Identification of basic residues involved in activation and calmodulin binding of rabbit smooth muscle myosin light chain kinase

Molecular Insights into the MLCK Activation by CaM

Calmodulin (CaM) is a universal regulatory protein that modulates numerous cellular processes by using calcium (Ca2+) as the signal. In smooth muscle cells (SMC), one major target of CaM is myosin light chain kinase (MLCK), a kinase that phosphorylates the myosin regulatory light chain and thereby regulates cell contraction. In the absence of CaM, MLCK remains inhibited by its autoinhibitory domain (AID). While it is well established that CaM activates MLCK, the molecular interactions between these two proteins remain elusive due to the lack of structural data. In this work, we constructed a molecular model of mammalian CaM (mCaM) in complex with MLCK leveraging AlphaFold, published biochemical data, and protein-protein docking. The model, along with a strategic set of CaM mutants including a inhibitory variant soybean CaM isoform 4 (sCaM-4), was subject to molecular dynamics (MD) simulations. Using principal component analysis (PCA), we mapped out the transition path for the removal of the AID from the MLCK kinase domain to provide molecular basis of MLCK activation. Additionally, we established MLCK conformations that correspond to the active and inactive states of the kinase. We showed that mCaM and sCaM-4 cause MLCK to undergo the transition to the active and inactive states, respectively. Using two structural metrics, we computed the probabilities of MLCK activation by different CaM variants, which were in good agreement with the experimental data. Distributions along these metrics revealed that different inhibitory CaM variants impair MLCK activation through unique mechanisms. We finally identified molecular contacts that contribute to the MLCK activation by CaM. Overall, we report a de novo molecular model of CaM-MLCK that provides insights into the molecular mechanism of MLCK activation by CaM. The mechanism requires effective removal of the AID while preserving an active configuration of the kinase domain. This mechanism may be shared by other MLCK isoforms and potentially other structurally similar kinases with CaM-mediated regulatory domains.

Calmodulin-Calcineurin Interaction beyond the Calmodulin-Binding Region Contributes to Calcineurin Activation

Calcineurin (CaN) is a calcium-dependent phosphatase involved in numerous signaling pathways. Its activation is in part driven by the binding of calmodulin (CaM) to a CaM recognition region (CaMBR) within CaN's regulatory domain (RD). However, secondary interactions between CaM and the CaN RD may be necessary to fully activate CaN. Specifically, it is established that the CaN RD folds upon CaM binding and a region C-terminal to CaMBR, the "distal helix", assumes an α-helix fold and contributes to activation [Dunlap, T. B., et al. (2013) Biochemistry 52, 8643-8651]. We hypothesized in that previous study that this distal helix can bind CaM in a region distinct from the canonical CaMBR. To test this hypothesis, we utilized molecular simulations, including replica-exchange molecular dynamics, protein-protein docking, and computational mutagenesis, to determine potential distal helix-binding sites on CaM's surface. We isolated a potential binding site on CaM (site D) that facilitates moderate-affinity interprotein interactions and predicted that mutation of site D residues K30 and G40 on CaM would weaken CaN distal helix binding. We experimentally confirmed that two variants (K30E and G40D) indicate weaker binding of a phosphate substrate p-nitrophenyl phosphate to the CaN catalytic site by a phosphatase assay. This weakened substrate affinity is consistent with competitive binding of the CaN autoinhibition domain to the catalytic site, which we suggest is due to the weakened distal helix-CaM interactions. This study therefore suggests a novel mechanism for CaM regulation of CaN that may extend to other CaM targets.

Interactions of calmodulin with death-associated protein kinase peptides: experimental and modeling studies

We have studied the interactions between calmodulin (CaM) and three target peptides from the death-associated protein kinase (DAPK) protein family using both experimental and modeling methods, aimed at determining the details of the underlying biological regulation mechanisms. Experimentally, calorimetric binding free energies were determined for the complexes of CaM with peptides representing the DAPK2 wild-type and S308D mutant, as well as DAPK1. The observed affinity of CaM was very similar for all three studied peptides. The DAPK2 and DAPK1 peptides differ significantly in sequence and total charge, while the DAPK2 S308D mutant is designed to model the effects of DAPK2 Ser308 phosphorylation. The crystal structure of the CaM-DAPK2 S308D mutant peptide is also reported. The structures of CaM-DAPK peptide complexes present a mode of CaM-kinase interaction, in which bulky hydrophobic residues at positions 10 and 14 are both bound to the same hydrophobic cleft. To explain the microscopic effects underlying these interactions, we performed free energy calculations based on the approximate MM-PBSA approach. For these highly charged systems, standard MM-PBSA calculations did not yield satisfactory results. We proposed a rational modification of the approach which led to reasonable predictions of binding free energies. All three complexes are strongly stabilized by two effects: electrostatic interactions and buried surface area. The strong favorable interactions are to a large part compensated by unfavorable entropic terms, in which vibrational entropy is the largest contributor. The electrostatic component of the binding free energy followed the trend of the overall peptide charge, with strongest interactions for DAPK1 and weakest for the DAPK2 mutant. The electrostatics was dominated by interactions of the positively charged residues of the peptide with the negatively charged residues of CaM. The nonpolar binding free energy was comparable for all three peptides, the largest contribution coming from the Trp305. About two-thirds of the buried surface area corresponds to nonpolar residues, showing that hydrophobic interactions play an important role in these CaM-peptide complexes. The simulation results agree with the experimental data in predicting a small effect of the S308D mutation on CaM interactions with DAPK2, suggesting that this mutation is not a good model for the S308 phosphorylation.

Structure of the dimeric autoinhibited conformation of DAPK2, a pro-apoptotic protein kinase

The death-associated protein kinase (DAPK) family has been characterized as a group of pro-apoptotic serine/threonine kinases that share specific structural features in their catalytic kinase domain. Two of the DAPK family members, DAPK1 and DAPK2, are calmodulin-dependent protein kinases that are regulated by oligomerization, calmodulin binding, and autophosphorylation. In this study, we have determined the crystal and solution structures of murine DAPK2 in the presence of the autoinhibitory domain, with and without bound nucleotides in the active site. The crystal structure shows dimers of DAPK2 in a conformation that is not permissible for protein substrate binding. Two different conformations were seen in the active site upon the introduction of nucleotide ligands. The monomeric and dimeric forms of DAPK2 were further analyzed for solution structure, and the results indicate that the dimers of DAPK2 are indeed formed through the association of two apposed catalytic domains, as seen in the crystal structure. The structures can be further used to build a model for DAPK2 autophosphorylation and to compare with closely related kinases, of which especially DAPK1 is an actively studied drug target. Our structures also provide a model for both homodimerization and heterodimerization of the catalytic domain between members of the DAPK family. The fingerprint of the DAPK family, the basic loop, plays a central role in the dimerization of the kinase domain.

Mutations in myosin light chain kinase cause familial aortic dissections

Mutations in smooth muscle cell (SMC)-specific isoforms of α-actin and β-myosin heavy chain, two major components of the SMC contractile unit, cause familial thoracic aortic aneurysms leading to acute aortic dissections (FTAAD). To investigate whether mutations in the kinase that controls SMC contractile function (myosin light chain kinase [MYLK]) cause FTAAD, we sequenced MYLK by using DNA from 193 affected probands from unrelated FTAAD families. One nonsense and four missense variants were identified in MYLK and were not present in matched controls. Two variants, p.R1480X (c.4438C>T) and p.S1759P (c.5275T>C), segregated with aortic dissections in two families with a maximum LOD score of 2.1, providing evidence of linkage of these rare variants to the disease (p = 0.0009). Both families demonstrated a similar phenotype characterized by presentation with an acute aortic dissection with little to no enlargement of the aorta. The p.R1480X mutation leads to a truncated protein lacking the kinase and calmodulin binding domains, and p.S1759P alters amino acids in the α-helix of the calmodulin binding sequence, which disrupts kinase binding to calmodulin and reduces kinase activity in vitro. Furthermore, mice with SMC-specific knockdown of Mylk demonstrate altered gene expression and pathology consistent with medial degeneration of the aorta. Thus, genetic and functional studies support the conclusion that heterozygous loss-of-function mutations in MYLK are associated with aortic dissections.

Characterization of the Munc13-calmodulin interaction by photoaffinity labeling

Sensing of and response to transient increases in the residual presynaptic Ca2+ levels are important adaptive mechanisms that define the short-term plasticity characteristics of neurons. Due to their essential function in synaptic vesicle priming and in the modulation of synaptic strength, Munc13 proteins have emerged as key regulators of these adaptive mechanisms. Indeed, Munc13-1 and ubMunc13-2 contain a conserved calmodulin (CaM) binding site and the Ca2+ -dependent interaction of these Munc13 isoforms with CaM constitutes a molecular mechanism that transduces residual Ca2+ signaling to the synaptic exocytotic machinery. Here, we used Munc13-derived model peptides in photoaffinity labeling (PAL) experiments to demonstrate the stoichiometric and Ca2+ -dependent CaM binding of the other members of the Munc13 family, bMunc13-2 and Munc13-3, via structurally distinct non-conserved binding sites. A PAL-based Ca2+ titration assay revealed that all Munc13 isoforms can form a complex with CaM already at low Ca2+ concentrations just above resting levels, underscoring the Ca2+ sensor/effector function of this interaction in short-term synaptic plasticity phenomena.

Rational design of new binding specificity by simultaneous mutagenesis of calmodulin and a target peptide

Calcium-saturated calmodulin (CaM) binds and influences the activity of a varied collection of target proteins in most cells. This promiscuity underlies the role of CaM as a shared participant in calcium-dependent signal transduction pathways but imposes a handicap on popular CaM-based calcium biosensors, which display an undesired tendency to cross-react with cellular proteins. Designed CaM/target pairs that retain high affinity for one another but lack affinity for wild-type CaM and its natural interaction partners would therefore be useful as sensor components and possibly also as elements of "synthetic" cellular-signaling networks. Here, we have adopted a rational approach to creating suitably modified CaM/target complexes by using computational design methods to guide parallel site-directed mutagenesis of both binding partners. A hierarchical design procedure was applied to suggest a small number of complementary mutations on CaM and on a peptide ligand derived from skeletal-muscle light-chain kinase (M13). Experimental analysis showed that the procedure was successful in identifying CaM and M13 mutants with novel specificity for one another. Importantly, the designed complexes retained an affinity comparable to the wild-type CaM/M13 complex. These results represent a step toward the creation of CaM and M13 derivatives with specificity fully orthogonal to the wild-type proteins and show that qualitatively accurate predictions may be obtained from computational methods applied simultaneously to two proteins involved in multiple-linked binding equilibria.

HRPAP20: a novel calmodulin-binding protein that increases breast cancer cell invasion

We previously reported the identification of HRPAP20 (hormone-regulated proliferation-associated protein 20), a novel hormone-regulated, proliferation-associated protein. In tumor cell lines, constitutive HRPAP20 expression enhanced proliferation and suppressed apoptosis, characteristics frequently associated with malignant progression. Here, we report that highly invasive breast cancer cell lines and human breast tumor specimens express elevated HRPAP20, which in transfection experiments in MCF-7 and MDA-MB-231 cells, increased invasion. Results from mechanistic studies revealed that HRPAP20 bound to calmodulin (CaM) via a conserved CaM-binding motif. Transfection of MCF-7 breast cancer cells with HRPAP20 harboring a mutated CaM-binding motif (HRPAP20K73A) inhibited its interaction with CaM and failed to increase invasion. Other experiments revealed that transfection with HRPAP20, but not HRPAP20K73A, increased secretion of matrix metalloproteinase-9 (MMP-9). Moreover, knockdown of HRPAP20 with small interfering RNA in MCF-7/HRPAP20 transfectants and wild-type MDA-MB-231 cells reduced invasion and inhibited secretion of MMP-9. Together these observations suggest that HRPAP20 may be an important regulator of breast tumor cell invasion by a CaM-mediated mechanism that leads to increased MMP-9 secretion. We conclude that dysregulation of HRPAP20 expression in tumor cells may contribute to the observed phenotypic changes associated with breast cancer progression.

Influence of mitochondrial inhibition on global and local [Ca(2+)](I) in rat tail artery

Inhibition of oxidative metabolism is often found to decrease contractility of systemic vascular smooth muscle, but not to reduce global [Ca(2+)](i). In the present study, we probe the hypothesis that it is associated with an altered pattern of intracellular Ca(2+) oscillations (waves) influencing force development. In the rat tail artery, mitochondrial inhibitors (rotenone, antimycin A, and cyanide) reduced alpha(1)-adrenoceptor-stimulated force by 50% to 80%, but did not reduce global [Ca(2+)](i). Less relaxation (about 30%) was observed after inhibition of myosin phosphatase activity with calyculin A, suggesting that part of the metabolic sensitivity involves the regulation of myosin 20-kDa light chain phosphorylation, although no decrease in phosphorylation was found in freeze-clamped tissue. Confocal imaging revealed that the mitochondrial inhibitors increased the frequency but reduced the amplitude of asynchronous cellular Ca(2+) waves elicited by alpha(1) stimulation. The altered wave pattern, in association with increased basal [Ca(2+)](i), accounted for the unchanged global [Ca(2+)](i). Inhibition of glycolytic ATP production by arsenate caused similar effects on Ca(2+) waves and global [Ca(2+)](i), developing gradually in parallel with decreased contractility. Inhibition of wave activity by the InsP(3) receptor antagonist 2-APB correlated closely with relaxation. Furthermore, abolition of waves with thapsigargin in the presence of verapamil reduced force by about 50%, despite unaltered global [Ca(2+)](i), suggesting that contraction may at least partly depend on Ca(2+) wave activity. This study therefore indicates that mitochondrial inhibition influences Ca(2+) wave activity, possibly due to a close spatial relationship of mitochondria and the sarcoplasmic reticulum and that this contributes to metabolic vascular relaxation.

Conformational requirements for Ca(2+)/calmodulin binding and activation of myosin light chain kinase

Myosin light chain kinase contains a regulatory segment consisting of an autoinhibitory region and a calmodulin-binding sequence that folds back on its catalytic core to inhibit kinase activity. It has been proposed that alpha-helix formation may be involved in displacement of the regulatory segment and activation of the kinase by Ca(2+)/calmodulin. Proline mutations were introduced at putative non-interacting residues in the regulatory segment to disrupt helix formation. Substitution of proline residues immediately N-terminal of the Trp in the calmodulin-binding sequence had most significant effects on Ca(2+)/calmodulin binding and activation. Formation of an alpha-helix in this region upon Ca(2+)/calmodulin binding may be necessary for displacement of the regulatory segment allowing phosphorylation of myosin regulatory light chain.

Ca(2+) binding and energy coupling in the calmodulin-myosin light chain kinase complex

We have previously shown that 3 Ca(2+) ions are released cooperatively and 1 independently from the complex between (Ca(2+))4-calmodulin and skeletal muscle myosin light chain kinase or a peptide containing its core calmodulin-binding sequence. We now have found that three Ca(2+)-binding sites also function cooperatively in equilibrium Ca(2+) binding to these complexes. Replacement of sites I and II in calmodulin by a copy of sites III and IV abolishes these cooperative effects. Energy coupling-dependent increases in Ca(2+)-binding affinity in the mutant and native calmodulin complexes with enzyme are considerably less than in the peptide complexes, although the complexes have similar affinities. Ca(2+) binding to three sites in the native calmodulin-enzyme complex is enhanced; the affinity of the remaining site is slightly reduced. In the mutant enzyme complex Ca(2+) binding to one pair of sites is enhanced; the other pair is unaffected. In this complex reversal of enzyme activation occurs when Ca(2+) dissociates from the pair of sites with enhanced affinity; more rapid dissociation from the other pair has no effect, although both pairs participate in activation. Ca(2+)-independent interactions with calmodulin clearly play a major role in the enzyme complex, and appear to weaken Ca(2+)-dependent interactions with the core calmodulin-binding sequence.

Structural examination of autoregulation of multifunctional calcium/calmodulin-dependent protein kinase II

Regulation of Ca(2+)/calmodulin-dependent protein kinase II is likely based on an auto-inhibitory mechanism in which a segment of the kinase occupies the catalytic site in the absence of calmodulin. We analyze potential auto-inhibitory associations by employing charge reversal and hydrophobic-to-charged residue mutagenesis. We identify interacting amino acid pairs by using double mutants to test which modification in the catalytic domain complements a given change in the auto-inhibitory domain. Our studies identify the core pseudosubstrate sequence (residues 297-300) but reveal that distinct sequences centered about the autophosphorylation site at Thr-286 are involved in the critical auto-inhibitory interactions. Individual changes in any of the residues Arg-274, His-282, Arg-283, Lys-291, Arg-297, Phe-293, and Asn-294 in the auto-inhibitory domain or their interacting partners in the catalytic domain produces an enhanced affinity for calmodulin or generates a constitutively active enzyme. A structural model of Ca(2+)/calmodulin-dependent protein kinase II that incorporates these interactions shows that Thr-286 is oriented inwardly into a hydrophobic channel. The model explains why calmodulin must bind to the auto-inhibitory domain in order for Thr-286 in that domain to be phosphorylated and why introduction of phospho-Thr-286 produces the important Ca(2+)-independent state of the enzyme.

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.

Inhibition of calmodulin-activated smooth-muscle myosin light-chain kinase by calmodulin-binding peptides and fluorescent (phosphodiesterase-activating) calmodulin derivatives

Aspects of the biochemistry of calmodulin have been addressed that bear on its cell biological role as a mediator of Ca2+ regulation. Calmodulin-binding peptides derived from the amino acid sequence of smooth-muscle myosin light-chain kinase (MLCK) were characterized as inhibitors of calmodulin activation of MLCK-catalyzed phosphorylation of the smooth-muscle regulatory light chain (MLC). MLCK activity was determined by measuring the rate of formation of one of the reaction products, ADP, in a coupled enzymatic assay by continuous fluorimetric monitoring of NADH removal in 100 microM CaCl2 at ionic strength 0.15 M, pH 7.0 and 21 degreesC. The Km value of calmodulin was 3.5 nM, a value 16-35-fold greater than the Kd value of calmodulin for MLCK [Török, K., and Trentham D. R. (1994) Biochemistry 33, 12807-12820]. The different Km and Kd values are most likely associated with the rate-limiting step in MLC phosphorylation being associated with product release from MLCK. The values of the inhibition constants, Ki, were the following: Ac-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-CONH2 (Trp peptide), 8.6 (+/-1. 4 sd) pM; Y4-analogue of Trp peptide (Tyr peptide), 7.3 (+/-0.1) nM; and A-R-R-K-W-Q-K-T-G-H-A-V-R-A-I-G-R-L-S-S (RS20-like peptide), 0. 11-0.39 nM. The Ki values were consistent with kinetically determined Kd values of the peptides to calmodulin. Kinetic determination of Kd values required the use of a fluorescently labeled calmodulin, 2-chloro-(epsilon-amino-Lys75)-[6-(4-N, N-diethylamino-phenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-calmodulin).1 Since, as here, Lys75 is a convenient labeling site on calmodulin for the introduction of fluorescent probes, the biological activity of the Lys-modified calmodulins was evaluated. TA-calmodulin and calmodulin selectively modified by 1-N, N-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-C1) at Lys75 (dansyl-calmodulin) were characterized as activators of cyclic AMP phosphodiesterase (PDE) and inhibitors of MLCK. The Km value for dansyl-calmodulin was equal to that of calmodulin, and that of TA-calmodulin was 3.5-fold greater. TA-calmodulin and Lys75-labeled dansyl-calmodulin thus distinguish between PDE and MLCK being agonists to the former and antagonists to the latter.

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.

Intermolecular tuning of calmodulin by target peptides and proteins: differential effects on Ca2+ binding and implications for kinase activation

Ca(2+)-activated calmodulin (CaM) regulates many target enzymes by docking to an amphiphilic target helix of variable sequence. This study compares the equilibrium Ca2+ binding and Ca2+ dissociation kinetics of CaM complexed to target peptides derived from five different CaM-regulated proteins: phosphorylase kinase. CaM-dependent protein kinase II, skeletal and smooth myosin light chain kinases, and the plasma membrane Ca(2+)-ATPase. The results reveal that different target peptides can tune the Ca2+ binding affinities and kinetics of the two CaM domains over a wide range of Ca2+ concentrations and time scales. The five peptides increase the Ca2+ affinity of the N-terminal regulatory domain from 14- to 350-fold and slow its Ca2+ dissociation kinetics from 60- to 140-fold. Smaller effects are observed for the C-terminal domain, where peptides increase the apparent Ca2+ affinity 8- to 100-fold and slow dissociation kinetics 13- to 132-fold. In full-length skeletal myosin light chain kinase the inter-molecular tuning provided by the isolated target peptide is further modulated by other tuning interactions, resulting in a CaM-protein complex that has a 10-fold lower Ca2+ affinity than the analogous CaM-peptide complex. Unlike the CaM-peptide complexes, Ca2+ dissociation from the protein complex follows monoexponential kinetics in which all four Ca2+ ions dissociate at a rate comparable to the slow rate observed in the peptide complex. The two Ca2+ ions bound to the CaM N-terminal domain are substantially occluded in the CaM-protein complex. Overall, the results indicate that the cellular activation of myosin light chain kinase is likely to be triggered by the binding of free Ca2(2+)-CaM or Ca4(2+)-CaM after a Ca2+ signal has begun and that inactivation of the complex is initiated by a single rate-limiting event, which is proposed to be either the direct dissociation of Ca2+ ions from the bound C-terminal domain or the dissociation of Ca2+ loaded C-terminal domain from skMLCK. The observed target-induced variations in Ca2+ affinities and dissociation rates could serve to tune CaM activation and inactivation for different cellular pathways, and also must counterbalance the variable energetic costs of driving the activating conformational change in different target enzymes.

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.

A role in enzyme activation for the N-terminal leader sequence in calmodulin

We have found that deletion of residues 2-8 from the N-terminal leader sequence: Ala1-Asp2-Gln-Leu4-Thr-Glu6-Glu-Gln8, in calmodulin abolishes calmodulin-dependent activation of skeletal muscle myosin light chain kinase activity and reduces calmodulin-dependent activation of smooth muscle myosin light chain kinase activity to approximately 50% of the maximum level measured at a saturating calmodulin concentration. Calmodulin-dependent activation of cerebellar nitric oxide synthase activity is not affected by this deletion. Overlapping tripeptide deletions from the leader sequence indicate that the acidic cluster, Glu6-Glu-Gln8, contains the determinants necessary for activation of myosin light chain kinase activity. Deletion of Asp2-Gln-Leu4 has no effect on activation of enzyme activity. Based on enzyme kinetic analyses, deletions in the leader sequence have little or no effect on the apparent affinities of calmodulin for the synthase or the two kinases. Since the N-terminal leader does not appear to play a significant structural role in the complexes between calmodulin and peptides representing the calmodulin-binding domains in the two kinases, our results indicate that it participates in secondary interactions with these enzymes that are important to activation, but not to recognition or binding of calmodulin.

Activation of myosin light chain kinase and nitric oxide synthase activities by engineered calmodulins with duplicated or exchanged EF hand pairs

We have constructed three engineered calmodulins (CaMs) in which the two EF hand pairs have been substituted for one another or exchanged: CaMNN, the C-terminal EF hand pair (residues 82-148) has been replaced by a duplication of the N-terminal pair (residues 9-75); CaMCC, the N-terminal pair has been replaced by a duplication of the C-terminal pair; CaMCN, the two EF had pairs have been exchanged. Skeletal muscle myosin light chain kinase (skMLCK) activity is activated to 75% of the maximum level by CaMCC and to 45% of the maximum level by CaMCN and is not significantly activated by CaMNN; Kact or Ki values for the engineered CaMs are 2-3.5 nM. Smooth muscle myosin light chain kinase activity (gMLCK) is fully activated by CaMCN and is not significantly activated by either CaMNN or CaMCC; the Kact value for CaMCN is 2 nM and the Ki values for CaMNN and CaMCC are 10 and 40 nM, respectively. Cerebellar nitric oxide synthase activity (nNOS) is fully activated by CaMNN and CaMCN and is not significantly activated by CaMCC; the engineered CaMs have Kact or Ki values for this enzyme activity of 2-8 nM. These results indicate that the EF hand pairs contain distinct but overlapping sets of determinants for binding and activation of enzymes, with the greater degree of overlap in determinants for binding. Furthermore, while the structural changes associated with swapping the EF hand pairs do not affect activation of nNOS or gMLCK activities, they significantly reduce activation of skMLCK activity, indicating that this process requires specific determinants in CaM outside the EF hand pairs.

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 regulatory region of calcium/calmodulin-dependent protein kinase I contains closely associated autoinhibitory and calmodulin-binding domains

The mechanism for the regulation of Ca2+/calmodulin-dependent protein kinase I (CaM kinase I) was investigated using a series of COOH-terminal truncated mutants. These mutants were expressed in bacteria as fusion proteins with glutathione S-transferase and purified by affinity chromatography using glutathione Sepharose 4B. A mutant (residues 1-332) showed complete Ca2+/CaM-dependent activity. Truncation mutants (residues 1-321, 1-314, and 1-309) exhibited decreasing affinities for Ca2+/CaM and also exhibited decreasing Ca2+/CaM-dependent activities. Truncation mutants (residues 1-305 or 1-299) were unable to bind Ca2+/CaM and were inactive. In contrast, truncation mutants (residues 1-293 or 1-277) were constitutively active at a slightly higher level (2-fold) than fully active CaM kinase I. These results indicate the location of the Ca2+/CaM-binding domain on CaM kinase I (residues 294-321) and predict the existence of an autoinhibitory domain near, or overlapping, the Ca2+/CaM-binding domain. These conclusions were supported by studies which showed that a synthetic peptide (CaM kinase I (294-321)) corresponding to residues 294-321 of CaM kinase I inhibited the fully active kinase in a manner that was competitive with Ca2+/CaM and also inhibited the constitutively active mutant (residues 1-293) in a manner that was competitive with Syntide-2, a peptide substrate, (Ki = 1.2 microM) but was non-competitive with ATP. Thus, these results suggest that CaM kinase I is regulated through an intrasteric mechanism common to other members of the family of Ca2+/CaM-dependent protein kinases.

Intrasteric regulation of myosin light chain kinase

Ca2+/calmodulin activates myosin light chain kinase by reversal of an autoinhibited state. The effects of substitution mutations on calmodulin activation properties implicate 4 of the 8 basic residues between the catalytic core and the calmodulin-binding domain in maintaining autoinhibition. These residues are further amino-terminal to the basic residues comprising the previously proposed pseudosubstrate sequence and suggest involvement of the connecting region in intrasteric autoinhibition. The pseudosubstrate model for autoinhibition proposes that basic residues within the autoinhibitory region mimic basic residues in the substrate and bind to defined acidic residues within the catalytic core. Charge reversal mutations of these specific acidic residues, however, had little or no effect on the Km value for regulatory light chain. From a total of 20 acidic residues on the surface of the substrate binding lobe of the catalytic core, 7 are implicated in binding directly or indirectly to the autoinhibitory domain but not to the light chain. Only 2 acidic residues near the catalytic site may bind to the autoinhibitory domain and the arginine at P-3 in the light chain. Exposure of these 2 residues upon calmodulin binding may be necessary and sufficient for light chain phosphorylation.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Evidence for calmodulin binding to the cytoplasmic domains of two C-CAM isoforms

C-CAM (cell-CAM 105) is a transmembrane cell adhesion molecule, belonging to the immunoglobulin superfamily. It is expressed in epithelia, vessel endothelia and leukocytes, and mediates intercellular adhesion in rat hepatocytes by homophilic binding. Two major isoforms (C-CAM1 and C-CAM2) that differ in their cytoplasmic domains occur. A previous study demonstrated that C-CAM can bind calmodulin in a Ca(2+)-dependent manner. In this study we have expressed the cytoplasmic domains of C-CAM1 and C-CAM2 in fusion proteins and measured calmodulin binding by a gel overlay assay, using 125I-labelled calmodulin. Our results indicate that the cytoplasmic domains of both C-CAM1 and C-CAM2 can bind calmodulin.