Myotonic dystrophy protein kinase domains mediate localization, oligomerization, novel catalytic activity, and autoinhibition

An overview of heart rhythm disorders and management in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is the most common adult form of muscular dystrophy, presenting with a constellation of systemic findings secondary to a CTG triplet expansion of the noncoding region of the DMPK gene. Cardiac involvement is frequent, with conduction disease and supraventricular and ventricular arrhythmias being the most prevalent cardiac manifestations, often developing from a young age. The development of cardiac arrhythmias has been linked to increased morbidity and mortality, with sudden cardiac death well described. Strategies to mitigate risk of arrhythmic death have been developed. In this review, we outline the current knowledge on the pathophysiology of rhythm abnormalities in patients with myotonic dystrophy and summarize available knowledge on arrhythmic risk stratification. We also review management strategies from an electrophysiological perspective, attempting to underline the substantial unmet need to address residual arrhythmic risks for this population.

Metabolic Alterations in Myotonic Dystrophy Type 1 and Their Correlation with Lipin

Myotonic dystrophy type 1 (DM1) is an autosomal dominant hereditary and multisystemic disease, characterized by progressive distal muscle weakness and myotonia. Despite huge efforts, the pathophysiological mechanisms underlying DM1 remain elusive. In this review, the metabolic alterations observed in patients with DM1 and their connection with lipin proteins are discussed. We start by briefly describing the epidemiology, the physiopathological and systemic features of DM1. The molecular mechanisms proposed for DM1 are explored and summarized. An overview of metabolic syndrome, dyslipidemia, and the summary of metabolic alterations observed in patients with DM1 are presented. Patients with DM1 present clinical evidence of metabolic alterations, namely increased levels of triacylglycerol and low-density lipoprotein, increased insulin and glucose levels, increased abdominal obesity, and low levels of high-density lipoprotein. These metabolic alterations may be associated with lipins, which are phosphatidate phosphatase enzymes that regulates the triacylglycerol levels, phospholipids, lipid signaling pathways, and are transcriptional co-activators. Furthermore, lipins are also important for autophagy, inflammasome activation and lipoproteins synthesis. We demonstrate the association of lipin with the metabolic alterations in patients with DM1, which supports further clinical studies and a proper exploration of lipin proteins as therapeutic targets for metabolic syndrome, which is important for controlling many diseases including DM1.

Phosphoproteome Profiling Reveals Molecular Mechanisms of Growth-Factor-Mediated Kinase Inhibitor Resistance in EGFR-Overexpressing Cancer Cells

Although substantial progress has been made regarding the use of molecularly targeted cancer therapies, resistance almost invariably develops and presents a major clinical challenge. The tumor microenvironment can rescue cancer cells from kinase inhibitors by growth-factor-mediated induction of pro-survival pathways. Here we show that epidermal growth factor receptor (EGFR) inhibition by Gefitinib is counteracted by growth factors, notably FGF2, and we assessed the global molecular consequences of this resistance at the proteome and phosphoproteome level in A431 cells. Tandem mass tag peptide labeling and quantitative mass spectrometry allowed the identification and quantification of 22 000 phosphopeptides and 8800 proteins in biological triplicates without missing values. The data show that FGF2 protects the cells from the antiproliferative effect of Gefitinib and largely prevents reprogramming of the proteome and phosphoproteome. Simultaneous EGFR/FGFR or EGFR/GSG2 (Haspin) inhibition overcomes this resistance, and the phosphoproteomic experiments further prioritized the RAS/MEK/ERK as well as the PI3K/mTOR axis for combination treatment. Consequently, the MEK inhibitor Trametinib prevented FGF2-mediated survival of EGFR inhibitor-resistant cells when used in combination with Gefitinib. Surprisingly, the PI3K/mTOR inhibitor Omipalisib reversed resistance mediated by all four growth factors tested, making it an interesting candidate for mitigating the effects of the tumor microenvironment.

Genome-wide identification and comprehensive analyses of the kinomes in four pathogenic microsporidia species

Microsporidia have attracted considerable attention because they infect a wide range of hosts, from invertebrates to vertebrates, and cause serious human diseases and major economic losses in the livestock industry. There are no prospective drugs to counteract this pathogen. Eukaryotic protein kinases (ePKs) play a central role in regulating many essential cellular processes and are therefore potential drug targets. In this study, a comprehensive summary and comparative analysis of the protein kinases in four microsporidia–Enterocytozoon bieneusi, Encephalitozoon cuniculi, Nosema bombycis and Nosema ceranae–was performed. The results show that there are 34 ePKs and 4 atypical protein kinases (aPKs) in E. bieneusi, 29 ePKs and 6 aPKs in E. cuniculi, 41 ePKs and 5 aPKs in N. bombycis, and 27 ePKs and 4 aPKs in N. ceranae. These data support the previous conclusion that the microsporidian kinome is the smallest eukaryotic kinome. Microsporidian kinomes contain only serine-threonine kinases and do not contain receptor-like and tyrosine kinases. Many of the kinases related to nutrient and energy signaling and the stress response have been lost in microsporidian kinomes. However, cell cycle-, development- and growth-related kinases, which are important to parasites, are well conserved. This reduction of the microsporidian kinome is in good agreement with genome compaction, but kinome density is negatively correlated with proteome size. Furthermore, the protein kinases in each microsporidian genome are under strong purifying selection pressure. No remarkable differences in kinase family classification, domain features, gain and/or loss, and selective pressure were observed in these four species. Although microsporidia adapt to different host types, the coevolution of microsporidia and their hosts was not clearly reflected in the protein kinases. Overall, this study enriches and updates the microsporidian protein kinase database and may provide valuable information and candidate targets for the design of treatments for pathogenic diseases.

Phosphorylation target site specificity for AGC kinases DMPK E and Lats2

Serine/threonine kinases of the AGC group are important regulators of cell growth and motility. To examine the candidate substrate profile for two members of this group, DMPK E and Lats2, we performed in vitro kinase assays on peptide arrays. Substrate peptides for both kinases exhibited a predominance of basic residues surrounding the phosphorylation target site. 3D homology modeling of the kinase domains of DMPK E and Lats2 indicated that presence of two negative pockets in the peptide binding groove provides an explanation for the substrate preference. These findings will aid future research toward signaling functions of Lats2 and DMPK E within cells.

Myotonic dystrophy protein kinase is critical for nuclear envelope integrity

Myotonic dystrophy 1 (DM1) is a multisystemic disease caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the gene coding for myotonic dystrophy protein kinase (DMPK). DMPK is a nuclear envelope (NE) protein that promotes myogenic gene expression in skeletal myoblasts. Muscular dystrophy research has revealed the NE to be a key determinant of nuclear structure, gene regulation, and muscle function. To investigate the role of DMPK in NE stability, we analyzed DMPK expression in epithelial and myoblast cells. We found that DMPK localizes to the NE and coimmunoprecipitates with Lamin-A/C. Overexpression of DMPK in HeLa cells or C2C12 myoblasts disrupts Lamin-A/C and Lamin-B1 localization and causes nuclear fragmentation. Depletion of DMPK also disrupts NE lamina, showing that DMPK is required for NE stability. Our data demonstrate for the first time that DMPK is a critical component of the NE. These novel findings suggest that reduced DMPK may contribute to NE instability, a common mechanism of skeletal muscle wasting in muscular dystrophies.

Structure of dystrophia myotonica protein kinase

Dystrophia myotonica protein kinase (DMPK) is a serine/threonine kinase composed of a kinase domain and a coiled-coil domain involved in the multimerization. The crystal structure of the kinase domain of DMPK bound to the inhibitor bisindolylmaleimide VIII (BIM-8) revealed a dimeric enzyme associated by a conserved dimerization domain. The affinity of dimerisation suggested that the kinase domain alone is insufficient for dimerisation in vivo and that the coiled-coil domains are required for stable dimer formation. The kinase domain is in an active conformation, with a fully-ordered and correctly positioned alphaC helix, and catalytic residues in a conformation competent for catalysis. The conserved hydrophobic motif at the C-terminal extension of the kinase domain is bound to the N-terminal lobe of the kinase domain, despite being unphosphorylated. Differences in the arrangement of the C-terminal extension compared to the closely related Rho-associated kinases include an altered PXXP motif, a different conformation and binding arrangement for the turn motif, and a different location for the conserved NFD motif. The BIM-8 inhibitor occupies the ATP site and has similar binding mode as observed in PDK1.

Myotonic dystrophy protein kinase (DMPK) and its role in the pathogenesis of myotonic dystrophy 1

Myotonic dystrophy 1 (DM1) is an autosomal, dominant inherited, neuromuscular disorder. The DM1 mutation consists in the expansion of an unstable CTG-repeat in the 3'-untranslated region of a gene encoding DMPK (myotonic dystrophy protein kinase). Clinical expression of DM1 is variable, presenting a progressive muscular dystrophy that affects distal muscles more than proximal and is associated with the inability to relax muscles appropriately (myotonia), cataracts, cardiac arrhythmia, testicular atrophy and insulin resistance. DMPK is a Ser/Thr protein kinase homologous to the p21-activated kinases MRCK and ROCK/rho-kinase/ROK. The most abundant isoform of DMPK is an 80 kDa protein mainly expressed in smooth, skeletal and cardiac muscles. Decreased DMPK protein levels may contribute to the pathology of DM1, as revealed by gene target studies. Here we review current understanding of the structural, functional and pathophysiological characteristics of DMPK.

Drosophila sticky/citron kinase is a regulator of cell-cycle progression, genetically interacts with Argonaute 1 and modulates epigenetic gene silencing

The sticky/citron kinase protein is a conserved regulator of cell-cycle progression from invertebrates to humans. While this kinase is essential for completion of cytokinesis, sticky/citron kinase phenotypes disrupting neurogenesis and cell differentiation suggest additional non-cell-cycle functions. However, it is not known whether these phenotypes are an indirect consequence of sticky mutant cell-cycle defects or whether they define a novel function for this kinase. We have isolated a temperature-sensitive allele of the Drosophila sticky gene and we show that sticky/citron kinase is required for histone H3-K9 methylation, HP1 localization, and heterochromatin-mediated gene silencing. sticky genetically interacts with Argonaute 1 and sticky mutants exhibit context-dependent Su(var) and E(var) activity. These observations indicate that sticky/citron kinase functions to regulate both actin-myosin-mediated cytokinesis and epigenetic gene silencing, possibly linking cell-cycle progression to heterochromatin assembly and inheritance of gene expression states.

ROCK enzymatic assay

We describe the protocols for measuring Rho-associated coiled-coil-containing kinase (ROCK) activity in vitro. A His-tagged, constitutively active form of the protein (lacking C-terminal inhibitory domains) is expressed in baculovirus. The protein is purified by a combination of metal affinity, ion exchange, and size exclusion chromatography. Enzymatic activity is measured spectrophotometrically in a coupled assay format wherein a molecule of NADH is oxidized to NAD+ each time a phosphate is transferred by ROCK.

Role of myotonic dystrophy protein kinase (DMPK) in glucose homeostasis and muscle insulin action

Myotonic dystrophy 1 (DM1) is caused by a CTG expansion in the 3′-unstranslated region of the DMPK gene, which encodes a serine/threonine protein kinase. One of the common clinical features of DM1 patients is insulin resistance, which has been associated with a pathogenic effect of the repeat expansions. Here we show that DMPK itself is a positive modulator of insulin action. DMPK-deficient (dmpk^−/−) mice exhibit impaired insulin signaling in muscle tissues but not in adipocytes and liver, tissues in which DMPK is not expressed. Dmpk^−/− mice display metabolic derangements such as abnormal glucose tolerance, reduced glucose uptake and impaired insulin-dependent GLUT4 trafficking in muscle. Using DMPK mutants, we show that DMPK is required for a correct intracellular trafficking of insulin and IGF-1 receptors, providing a mechanism to explain the molecular and metabolic phenotype of dmpk^−/− mice. Taken together, these findings indicate that reduced DMPK expression may directly influence the onset of insulin-resistance in DM1 patients and point to dmpk as a new candidate gene for susceptibility to type 2-diabetes.

Molecular mechanism for the regulation of rho-kinase by dimerization and its inhibition by fasudil

Rho-kinase is a key regulator of cytoskeletal events and a promising drug target in the treatment of vascular diseases and neurological disorders. Unlike other protein kinases, Rho-kinase requires both N- and C-terminal extension segments outside the kinase domain for activity, although the details of this requirement have been elusive. The crystal structure of an active Rho-kinase fragment containing the kinase domain and both the extensions revealed a head-to-head homodimer through the N-terminal extension forming a helix bundle that structurally integrates the C-terminal extension. This structural organization enables binding of the C-terminal hydrophobic motif to the N-terminal lobe, which defines the correct disposition of helix alphaC that is important for the catalytic activity. The bound inhibitor fasudil significantly alters the conformation and, consequently, the mode of interaction with the catalytic cleft that contains local structural changes. Thus, both kinase and drug conformational pliability and stability confer selectivity.

Myotonic dystrophy protein kinase monoclonal antibody generation from a coiled-coil template

Myotonic dystrophy protein kinase (DMPK) was the initial representative of a ubiquitous protein kinase family that regulates cell size and shape. DMPK is highly expressed in heart and skeletal muscle and transgenic over-expression induces cardiac hypertrophy. The characterization of DMPK has been limited by the paucity of immunological reagents with high affinity and well-defined specificity. Amino acid sequence data was used to predict the surface exposure of the coil-coiled domain of DMPK. These exposed amino acids were substituted into an extremely stable coiled-coil template to produce a peptide antigen. Sera from mice immunized with the peptide conjugated to keyhole limpet hemocyanin were screened against recombinant DMPK using Western blots. Murine spleens expressing DMPK antibodies were used to produce hybridoma cell lines. Hybridoma supernatants were further screened against recombinant DMPK and four clonal hybridoma cell lines expressing DMPK antibodies were generated. These four monoclonal antibodies recognized recombinant DMPK in Western blots of COS-1 cell lysates expressing high levels of recombinant DMPK and immunoprecipitated recombinant DMPK from COS-1 cell lysates. The identity of the immunoprecipitated DMPK was confirmed by MALDI-TOF mass spectrometry and peptide mass fingerprinting. DMPK was the only protein detected in the immunoprecipitates, indicating the high specificity of the antibodies. Western blots immunostained with two of the monoclonal antibodies specifically recognized the two isoforms of endogenous DMPK, DMPK-1 and DMPK-2, that are expressed at low levels in the human heart. The recognition of low amounts of DMPK-1 and DMPK-2 indicates the high affinity of these antibodies. A human heart lysate was subjected to ammonium sulfate precipitation and column chromatography to produce a fraction that was enriched in DMPK. One of the monoclonal antibodies immunoprecipitated endogenous DMPK from this fraction. This antibody was used for immuno-localization studies of an adenoviral DMPK construct, expressed in adult mouse cardiac myocytes. This construct was localized to the intercalated disc, the site of endogenous DMPK, indicating that this antibody is applicable to immuno-localization studies. This study demonstrates the utility of the described procedure for generation of specific monoclonal antibodies with high affinity for epitopes in coiled-coiled domains of mammalian proteins expressed at low levels.

Coiled-coil interactions modulate multimerization, mitochondrial binding and kinase activity of myotonic dystrophy protein kinase splice isoforms

The myotonic dystrophy protein kinase polypeptide repertoire in mice and humans consists of six different splice isoforms that vary in the nature of their C-terminal tails and in the presence or absence of an internal Val-Ser-Gly-Gly-Gly motif. Here, we demonstrate that myotonic dystrophy protein kinase isoforms exist in high-molecular-weight complexes controlled by homo- and heteromultimerization. This multimerization is mediated by coiled-coil interactions in the tail-proximal domain and occurs independently of alternatively spliced protein segments or myotonic dystrophy protein kinase activity. Complex formation was impaired in myotonic dystrophy protein kinase mutants in which three leucines at positions a and d in the coiled-coil heptad repeats were mutated to glycines. These coiled-coil mutants were still capable of autophosphorylation and transphosphorylation of peptides, but the rates of their kinase activities were significantly lowered. Moreover, phosphorylation of the natural myotonic dystrophy protein kinase substrate, myosin phosphatase targeting subunit, was preserved, even though binding of the myotonic dystrophy protein kinase to the myosin phosphatase targeting subunit was strongly reduced. Furthermore, the association of myotonic dystrophy protein kinase isoform C to the mitochondrial outer membrane was weakened when the coiled-coil interaction was perturbed. Our findings indicate that the coiled-coil domain modulates myotonic dystrophy protein kinase multimerization, substrate binding, kinase activity and subcellular localization characteristics.

The structure of dimeric ROCK I reveals the mechanism for ligand selectivity

ROCK or Rho-associated kinase, a serine/threonine kinase, is an effector of Rho-dependent signaling and is involved in actin-cytoskeleton assembly and cell motility and contraction. The ROCK protein consists of several domains: an N-terminal region, a kinase catalytic domain, a coiled-coil domain containing a RhoA binding site, and a pleckstrin homology domain. The C-terminal region of ROCK binds to and inhibits the kinase catalytic domains, and this inhibition is reversed by binding RhoA, a small GTPase. Here we present the structure of the N-terminal region and the kinase domain. In our structure, two N-terminal regions interact to form a dimerization domain linking two kinase domains together. This spatial arrangement presents the kinase active sites and regulatory sequences on a common face affording the possibility of both kinases simultaneously interacting with a dimeric inhibitory domain or with a dimeric substrate. The kinase domain adopts a catalytically competent conformation; however, no phosphorylation of active site residues is observed in the structure. We also determined the structures of ROCK bound to four different ATP-competitive small molecule inhibitors (Y-27632, fasudil, hydroxyfasudil, and H-1152P). Each of these compounds binds with reduced affinity to cAMP-dependent kinase (PKA), a highly homologous kinase. Subtle differences exist between the ROCK- and PKA-bound conformations of the inhibitors that suggest that interactions with a single amino acid of the active site (Ala215 in ROCK and Thr183 in PKA) determine the relative selectivity of these compounds. Hydroxyfasudil, a metabolite of fasudil, may be selective for ROCK over PKA through a reversed binding orientation.

Rho kinase, a promising drug target for neurological disorders

Rho kinases (ROCKs), the first Rho effectors to be described, are serine/threonine kinases that are important in fundamental processes of cell migration, cell proliferation and cell survival. Abnormal activation of the Rho/ROCK pathway has been observed in various disorders of the central nervous system. Injury to the adult vertebrate brain and spinal cord activates ROCKs, thereby inhibiting neurite growth and sprouting. Inhibition of ROCKs results in accelerated regeneration and enhanced functional recovery after spinal-cord injury in mammals, and inhibition of the Rho/ROCK pathway has also proved to be efficacious in animal models of stroke, inflammatory and demyelinating diseases, Alzheimer's disease and neuropathic pain. ROCK inhibitors therefore have potential for preventing neurodegeneration and stimulating neuroregeneration in various neurological disorders.

New insights into the structure-function relationships of Rho-associated kinase: a thermodynamic and hydrodynamic study of the dimer-to-monomer transition and its kinetic implications

The effect of the length of ROCK (Rho-associated kinase) on its oligomerization state has been investigated by analysing full-length protein and four truncated constructs using light-scattering and analytical ultracentrifugation methods. Changes in size correlate with the kinetic properties of the kinase. Sedimentation velocity, sedimentation equilibrium and light-scattering data analyses revealed that protein constructs of size Ser6-Arg415 and larger exist predominantly as dimers, while smaller constructs are predominantly monomeric. The amino acid segments comprising residues 379-415 and 47-78 are shown to be necessary to maintain the dimeric ROCK structure. kcat values ranged from 0.7 to 2.1 s(-1) and from 1.0 to 5.9 s(-1) using ROCK peptide (KKRNRTLSV) and the 20000 Da subunit of myosin light chain respectively as substrate, indicating that the effect of the ROCK oligomerization state on the kcat is minor. Values of ATP K(m) for monomeric constructs were increased by 50-80-fold relative to the dimeric constructs, and K(i) comparisons using the specific competitive ROCK inhibitor Y-27632 also showed increases of at least 120-fold, demonstrating significant perturbations in the ATP binding site. The corresponding K(m) values for the ROCK peptide and myosin light chain substrates increased in the range 1.4-16-fold, demonstrating that substrate binding is less sensitive to the ROCK oligomerization state. These results show that the oligomerization state of ROCK may influence both its kinase activity and its interactions with inhibitors, and suggest that the dimeric structure is essential for normal in vivo function.

Human myotonic dystrophy protein kinase effect in S. cerevisiae

Human myotonic dystrophy protein kinase (DMPK), the product of the myotonic dystrophy (DM) locus, is a member of a novel class of multidomain serine-threonine protein kinases, which interacts with members of the Rho family of small GTPases. DMPK has been shown to affect the cell growth, size and shape in different organisms, from fission yeast to man, but its physiological role is still unclear. We examined the effect of the overexpression of two forms of human DMPK, full-length (DMFL) and a C-terminal truncated form (DMT) on the growth and cell morphology of S. cerevisiae, which possesses a DMPK homologous gene (CBK1) important for polarized growth and cell division. We report that the overexpression of either forms of human DMPK did not complement the CBK1 function in the haploid strain WR208-1a, deleted for CBK1. The truncated form, but not the full length one, slowed down growth rate and induced elongation of the haploid wild type strain CBK1. Similar results were obtained in the diploid wild type strain RS112 of S. cerevisiae where also the full-length form was effective. These effects were abolished when either DMFL or DMT were mutated in the ATP binding site (K100R mutation), suggesting that the kinase activity of DMPK is required. Interestingly, DMPK localization in yeast is similar to that of Cbk1 protein suggesting that it might affect a pathway, which regulates cell morphogenesis and progression through cell cycle, possibly involving CBK1.

Divergent mitochondrial and endoplasmic reticulum association of DMPK splice isoforms depends on unique sequence arrangements in tail anchors

Myotonic dystrophy protein kinase (DMPK) is a Ser/Thr-type protein kinase with unknown function, originally identified as the product of the gene that is mutated by triplet repeat expansion in patients with myotonic dystrophy type 1 (DM1). Alternative splicing of DMPK transcripts results in multiple protein isoforms carrying distinct C termini. Here, we demonstrate by expressing individual DMPKs in various cell types, including C(2)C(12) and DMPK(-/-) myoblast cells, that unique sequence arrangements in these tails control the specificity of anchoring into intracellular membranes. Mouse DMPK A and C were found to associate specifically with either the endoplasmic reticulum (ER) or the mitochondrial outer membrane, whereas the corresponding human DMPK A and C proteins both localized to mitochondria. Expression of mouse and human DMPK A-but not C-isoforms in mammalian cells caused clustering of ER or mitochondria. Membrane association of DMPK isoforms was resistant to alkaline conditions, and mutagenesis analysis showed that proper anchoring was differentially dependent on basic residues flanking putative transmembrane domains, demonstrating that DMPK tails form unique tail anchors. This work identifies DMPK as the first kinase in the class of tail-anchored proteins, with a possible role in organelle distribution and dynamics.

Myotonic dystrophy protein kinase phosphorylates phospholamban and regulates calcium uptake in cardiomyocyte sarcoplasmic reticulum

Myotonic dystrophy (DM) is caused by a CTG expansion in the 3'-untranslated region of a protein kinase gene (DMPK). Cardiovascular disease is one of the most prevalent causes of death in DM patients. Electrophysiological studies in cardiac muscles from DM patients and from DMPK(-/-) mice suggested that DMPK is critical to the modulation of cardiac contractility and to the maintenance of proper cardiac conduction activity. However, there are no data regarding the molecular signaling pathways involved in DM heart failure. Here we show that DMPK expression in cardiac myocytes is highly enriched in the sarcoplasmic reticulum (SR) where it colocalizes with the ryanodine receptor and phospholamban (PLN), a muscle-specific SR Ca(2+)-ATPase (SERCA2a) inhibitor. Coimmunoprecipitation studies showed that DMPK and PLN can physically associate. Furthermore, purified wild-type DMPK, but not a kinase-deficient mutant (K110A DMPK), phosphorylates PLN in vitro. Subsequent studies using the DMPK(-/-) mice demonstrated that PLN is hypo-phosphorylated in SR vesicles from DMPK(-/-) mice compared with wild-type mice both in vitro and in vivo. Finally, we show that Ca(2+) uptake in SR is impaired in ventricular homogenates from DMPK(-/-) mice. Together, our data suggest the existence of a novel regulatory DMPK pathway for cardiac contractility and provide a molecular mechanism for DM heart pathology.

Models of the cooperative mechanism for Rho effector recognition: implications for RhoA-mediated effector activation

Activated GTPases of the Rho family regulate a spectrum of functionally diverse downstream effectors, initiating a network of signal transduction pathways by interaction and activation of effector proteins. Although effectors are defined as proteins that selectively bind the GTP-bound state of the small GTPases, there have been also several indications for a nucleotide-independent binding mode. By characterizing the molecular mechanism of RhoA interaction with its effectors, we have determined the equilibrium dissociation constants of several Rho-binding domains of three different effector proteins (Rhotekin, ROCKI/ROK beta/p160ROCK, PRK1/PKNalpha where ROK is RhoA-binding kinase) for both RhoA.GDP and RhoA.GTP using fluorescence spectroscopy. In addition, we have identified two novel Rho-interacting domains in ROCKI, which bind RhoA with high affinity but not Cdc42 or Rac1. Our results, together with recent structural data, support the notion of multiple effector-binding sites in RhoA and strongly indicate a cooperative binding mechanism for PRK1 and ROCKI that may be the molecular basis of Rho-mediated effector activation.

Expression of the human myotonic dystrophy kinase-related Cdc42-binding kinase gamma is regulated by promoter DNA methylation and Sp1 binding

Myotonic dystrophy kinase-related Cdc42 binding kinases (MRCKs) are family members most related to the myotonic dystrophy kinase (DMPK), RhoA-binding kinase (ROK), and citron kinase. Two highly conserved members, MRCKalpha and -beta, have been previously identified and characterized. We now describe a novel isoform, MRCKgamma, which is functionally and structurally related to members of this kinase family. We show these kinases to have marked similarities in their genomic organization, substrate phosphorylation, and catalytic autoinhibition. Unlike MRCKalpha and -beta, which are expressed ubiquitously, MRCKgamma mRNA was only expressed in heart and skeletal muscle. In cultured cells, MRCKgamma showed differential expression with high levels of expression only in certain cell lines. DNA analysis showed that lack of expression is correlated with promoter DNA methylation. We have mapped the methylation sites in the MRCKgamma promoter. Significantly, agents that suppressed DNA methylation caused increases in the expression of the kinase in low-expressing cells, further supporting the notion that promoter DNA methylation plays an important role in the expression of MRCKgamma. Analysis of the MRCKgamma promoter has also revealed two proximal Sp1 sites that are essential for transcriptional activity. We conclude that both promoter DNA methylation and Sp1 binding are important regulators for MRCKgamma expression.

Parallel coiled-coil association of the RhoA-binding domain in Rho-kinase

Rho-kinase is a serine/threonine protein kinase that regulates cytoskeletal events in cells. The enzyme activity of Rho-kinase is auto-inhibited in the free state but is activated through direct binding to the small GTPase Rho in the GTP-bound form. The crystal structure of the Rho-binding domain (RhoBD) of Rho-kinase has been determined at 1.8-A resolution by the multi-wavelength anomalous dispersion technique. The structure shows that RhoBD dimerizes to form a parallel coiled-coil with long consecutive alpha-helices extended to approximately 97 A and suggests that free Rho-kinase can also form a dimer through parallel self-association. At the middle region of the coiled-coil, the polypeptide chains are flexible and display loose "knobs-into-holes" packing of the side chains from both chains. RhoBD residues that have been shown to be critical for Rho-binding are spread in the positively charged C-terminal region. The parallel coiled-coil structure of our Rho-kinase RhoBD in the free form is different from the anti-parallel coiled-coil structure of RhoBD of protein kinase N when complexed with RhoA. Implications derived from these structural studies in relation to the mechanism of Rho-kinase activation will be addressed with previously reported experimental data.

Alternative splicing controls myotonic dystrophy protein kinase structure, enzymatic activity, and subcellular localization

Transcripts of the myotonic dystrophy protein kinase (DMPK) gene, a member of the Rho kinase family, are subject to cell-type specific alternative splicing. An imbalance in the splice isoform profile of DMPK may play a role in the pathogenesis of DM1, a severe multisystemic disorder. Here, we report how structural subdomains determine biochemical properties and subcellular distribution of DMPK isoforms. A newly developed kinase assay revealed that DMPK is a Lys/Arg-directed kinase. Individual DMPK isoforms displayed comparable transphosphorylation activity and sequence preference for peptide substrates. However, DMPK autophosphorylation and phosphorylation of MYPT1 (as putative in vivo target of DMPK), were dependent on presence of an alternatively spliced VSGGG motif and the nature of the C terminus. In-gel effects of the VSGGG motif on the migration behavior of full-length kinase provide evidence for a model in which this motif mediates 3-D-conformational changes in DMPK isoforms. Finally, different C termini endow DMPK with the ability to bind to either endoplasmic reticulum or mitochondria or to adopt a cytosolic location. Our results suggest that DMPK isoforms have cell-type and location dependent substrate specificities with a role in organellar and cytoarchitectural dynamics.

Homodimerization through coiled-coil regions enhances activity of the myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) is the protein product of the human DM-1 locus on chromosome 19q13.1 and has been implicated in the cardiac and behavioral dysfunctions of the disorder. DMPK contains four distinct regions: a leucine-rich repeat (L), a serine-threonine protein kinase catalytic domain (PK), an alpha-helical coiled-coil region (H), and a putative transmembrane-spanning tail (T). Multiple protein kinases that participate in cytoskeletal and cell cycle functions share homology with DMPK in the PK and H regions. Here we show that the LPKH and PKH subfragments of DMPK formed dimers of 140000 molecular weight, whereas the LPK subfragment remained a monomer of 62000 apparent molecular weight. The H domain thus appeared to be required for dimerization of DMPK subfragments. Caspase 1 cleaved LPKH between the PK and H regions. After cleavage, LPKH dimers became LPK-like monomers, consistent with the H domain mediating dimerization. The V(max) and k(cat)/K(m) of LPKH with a synthetic peptide kinase substrate were over 10-fold greater than either LPK or caspase-cleaved LPKH. The K(m) of dimeric LPKH was over three-fold greater than those of the monomeric proteins. Dimerization appeared to significantly affect the catalytic efficiency and substrate binding of DMPK. These interactions are likely to be functionally significant in other members of the myotonic dystrophy family of protein kinases with extensive coiled-coil domains.

Rocks: multifunctional kinases in cell behaviour

ROCKs, or Rho kinases, are serine/threonine kinases that are involved in many aspects of cell motility, from smooth-muscle contraction to cell migration and neurite outgrowth. Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.

Genomic organization of human myotonic dystrophy kinase-related Cdc42-binding kinase alpha reveals multiple alternative splicing and functional diversity

Myotonic dystrophy kinase-related Cdc42-binding kinase alpha (MRCKalpha) is a Cdc42/Rac interactive binding-containing serine/threonine kinase with multiple functional domains. Its roles in the regulation of peripheral actin reorganization in HeLa cells and NGF-induced neurite outgrowth in PC12 cells have been documented. Here we report the characterization of the genomic structure and alternative splicing of the human counterpart. Human MRCKalpha gene is located on chromosome 1q42.1, spanning a genomic region of 250-300 kb and is composed of 41 exons. Four exons in the internal variable region and six in the 3' end were found to undergo extensive alternative splicing, giving rise to 96 possible transcripts of different combinations. The region of the internal splice site that defines a variable region in between two functional domains of opposite regulatory effects on MRCKalpha catalytic activity, and the 3' end splice site that generates variants with differential GTPase binding activity suggest a role for these alternative splicing events in MRCKalpha regulation.

Characterization of RhoA-binding kinase ROKalpha implication of the pleckstrin homology domain in ROKalpha function using region-specific antibodies

Rho-binding kinase alpha (ROKalpha) is a serine/threonine kinase with multiple functional domains involved in actomyosin assembly. It has previously been documented that the C terminus part of ROKalpha interacts with the N-terminal kinase domain and thereby regulates its catalytic activity. Here we used antibodies against different domains of ROKalpha and were able to reveal some structural aspects that are essential for the specific functions of ROKalpha. Antibodies against the kinase domain revealed that this part of the protein is highly complex and inaccessible. Further experiments confirmed that this domain could undergo inter- and intramolecular interactions in a complex manner, which regulates the kinase catalytic activity. Other antibodies that raised against the coiled-coil domain, Rho binding domain, and the pleckstrin homology (PH) domain were all effective in recognizing the native proteins in an immunoprecipitation assay. Only the anti-Rho binding domain antibodies could activate the kinase independent of RhoA. The PH antibodies had no apparent effects on the catalytic activity but were effective in blocking actomyosin assembly and cell contractility. Likewise, mutations of the PH domains can abrogate its dominant negative effects on actin morphology. The subsequent disruption of endogenous ROK localization to the actomyosin network by overexpressing the PH domain is supportive of a role of the PH domain of ROK in targeting the kinase to these structures.

Intermolecular and intramolecular interactions regulate catalytic activity of myotonic dystrophy kinase-related Cdc42-binding kinase alpha

Myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK) is a Cdc42-binding serine/threonine kinase with multiple functional domains. We had previously shown MRCKalpha to be implicated in Cdc42-mediated peripheral actin formation and neurite outgrowth in HeLa and PC12 cells, respectively. Here we demonstrate that native MRCK exists in high-molecular-weight complexes. We further show that the three independent coiled-coil (CC) domains and the N-terminal region preceding the kinase domain are responsible for intermolecular interactions leading to MRCKalpha multimerization. N terminus-mediated dimerization and consequent transautophosphorylation are critical processes regulating MRCKalpha catalytic activities. A region containing the two distal CC domains (CC2 and CC3; residues 658 to 930) was found to interact intramolecularly with the kinase domain and negatively regulates its activity. Its deletion also resulted in an active kinase, confirming a negative autoregulatory role. We provide evidence that the N terminus-mediated dimerization and activation of MRCK and the negative autoregulatory kinase-distal CC interaction are two mutually exclusive events that tightly regulate the catalytic state of the kinase. Disruption of this interaction by a mutant kinase domain resulted in increased kinase activity. MRCK kinase activity was also elevated when cells were treated with phorbol ester, which can interact directly with a cysteine-rich domain next to the distal CC domain. We therefore suggest that binding of phorbol ester to MRCK releases its autoinhibition, allowing N-terminal dimerization and subsequent kinase activation.

Myotonic dystrophy protein kinase phosphorylates the myosin phosphatase targeting subunit and inhibits myosin phosphatase activity

Myotonic dystrophy protein kinase (DMPK) and Rho-kinase are related. An important function of Rho-kinase is to phosphorylate the myosin-binding subunit of myosin phosphatase (MYPT1) and inhibit phosphatase activity. Experiments were carried out to determine if DMPK could function similarly. MYPT1 was phosphorylated by DMPK. The phosphorylation site(s) was in the C-terminal part of the molecule. DMPK was not inhibited by the Rho-kinase inhibitors, Y-27632 and HA-1077. Several approaches were taken to determine that a major site of phosphorylation was T654. Phosphorylation at T654 inhibited phosphatase activity. Thus both DMPK and Rho-kinase may regulate myosin II phosphorylation.