Chimeric calmodulin-cardiac troponin C proteins differentially activate calmodulin target enzymes

La3+ stimulate the activity of calcineurin in two different ways

It is well known that the activity of calcineurin (CaN) could be modulated by several transitional metal ions. In the present work, the effects of a calcium analog, lanthanum ion (La(3+)), on the activity of CaN were studied. It was found that La(3+) exerted multiple effects on CaN activity. La(3+) could stimulate CaN in the absence of calmodulin (CaM); whereas at low concentrations of La(3+), there was a slight inhibition of activation of CaN in the presence of CaM. Competitive experiments and limited trypsin proteolysis confirmed that La(3+) did not act on the catalytic core of CaN, but exerted its effect through direct action on the CaN regulatory domain similar to Mg(2+). In activity titration and spot blotting studies, La(3+)-containing CaM complexes were less effective in stimulating CaN than Ca(2+) or Mn(2+)-containing CaM; however, the binding affinity of these metal-CaM complexes to CaN was similar. These effects of La(3+) on CaN activity are unique among metal ions and may provide clues to understand the biological effects of La(3+).

Differential activation of nitric-oxide synthase isozymes by calmodulin-troponin C chimeras

The interactions of neuronal nitric-oxide synthase (nNOS) with calmodulin (CaM) and mutant forms of CaM, including CaM-troponin C chimeras, have been previously reported, but there has been no comparable investigation of CaM interactions with the other constitutively expressed NOS (cNOS), endothelial NOS (eNOS), or the inducible isoform (iNOS). The present study was designed to evaluate the role of the four CaM EF hands in the activation of eNOS and iNOS. To assess the role of CaM regions on aspects of enzymatic function, three distinct activities associated with NOS were measured: NADPH oxidation, cytochrome c reduction, and nitric oxide (*NO) generation as assessed by the oxyhemoglobin capture assay. CaM activates the cNOS enzymes by a mechanism other than stimulating electron transfer into the oxygenase domain. Interactions with the reductase moiety are dominant in cNOS activation, and EF hand 1 is critical for activation of both nNOS and eNOS. Although the activation patterns for nNOS and eNOS are clearly related, effects of the chimeras on all the reactions are not equivalent. We propose that cytochrome c reduction is a measure of the release of the FMN domain from the reductase complex. In contrast, cytochrome c reduction by iNOS is readily activated by each of the chimeras examined here and may be constitutive. Each of the chimeras were co-expressed with the human iNOS enzyme in Escherichia coli and subsequently purified. Domains 2 and 3 of CaM contain important elements required for the Ca2+/CaM independence of *NO production by the iNOS enzyme. The disparity between cytochrome c reduction and *NO production at low calcium can be attributed to poor association of heme and FMN domains when the bound CaM constructs are depleted of Ca2+. In general cNOSs are much more difficult to activate than iNOS, which can be attributed to their extra sequence elements, which are adjacent to the CaM-binding site and associated with CaM control.

Myosin light chain kinase: functional domains and structural motifs

Conventional myosin light chain kinase found in differentiated smooth and non-muscle cells is a dedicated Ca2+/calmodulin-dependent protein kinase which phosphorylates the regulatory light chain of myosin II. This phosphorylation increases the actin-activated myosin ATPase activity and is thought to play major roles in a number of biological processes, including smooth muscle contraction. The catalytic domain contains residues on its surface that bind a regulatory segment resulting in autoinhibition through an intrasteric mechanism. When Ca2+/calmodulin binds, there is a marked displacement of the regulatory segment from the catalytic cleft allowing phosphorylation of myosin regulatory light chain. Kinase activity depends upon Ca2+/calmodulin binding not only to the canonical calmodulin-binding sequence but also to additional interactions between Ca2+/calmodulin and the catalytic core. Previous biochemical evidence shows myosin light chain kinase binds tightly to actomyosin containing filaments. The kinase has low-affinity myosin and actin binding sites in Ig-like motifs at the N- and C-terminus, respectively. Recent results show the N-terminus of myosin light chain kinase is responsible for filament binding in vivo. However, the apparent binding affinity is greater for smooth muscle myofilaments, purified thin filaments, or actin-containing filaments in permeable cells than for purified smooth muscle F-actin or actomyosin filaments from skeletal muscle. These results suggest a protein on actin thin filaments that may facilitate kinase binding. Myosin light chain kinase does not dissociate from filaments in the presence of Ca2+/calmodulin raising the interesting question as to how the kinase phosphorylates myosin in thick filaments if it is bound to actin-containing thin filaments.

Identification of troponin C antagonists from a phage-displayed random peptide library

Affinity purification of a phage-displayed library, expressing random peptide 12-mers at the N terminus of protein III, has identified 10 distinct novel sequences which bind troponin C specifically. The troponin C-selected peptides yield a consensus binding sequence of (V/L)(D/E)XLKXXLXXLA. Sequence comparison revealed as much as a 62.5% similarity between phiT5, the peptide sequence of the phage clone with the highest level of binding to troponin C, and the N-terminal region of troponin I isoforms. Biotinylated peptides corresponding to library-derived sequences and similar sequences from various isoforms of troponin I were synthesized shown to bind troponin C specifically. Alkaline phosphatase fusion proteins of two of the phage clone sequences bound troponin C specifically, and were specifically competed by both library-derived and native troponin I peptides. Measurement of equilibrium dissociation constants of the peptides by surface plasmon resonance yielded dissociation constants for troponin C as low as 0.43 microM for pT5; in contrast, dissociation constants for calmodulin were greater than 6 microM for all peptides studied. Nondenaturing polyacrylamide gel electrophoresis demonstrated that pT5 formed a stable complex with troponin C in the presence of calcium. We also found that the pT5 peptide inhibited the maximal calcium-activated tension of rabbit psoas muscle fibers.

Regulatory segments of Ca2+/calmodulin-dependent protein kinases

Catalytic cores of skeletal and smooth muscle myosin light chain kinases and Ca2+/calmodulin-dependent protein kinase II are regulated intrasterically by different regulatory segments containing autoinhibitory and calmodulin-binding sequences. The functional properties of these regulatory segments were examined in chimeric kinases containing either the catalytic core of skeletal muscle myosin light chain kinase or Ca2+/calmodulin-dependent protein kinase II with different regulatory segments. Recognition of protein substrates by the catalytic core of skeletal muscle myosin light chain kinase was altered with the regulatory segment of protein kinase II but not with smooth muscle myosin light chain kinase. Similarly, the catalytic properties of the protein kinase II were altered with regulatory segments from either myosin light chain kinase. All chimeric kinases were dependent on Ca2+/calmodulin for activity. The apparent Ca2+/calmodulin activation constant was similarly low with all chimeras containing the skeletal muscle catalytic core. The activation constant was greater with chimeric kinases containing the catalytic core of Ca2+/calmodulin-dependent protein kinase II with its endogenous or myosin light chain kinase regulatory segments. Thus, heterologous regulatory segments affect substrate recognition and kinase activity. Furthermore, the sensitivity to calmodulin activation is determined primarily by the respective catalytic cores, not the calmodulin-binding sequences.

Interactions between domains of apo calmodulin alter calcium binding and stability

Calmodulin (CaM) is an essential protein that exerts exquisite spatial and temporal control over diverse eukaryotic processes. Although the two half-molecule domains of CaM each have two EF-hands and bind two calcium ions cooperatively, they have distinct roles in activation of some targets. Interdomain interactions may mediate coordination of their actions. Proteolytic footprinting titrations of CaM [Pedigo and Shea (1995) Biochemistry 34, 1179-1196; Shea, Verhoeven, and Pedigo (1996) Biochemistry 35, 2943-2957] showed that calcium binding to the high-affinity sites (III and IV in the C-domain) alters the conformation of helix B in the N-domain despite sites I and II being vacant. This may arise from calcium-induced disruption of interactions between the apo domains. In this study, comparing the cloned domains (residues 1-75, 76-148) to whole CaM, the proteolytic susceptibility of helix B in the apo isolated N-domain was higher than in apo CaM. The isolated N-domain was monotonically protected by calcium binding and had a higher calcium affinity than when part of whole CaM. The change in affinity was small (1-1.5 kcal/mol) but acted to separate the domain saturation curves of whole CaM. Unfolding enthalpies and melting temperatures of the apo isolated domains did not correspond to the two transitions resolved for apo CaM. In summary, the interactions between domains of apo CaM protected the N-domain from proteolysis and raised its Tm by 10 degrees C, demonstrating that CaM is not the sum of its parts.

Interaction of the recombinant S100A1 protein with twitchin kinase, and comparison with other Ca2+-binding proteins

The giant myosin-associated twitchin kinase, a member of the Ca2+-regulated protein kinase superfamily, is activated by the EF-hand protein S100A1 in a Ca2+-dependent and Zn2+-enhanced manner. We used recombinant S100A1 to further characterize the interaction between the two proteins. Zn2+ enhanced the binding of Ca2+/S100A1 to twitchin kinase fragments (Kd < 50 nM) in assays using a BIAcore biosensor by reducing the S100A1 off rate. Other Ca2+-binding proteins (S100A6, calmodulin, and the calmodulin-like domain of Ca2+-dependent protein kinase alpha) bound to the kinase but did not activate it. These results indicate that binding of Ca2+-binding proteins alone is insufficient to trigger the intramolecular rearrangement of kinase autoinhibitory contacts required for twitchin kinase activation that is specifically elicited by the S100A1 protein. Kinase fragments that contained only the autoinhibited catalytic sequence or an additional immunoglobulin-like domain had very similar properties, indicating that the tethered immunoglobulin-like domain does not modulate kinase regulation.

Differential activation of NAD kinase by plant calmodulin isoforms. The critical role of domain I

NAD kinase is a Ca2+/calmodulin (CaM)-dependent enzyme capable of converting cellular NAD to NADP. The enzyme purified from pea seedlings can be activated by highly conserved soybean CaM, SCaM-1, but not by the divergent soybean CaM isoform, SCaM-4 (Lee, S. H., Kim, J. C., Lee, M. S., Heo, W. D., Seo, H. Y., Yoon, H. W., Hong, J. C., Lee, S. Y., Bahk, J. D., Hwang, I., and Cho, M. J. (1995) J. Biol. Chem. 270, 21806-21812). To determine which domains were responsible for this differential activation of NAD kinase, a series of chimeric SCaMs were generated by exchanging functional domains between SCaM-4 and SCaM-1. SCaM-4111, a chimeric SCaM-1 that contains the first domain of SCaM-4, was severely impaired (only 40% of maximal) in its ability to activate NAD kinase. SCaM-1444, a chimeric SCaM-4 that contains the first domain of SCaM-1 exhibited nearly full ( approximately 70%) activation of NAD kinase. Only chimeras containing domain I of SCaM-1 produced greater than half-maximal activation of NAD kinase. To define the amino acid residue(s) in domain I that were responsible for this differential activation, seven single residue substitution mutants of SCaM-1 were generated and tested for NAD kinase activation. Among these mutants, only K30E and G40D showed greatly reduced NAD kinase activation. Also a double residue substitution mutant, K30E/G40D, containing these two mutations in combination was severely impaired in its NAD kinase-activating potential, reaching only 20% of maximal activation. Furthermore, a triple mutation, K30E/M36I/G40D, completely abolished NAD kinase activation. Thus, our data suggest that domain I of CaM plays a key role in the differential activation of NAD kinase exhibited by SCaM-1 and SCaM-4. Further, the residues Lys30 and Glu40 of SCaM-1 are critical for this function.

Functional consequences of truncating amino acid side chains located at a calmodulin-peptide interface

To test the relevance of the calmodulin-peptide crystal structures to their respective calmodulin-enzyme interactions, amino acid side chains in calmodulin were altered at positions that interact with the calmodulin-binding peptide of smooth muscle myosin light chain kinase but not with the calmodulin kinase IIalpha peptide. Since shortening the side chains of Trp-800, Arg-812, and Leu-813 in smooth muscle myosin light chain kinase abrogated calmodulin-dependent activation (Bagchi, I. C., Huang, Q., and Means, A. R. (1992) J. Biol. Chem. 267, 3024-3029), substitutions were introduced at positions in calmodulin which contact residues corresponding to Arg-812 and Leu-813 in the smooth muscle myosin light chain kinase peptide. Assays of smooth muscle myosin light chain kinase with the calmodulin mutants M51A,V55A, L32A,M51A,V55A, and L32A,M51A,V55A,F68L, M71A exhibited 60%, 25%, and less than 1% of maximal activity respectively, whereas the mutants fully activated calmodulin kinase IIalpha. Alanine substitutions at positions on the smooth muscle myosin light chain kinase peptide, corresponding to Trp-800 and Arg-812 in the enzyme, produced an 8-fold increase in the enzyme inhibition constant in contrast with the abolition of calmodulin binding by similar mutations in the parent enzyme.

Localization of unique functional determinants in the calmodulin lobes to individual EF hands

We have investigated the functional interchangeability of EF hands I and III or II and IV, which occupy structurally analogous positions in the native I-II and III-IV EF hand pairs of calmodulin. Our approach was to functionally characterize four engineered proteins, made by replacing in turn each EF hand in one pair by a duplicate of its structural analog in the other. In this way functional determinants we define as unique were localized to the component EF hands in each pair. Replacement of EF hand I by III reduces calmodulin-dependent activation of cerebellar nitric oxide synthase activity by 50%. Replacement of EF hand IV by II reduces by 60% activation of skeletal muscle myosin light chain kinase activity. There appear to be no major unique determinants for activation of these enzyme activities in the other EF hands. Replacement of EF hand III by I or IV by II reduces by 50-80% activation of smooth muscle myosin light chain kinase activity, and replacement of EF hand I by III or II by IV reduces by 90% activation of this enzyme activity. Thus, calmodulin-dependent activation of each of the enzyme activities examined, even the closely related kinases, is dependent upon a distinct pattern of unique determinants in the four EF hands of calmodulin. All the engineered proteins examined bind four Ca2+ ions with high affinity. Comparison of the Ca2+-binding properties of native and engineered CaMs indicates that the Ca2+-binding affinity of an engineered I-IV EF hand pair and a native I-II pair are similar, but an engineered III-II EF hand pair is intermediate in affinity to the native III-IV and I-II pairs, minimally suggesting that EF hands I and III contain unique determinants for the formation and function of EF hand pairs. The residues directly coordinating Ca2+ ion appear to play little or no role in establishing the different Ca2+-binding properties of the EF hand pairs in calmodulin.

Methionine to glutamine substitutions in the C-terminal domain of calmodulin impair the activation of three protein kinases

The 9 methionine residues of vertebrate calmodulin (CaM) were individually changed to glutamine residues in order to investigate their roles in enzyme binding and activation. The mutant proteins showed three classes of effect on the activation of smooth muscle myosin light chain kinase, CaM-dependent protein kinase IIalpha, and CaM-dependent protein kinase IV. First, some mutations had no appreciable effect on the ability of CaM to activate the three protein kinases. Included in this category were glutamine substitutions at residues 36 and 51 in the N-terminal domain, at residue 76 in the domain linker sequence, and at residues 144 and 145 in the C-terminal domain. Second, glutamine substitutions in the N-terminal domain of CaM, particularly those at positions 71 and 72, lowered the maximal activity of smooth muscle myosin light chain kinase while having no effect on the other two enzymes. Finally the affinity of CaM for all three enzymes was lowered by glutamine mutations at the neighboring methionines 109 and 124, located on a solvent-accessible surface of the C-terminal domain of Ca2+/CaM. This last result provides the first demonstration of the involvement of the same hydrophobic groups in the high affinity binding of CaM to three different enzymes.

Different mechanisms for Ca2+ dissociation from complexes of calmodulin with nitric oxide synthase or myosin light chain kinase

We have determined the stoichiometry and rate constants for the dissociation of Ca2+ ion from calmodulin (CaM) complexes with rabbit skeletal muscle myosin light chain kinase (skMLCK), rat brain nitric oxide synthase (nNOS) or with the respective peptides (skPEP and nPEP) representing the CaM-binding domains in these enzymes. Ca2+ dissociation kinetics determined by stopped-flow fluorescence using the Ca2+ chelator quin-2 MF are as follows. 1) Two sites in the CaM-nNOS and CaM-nPEP complexes have a rate constant of 1 s-1. 2) The remaining two sites have a rate constant of 18 s-1 for CaM-nPEP and > 1000 s-1 for CaM-nNOS. 3) Three sites have a rate constant of 1.6 s-1 for CaM-skMLCK and 0.15 s-1 for CaM-skPEP. 4) The remaining site has a rate constant of 2 s-1 for CaM-skPEP and > 1000 s-1 for CaM-skMLCK. Comparison of these rate constants with those determined for complexes between the peptides and tryptic fragments representing the C- or N-terminal lobes of CaM indicate a mechanism for Ca2+ dissociation from CaM-nNOS of 2C slow + 2N fast and from CaM-skMLCK of (2C + 1N) slow + 1N fast. Ca2+ removal inactivates CaM-nNOS and CaM-skMLCK activities with respective rate constants of > 10 s-1 and 1 s-1. CaM-nNOS inactivation is fit by a model in which rapid Ca2+ dissociation from the N-terminal lobe of CaM is coupled to enzyme inactivation and slower Ca2+ dissociation from the C-terminal lobe is coupled to dissociation of the CaM-nNOS complex. CaM-skMLCK inactivation is fit by a model in which the three slowly dissociating Ca(2+)-binding sites are coupled to both dissociation of the complex and enzyme inactivation.

The calmodulin-nitric oxide synthase interaction. Critical role of the calmodulin latch domain in enzyme activation

The neuronal isoform of nitric oxide synthase (nNOS) requires calmodulin for nitric oxide producing activity. Calmodulin functions as a molecular switch, allowing electron transport from the carboxyl-terminal reductase domain of nitric oxide synthase to its heme-containing amino-terminal domain. Available evidence suggests that calmodulin binds to a site between the two domains of nNOS, but it is not known how calmodulin then executes its switch function. To study the calmodulin-nNOS interaction, we created a series of chimeras between calmodulin and cardiac troponin C (cTnC, a homologue of calmodulin that does not activate nNOS). Although a few chimeras showed good ability to activate nNOS, most failed to activate. A subset of the inactive chimeras retained the ability to bind to nNOS and therefore functioned as potent competitive inhibitors of nNOS activation by calmodulin (CaM). The observed inhibition was additive with the arginine antagonists NG-monomethyl-L-arginine and 7-nitroindazole, indicating a distinct and independent mechanism of nNOS inhibition. To localize the calmodulin residues that account for impaired activation in the inhibitory CaM-cTnC chimeras, we conducted a detailed mutagenesis study, replacing CaM subdomains and individual amino acid residues with the corresponding residues from cTnC. This revealed that mutations in CaM helices 2 and 6 (its latch domain) have a disproportionate negative effect on nNOS activation. Thus, our evidence suggests that the CaM latch domain plays a critical role in its molecular switch function.

Selective activation and inhibition of calmodulin-dependent enzymes by a calmodulin-like protein found in human epithelial cells

A calmodulin-like protein, which is identical in size and 85% identical to vertebrate calmodulin, was recently identified by 'subtractive hybridization' comparison of transcripts expressed in normal versus transformed human mammary epithelial cells. Unlike the ubiquitous distribution of calmodulin, calmodulin-like protein expression is restricted to certain epithelial cells, and appears to be modulated during differentiation. In addition, calmodulin-like protein levels are often significantly reduced in malignant tumor cells as compared to corresponding normal epithelial cells. The current studies compare calmodulin-like protein functions with those of calmodulin. We find that calmodulin-like protein activation of multifunctional Ca2+/calmodulin-dependent protein kinase II (calmodulin kinase II) is equivalent to activation by calmodulin, but that four other calmodulin-dependent enzymes, cGMP phosphodiesterase, calcineurin, nitric-oxide synthase, and myosin-light-chain kinase, display much weaker activation by calmodulin-like protein than by calmodulin. In the case of myosin-light-chain kinase, calmodulin-like protein competitively inhibits calmodulin activation of the enzyme with a Ki value of 170 nM. Thus, calmodulin-like protein may have evolved to function as a specific agonist of certain calmodulin-dependent enzymes, and/or as a specific competitive antagonist of other calmodulin-dependent enzymes.

The latch region of calcineurin B is involved in both immunosuppressant-immunophilin complex docking and phosphatase activation

The immunosuppressants cyclosporin A and FK506, when complexed with their intracellular receptors, prevent T cell activation by directly binding to the phosphatase calcineurin. We have used molecular modeling and mutagenesis to identify sites on calcineurin important for this interaction. We have created calcineurins that are resistant to both cyclosporin A and FK506 by mutating specific residues in CnB, a calcium-binding protein that regulates the catalytic subunit, CnA. Significantly, on a model of CnB, these mutations map to the latch region, an element of tertiary structure that forms when CnB binds CnA. In addition, we show that this latch region plays an important role in activating the catalytic subunit CnA. These results suggest a molecular mechanism for suppression of calcineurin by cyclosporin A and FK506 involving their binding to the same region of CnB used for allosterically activating CnA.

Calmodulin is intrinsically LESS effective than troponin C in activating skeletal muscle contraction

Calmodulin (CaM) and troponin C (TnC) are evolutionarily and structurally homologous, yet they are not functionally interchangeable. In particular, CaM cannot effectively substitute for TnC as an activator of skeletal muscle contraction. To determine if this is a consequence of CaM's weak association with troponin T and I or the result of a more fundamental mechanistic defect, we have used CaM and a CaM[TnC] chimera, CaM[3,4 TnC], that stably associates with the thin filament. Replacement of TnC with CaM or CaM[3,4 TnC] reveals that CaM-like molecules reduce the Ca(2+)-sensitivity and cooperativity of activation, as well as the maximal Ca(2+)-activated tension. These observations indicate that CaM-like molecules are unable to continuously maintain the activated state of the thin filament.

Gain of function mutations for yeast calmodulin and calcium dependent regulation of protein kinase activity

Yeast calmodulin binds only three calcium ions in the presence of millimolar concentrations of magnesium due to a defective calcium-binding sequence in its carboxyl terminal domain. Yeast calmodulin's diminished calcium-binding activity can be restored to that of other calmodulins by the use of site-directed mutagenesis to substitute its fourth calcium-binding domain with that of a vertebrate calmodulin sequence. However, the repair of yeast calmodulin's calcium-binding activity is not sufficient to repair quantitatively yeast calmodulin's defective protein kinase activator activity. Yeast calmodulin's activator activity with smooth muscle and skeletal muscle myosin light chain kinases and brain calmodulin-dependent protein kinase II can be progressively repaired by additional substitutions of vertebrate calmodulin sequences, provided that the four calcium-binding sites remain intact. An unexpected result obtained during the course of these studies was the observation that myosin light chain kinases from smooth and skeletal muscle tissues can respond differently to mutations in calmodulin. These and previous results indicate that the binding of four calcium ions by calmodulin is necessary but not sufficient to bring about quantitative activation of protein kinases, and are consistent with the conformational selection/restriction model of the dynamic equilibrium among calcium, calmodulin and each calmodulin regulated enzyme.

Structure of Paramecium tetraurelia calmodulin at 1.8 A resolution

The crystal structure of calmodulin (CaM; M(r) 16,700, 148 residues) from the ciliated protozoan Paramecium tetraurelia (PCaM) has been determined and refined using 1.8 A resolution area detector data. The crystals are triclinic, space group P1, a = 29.66, b = 53.79, c = 25.49 A, alpha = 92.84, beta = 97.02, and gamma = 88.54 degrees with one molecule in the unit cell. Crystals of the mammalian CaM (MCaM; Babu et al., 1988) and Drosophila CaM (DCaM; Taylor et al., 1991) also belong to the same space group with very similar cell dimensions. All three CaMs have 148 residues, but there are 17 sequence changes between PCaM and MCaM and 16 changes between PCaM and DCaM. The initial difference in the molecular orientation between the PCaM and MCaM crystals was approximately 7 degrees as determined by the rotation function. The reoriented Paramecium model was extensively refitted using omit maps and refined using XPLOR. The R-value for 11,458 reflections with F > 3 sigma is 0.21, and the model consists of protein atoms for residues 4-147, 4 calcium ions, and 71 solvent molecules. The root mean square (rms) deviations in the bond lengths and bond angles in the model from ideal values are 0.016 A and 3 degrees, respectively. The molecular orientation of the final PCaM model differs from MCaM by only 1.7 degrees. The overall Paramecium CaM structure is very similar to the other calmodulin structures with a seven-turn long central helix connecting the two terminal domains, each containing two Ca-binding EF-hand motifs. The rms deviation in the backbone N, Ca, C, and O atoms between PCaM and MCaM is 0.52 A and between PCaM and DCaM is 0.85 A. The long central helix regions differ, where the B-factors are also high, particularly in PCaM and MCaM. Unlike the MCaM structure, with one kink at D80 in the middle of the linker region, and the DCaM structure, with two kinks at K75 and I85, in our PCaM structure there are no kinks in the helix; the distortion appears to be more gradually distributed over the entire helical region, which is bent with an apparent radius of curvature of 74.5(2) A. The different distortions in the central helical region probably arise from its inherent mobility.

The heterodimer calmodulin: myosin light-chain kinase as a prototype vertebrate calcium signal transduction complex

The heterodimer complex of calmodulin (CaM) and the protein kinase catalytic subunit of myosin light chain kinase from vertebrate smooth muscle and non-muscle tissues (sm/nmMLCK) is one of the most extensively characterized CaM-regulated enzyme complexes and it has an established in vivo role in the transduction of calcium signals into biological responses. We have used a combination of approaches to the study of CaM and sm/nmMLCK in order to derive initial insight into the key features of each protein and of the CaM-MLCK heterodimeric complex that are involved in protein-protein and calcium-protein recognition and regulation of enzyme activity. On-going studies are described here that include site-specific mutagenesis, fluorescence spectroscopy, enzymology and peptide analog analysis. These and previous results indicate that: (1), both electrostatic and hydrophobic features are important in the functionally correct interactions between CaM and MLCK; (2), even the interactions between CaM and peptide analogs of the CaM binding site of MLCK are heterogeneous and non-trivial in nature; (3), amino-acid residues that have been conserved in CaM across millions of years of evolution and that are conserved in CaMs with quantitative MLCK activator activity can be mutated without any detectable effect on activity and (4), structures different from the prototypical EF-hand domain of CaM can have similar calcium-binding activity in the presence of a CaM binding structure.

Calcium binding induces conformational changes in muscle regulatory proteins

Calcium binding to proteins containing the 'EF-hand' structural motif regulates a variety of biochemical processes including muscle contraction. Techniques such as protein crystallography, site-directed mutagenesis and domain transplantation experiments are being used to unravel the conformational changes induced by calcium binding.

Regulation of smooth muscle myosin light chain kinase by calmodulin

The mutagenesis work described in this paper has been instrumental in furthering our understanding of how CaM binds to and activates MLCK. Figure 2 schematically represents this interaction. The inactive MLCK appears to have a catalytic domain that is repressed by a substrate inhibitory domain that overlaps with the CaM binding domain, a basic amphipathic helix. In the presence of Ca2+, CaM undergoes a conformational change that exposes two hydrophobic pockets, one in each globular lobe, that are important for binding to MLCK. Upon binding CaM, MLCK undergoes a conformational change that derepresses the catalytic site, allows substrate access and light chain phosphorylation. Calmodulin antagonist drugs intercalate within these hydrophobic pockets to interfere with target enzyme binding. The total loss of activity if W800 is altered to A illustrates the importance of these hydrophobic interactions within the enzyme. The basic residues are also important; most of the basic residues in the binding domain of MLCK appear to aid in CaM binding but are not in themselves crucial, this includes the RRK triad. However, a specific electrostatic interaction between R812 of MLCK and CaM is suggested by the complete failure in MLCK activation if this residue is changed to an A. Electrostatic interactions between MLCK and CaM are also indicated by the TaM-BM1 mutant. This mutant can bind to but not activate MLCK. It is hypothesized that TaM-BM1 will bind to the basic amphipathic helix of MLCK but that the alterations in the surface charges (especially E14 and T34) and/or hydrophobicity (S38) prevent the proper conformational change in MLCK necessary for light chain phosphorylation. The resulting MLCK-CaM complex is therefore, inactive but can bind TaM-BM1. The exact interaction of these amino acids in CaM with MLCK will have to await the elucidation of a CaM-MLCK co-crystal.

Regulatory functions of calmodulin

Calmodulin is a Ca2+ binding protein present in all eukaryotic cells that serves as the primary intracellular receptor for Ca2+. This 148 amino acid protein is involved in activation of more than 20 enzymes which mediate a wide variety of physiological processes. Many of these enzymes are inhibited in an intramolecular manner and the Ca(2+)-calmodulin complex relieves this inhibition. Calmodulin is essential for life as disruption of the gene in genetically tractable organisms is lethal. This protein plays important regulatory roles in cell proliferation and is required at multiple points in the cell cycle. The mechanism of enzyme activation by calmodulin and its importance in cell growth regulation are reviewed.