Activation of skeletal muscle myosin light chain kinase by calcium(2+) and calmodulin

Many biological processes are now known to be regulated by Ca2+ via calmodulin (CM). Although a general mechanistic model by which Ca2+ and calmodulin modulate many of these activities has been proposed, an accurate quantitative model is not available. A detailed analysis of skeletal muscle myosin light chain kinase activation was undertaken in order to determine the stoichiometries and equilibrium constants of Ca2+, calmodulin, and enzyme catalytic subunit in the activation process. The analysis indicates that activation is a sequential, fully reversible process requiring both Ca2+ and calmodulin. The first step of the activation process appears to require binding of Ca2+ to all four divalent metal binding sites on calmodulin for form the complex, Ca42+-calmodulin. This complex then interacts with the inactive catalytic subunit of the enzyme to form the active holoenzyme complex, Ca42+-calmodulin-enzyme. Formation of the holoenzyme follows simply hyperbolic kinetics, indicating 1:1 stoichiometry of Ca42+-calmodulin to catalytic subunit. The rate equation derived from the mechanistic model was used to determine the values of KCa2+ and KCM, the intrinsic activation constants for each step of the activation process. KCa2+ and KCM were found to have values of 10 microM and 0.86 nM, respectively, at 10 mM Mg2+. The rate equation using these equilibrium constants accurately predicts the extent of enzyme activation over a wide range of Ca2+ and calmodulin concentrations. The kinetic model and analytical techniques employed herein may be generally applicable to other enzymes with similar regulatory schemes.     

Free-energy simulations of the oxidation of c-terminal methionines in calmodulin

In the course of aging or under conditions of oxidative stress, methionine residues of calmodulin undergo oxidation, leading to loss of biological activity of the protein. We have performed free-energy simulations of the effects of C-terminal methionine side-chain oxidation on the properties of calmodulin. The simulation results indicate that oxidation should have a destabilizing effect on all three protein functional states: calcium free, calcium loaded, and with both calcium and target peptide bound. Because the different states are destabilized by different amounts, this leads to a more complex pattern in the observable effects on protein thermal stability, calcium affinity, and binding of a target peptide. The influence of oxidation on the free energy of CaM unfolding is estimated by comparing the free-energy cost of oxidizing a Met residue in a Gly-Met-Gly peptide and in the protein. The protein thermal stability of the oxidized forms is lowered by a moderate amount 1-3 kcal/mol, in qualitative agreement with experimental results of 0.3 kcal/mol. The calculated changes in affinity for calcium and for the target peptide show opposing trends. Oxidation at position 144 is predicted to enhance peptide binding and weaken calcium binding, whereas oxidation at 145 weakens peptide binding and enhances affinity for calcium. The lower affinity of Met 145-oxidized calmodulin toward the target peptide correlates with experimentally observed lowering of calmodulin-activated Ca-ATPase activity when oxidized calmodulin from aged rat brains is used. Thus, our simulations suggest that Met 145 is the oxidation site in the C-terminal fragment of calmodulin. The microscopic mechanism behind the calculated free energy changes appears to be a greater affinity for water of the oxidized Met side-chain relative to normal Met. Structures with Met exposed to solvent had consistently lower free energies than those with buried Met sidechains.     

Structural and dynamic characterization of a neuron-specific protein kinase C substrate,neurogranin

Neurogranin/RC3 is a neuron-specific, Ca(2+)-sensitive calmodulin binding protein and a specific protein kinase C substrate. Neurogranin may function to regulate calmodulin levels at specific sites in neurons through phosphorylation at serine residue within its IQ motif, oxidation outside the IQ motif, or changes in local cellular Ca(2+) concentration. To gain insight into the functional role of neurogranin in the regulation of calmodulin-dependent activities, we investigated the structure and dynamics of a full-length rat neurogranin protein with 78 amino acids using triple resonance NMR techniques. In the absence of calmodulin or PKC, neurogranin exists in an unfolded form as evidenced by high backbone mobility and the absence of long-range nuclear Overhauser effect (NOE). Analyses of the chemical shifts (13)C(alpha), (13)C(beta), and (1)H(alpha) reveal the presence of a local alpha-helical structure for the region between residues G25-A42. Three-bond (1)H(N)-(1)H(alpha) coupling constants support the finding that the sequence between residues G25 and A42 populates a non-native helical structure in the unfolded neurogranin. Homonuclear NOE results are consistent with the conclusions drawn from chemical shifts and coupling constants. (15)N relaxation data indicate motional restrictions on a nanosecond time scale in the region from D15 to S48. Spectral densities and order parameters data further confirm that the unfolded neurogranin exists in conformation with residual secondary structures. The medium mobility of the nascent helical region may help to reduce the entropy loss when neurogranin binds to its targets, but the complex between neurogranin and calmodulin is not stable enough for structural determination by NMR. Calmodulin titration of neurogranin indicates that residues D15-G52 of neurogranin undergo significant structural changes upon binding to calmodulin.     

Isolation, characterization and Ca2+ and calmodulin regulation of cyclic nucleotide phosphodiesterases from human placentae of different ages

Two kinds of phosphodiesterases were isolated from human placenta by DEAE chromatography and characterized: one Ca2+ and calmodulin dependent, the other stimulated by Ca2+ but not by calmodulin. Both hydrolyzed cAMP and cGMP. The first one exhibited a higher affinity for cGMP. Half maximal activation by calmodulin was attained at 10(-8)M of calmodulin concentration independently of the hydrolyzed substrate (cGMP or cAMP). This phosphodiesterase appears to be almost homogeneous by molecular sieve chromatography on Ultragel AcA 34. The second phosphodiesterase exhibited similar affinities for cAMP and cGMP and could be resolved into three active isoforms with different molecular weight on Ultrogel AcA 34. Only minor differences were observed in the characteristics of these enzymes when the phosphodiesterases were prepared from placentae of 7-8 weeks of pregnancy or from normal term placenta.     

Free energy and the kinetics of biochemical diagrams, including active transport.

In earlier papers on muscle contraction it was found very useful to relate the actual (not standard) free energy levels of the different states in the biochemical diagram of the myosin cross-bridge to the first-order rate constants governing transitions between these states and to the details of the conversion of ATP free energy into mechanical work. This same approach is applied here to other macromolecular biochemical systems, for example, carriers in active transport, and simple enzyme reactions. With the definition of free energy changes between states of diagram used here (and in the muscle papers), the rate constants of the diagram are firat order, the macromolecular transitions are effectively isomeric, the equilibrium constants are dimensionless, the free energy changes are directly related to first-order rate constant ratios, and the ratio of products of forward and backward rate constants around any cycle of the diagram is related to operational free energy changes (e.g. the in vivo free energy of ADP HYDROLYSIS). These general points are illustrated by means of particular arbitrary models, especially transport models. In contrast to the muscle case, the free energy conversion question in other biochemical systems can be handled at the less detailed, complete-cycle level rather than at the elementary transition level. There is a corresponding complete-cycle kinetics, with composite first-order rate constants for the different possible cycles (in both directions). An introductory stochastic treatment of cycle kinetics is included.     

Improved hidden Markov models for molecular motors, part 1: basic theory

Hidden Markov models (HMMs) provide an excellent analysis of recordings with very poor signal/noise ratio made from systems such as ion channels which switch among a few states. This method has also recently been used for modeling the kinetic rate constants of molecular motors, where the observable variable—the position—steadily accumulates as a result of the motor's reaction cycle. We present a new HMM implementation for obtaining the chemical-kinetic model of a molecular motor's reaction cycle called the variable-stepsize HMM in which the quantized position variable is represented by a large number of states of the Markov model. Unlike previous methods, the model allows for arbitrary distributions of step sizes, and allows these distributions to be estimated. The result is a robust algorithm that requires little or no user input for characterizing the stepping kinetics of molecular motors as recorded by optical techniques.     

Interaction of a fluorescent N-dansylaziridine derivative of troponin I with calmodulin in the absence and presence of calcium

Rabbit skeletal muscle troponin I was covalently labeled with N-dansylaziridine, resulting in a fluorescent labeled protein. This derivative (DANZTnI) and native troponin I (TnI) inhibited calmodulin (CaM) stimulation of bovine heart Ca2+-sensitive cyclic nucleodite phosphodiesterase with identical inhibition constants. Association of DANZTnI with calmodulin was monitored directly by changes in flourescence intensity in the presence of Ca2+ and by changes in fluorescence anisotropy in the absence of Ca2+. Quantitation of the affinity of calmodulin for calmodulin-binding proteins in both the presence and absence of Ca2+ is necessary for prediction of the extent of interaction of both Ca2+ and calmodulin-binding proteins with calmodulin in vivo. The dissociation constants for the DANZTnI-calmodulin-l4Ca2+ and DANZTnI-calmodulin complexes were 20 nM and 70 micrometers, respectively. These dissociation constants define a free energy coupling of-4.84 kcal/mol of troponin I for binding of Ca2+ and troponin I to calmodulin. The Ca2+ dependence for troponin I-calmodulin complex formation predicted from these experimentally determined parameters was closely approximated by the Ca2+ dependence for complex formation between troponin I and fluorescent 5-[[[(iodoacetyl)amino]ethyl]-amino]-1-napthalenesulfonic acid derivatized calmodulin as determined by fluorescence anisotropy. Complex formation occurred over a relatively narrow range of Ca2+ concentration, indicative of positive heterotropic cooperativity for Ca2+ and troponin I binding to calmodulin.     

Structural requirements for calmodulin binding to membrane-associated guanylate kinase homologs

Effector molecules such as calmodulin modulate the interactions of membrane-associated guanylate kinase homologs (MAGUKs) and other scaffolding proteins of the membrane cytoskeleton by binding to the Src homology 3 (SH3) domain, the guanylate kinase (GK) domain, or the connecting HOOK region of MAGUKs. Using surface plasmon resonance, we studied the interaction of members of all four MAGUK subfamilies--synapse-associated protein 97 (SAP97), calcium/calmodulin-dependent serine protein kinase (CASK), membrane palmitoylated protein 2 (MPP2), and zona occludens (ZO) 1--and calmodulin to determine interaction affinities and localize the binding site. The SH3-GK domains of the proteins and derivatives thereof were expressed in E. coli and purified. In all four proteins, high-affinity calmodulin binding was identified. CASK was shown to contain a Ca2+-dependent calmodulin binding site within the HOOK region, overlapping with a protein 4.1 binding site. In ZO1, a Ca2+-dependent calmodulin binding site was detected within the GK domain. The equilibrium dissociation constants for MAGUK-calmodulin interaction were found to range from 50 nM to 180 nM. Sequence analyses suggest that binding sites for calmodulin have evolved independently in at least three subfamilies. For ZO1, pulldown of GST-calmodulin was shown to occur in a calcium-dependent manner; moreover, molecular modeling and sequence analyses predict conserved basic residues to be exposed on one side of a helix. Thus, calmodulin binding appears to be a common feature of MAGUKs, and Ca2+-activated calmodulin may serve as a general regulator to affect the interactions of MAGUKs and various components of the cytoskeleton.     

Quantitative analysis of the free energy coupling in the system calmodulin, calcium, smooth muscle myosin light chain kinase

Interactions between Ca2+, calmodulin and turkey gizzard myosin light chain kinase have been studied by equilibrium gel filtration and analyzed in terms of the theory of free energy coupling as formulated by Huang and King for calmodulin-regulated systems (Current Topics in Cellular Regulation 27, 1966-1971, 1985). Direct binding studies revealed that upon interaction with the enzyme, calmodulin acquires strong positive cooperativity in Ca2+-binding. The determination of the Ca2+-binding constants is inherently approximative due to the apparent homotropic cooperativity; therefore a statistical chi 2 analysis was carried out to delimit the formation-, and subsequently the stoichiometric Ca2+-binding constants. Whereas the first two stoichiometric Ca2+-binding constants of enzyme-bound CaM do not differ or are at the upmost 10-fold higher than those in free calmodulin, the third Ca2+ ion binds with an at least 70-fold and more likely 3000-fold higher affinity constant. The binding constant for the fourth Ca2+ is only 5-fold higher than the corresponding one in free calmodulin, thus creating a plateau at 3 bound Ca2+ in the isotherm. Direct binding of Ca2+-free calmodulin to myosin light chain kinase at 10(-7) M free Ca2+ yielded a l/l stoichiometry and an affinity constant of 2.2 x 10(5) M-1. It is thus anticipated that in resting smooth muscle ([Ca2+] less than or equal to 10(-7) M) more than half of the enzyme is bound to metal-free calmodulin. Analysis of the enzymatic activation of myosin light chain kinase at different concentrations of calmodulin and Ca2+ revealed that this Ca2+-free complex is inactive and that activation is concomitant with the formation of the enzyme.calmodulin.Ca3 complex.     

Binding of simple peptides, hormones, and neurotransmitters by calmodulin

We have prepared a fluorescent conjugate of porcine calmodulin with 5-(dimethylamino)-1-naphthalene-sulfonyl chloride that is highly sensitive to both calcium binding and protein binding. We have used the fluorescence of this conjugate in addition to the intrinsic peptide fluorescence to show that adrenocorticotropic hormone (ACTH), beta-endorphin, glucagon, and substance P undergo calcium-dependent binding by calmodulin, with competition for common binding sites. The dissociation constants determined in the presence of 0.85 mM CaCl2 and 0.2 N KC1, pH 7.3 at 25 degrees C, range from 1.5 muM to 3.4 muM. The alpha-melanocyte-stimulating hormone, bombesin, and somatostatin also bind, with dissociation constants between 60 muM and 90 muM. Angiotensins I and III, bradykinin, neurotensin, physalaemin, substance P octapeptide, insulin, and Leu- and Met-enkephalin show little or no binding. Sequence comparisons show that the peptides that bind calmodulin well contain regions structurally similar to the recognition sequence for the cAMP-dependent protein kinase and to the sequences surrounding phosphorylated serine residues in several calmodulin binding proteins. This result suggests that modification of calmodulin binding sites in calmodulin-dependent proteins is one of the functions of protein kinase. Calcium has a dual role in peptide binding by calmodulin. The occupation of calcium binding sites having a pK approximately 4 results in a 2-fold increase in peptide binding affinity.     

Backbone dynamics and energetics of a calmodulin domain mutant exchanging between closed and open conformations.

Previous studies have suggested that the Ca2+-saturated E140Q mutant of the C-terminal domain of calmodulin exhibits equilibrium exchange between "open" and "closed" conformations similar to those of the Ca2+-free and Ca2+-saturated states of wild-type calmodulin. The backbone dynamics of this mutant were studied using15N spin relaxation experiments at three different temperatures. Measurements at each temperature of the15N rate constants for longitudinal and transverse auto-relaxation, longitudinal and transverse cross-correlation relaxation, and the1H-15N cross-relaxation afforded unequivocal identification of conformational exchange processes on microsecond to millisecond time-scales, and characterization of fast fluctuations on picosecond to nanosecond time-scales using model-free approaches. The results show that essentially all residues of the protein are involved in conformational exchange. Generalized order parameters of the fast internal motions indicate that the conformational substates are well folded, and exclude the possibility that the exchange involves a significant population of unfolded or disordered species. The temperature dependence of the order parameters offers qualitative estimates of the contribution to the heat capacity from fast fluctuations of the protein backbone, revealing significant variation between the well-ordered secondary structure elements and the more flexible regions. The temperature dependence of the conformational exchange contributions to the transverse auto-relaxation rate constants directly demonstrates that the microscopic exchange rate constants are greater than 2.7x10(3)s-1at 291 K. The conformational exchange contributions correlate with the chemical shift differences between the Ca2+-free and Ca2+-saturated states of the wild-type protein, thereby substantiating that the conformational substates are similar to the open and closed states of wild-type calmodulin. Taking the wild-type chemical shifts to represent the conformational substates of the mutant and populations estimated previously, the microscopic exchange rate constants could be estimated as 2x10(4)to 3x10(4)s-1at 291 K for a subset of residues. The temperature depen dence of the exchange allows the characterization of apparent energy barriers of the conformational transition, with results suggesting a complex process that does not correspond to a single global transition between substates.     

Interactions of flavins with melanin. Studies on equilibrium binding of riboflavin to dopa-melanin and some spectroscopic characteristics of flavin-melanin complex

Natural melanins are photoprotective pigments that in mammals are principally found in the skin, hair, and eyes. Although the molecular mechanism of photoprotection of pigmented cells has not yet been established, several hypotheses have been proposed with melanin acting as a light filter, free radical scavenger, and quencher of electronically excited states of reactive intermediates. It can be expected that the detoxicating efficiency of melanin should be enhanced if the melanin and potentially cytotoxic species are brought close together. Such a situation may occur for a number of photosensitizing dyes that have the ability to bind to melanin. The interaction of melanin with flavins has been studied under strictly controlled experimental conditions. The equilibrium dialysis method has been employed to determine dissociation constants and the number of binding sites in melanin at pH 5-9. The data reveal that synthetic DOPA-melanin has two different classes of binding sites with dissociation constants of 10(-6) and 10(-5) M, respectively. The overall binding capacity of melanin, at pH 7, is 250 nmol RF/mg melanin. The amount of bound-to-melanin RF increases with pH. The absorption spectra of melanin complexes with RF and lumiflavin indicate that hydrophobic interaction may be involved in the binding of these flavins by melanin. No changes in flavin fluorescence have been detected after binding of flavin to melanin. It appears that, contrary to cationic photosensitizing dyes, the singlet excited state of flavin molecules is not quenched by melanin.     

Irrelevance of the Power Stroke for the Directionality,Stopping Force,and Optimal Efficiency of Chemically Driven Molecular Machines

A simple model for a chemically driven molecular walker shows that the elastic energy stored by the molecule and released during the conformational change known as the power-stroke (i.e., the free-energy difference between the pre- and post-power-stroke states) is irrelevant for determining the directionality, stopping force, and efficiency of the motor. Further, the apportionment of the dependence on the externally applied force between the forward and reverse rate constants of the power-stroke (or indeed among all rate constants) is irrelevant for determining the directionality, stopping force, and efficiency of the motor. Arguments based on the principle of microscopic reversibility demonstrate that this result is general for all chemically driven molecular machines, and even more broadly that the relative energies of the states of the motor have no role in determining the directionality, stopping force, or optimal efficiency of the machine. Instead, the directionality, stopping force, and optimal efficiency are determined solely by the relative heights of the energy barriers between the states. Molecular recognition—the ability of a molecular machine to discriminate between substrate and product depending on the state of the machine—is far more important for determining the intrinsic directionality and thermodynamics of chemo-mechanical coupling than are the details of the internal mechanical conformational motions of the machine. In contrast to the conclusions for chemical driving, a power-stroke is very important for the directionality and efficiency of light-driven molecular machines and for molecular machines driven by external modulation of thermodynamic parameters.     

The design, synthesis, and characterization of tight-binding inhibitors of calmodulin

Based on a consideration of the probable structure of calmodulin and some natural peptides known to interact with it, two calmodulin-binding peptides were designed. These peptides bind to calmodulin in helical conformations and are capable of forming electrostatic and hydrophobic interactions with calmodulin. Their dissociation constants for binding (less than or equal to 210 and 400 pM) place them as the tightest-binding inhibitors of calmodulin thus far reported. The study of the interactions of these and similar peptides with calmodulin will provide valuable insights into the mechanisms whereby calmodulin binds to target enzymes, and it also serves as an excellent model system for exploring the physical chemistry of protein-protein interaction.     

Purification and characterization of calmodulin from porcine renal medulla

A calcium sensitive phosphodiesterase (PDE) activated by an endogenous calmodulin was identified in the cytosolic fraction of porcine renal medulla. The PDE and calmodulin were separated from each other by DEAE-cellulose column chromatography. Calmodulin was purified from a heat-treated supernatant by column chromatography with DEAE-cellulose and hydroxylapatite. The purified renal calmodulin has a molecular weight of 17,500, is heatstable, and has a pI of 4.2. Activation of the renal PDE by calmodulin was immediate and stoichiometric. The renal calmodulin and PDE cross react with bovine brain calmodulin and PDE, indicating a lack of tissue and species specificity. Thus, renal calmodulin is very similar to bovine brain calmodulin. However, renal calmodulin did not affect detergent-solubilized or membrane-bound renal adenylate cyclase or the antidiuretic hormone-stimulated activity of the enzyme. These results suggest that calmodulin may function in the renal medulla to regulate cAMP levels by stimulation of PDE but not adenylate cyclase. However, the ubiquitous distribution of calmodulin in eukaryotic cells and its effects on a number of other enzymes allow the possibility that calmodulin may have a role in renal function other than cAMP metabolism.     

Rates of cyanide binding to the catalytic intermediates of mammalian cytochrome c oxidase, and the effects of cytochrome c and poly(L-lysine).

Rate constants of cyanide binding to 'fast' oxidase have been measured in the fully-oxidised (O), peroxy (P) and ferryl (F) states at pH 8.0. Values of 2.2, 8 and 10 M-1 s-1, respectively, were obtained. Thus, none of these states appears to exhibit a rate that would identify it as the species responsible for the extremely rapid cyanide binding observed during turnover. On the other hand, with 'oxidised' enzyme as prepared, containing a very small fraction of one-electron-reduced (E state) oxidase, a corresponding fraction of enzyme exhibited spectral changes consistent with cyanide binding with a rate constant in excess of 10(4) M-1 s-1. Evidence is presented suggesting that mediation of electron transfer from one-electron-reduced, cyanide-liganded enzyme to free, ferric oxidase, rather than a global protein conformational change of the enzyme, is responsible for the greatly enhanced cyanide binding rates seen in the presence of cytochrome c or poly(L-lysine). Inter-oxidase electron exchange in 'oxidised' enzyme can result in a complicated dependence of the binding rate on cyanide concentration. We have demonstrated that this may give rise to a saturation of the rate of cyanide binding.     

Effects of chronic administration of doxorubicin on myocardial alpha-adrenergic receptors, histamine, cyclic nucleotides, calcium, norepinephrine, calmodulin, and guanylate cyclase activity, and plasma catecholamines in rats

The present study examined changes in the levels of plasma catecholamines and myocardial histamine, guanylate cyclase activity, cyclic nucleotides, calcium, calmodulin, and norepinephrine following chronic administration of doxorubicin (DXR). In addition, changes in myocardial alpha 1-adrenergic receptor density and dissociation constant were measured. Rats received DXR (2 mg/kg) or vehicle weekly by the SC route for 2, 4, 8, and 13 weeks. Rats were sacrificed one week after their last dose. One group of rats treated for 13 weeks was sacrificed at 19 weeks, six weeks after the last dose. Heart histamine was unchanged at 3, 5, 9, and 19 weeks, yet at 14 weeks it was significantly elevated in DXR-treated rats over controls. Cardiac calcium, norepinephrine, and cyclic GMP levels were unchanged throughout the course of the study. Cardiac cAMP and calmodulin levels were unchanged at 3, 5, 9, and 14 weeks. At 19 weeks in DXR-treated rats, cAMP was depressed while calmodulin was elevated. Plasma catecholamines and myocardial guanylate cyclase activity examined at 14 weeks were unchanged. In contrast, alpha 1 receptor density examined at 14 weeks in DXR-treated rats was significantly depressed while the dissociation constant was unchanged. Changes in cAMP and calmodulin are suggestive of a redistribution of calcium, although total levels of calcium were unchanged. The depression of cAMP indicates damage to the membrane bound enzyme, adenylate cyclase, and that the membrane interaction of doxorubicin appears to be an integral part of the biochemical mechanism of its toxicity.     

Fluorescence correlation spectroscopy: past, present, future

In recent years fluorescence correlation spectroscopy (FCS) has become a routine method for determining diffusion coefficients, chemical rate constants, molecular concentrations, fluorescence brightness, triplet state lifetimes, and other molecular parameters. FCS measures the spatial and temporal correlation of individual molecules with themselves and so provides a bridge between classical ensemble and contemporary single-molecule measurements. It also provides information on concentration and molecular number fluctuations for nonlinear reaction systems that complement single-molecule measurements. Typically implemented on a fluorescence microscope, FCS samples femtoliter volumes and so is especially useful for characterizing small dynamic systems such as biological cells. In addition to its practical utility, however, FCS provides a window on mesoscopic systems in which fluctuations from steady states not only provide the basis for the measurement but also can have important consequences for the behavior and evolution of the system. For example, a new and potentially interesting field for FCS studies could be the study of nonequilibrium steady states, especially in living cells.     

Calmodulin N-methyltransferase. Partial purification and characterization

The distribution, properties, and substrate specificity of S-adenosylmethionine:calmodulin (lysine) N-methyltransferse (EC 2.1.1.60, calmodulin N-methyltransferase) of the rat have been studied. This enzyme is cytosolic and is found at high levels in tissues with high levels of calmodulin and at low levels in tissues with little calmodulin. In liver, heart, and skeletal muscle, which have low levels of calmodulin and very low calmodulin N-methyltransferase activity (a low ratio of calmodulin N-methyltransferase to calmodulin), calmodulin was found to be incompletely methylated, as judged by its ability to act as a substrate for purified calmodulin N-methyltransferase. Calmodulin N-methyltransferase was purified 470-fold with a 33% yield from rat testis cytosol, using ammonium sulfate precipitation and chromatography on DEAE-cellulose, CM-Sepharose, and Sephadex G-100. At pH 7.4, calmodulin N-methyltransferase did not bind to DEAE-cellulose, but bound strongly to CM-Sepharose. The enzyme eluted from Sephadex G-100 with an apparent molecular weight of 55,000. Purified calmodulin N-methyltransferase was incubated with extracts of rat tissues, and [methyl-3H]AdoMet and methylated proteins were resolved by electrophoresis in an attempt to discover substances other than calmodulin, but this enzyme only catalyzed the methylation of calmodulin, indicating a high degree of substrate specificity. Conditions were established for the in vitro preparative methylation of des(methyl)-calmodulin from Dictyostelium discoideum. Three moles of methyl/mol of calmodulin were incorporated into lysine 115 of des(methyl)calmodulin, resulting in the formation of 1 mol of trimethyllysine at the site normally methylated in calmodulins from most species. Activation of cyclic nucleotide phosphodiesterase by des(methyl)calmodulin was indistinguishable from activation by in vitro methylated or sham methylated Dictyostelium calmodulin, indicating that methylation does not affect the ability of calmodulin to activate this enzyme.     

Sequential assignment of 1H, 15N, 13C resonances and secondary structure of human calmodulin-like protein determined by NMR spectroscopy.

Human calmodulin-like protein (CLP) is closely related to vertebrate calmodulin, yet its unique cell specific expression pattern, overlapping but divergent biochemical properties, and specific target proteins suggest that it is not an isoform of calmodulin. To gain insight into the structural differences that may underlie the difference target specificities and biochemical properties of CLP when compared to calmodulin, we determined the sequential backbone assignment and associated secondary structure of 144 out of the 148 residues of Ca2+-CLP by using multinuclear multidimensional NMR spectroscopy. Despite a very high overall degree of structural similarity between CLP and calmodulin, a number of significant differences were found mainly in the length of alpha-helices and in the central nonhelical flexible region. Interestingly, the regions of greatest primary sequence divergence between CLP and calmodulin in helices III and VIII displayed only minor secondary structure differences. The data suggest that the distinct differences in target specificity and biochemical properties of CLP and calmodulin result from the sum of several minor structural and side-chain changes spread over multiple domains in these proteins.