recognition. Substrate conformation could be an equally important parameter in substrate recognition as specific preferences in sequence position. Taking the data together, this kinase branch encapsulates a treasure trove of features that renders it distinct from many other protein kinases and calls for future research activities in this field.

The length of the calmodulin linker determines the extent of transient interdomain association and target affinity

Calmodulin (CaM), the prototypical calcium sensing protein in eukaryotes, comprises two domains separated by a short flexible linker, which allows CaM to assume a wide range of extended and compact conformations. Here we use NMR relaxation measurements to explore the role of the linker in CaM function and dynamics. Using paramagnetic relaxation enhancement (PRE) measurements, we examine the effect of changes in the length and rigidity of the linker on the transient association between the two domains of Ca(2+)-bound CaM (CaM-4Ca(2+)). We observe that transient interdomain association, represented by an effective molarity (M(eff)), is maximal for a linker extended by one residue from the wild-type length and decreases for lengths longer or shorter than that. The results can be quantitatively rationalized using a simplified model of a random coil whose two ends must be a specific distance apart for an interaction to occur. The results correlate well with the affinity of CaM-4Ca(2+) for a target peptide, suggesting that the transient compact states adopted by CaM-4Ca(2+) in the absence of peptide play a direct role in facilitating target binding.

Structural and biophysical characterization of the interactions between the death domain of Fas receptor and calmodulin

The extrinsic apoptotic pathway is initiated by cell surface death receptors such as Fas. Engagement of Fas by Fas ligand triggers a conformational change that allows Fas to interact with adaptor protein Fas-associated death domain (FADD) via the death domain, which recruits downstream signaling proteins to form the death-inducing signaling complex (DISC). Previous studies have shown that calmodulin (CaM) is recruited into the DISC in cholangiocarcinoma cells, suggesting a novel role of CaM in Fas-mediated signaling. CaM antagonists induce apoptosis through a Fas-related mechanism in cholangiocarcinoma and other cancer cell lines possibly by inhibiting Fas-CaM interactions. The structural determinants of Fas-CaM interaction and the underlying molecular mechanisms of inhibition, however, are unknown. Here we employed NMR and biophysical techniques to elucidate these mechanisms. Our data show that CaM binds to the death domain of Fas (FasDD) with an apparent dissociation constant (Kd) of ~2 μM and 2:1 CaM:FasDD stoichiometry. The interactions between FasDD and CaM are endothermic and entropically driven, suggesting that hydrophobic contacts are critical for binding. We also show that both the N- and C-terminal lobes of CaM are important for binding. NMR and surface plasmon resonance data show that three CaM antagonists (N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide, tamoxifen, and trifluoperazine) greatly inhibit Fas-CaM interactions by blocking the Fas-binding site on CaM. Our findings provide the first structural evidence for Fas-CaM interactions and mechanism of inhibition and provide new insight into the molecular basis for a novel role of CaM in regulating Fas-mediated apoptosis.

Structure-function analysis of the EF-hand protein centrin-2 for its intracellular localization and nucleotide excision repair

Centrin-2 is an evolutionarily conserved, calmodulin-related protein, which is involved in multiple cellular functions including centrosome regulation and nucleotide excision repair (NER) of DNA. Particularly to exert the latter function, complex formation with the XPC protein, the pivotal NER damage recognition factor, is crucial. Here, we show that the C-terminal half of centrin-2, containing two calcium-binding EF-hand motifs, is necessary and sufficient for both its localization to the centrosome and interaction with XPC. In XPC-deficient cells, nuclear localization of overexpressed centrin-2 largely depends on co-overexpression of XPC, and mutational analyses of the C-terminal domain suggest that XPC and the major binding partner in the centrosome share a common binding surface on the centrin-2 molecule. On the other hand, the N-terminal domain of centrin-2 also contains two EF-hand motifs but shows only low-binding affinity for calcium ions. Although the N-terminal domain is dispensable for enhancement of the DNA damage recognition activity of XPC, it contributes to augmenting rather weak physical interaction between XPC and XPA, another key factor involved in NER. These results suggest that centrin-2 may have evolved to bridge two protein factors, one with high affinity and the other with low affinity, thereby allowing delicate regulation of various biological processes.

Structural diversity of calmodulin binding to its target sites

Calmodulin (CaM) is a ubiquitous, highly conserved, eukaryotic protein that binds to and regulates a number of diverse target proteins involved in different functions such as metabolism, muscle contraction, apoptosis, memory, inflammation and the immune response. In this minireview, we analyze the large number of CaM-complex structures deposited in the Protein Data Bank (i.e. crystal and nuclear magnetic resonance structures) to gain insight into the structural diversity of CaM-binding sites and mechanisms, such as those for CaM-activated protein kinases and phosphatases, voltage-gated Ca(2+)-channels and the plasma membrane Ca(2+)-ATPase.

Calcium in red blood cells-a perilous balance

Ca2+ is a universal signalling molecule involved in regulating cell cycle and fate, metabolism and structural integrity, motility and volume. Like other cells, red blood cells (RBCs) rely on Ca2+ dependent signalling during differentiation from precursor cells. Intracellular Ca2+ levels in the circulating human RBCs take part not only in controlling biophysical properties such as membrane composition, volume and rheological properties, but also physiological parameters such as metabolic activity, redox state and cell clearance. Extremely low basal permeability of the human RBC membrane to Ca2+ and a powerful Ca2+ pump maintains intracellular free Ca2+ levels between 30 and 60 nM, whereas blood plasma Ca2+ is approximately 1.8 mM. Thus, activation of Ca2+ uptake has an impressive impact on multiple processes in the cells rendering Ca2+ a master regulator in RBCs. Malfunction of Ca2+ transporters in human RBCs leads to excessive accumulation of Ca2+ within the cells. This is associated with a number of pathological states including sickle cell disease, thalassemia, phosphofructokinase deficiency and other forms of hereditary anaemia. Continuous progress in unravelling the molecular nature of Ca2+ transport pathways allows harnessing Ca2+ uptake, avoiding premature RBC clearance and thrombotic complications. This review summarizes our current knowledge of Ca2+ signalling in RBCs emphasizing the importance of this inorganic cation in RBC function and survival.

Calmodulin adopts an extended conformation when interacting with L-selectin in membranes

Calmodulin, an intracellular calcium-binding protein, is thought to regulate ectodomain shedding of many membrane proteins, but the underlying molecular mechanism has remained unclear. Basing on a solution structure of calcium-loaded calmodulin in complex with a L-selectin fragment that contains a portion of its transmembrane domain, Gifford et al. (University of Calgary) recently suggested that calmodulin regulates L-selectin shedding by binding directly to a portion of the L-selectin transmembrane domain in a compact conformation. Using fluorescently labeled calmodulin, we show however that calmodulin adopts a distinctly different and much more extended conformation when it binds to the CLS peptide (i.e. the entire transmembrane and cytoplasmic domains of L-selectin) reconstituted in the phosphatidylcholine liposome with micromolar dissociation constant and in a calcium-independent manner. Calmodulin adopts a similarly extended conformation in a ternary complex with the N-terminal FERM domain of moesin and CLS reconstituted in the phospholipid liposome that mimics the native membrane environment. These results indicate that calmodulin does not bind directly to the transmembrane domain of L-selectin. Understanding the association of calmodulin with L-selectin helps to shed light on the mechanisms underlying regulation of ectodomain shedding.

Calmodulin transduces Ca2+ oscillations into differential regulation of its target proteins

Diverse physiological processes are regulated differentially by Ca(2+) oscillations through the common regulatory hub calmodulin. The capacity of calmodulin to combine specificity with promiscuity remains to be resolved. Here we propose a mechanism based on the molecular properties of calmodulin, its two domains with separate Ca(2+) binding affinities, and target exchange rates that depend on both target identity and Ca(2+) occupancy. The binding dynamics among Ca(2+), Mg(2+), calmodulin, and its targets were modeled with mass-action differential equations based on experimentally determined protein concentrations and rate constants. The model predicts that the activation of calcineurin and nitric oxide synthase depends nonmonotonically on Ca(2+)-oscillation frequency. Preferential activation reaches a maximum at a target-specific frequency. Differential activation arises from the accumulation of inactive calmodulin-target intermediate complexes between Ca(2+) transients. Their accumulation provides the system with hysteresis and favors activation of some targets at the expense of others. The generality of this result was tested by simulating 60 000 networks with two, four, or eight targets with concentrations and rate constants from experimentally determined ranges. Most networks exhibit differential activation that increases in magnitude with the number of targets. Moreover, differential activation increases with decreasing calmodulin concentration due to competition among targets. The results rationalize calmodulin signaling in terms of the network topology and the molecular properties of calmodulin.