Structural requirements for phosphorylation of myosin regulatory light chain from smooth muscle

Cardiac myosin regulatory light chain kinase modulates cardiac contractility by phosphorylating both myosin regulatory light chain and troponin I

Heart muscle contractility and performance are controlled by posttranslational modifications of sarcomeric proteins. Although myosin regulatory light chain (RLC) phosphorylation has been studied extensively in vitro and in vivo, the precise role of cardiac myosin light chain kinase (cMLCK), the primary kinase acting upon RLC, in the regulation of cardiomyocyte contractility remains poorly understood. In this study, using recombinantly expressed and purified proteins, various analytical methods, in vitro and in situ kinase assays, and mechanical measurements in isolated ventricular trabeculae, we demonstrate that human cMLCK is not a dedicated kinase for RLC but can phosphorylate other sarcomeric proteins with well-characterized regulatory functions. We show that cMLCK specifically monophosphorylates Ser²³ of human cardiac troponin I (cTnI) in isolation and in the trimeric troponin complex in vitro and in situ in the native environment of the muscle myofilament lattice. Moreover, we observed that human cMLCK phosphorylates rodent cTnI to a much smaller extent in vitro and in situ, suggesting species-specific adaptation of cMLCK. Although cMLCK treatment of ventricular trabeculae exchanged with rat or human troponin increased their cross-bridge kinetics, the increase in sensitivity of myofilaments to calcium was significantly blunted by human TnI, suggesting that human cTnI phosphorylation by cMLCK modifies the functional consequences of RLC phosphorylation. We propose that cMLCK-mediated phosphorylation of TnI is functionally significant and represents a critical signaling pathway that coordinates the regulatory states of thick and thin filaments in both physiological and potentially pathophysiological conditions of the heart.

Functional links between Disabled-2 Ser723 phosphorylation and thrombin signaling in human platelets

Essentials Disabled-2 (Dab2) phosphorylation status in thrombin signaling of human platelet was investigated. Ser723 was the major Dab2 phosphorylation site in human platelets stimulated by thrombin. Dab2 S723 phosphorylation (pS723) caused the dissociation of Dab2-CIN85 protein complex. Dab2-pS723 regulated ADP release and integrin αIIbβ3 activation in thrombin-treated platelets.

SUMMARY

Background Disabled-2 (Dab2) is a platelet protein that is functionally involved in thrombin signaling in mice. It is unknown whether or not Dab2 undergoes phosphorylation during human platelet activation. Objectives To investigate the phosphorylation status of Dab2 and its functional consequences in thrombin-stimulated human platelets. Methods Dab2 was immunoprecipitated from resting and thrombin-stimulated platelet lysates for differential isotopic labeling. After enrichment of the phosphopeptides, the phosphorylation sites were analyzed by mass spectrometry. The corresponding phospho-specific antibody was generated. The protein kinases responsible for and the functional significance of Dab2 phosphorylation were defined by the use of signaling pathway inhibitors/activators, protein kinase assays, and various molecular approaches. Results Dab2 was phosphorylated at Ser227, Ser394, Ser401 and Ser723 in thrombin-stimulated platelets, with Ser723 phosphorylation being the most significantly increased by thrombin. Dab2 was phosphorylated by protein kinase C at Ser723 in a Gαq -dependent manner. ADP released from the stimulated platelets further activated the Gβγ -dependent pathway to sustain Ser723 phosphorylation. The Cbl-interacting protein of 85 kDa (CIN85) bound to Dab2 at a motif adjacent to Ser723 in resting platelets. The consequence of Ser723 phosphorylation was the dissociation of CIN85 from the Dab2-CIN85 complex. These molecular events led to increases in fibrinogen binding and platelet aggregation in thrombin-stimulated platelets by regulating αIIb β3 activation and ADP release. Conclusions Dab2 Ser723 phosphorylation is a key molecular event in thrombin-stimulated inside-out signaling and platelet activation, contributing to a new function of Dab2 in thrombin signaling.

Myosin light chain kinase steady-state kinetics: comparison of smooth muscle myosin II and nonmuscle myosin IIB as substrates

Myosin light chain kinase (MLCK) phosphorylates S19 of the myosin regulatory light chain (RLC), which is required to activate myosin's ATPase activity and contraction. Smooth muscles are known to display plasticity in response to factors such as inflammation, developmental stage, or stress, which lead to differential expression of nonmuscle and smooth muscle isoforms. Here, we compare steady-state kinetics parameters for phosphorylation of different MLCK substrates: (1) nonmuscle RLC, (2) smooth muscle RLC, and heavy meromyosin subfragments of (3) nonmuscle myosin IIB, and (4) smooth muscle myosin II. We show that MLCK has a ~2-fold higher k_cat for both smooth muscle myosin II substrates compared with nonmuscle myosin IIB substrates, whereas K_m values were very similar. Myosin light chain kinase has a 1.6-fold and 1.5-fold higher specificity (k_cat /K_m ) for smooth versus nonmuscle-free RLC and heavy meromyosin, respectively, suggesting that differences in specificity are dictated by RLC sequences. Of the 10 non-identical RLC residues, we ruled out 7 as possible underlying causes of different MLCK kinetics. The remaining 3 residues were found to be surface exposed in the N-terminal half of the RLC, consistent with their importance in substrate recognition. These data are consistent with prior deletion/chimera studies and significantly add to understanding of MLCK myosin interactions.

SIGNIFICANCE OF THE STUDY

Phosphorylation of nonmuscle and smooth muscle myosin by myosin light chain kinase (MLCK) is required for activation of myosin's ATPase activity. In smooth muscles, nonmuscle myosin coexists with smooth muscle myosin, but the two myosins have very different chemo-mechanical properties relating to their ability to maintain force. Differences in specificity of MLCK for different myosin isoforms had not been previously investigated. We show that the MLCK prefers smooth muscle myosin by a significant factor. These data suggest that nonmuscle myosin is phosphorylated more slowly than smooth muscle myosin during a contraction cycle.

S100A1 transgenic treatment of acute heart failure causes proteomic changes in rats

S100 Ca²⁺-binding protein A1 (S100A1) is an important regulator of myocardial contractility. The aim of the present study was to identify the underlying mechanisms of S100A1 activity via profiling the protein expression in rats administered with an S100A1 adenovirus (Ad-S100A1-EGFP) following acute myocardial infarction (AMI). LTQ OrbiTrap mass spectrometry was used to profile the protein expression in the Ad-S100A1-EGFP and control groups post-AMI. Using Protein Analysis Through Evolutionary Relationships (PANTHER) analysis, 134 energy metabolism-associated proteins, which comprised 20 carbohydrate metabolism-associated and 27 lipid metabolism associated proteins, were identified as differentially expressed in the Ad-S100A1-EGFP hearts compared with controls. The majority of the differentially expressed proteins identified were important enzymes involved in energy metabolism. The present study identified 12 Ca²⁺-binding proteins and 22 cytoskeletal proteins. The majority of the proteins expressed in the Ad-S100A1-EGFP group were upregulated compared with the control group. These results were further validated using western blot analysis. Following AMI, Ca²⁺ is crucial for the recovery of myocardial function in S100A1 transgenic rats as indicated by the upregulation of proteins associated with energy metabolism and Ca²⁺-binding. Thus, the current study ascertained that energy production and contractile ability were enhanced after AMI in the ventricular myocardium of the Ad-S100A1-EGFP group.

Cardiac myosin light chain is phosphorylated by Ca2+/calmodulin-dependent and -independent kinase activities

The well-known, muscle-specific smooth muscle myosin light chain kinase (MLCK) (smMLCK) and skeletal muscle MLCK (skMLCK) are dedicated protein kinases regulated by an autoregulatory segment C terminus of the catalytic core that blocks myosin regulatory light chain (RLC) binding and phosphorylation in the absence of Ca(2+)/calmodulin (CaM). Although it is known that a more recently discovered cardiac MLCK (cMLCK) is necessary for normal RLC phosphorylation in vivo and physiological cardiac performance, information on cMLCK biochemical properties are limited. We find that a fourth uncharacterized MLCK, MLCK4, is also expressed in cardiac muscle with high catalytic domain sequence similarity with other MLCKs but lacking an autoinhibitory segment. Its crystal structure shows the catalytic domain in its active conformation with a short C-terminal "pseudoregulatory helix" that cannot inhibit catalysis as a result of missing linker regions. MLCK4 has only Ca(2+)/CaM-independent activity with comparable Vmax and Km values for different RLCs. In contrast, the Vmax value of cMLCK is orders of magnitude lower than those of the other three MLCK family members, whereas its Km (RLC and ATP) and KCaM values are similar. In contrast to smMLCK and skMLCK, which lack activity in the absence of Ca(2+)/CaM, cMLCK has constitutive activity that is stimulated by Ca(2+)/CaM. Potential contributions of autoregulatory segment to cMLCK activity were analyzed with chimeras of skMLCK and cMLCK. The constitutive, low activity of cMLCK appears to be intrinsic to its catalytic core structure rather than an autoinhibitory segment. Thus, RLC phosphorylation in cardiac muscle may be regulated by two different protein kinases with distinct biochemical regulatory properties.

Vascular smooth muscle myosin light chain diphosphorylation: mechanism, function, and pathological implications

Smooth muscle contraction is activated primarily by phosphorylation at S19 of the 20-kDa regulatory light chain subunits of myosin II (LC(20) ) catalyzed by Ca(2+) /calmodulin-dependent myosin light chain kinase. Other kinases, for example, integrin-linked kinase (ILK), Rho-associated kinase (ROCK), and zipper-interacting protein kinase (ZIPK), can phosphorylate T18 in addition to S19, which increases the actin-activated myosin MgATPase activity at subsaturating actin concentrations ∼3-fold. These phosphorylatable residues and the amino acid sequence surrounding them are highly conserved throughout the animal kingdom; they are also found in an LC(20) homolog within the genome of Monosiga brevicollis, the closest living relative of metazoans. LC(20) diphosphorylation has been detected in mammalian vascular smooth muscle tissues in response to specific contractile stimuli and in pathophysiological situations associated with hypercontractility. LC(20) diphosphorylation has also been observed frequently in cultured cells where it activates force generation. Kinases such as ILK, ROCK, and ZIPK, therefore, are potential therapeutic targets in the treatment of, for example, cerebral vasospasm following subarachnoid hemorrhage and atherosclerosis.

Myosin light chain kinase and the role of myosin light chain phosphorylation in skeletal muscle

Skeletal muscle myosin light chain kinase (skMLCK) is a dedicated Ca(2+)/calmodulin-dependent serine-threonine protein kinase that phosphorylates the regulatory light chain (RLC) of sarcomeric myosin. It is expressed from the MYLK2 gene specifically in skeletal muscle fibers with most abundance in fast contracting muscles. Biochemically, activation occurs with Ca(2+) binding to calmodulin forming a (Ca(2+))(4)•calmodulin complex sufficient for activation with a diffusion limited, stoichiometric binding and displacement of a regulatory segment from skMLCK catalytic core. The N-terminal sequence of RLC then extends through the exposed catalytic cleft for Ser15 phosphorylation. Removal of Ca(2+) results in the slow dissociation of calmodulin and inactivation of skMLCK. Combined biochemical properties provide unique features for the physiological responsiveness of RLC phosphorylation, including (1) rapid activation of MLCK by Ca(2+)/calmodulin, (2) limiting kinase activity so phosphorylation is slower than contraction, (3) slow MLCK inactivation after relaxation and (4) much greater kinase activity relative to myosin light chain phosphatase (MLCP). SkMLCK phosphorylation of myosin RLC modulates mechanical aspects of vertebrate skeletal muscle function. In permeabilized skeletal muscle fibers, phosphorylation-mediated alterations in myosin structure increase the rate of force-generation by myosin cross bridges to increase Ca(2+)-sensitivity of the contractile apparatus. Stimulation-induced increases in RLC phosphorylation in intact muscle produces isometric and concentric force potentiation to enhance dynamic aspects of muscle work and power in unfatigued or fatigued muscle. Moreover, RLC phosphorylation-mediated enhancements may interact with neural strategies for human skeletal muscle activation to ameliorate either central or peripheral aspects of fatigue.

A single serine in the carboxyl terminus of cardiac essential myosin light chain-1 controls cardiomyocyte contractility in vivo

Although it is well known that mutations in the cardiac essential myosin light chain-1 (cmlc-1) gene can cause hypertrophic cardiomyopathy, the precise in vivo structural and functional roles of cMLC-1 in the heart are only poorly understood. We have isolated the zebrafish mutant lazy susan (laz), which displays severely reduced contractility of both heart chambers. By positional cloning, we identified a nonsense mutation within the zebrafish cmlc-1 gene to be responsible for the laz phenotype, leading to expression of a carboxyl-terminally truncated cMLC-1. Whereas complete loss of cMLC-1 leads to cardiac acontractility attributable to impaired cardiac sarcomerogenesis, expression of a carboxyl-terminally truncated cMLC-1 in laz mutant hearts is sufficient for normal cardiac sarcomerogenesis but severely impairs cardiac contractility in a cell-autonomous fashion. Whereas overexpression of wild-type cMLC-1 restores contractility of laz mutant cardiomyocytes, overexpression of phosphorylation site serine 195-deficient cMLC-1 (cMLC-1(S195A)) does not reconstitute cardiac contractility in laz mutant cardiomyocytes. By contrast, introduction of a phosphomimetic amino acid on position 195 (cMLC-1(S195D)) rescues cardiomyocyte contractility, demonstrating for the first time an essential role of the carboxyl terminus and especially of serine 195 of cMLC-1 in the regulation of cardiac contractility.

Myosin regulatory light chain phosphorylation attenuates cardiac hypertrophy

Hyperphosphorylation of myosin regulatory light chain (RLC) in cardiac muscle is proposed to cause compensatory hypertrophy. We therefore investigated potential mechanisms in genetically modified mice. Transgenic (TG) mice were generated to overexpress Ca2+/calmodulin-dependent myosin light chain kinase specifically in cardiomyocytes. Phosphorylation of sarcomeric cardiac RLC and cytoplasmic nonmuscle RLC increased markedly in hearts from TG mice compared with hearts from wild-type (WT) mice. Quantitative measures of RLC phosphorylation revealed no spatial gradients. No significant hypertrophy or structural abnormalities were observed up to 6 months of age in hearts of TG mice compared with WT animals. Hearts and cardiomyocytes from WT animals subjected to voluntary running exercise and isoproterenol treatment showed hypertrophic cardiac responses, but the responses for TG mice were attenuated. Additional biochemical measurements indicated that overexpression of the Ca2+/calmodulin-binding kinase did not perturb other Ca2+/calmodulin-dependent processes involving Ca2+/calmodulin-dependent protein kinase II or the protein phosphatase calcineurin. Thus, increased myosin RLC phosphorylation per se does not cause cardiac hypertrophy and probably inhibits physiological and pathophysiological hypertrophy by contributing to enhanced contractile performance and efficiency.

Identification of cardiac-specific myosin light chain kinase

Two myosin light chain (MLC) kinase (MLCK) proteins, smooth muscle (encoded by mylk1 gene) and skeletal (encoded by mylk2 gene) MLCK, have been shown to be expressed in mammals. Even though phosphorylation of its putative substrate, MLC2, is recognized as a key regulator of cardiac contraction, a MLCK that is preferentially expressed in cardiac muscle has not yet been identified. In this study, we characterized a new kinase encoded by a gene homologous to mylk1 and -2, named cardiac MLCK, which is specifically expressed in the heart in both atrium and ventricle. In fact, expression of cardiac MLCK is highly regulated by the cardiac homeobox protein Nkx2-5 in neonatal cardiomyocytes. The overall structure of cardiac MLCK protein is conserved with skeletal and smooth muscle MLCK; however, the amino terminus is quite unique, without significant homology to other known proteins, and its catalytic activity does not appear to be regulated by Ca(2+)/calmodulin in vitro. Cardiac MLCK is phosphorylated and the level of phosphorylation is increased by phenylephrine stimulation accompanied by increased level of MLC2v phosphorylation. Both overexpression and knockdown of cardiac MLCK in cultured cardiomyocytes revealed that cardiac MLCK is likely a new regulator of MLC2 phosphorylation, sarcomere organization, and cardiomyocyte contraction.

Novel sensors of the regulatory switch on the regulatory light chain of smooth muscle Myosin

Smooth muscle myosin can be switched on by phosphorylation of Ser-19 of the regulatory light chain. Our previous photocross-linking results suggested that an element of the structural mechanism for the regulatory switch was a phosphorylation-induced motion of the regulatory light chain N terminus (Wahlstrom, J. L., Randall, M. A., Jr., Lawson, J. D., Lyons, D. E., Siems, W. F., Crouch, G. J., Barr, R., Facemyer, K. C., and Cremo, C. R. (2003) J. Biol. Chem. 278, 5123-5131). Here we used three different approaches to test this notion, which are reactivity of cysteine thiols, pyrene and acrylodan spectral analysis, and pyrene fluorescence quenching. All methods detected significant differences between the unphosphorylated and phosphorylated regulatory light chain N termini in heavy meromyosin, a double-headed subfragment with an intact regulatory switch. These differences were not observed for subfragment-1, a single-headed, unregulated subfragment. In the presence of either ATP or ADP, phosphorylation increased the solvent exposure and decreased the polarity of the environment about position 23 of the regulatory light chain of heavy meromyosin. These phosphorylation-induced structural changes were not as evident in the absence of nucleotides. Nucleotide binding to unphosphorylated heavy meromyosin caused a decrease in exposure and an increase in polarity of the N terminus, whereas the effects of nucleotide on phosphorylated heavy meromyosin were the opposite. We showed a direct correlation between the kinetics of nucleotide binding/turnover and the conformational change reported by acrylodan at position 23 of the regulatory light chain. Acrylodan-A23C also reports the heads up (extended) to flexed (folded) transition in unphosphorylated heavy meromyosin. This is the first demonstration of direct coupling of nucleotide binding to conformational changes in the N terminus of the regulatory light chain.

Molecular mechanisms of nonmuscle myosin-II regulation

Myosin II, the conventional two-headed myosin that forms bipolar filaments, is directly involved in regulating cytokinesis, cell motility and cell morphology in nonmuscle cells. To understand the mechanisms by which nonmuscle myosin-II regulates these processes, investigators are now looking at the regulation of this molecule in vertebrate nonmuscle cells. The identification of multiple isoforms of nonmuscle myosin-II, whose activities and regulation differ from that of smooth muscle myosin-II, suggests that, in addition to regulatory light chain phosphorylation, other regulatory mechanisms control vertebrate nonmuscle myosin-II activity.

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.

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.

Modulation of contractility in human cardiac hypertrophy by myosin essential light chain isoforms

Cardiac hypertrophy is an adaptive response that normalizes wall stress and compensates for increased workload. It is accompanied by distinct qualitative and quantitative changes in the expression of protein isoforms concerning contractility, intracellular Ca(2+)-homeostasis and metabolism. Changes in the myosin subunit isoform expression improves contractility by an increase in force generation at a given Ca(2+)-concentration (increased Ca(2+)-sensitivity) and by improving the economy of the chemo-mechanical transduction process per amount of utilised ATP (increased duty ratio). In the human atrium this is achieved by partial replacement of the endogenous fast myosin by the ventricular slow-type heavy and light chains. In the hypertrophic human ventricle the slow-type beta-myosin heavy chains remain unchanged, but the ectopic expression of the atrial myosin essential light chain (ALC1) partially replaces the endogenous ventricular isoform (VLC1). The ventricular contractile apparatus with myosin containing ALC1 is characterised by faster cross-bridge kinetics, a higher Ca(2+)-sensitivity of force generation and an increased duty ratio. The mechanism for cross-bridge modulation relies on the extended Ala-Pro-rich N-terminus of the essential light chains of which the first eleven residues interact with the C-terminus of actin. A change in charge in this region between ALC1 and VLC1 explains their functional difference. The intracellular Ca(2+)-handling may be impaired in heart failure, resulting in either higher or lower cytosolic Ca(2+)-levels. Thus the state of the cardiomyocyte determines whether this hypertrophic adaptation remains beneficial or becomes detrimental during failure. Also discussed are the effects on contractility of long-term changes in isoform expression of other sarcomeric proteins. Positive and negative modulation of contractility by short-term phosphorylation reactions at multiple sites in the myosin regulatory light chain, troponin-I, troponin-T, alpha-tropomyosin and myosin binding protein-C are considered in detail.

Construction of an epitope-tagged calmodulin useful for the analysis of calmodulin-binding proteins: addition of a hemagglutinin epitope does not affect calmodulin-dependent activation of smooth muscle myosin light chain kinase

An epitope-tagged calmodulin (CaM), capable of interacting with CaM-binding proteins in cellular extracts, would be a valuable tool for identifying proteins in signal transduction pathways involving calcium. A bacterial overexpression vector for epitope-tagged CaM has been constructed by inserting the coding sequence for a nine amino acid portion of the influenza virus hemagglutinin (HA) protein into the initiation site of an overexpression vector for chicken CaM. The HA-CaM fusion produced in bacteria was compared to native CaM for its ability to activate smooth muscle myosin light chain kinase (MLCK), one of the best understood CaM-dependent enzymes. MLCK activity was tested in both a purified system and a CaM-depleted "native actomyosin" preparation maintaining many of the regulatory properties of the intact smooth muscle. HA-CaM behaves identically to unmodified CaM in both systems, indicating that the HA epitope does not adversely affect CaM function. The recombinant HA-CaM was used to sensitively detect CaM interactions with smooth muscle proteins in a modified gel overlay assay, using a monoclonal antibody against the HA epitope as the secondary reagent. Enzymatically active complexes of HA-CaM and MLCK could be immunoprecipitated from actomyosin preparations using the same monoclonal antibody and protein G-Sepharose beads.

Binding of myosin light chain kinase to cellular actin-myosin filaments

Myosin light chain kinase binds to the actomyosin-containing filaments in smooth and nonmuscle cells. However, the region of the kinase necessary for this high affinity binding in vivo is not known, although it has been proposed that the N and C termini bind to actin and myosin in vitro, respectively. Truncated myosin light chain kinases containing the catalytic core and calmodulin-binding domain but lacking N (amino acids 1-655) and/or C (amino acids 1004-1147) termini were expressed in the baculovirus system and purified. All enzymes were catalytically active and Ca2+/calmodulin-dependent. The C-terminal truncated myosin light chain kinase bound to detergent-washed smooth muscle contractile proteins similar to recombinant full-length myosin light chain kinase or enzyme purified from smooth muscle. The apparent affinity of the full-length kinase was greater for the actomyosin-containing filaments with associated proteins than for purified smooth muscle F-actin or actomyosin filaments from skeletal muscle. In contrast, truncations at the N terminus alone or at both N and C termini resulted in no significant binding. Similar effects were observed by two other assays: binding of fluorescently labeled myosin light chain kinases to actin-containing stress fibers in detergent-treated fibroblasts and localization of fluorescently labeled kinases after microinjection into primary smooth muscle cells in culture. The full-length and the C-terminal truncated myosin light chain kinases, but not myosin light chain kinases truncated at the N terminus or both N and C termini, associated with filaments in cells. Thus, the N terminus and not the C terminus of myosin light chain kinase is necessary for high affinity binding to actomyosin-containing filaments in smooth and nonmuscle cells.

Analysis of the specificity of the AMP-activated protein kinase by site-directed mutagenesis of bacterially expressed 3-hydroxy 3-methylglutaryl-CoA reductase, using a single primer variant of the unique-site-elimination method

The specificity of protein kinases is usually examined using synthetic peptide substrates, either designed variants, or, more recently random peptide libraries. However not all protein kinases utilize synthetic peptides efficiently as substrates. Even among those that do, these approaches neglect effects caused by three-dimensional protein conformation, or the existence of determinants remote from the phosphorylation site. To follow up our previous peptide studies on the specificity of the AMP-activated protein kinase (AMPK) [Dale, S., Wilson, W. A., Edelman, A.M., & Hardie, D. G. (1995) FEBS Lett. 361, 191-195], we have expressed the C-terminal, catalytic domain of Chinese hamster hydroxymethylglutaryl-CoA reductase in Escherichia coli. The domain was expressed with an N-terminal His6 tag which allowed rapid purification on Nj(2+)-agarose. The purified protein retained full enzymic activity, and as with the native enzyme, was totally inactivated by phosphorylation by AMPK at a single site corresponding to Ser871. Using a novel modification of the unique-site elimination method (which allowed direct mutagenesis of the double-stranded expression vector using a single oligonucleotide primer) we expressed 18 mutations involving residues around Ser871. The results broadly confirmed the recognition motif previously proposed on the basis of peptide studies. Three of the mutants were better substrates for AMPK than the wild type, and one of these (K872A) had hydroxymethylglutaryl-CoA reductase kinetic parameters virtually indistinguishable from the wild type. This suggests that hydroxymethylglutaryl-CoA reductase may have been selected to be a sub-optimal substrate for AMPK.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Properties of Acanthamoeba myosin I heavy chain kinase bound to phospholipid vesicles

The actin-activated Mg(2+)-ATPase and in vitro motility activities of the three Acanthamoeba myosin I isozymes depend upon phosphorylation of their single heavy chains by myosin I heavy chain kinase. Previously, the kinase had been shown to be activated by autophosphorylation, which is enhanced by acidic phospholipids, or simply by binding to purified plasma membranes in the absence of significant autophosphorylation. In this paper, we show that the rate of phosphorylation of myosin I by unphosphorylated kinase is approximately 20-fold faster when both the myosin I and the kinase are bound to acidic phospholipid vesicles than when both are soluble. This activation is not due to an increase in the local concentrations of vesicle-bound kinase and myosin I. Thus, acidic phospholipids, like membranes, can activate myosin I heavy chain kinase in the absence of significant autophosphorylation, i.e. membrane proteins are not required. Kinetic studies show that both binding of kinase to phospholipid vesicles and autophosphorylation of kinase in the absence of phospholipid increase the Vmax relative to soluble, unphosphorylated kinase with either an increase in the apparent Km (when myosin I is the substrate) or no significant change in Km (when a synthetic peptide is the substrate). Kinetic data showed that autophosphorylation of phospholipid-bound kinase is both intermolecular and intervesicular, and that phosphorylation of phospholipid-bound myosin I by phospholipid-bound kinase is also intervesicular even when the kinase and myosin are bound to the same vesicles. The relevance of these results to the activation of myosin I heavy chain kinase and phosphorylation of myosin I isozymes in situ are discussed.

The role of the skeletal muscle myosin light chains N-terminal fragments

The myosin regulatory and essential light chains in skeletal muscle do not play a role as significant as in scallop or smooth muscle, however, there are some data suggesting that the skeletal myosin light chains and their N-terminal parts may have a modulatory function in the interaction of actin with myosin heads. In this paper four conformational states of the myosin head with respect to the regulatory light chain bound cation (magnesium or calcium) and phosphorylation are proposed. Communication between regulatory and essential light chains and putative binding of the N-terminus of A1 essential light chain to actin is discussed.

Phosphorylation of myosin regulatory light chains by the molluscan twitchin kinase

The unusually large (approximately 600 to > 3000 kDa) myosin-associated proteins of the titin/twitchin superfamily are considered to be important cytoskeletal rulers for thick filament assembly in muscle. This function is maintained by approximately 60-240 modular fibronectin-type-III and immunoglobulin-C2 repeats in these proteins which further contain a protein serine/threonine kinase domain of unknown function. In this study, the bacterially expressed kinase domain of Aplysia twitchin was used in order to identify a potential physiological substrate. Addition of the recombinant kinase to Aplysia actomyosin preparations resulted in the specific phosphorylation of the 19-kDa myosin regulatory light chains. The twitchin kinase phosphorylated purified light chains on Thr15 in a region which shared a high degree of similarity with the phosphorylation site for vertebrate smooth muscle myosin light chain kinase. Peptide analogs of the twitchin substrate sequence and the similar sequence in vertebrate smooth muscle myosin light chains were phosphorylated with good kinetic properties. These data reveal the first potential substrate for any of the giant protein kinases and support a dual role of twitchin in molluscan muscle as a cytoskeletal protein as well as a myosin light chain kinase.

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.

Photoaffinity labeling of a peptide substrate to myosin light chain kinase

The substrate binding properties of skeletal muscle myosin light chain kinase were investigated with a synthetic peptide containing the photoreactive amino acid p-benzoylphenylalanine (Bpa) incorporated amino-terminal of the phosphoacceptor serine (BpaKKRAARATSNVFA). When photolyzed at 350 nm, the peptide was cross-linked stoichiometrically to myosin light chain kinase in a Ca2+/calmodulin-dependent manner. Peptide incorporation into kinase inhibited light chain phosphorylation, and the loss of kinase activity was proportional to the extent of peptide incorporated. After peptide I was incorporated into myosin light chain kinase, it was partially phosphorylated in the absence of Ca2+/calmodulin. The extent of phosphorylation increased in the presence of Ca2+/calmodulin. The cross-linked photoadduct was digested, labeled peptides were purified by high performance liquid chromatography, and sites of covalent modification were determined by amino acid sequencing and analysis. The covalent modification in the catalytic core occurred on Ile-373 (66%) and in a peptide containing residues Asn-422 to Met-437 (14%), respectively. Lys-572 in the autoinhibitory region accounted for 20% of the incorporated label. The coincident covalent modification of the autoinhibitory domain suggests that it is located near the catalytic site. Based upon a model of the catalytic core, the substrate peptide is predicted to bind in the cleft between the two lobes of the kinase. The orientation of the substrate peptide on myosin light chain kinase is similar to the orientation of the substrate recognition fragment, but not the high affinity binding fragment, of inhibitor peptide of cAMP-dependent protein kinase in the catalytic subunit of the cAMP-dependent protein kinase.