Calmodulin Directly Interacts with the Cx43 Carboxyl-Terminus and Cytoplasmic Loop Containing Three ODDD-Linked Mutants (M147T, R148Q, and T154A) that Retain α-Helical Structure, but Exhibit Loss-of-Function and Cellular Trafficking Defects

The autosomal-dominant pleiotropic disorder called oculodentodigital dysplasia (ODDD) is caused by mutations in the gap junction protein Cx43. Of the 73 mutations identified to date, over one-third are localized in the cytoplasmic loop (Cx43CL) domain. Here, we determined the mechanism by which three ODDD mutations (M147T, R148Q, and T154A), all of which localize within the predicted 1-5-10 calmodulin-binding motif of the Cx43CL, manifest the disease. Nuclear magnetic resonance (NMR) and circular dichroism revealed that the three ODDD mutations had little-to-no effect on the ability of the Cx43CL to form α-helical structure as well as bind calmodulin. Combination of microscopy and a dye-transfer assay uncovered these mutations increased the intracellular level of Cx43 and those that trafficked to the plasma membrane did not form functional channels. NMR also identify that CaM can directly interact with the Cx43CT domain. The Cx43CT residues involved in the CaM interaction overlap with tyrosines phosphorylated by Pyk2 and Src. In vitro and in cyto data provide evidence that the importance of the CaM interaction with the Cx43CT may lie in restricting Pyk2 and Src phosphorylation, and their subsequent downstream effects.

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.

Elucidating the mechanisms of cooperative calcium-calmodulin interactions: a structural systems biology approach

BACKGROUND

Calmodulin is an important multifunctional molecule that regulates the activities of a large number of proteins in the cell. Calcium binding induces conformational transitions in calmodulin that make it specifically active to particular target proteins. The precise mechanisms underlying calcium binding to calmodulin are still, however, quite poorly understood.

RESULTS

In this study, we adopt a structural systems biology approach and develop a mathematical model to investigate various types of cooperative calcium-calmodulin interactions. We compare the predictions of our analysis with physiological dose-response curves taken from the literature, in order to provide a quantitative comparison of the effects of different mechanisms of cooperativity on calcium-calmodulin interactions. The results of our analysis reduce the gap between current understanding of intracellular calmodulin function at the structural level and physiological calcium-dependent calmodulin target activation experiments.

CONCLUSION

Our model predicts that the specificity and selectivity of CaM target regulation is likely to be due to the following factors: variations in the target-specific Ca2+ dissociation and cooperatively effected dissociation constants, and variations in the number of Ca2+ ions required to bind CaM for target activation.

Hydrogen peroxide-mediated oxidative stress disrupts calcium binding on calmodulin: More evidence for oxidative stress in vitiligo

Patients with acute vitiligo have low epidermal catalase expression/activities and accumulate 10(-3) M H(2)O(2). One consequence of this severe oxidative stress is an altered calcium homeostasis in epidermal keratinocytes and melanocytes. Here, we show decreased epidermal calmodulin expression in acute vitiligo. Since 10(-3)M H(2)O(2) oxidises methionine and tryptophan residues in proteins, we examined calcium binding to calmodulin in the presence and absence of H(2)O(2) utilising (45)calcium. The results showed that all four calcium atoms exchanged per molecule of calmodulin. Since oxidised calmodulin looses its ability to activate calcium ATPase, enzyme activities were followed in full skin biopsies from lesional skin of patients with acute vitiligo (n=6) and healthy controls (n=6). The results yielded a 4-fold decrease of ATPase activities in the patients. Computer simulation of native and oxidised calmodulin confirmed the loss of all four calcium ions from their specific EF-hand domains. Taken together H(2)O(2)-mediated oxidation affects calcium binding in calmodulin leading to perturbed calcium homeostasis and perturbed l-phenylalanine-uptake in the epidermis of acute vitiligo.

Computational studies of the binding mechanism of calmodulin with chrysin

Calmodulin (CaM) plays a crucial role in metabolism and physiology of eukaryotes by regulating biological activities. Multiple lines of evidences indicate that the phosphorylated flavonoids possess relatively stronger affinities for proteins by forming non-covalent complexes with them, and that the cellular functions are often triggered by this kind of interactions. Chrysin is one of the phosphorylated flavonoids that exist ubiquitously in plants and have remarkably beneficial pharmacological effects. In this study, the molecular docking tools were utilized to investigate the interactions of CaM with chrysin. Two different favorable binding modes have been observed. To complement the results obtained by the molecular docking study, an in-depth investigation into the binding modes was conducted with the molecular dynamics (MD) simulation to explore the binding profile and energy landscape. Based on the results thus obtained, a clear definition of the binding pocket for each of the two binding modes has been revealed. These findings may shed light upon the binding interactions of CaM with chrysin, providing a solid molecular basis for subset analysis of its pharmacological benefits.

Electrostatic control of the overall shape of calmodulin: numerical calculations

The paper reports the results of numerical calculations of the pKa's of the ionizable groups and the electrostatic interactions between calmodulin lobes in three different states of calmodulin: calcium-free, peptide-free; calcium-loaded, peptide-free; and calcium-loaded, peptide-bound. NMR and X-ray studies revealed that in these states the overall structure of calmodulin adopts various conformations referred as: disordered semi-compact, extended and compact conformations, respectively. In addition, a new X-ray structure was recently reported (Structure, 2003, 11, 1303) showing that calcium-loaded, peptide-free calmodulin can also adopt a compact conformation in addition to the well known extended conformation. The calculated energy changes of calcium-loaded, peptide-free calmodulin along the pathway connecting these two conformations provide a possible explanation for this structural plasticity. The effect of pH and organic compounds in the solution phase on the preference of calmodulin to adopt compact or extended conformations may be thus rationalized. Analysis of the contribution of the ionization changes to the energy of association of calmodulin lobes suggested that the formation of the compact forms requires protonation of several acidic residues. However, two different protonation scenarios are revealed: a protonation due to internal lobe organization and thus independent of the lobes association, and a protonation induced by the lobes association resulting to a proton uptake. In addition, the role of the individual residues on the energy of association of calmodulin lobes is calculated in two compact conformations (peptide-free and peptide-bound) and is shown that a set of residues always plays a dominant role in inter-domain interactions.

The formation of a complex between calmodulin and neuronal nitric oxide synthase is determined by ESI-MS

Calmodulin (CaM) is an acidic ubiquitous calcium binding protein, involved in many intracellular processes, which often involve the formation of complexes with a variety of protein and peptide targets. One such system, activated by Ca2+ loaded CaM, is regulation of the nitric oxide synthase (NOS) enzymes, which in turn control the production of the signalling molecule and cytotoxin NO. A recent crystallographic study mapped the interaction of CaM with endothelial NOS (eNOS) using a 20 residue peptide comprising the binding site within eNOS. Here the interaction of CaM to the FMN domain of neuronal nitric oxide synthase (nNOS) has been investigated using electrospray ionization mass spectrometry (ESI-MS). The 46 kDa complex formed by CaM-nNOS has been retained in the gas-phase, and is shown to be exclusively selective for CaM.4Ca2+. Further characterization of this important biological system has been afforded by examining a complex of CaM with a 22 residue synthetic peptide, which represents the linker region between the reductase and oxygenase domains of nNOS. This nNOS linker peptide, which is found to be random coil in aqueous solution by both circular dichroism and molecular modelling, also exhibits great discrimination for the form of CaM loaded with 4[Ca2+]. The peptide binding loop is presumed to be configured to an alpha-helix on binding to CaM as was found for the related eNOS binding peptide. Our postulate is supported by gas-phase molecular dynamics calculations performed on the isolated nNOS peptide. Collision induced dissociation was employed to probe the strength of binding of the nNOS binding peptide to CaM.4Ca2+. The methodology taken here is a new approach in understanding the CaM-nNOS binding site, which could be employed in future to inform the specificity of CaM binding to other NOS enzymes.

Microautophagic Vacuole Invagination Requires Calmodulin in a Ca2+-independent Function

Microautophagy is the uptake of cytosolic compounds by direct invagination of the vacuolar/lysosomal membrane. In Saccharomyces cerevisiae microautophagic uptake of soluble cytosolic proteins occurs via an autophagic tube, a highly specialized vacuolar membrane invagination. Autophagic tubes are topologically equivalent to the invaginations at multivesicular endosomes. At the tip of an autophagic tube, vesicles (autophagic bodies) pinch off into the vacuolar lumen for degradation. In this study we have identified calmodulin (Cmd1p) as necessary for microautophagy. Temperature-sensitive mutants for Cmd1p displayed reduced frequencies of vacuolar tube formation and/or abnormal tube morphologies. Microautophagic vacuole invagination was sensitive to Cmd1p antagonists as well as to antibodies to Cmd1p. cmd1 mutants with substitutions in the Ca2+-binding domains showed full invagination activity, and vacuolar membrane invagination was independent of the free Ca2+ concentration. Thus, rather than acting as a calcium-triggered switch, Cmd1p has a constitutive Ca2+-independent role in the formation of autophagic tubes. Kinetic analysis indicates that calmodulin is required for autophagic tube formation rather than for the final scission of vesicles from the tip of the tube.

Backbone dynamics of a symmetric calmodulin dimer in complex with the calmodulin-binding domain of the basic-helix-loop-helix transcription factor SEF2-1/E2-2: a highly dynamic complex

Calmodulin (CaM) interacts specifically as a dimer with some dimeric basic-Helix-Loop-Helix (bHLH) transcription factors via a novel high affinity binding mode. Here we report a study of the backbone dynamics by (15)N-spin relaxation on the CaM dimer in complex with a dimeric peptide that mimics the CaM binding region of the bHLH transcription factor SEF2-1. The relaxation data were measured at multiple magnetic fields, and analyzed in a model-free manner using in-house written software designed to detect nanosecond internal motion. Besides picosecond motions, all residues also experience internal motion with an effective correlation time of approximately 2.5 ns with squared order parameter (S(2)) of approximately 0.75. Hydrodynamic calculations suggest that this can be attributed to motions of the N- and C-terminal domains of the CaM dimer in the complex. Moreover, residues with significant exchange broadening are found. They are clustered in the CaM:SEF2-1mp binding interface, the CaM:CaM dimer interface, and in the flexible helix connecting the CaM N- and C-terminal domains, and have similar exchange times (approximately 50 micros), suggesting a cooperative mechanism probably caused by protein:protein interactions. The dynamic features presented here support the conclusion that the conformationally heterogeneous bHLH mimicking peptide trapped inside the CaM dimer exchanges between different binding sites on both nanosecond and microsecond timescales. Nature has thus found a way to specifically recognize a relatively ill-fitting target. This novel mode of target-specific binding, which neither belongs to lock-and-key nor induced-fit binding, is characterized by dimerization and continuous exchange between multiple flexible binding alternatives.

Nanometre localization of single ReAsH molecules

ReAsH is a red-emitting dye that binds to the unique sequence Cys-Cys-Xaa-Xaa-Cys-Cys (where Xaa is a noncysteine amino acid) in the protein. We attached a single ReAsH to a calmodulin with an inserted tetracysteine motif and immobilized individual calmodulins to a glass surface at low density. Total internal reflection fluorescence microscopy was used to image individual ReAsH molecules. We determined the centre of the distribution of photons in the image of a single molecule in order to determine the position of the dye within 5 nm precision and with an image integration time of 0.5 s. The photostability of ReAsH was also characterized and observation times ranging from several seconds to over a minute were observed. We found that 2-mercaptoethanesulphonic acid increased the number of collected photons from ReAsH molecules by a factor of two. Individual ReAsH molecules were then moved via a nanometric stage in 25 or 40 nm steps, either at a constant rate or at a Poisson-distributed rate. Individual steps were clearly seen, indicating that the observation of translational motion on this scale, which is relevant for many biomolecular motors, is possible with ReAsH.

Direct visualization of the calmodulin subunit of phosphorylase kinase via electron microscopy following subunit exchange

Calmodulin is a tightly bound, intrinsic subunit (delta) of the hexadecameric phosphorylase-b kinase holoenzyme, (alphabetagammadelta)4. To introduce specifically labeled calmodulin into the phosphorylase-b kinase complex for its eventual visualization by electron microscopy, we have developed a method for rapidly exchanging exogenous calmodulin for the intrinsic delta subunit. This method exploits previous findings that low concentrations of urea in the absence of Ca(2+) ions cause the specific dissociation of only the delta subunit from the holoenzyme [Paudel, H. K., and Carlson, G. M. (1990) Biochem. J. 268, 393-399]. In the current study, phosphorylase-b kinase was incubated with excess exogenous calmodulin and a threshold concentration of urea to promote exchange of its delta subunit with the exogenous calmodulin. Size exclusion HPLC was then used to remove the excess calmodulin from the holoenzyme containing exchanged delta subunits. Using metabolically labeled [35S]calmodulin to allow quantification and optimization of exchange conditions, we achieved exchange of approximately 10% of all delta subunits within 1 h, with the exchanged holoenzyme retaining full catalytic activity. Calmodulins derivatized with Nanogold for visualization by scanning transmission electron microscopy were then exchanged for delta, which for the first time allowed localization of the delta subunit within the bridged, bilobal phosphorylase b kinase holoenzyme complex. The delta subunits were determined to be near the edge of the lobes, just distal to the interlobal bridges and proximal to a previously identified region of the enzyme's catalytic gamma subunit.

The three-dimensional structure of the Limulus acrosomal process: a dynamic actin bundle

Limulus sperm contains a dynamic macromolecular structure that rapidly extends a 50 microm process called the true discharge. The core of this structure is a bundle of ordered filaments composed of a complex of actin, scruin and calmodulin. We determined its structure by electron crystallographic reconstruction. The three-dimensional map reveals an actin-scruin helix that is azimuthally modulated by the influence of the interactions of a filament with its neighbors. There are a variety of density connections with neighboring filaments involving scruin. Scruin commonly contacts one neighbor, but we observe up to three interfilament connections involving both domains of the 28 scruin molecules in the unit cell. Our structure indicates that promiscuous scruin-scruin contacts are the major determinants of bundle stability in the true discharge. It also suggests that rearrangements would be permitted, which can facilitate the transition from the coiled to the true discharge form.

Crystal structure of troponin C in complex with troponin I fragment at 2.3-A resolution

Troponin (Tn), the complex of three subunits (TnC, TnI, and TnT), plays a key role in Ca2+-dependent regulation of muscle contraction. To elucidate the interactions between the Tn subunits and the conformation of TnC in the Tn complex, we have determined the crystal structure of TnC (two Ca2+ bound state) in complex with the N-terminal fragment of TnI (TnI1-47). The structure was solved by the single isomorphous replacement method in combination with multiple wavelength anomalous dispersion data. The refinement converged to a crystallographic R factor of 22.2% (Rfree = 32.6%). The central, connecting alpha-helix observed in the structure of uncomplexed TnC (TnCfree) is unwound at the center (residues Ala-87, Lys-88, Gly-89, Lys-90, and Ser-91) and bent by 90 degrees. As a result, TnC in the complex has a compact globular shape with direct interactions between the N- and C-terminal lobes, in contrast to the elongated dumb-bell shaped molecule of uncomplexed TnC. The 31-residue long TnI1-47 alpha-helix stretches on the surface of TnC and stabilizes its compact conformation by multiple contacts with both TnC lobes. The amphiphilic C-end of the TnI1-47 alpha-helix is bound in the hydrophobic pocket of the TnC C-lobe through 38 van der Waals interactions. The results indicate the major difference between Ca2+ receptors integrated with the other proteins (TnC in Tn) and isolated in the cytosol (calmodulin). The TnC/TnI1-47 structure implies a mechanism of how Tn regulates the muscle contraction and suggests a unique alpha-helical regulatory TnI segment, which binds to the N-lobe of TnC in its Ca2+ bound conformation.

Ultrastructural co-localization of calmodulin and B-50/growth-associated protein-43 at the plasma membrane of proximal unmyelinated axon shafts studied in the model of the regenerating rat sciatic nerve

Calmodulin and de-phosphorylated B-50/growth-associated protein-43 (GAP-43) have been shown to bind in vitro in a molecular complex, but evidence for an in situ association in the nervous system does not exist. Previously, we have reported that, in the model of the regenerating rat sciatic nerve, the B-50/GAP-43 immunoreactivity is increased and concentrated at the axolemma of unmyelinated axons located proximal to the site of injury and axon outgrowth. To explore a putative function of B-50/GAP-43, namely, the capacity of binding calmodulin to the plasma membrane, we examined the ultrastructural distribution of calmodulin in the proximal unmyelinated axon shafts of this model, using double immunolabelling and detection by fluorescent or gold probes conjugated to second antibodies. Immunofluorescence showed that seven days post-sciatic nerve crush the calmodulin immunoreactivity, similar to B-50/GAP-43 immunoreactivity, was intense in unmyelinated axon shafts located proximal to the site of injury of the regenerating nerve. Ultrastructurally, calmodulin was located at the axolemma of these regenerating unmyelinated axon shafts and inside the axoplasm, where it was associated with vesicles and microtubules. The plasma membrane labelling (approximately 69%) was significantly higher than the axoplasmic labelling. Over 60% of the plasma membrane-associated calmodulin co-localized with B-50/GAP-43 in a non-random distribution. Since normally calmodulin is largely present in the cytoplasm, these data suggest that calmodulin has been concentrated at the plasma membrane of unmyelinated axons, most probably by B-50/GAP-43. If the concentrating effect is due to B-50/GAP-43, then there is a possibility that these proteins may be present as a molecular complex in situ. The physiological significance could be that this association regulates the local availability of both B-50/GAP-43 and calmodulin for other interactions.

Recovery of native structure by calcium binding site mutants of calmodulin upon binding of sk-MLCK target peptides

The calcium-dependent binding of two synthetic 18-residue peptides derived from the calmodulin binding region of skeletal myosin light chain kinase to wild-type Drosophila melanogaster calmodulin and four calcium binding site calmodulin mutants has been investigated using optical spectroscopy. The WFF peptide (with W4 and F17) and the FFW peptide (with F4 and W17) both bind to wild-type calmodulin with 1:1 stoichiometry and Kd values of < or = 0.2 and 1.6 nM, respectively. Near-UV CD spectra of the protein-peptide complexes suggest that both peptides bind in the same orientation, with the side chain of residue 4 interacting with the C-domain of calmodulin and that of residue 17 with the N-domain [as in the structure of the calmodulin-M13 peptide complex determined by Ikura et al. [Ikura, M., Clore, G. M., Gronenborn, A. M., Zhu, G., Klee, C. B., & Bax, A. (1992) Science 256, 632-638]]. Both peptides have lower affinities for all the mutant calmodulins than for the wild-type protein. Fluorescence measurements suggest that mutation of calcium binding site 2 in the N-domain does not affect the interaction of the W4 side chain of the WFF peptide with the C-domain of calmodulin. However, the E67Q (B2Q) but not the E67K (B2K) mutation (site 2, N-domain) alters the interaction of W17 of the FFW peptide with the protein. In contrast, the E140K (B4K) mutation has a much greater effect than the E140Q (B4Q) mutation (site 4, C-domain) on the interaction of calmodulin with both peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution of calmodulin/calmodulin binding proteins in the rat tapeworm, Hymenolepis diminuta

Live tapeworms have been fixed to retain antigenicity of their proteins, and subsequently prepared for electron microscopy. Thin sections of tapeworms were prepared from resin blocks. Sections were immunocytochemically labelled using a colloidal gold probe and viewed using transmission electron microscopy. Calmodulin was detected associated with cellular structures to which calmodulin has previously been linked in other higher eukaryotes. Calmodulin would appear to have a similar role of importance in tapeworms, as it does in higher eukaryotes although tapeworms are prevalently a syncitium.

Resolution of structural changes associated with calcium activation of calmodulin using frequency domain fluorescence spectroscopy

Structural changes associated with the calcium-dependent activation of wheat germ calmodulin (CaM) were assessed through measurements of steady-state and time-resolved changes in the fluorescence associated with (1) the unique tyrosine (Tyr139) located in calcium binding loop IV or (2) N-(1-pyrenyl)-maleimide (PM) or 4-(iodoacetamido)salicylic acid (IASA) covalently attached to Cys27 present in calcium binding loop I. These fluorophores permit the measurement of calcium-dependent changes in (i) the solvent accessibility and rotational dynamics associated with calcium binding loops I and IV and (ii) the hydrodynamic properties of the entire protein. Specific nitration of the unique tyrosine (Tyr139) in calcium binding loop IV permits the use of fluorescence resonance energy transfer to measure both the average spatial separation and distance heterogeneity between Cys27 and Tyr139, providing a direct measurement of the conformational flexibility of the central helix. Upon calcium binding, (i) the solvent accessibility and rotational dynamics of both PM and IASA (covalently bound to Cys27) and Tyr139 increase, (ii) overall protein rotational motion decreases, (iii) the average separation between the chromophores at Cys27 and nitrotyrosine 139 decreases, and (iv) the conformational flexibility associated with the central helix decreases. Therefore, upon calcium binding, the central helix becomes more extended and rigid, while the globular domains adopt a more open tertiary conformation that brings Cys27 and Tyr139 into closer proximity. This calcium-dependent structural change functions to expose the hydrophobic binding sites located within the globular domains, and to enhance the probability of binding target sequences through a reduction in conformational heterogeneity.

Backbone dynamics of calmodulin studied by 15N relaxation using inverse detected two-dimensional NMR spectroscopy: the central helix is flexible

The backbone dynamics of Ca(2+)-saturated recombinant Drosophila calmodulin has been studied by 15N longitudinal and transverse relaxation experiments, combined with 15N(1H) NOE measurements. Results indicate a high degree of mobility near the middle of the central helix of calmodulin, from residue K77 through S81, with order parameters (S2) in the 0.5-0.6 range. The anisotropy observed in the motion of the two globular calmodulin domains is much smaller than expected on the basis of hydrodynamic calculations for a rigid dumbbell type structure. This indicates that, for the purposes of 15N relaxation, the tumbling of the N-terminal (L4-K77) and C-terminal (E82-S147) lobes of calmodulin is effectively independent. A slightly shorter motional correlation time (tau c approximately 6.3 ns) is obtained for the C-terminal domain compared to the N-terminal domain (tau c approximately 7.1 ns), in agreement with the smaller size of the C-terminal domain. A high degree of mobility, with order parameters of approximately 0.5, is also observed in the loop that connects the first with the second EF-hand type calcium binding domain and in the loop connecting the third and fourth calcium binding domain.

Calcium binding to calmodulin and its globular domains

The macroscopic Ca(2+)-binding constants of bovine calmodulin have been determined from titrations with Ca2+ in the presence of the chromophoric chelator 5,5'-Br2BAPTA in 0, 10, 25, 50, 100, and 150 mM KCl. Identical experiments have also been performed for tryptic fragments comprising the N-terminal and C-terminal domains of calmodulin. These measurements indicate that the separated globular domains retain the Ca2+ binding properties that they have in the intact molecule. The Ca2+ affinity is 6-fold higher for the C-terminal domain than for the N-terminal domain. The salt effect on the free energy of binding two Ca2+ ions is 20 and 21 kJ. mol-1 for the N- and C-terminal domain, respectively, comparing 0 and 150 mM KCl. Positive cooperativity of Ca2+ binding is observed within each globular domain at all ionic strengths. No interaction is observed between the globular domains. In the N-terminal domain, the cooperativity amounts to 3 kJ.mol-1 at low ionic strength and greater than or equal to 10 kJ.mol-1 at 0.15 M KCl. For the C-terminal domain, the corresponding figures are 9 +/- 2 kJ.mol-1 and greater than or equal to 10 kJ.mol-1. Two-dimensional 1H NMR studies of the fragments show that potassium binding does not alter the protein conformation.

Electron microscopic studies of chicken gizzard caldesmon and its complex with calmodulin

Caldesmon samples mounted on a stage rotating about a horizontal axis were shadowed keeping the shadow angle at about 3 degrees. This technique minimizes background metal deposits compared with the conventional method. The identity of caldesmon was confirmed by comparing the images of caldesmon alone with those of the caldesmon-calmodulin complex. In these samples the caldesmon molecules appeared to be elongated; most were between 30 and 80 nm in length. The maximum length was in good agreement with the earlier estimate of 74 nm based on hydrodynamic studies. Our observations also suggested the presence of a rather rigid 30-40 nm stretch in the middle of the caldesmon molecule, which was always visible under rotary shadowing, and a flexible structure of about 20 nm in length at each end of the molecule, which may or may not be visible depending on their orientation on the mica surface. In the samples of caldesmon crosslinked with calmodulin, we noticed the existence of complexes containing two calmodulin molecules per caldesmon molecule, separated by a distance of 60 nm, consistent with the suggestion that each end of caldesmon can interact with calmodulin.

A joint 2D NMR and theoretical investigation of Ca2+ binding loops III and IV of calmodulin

A 2D NMR NOESY spectrum of integral CaM in water(148 residues) reveals a series of downfield-shifted crosspeaks stemming from the NH protons of the Ca2(+)-binding loops III and IV. Their attribution, with the help of already assigned proton resonances of isolated tryptic fragments, was complemented by means of energy-minimizations on the Ca2+ complexes of loops III and IV. From these calculations, a set of two alternative, related conformations was obtained for each loop. The first type of conformation provides a coordination pattern for Ca2+ that is similar to that found in loop EF of parvalbumin. The computed interproton distances in both loops are fully compatible with the inferences from the sets of NOESY cross-peaks. Evidence is also provided for interloop interactions.

Alpha-actinin and calmodulin interact with distinct sites on the arms of the clathrin trimer

In the present study, the interaction of alpha-actinin and calmodulin with clathrin heavy chain are demonstrated using Western blot analysis and rotary shadowing electron microscopy. The results show that alpha-actinin and calmodulin bind the clathrin heavy chain. The interaction is specific and affected by calcium. However, the interaction of both proteins with the clathrin heavy chain is distinct; the proteins do not block each other's ability to bind, and they interact with different protein fragments of the clathrin heavy chain. Furthermore, using rotary shadowing the results show that alpha-actinin differentially affected the terminal region of the clathrin trimer. Whereas, the effects of calmodulin were most noticeably detected along the length of trimer arms. The possible existence of distinct binding sites on the arms of the clathrin trimer for these cytosolic proteins supports the contention that these cytosolic proteins play an important role in cellular trafficking.