Heart myosin light chain 2 gene. Nucleotide sequence of full length cDNA and expression in normal and hypertensive rat

Developmental, Physiological and Phylogenetic Perspectives on the Expression and Regulation of Myosin Heavy Chains in Craniofacial Muscles

This review deals with the developmental origins of extraocular, jaw and laryngeal muscles, the expression, regulation and functional significance of sarcomeric myosin heavy chains (MyHCs) that they express and changes in MyHC expression during phylogeny. Myogenic progenitors from the mesoderm in the prechordal plate and branchial arches specify craniofacial muscle allotypes with different repertoires for MyHC expression. To cope with very complex eye movements, extraocular muscles (EOMs) express 11 MyHCs, ranging from the superfast extraocular MyHC to the slowest, non-muscle MyHC IIB (nmMyH IIB). They have distinct global and orbital layers, singly- and multiply-innervated fibres, longitudinal MyHC variations, and palisade endings that mediate axon reflexes. Jaw-closing muscles express the high-force masticatory MyHC and cardiac or limb MyHCs depending on the appropriateness for the acquisition and mastication of food. Laryngeal muscles express extraocular and limb muscle MyHCs but shift toward expressing slower MyHCs in large animals. During postnatal development, MyHC expression of craniofacial muscles is subject to neural and hormonal modulation. The primary and secondary myotubes of developing EOMs are postulated to induce, via different retrogradely transported neurotrophins, the rich diversity of neural impulse patterns that regulate the specific MyHCs that they express. Thyroid hormone shifts MyHC 2A toward 2B in jaw muscles, laryngeal muscles and possibly extraocular muscles. This review highlights the fact that the pattern of myosin expression in mammalian craniofacial muscles is principally influenced by the complex interplay of cell lineages, neural impulse patterns, thyroid and other hormones, functional demands and body mass. In these respects, craniofacial muscles are similar to limb muscles, but they differ radically in the types of cell lineage and the nature of their functional demands.

Generation of MLC-2v-tdTomato knock-in reporter mouse line

MLC-2v is a myosin light chain regulatory protein which is specifically expressed in ventricular cardiomyocytes and slow twitch skeletal muscle cells. MLC-2v plays critical roles in ventricular maturation during heart development. Mice lacking MLC-2v are embryonic lethal due to heart failure associated with abnormal myofibrillar organization of ventricular cardiomyocytes. To study the development of ventricular cardiac muscle and slow twitch skeletal muscle, we generated a new MLC-2v reporter mouse line by knocking-in a tdTomato reporter cassette into 3' UTR of the MLC-2v gene without disrupting the endogenous gene. Our results demonstrated specific MLC-2v-tdTomato knock-in reporter expression in ventricular cardiomyocytes and slow twitch muscle during myogenesis, precisely recapitulating the spatiotemporal expression pattern of endogenous MLC-2v. No tdTomato expression was observed in the atria, fast twitch muscle or other organs throughout development into adulthood. Isolated neonatal and adult ventricular cardiomyocytes uniformly express tdTomato. Taken together, MLC-2v-tdTomato knock-in reporter mouse model described in this article will serve as a valuable tool to study cardiac chamber and skeletal muscle specification during development and regeneration by overcoming the pitfalls of transgenic strategies.

Phosphorylation of the regulatory light chain of myosin in striated muscle: methodological perspectives

Phosphorylation of the regulatory light chain (RLC) of myosin modulates cellular functions such as muscle contraction, mitosis, and cytokinesis. Phosphorylation defects are implicated in a number of diseases. Here we focus on striated muscle where changes in RLC phosphorylation relate to diseases such as hypertrophic cardiomyopathy and muscular dystrophy, or age-related changes. RLC phosphorylation in smooth muscle and non-muscle cells are covered briefly where relevant. There is much scientific interest in controlling the phosphorylation levels of RLC in vivo and in vitro in order to understand its physiological function in striated muscles. A summary of available and emerging in vivo and in vitro methods is presented. The physiological role of RLC phosphorylation and novel pathways are discussed to highlight the differences between muscle types and to gain insights into disease processes.

Functions of myosin light chain-2 (MYL2) in cardiac muscle and disease

Myosin light chain-2 (MYL2, also called MLC-2) is an ~19kDa sarcomeric protein that belongs to the EF-hand calcium binding protein superfamily and exists as three major isoforms encoded by three distinct genes in mammalian striated muscle. Each of the three different MLC-2 genes (MLC-2f; fast twitch skeletal isoform, MLC-2v; cardiac ventricular and slow twitch skeletal isoform, MLC-2a; cardiac atrial isoform) has a distinct developmental expression pattern in mammals. Genetic loss-of-function studies in mice demonstrated an essential role for cardiac isoforms of MLC-2, MLC-2v and MLC-2a, in cardiac contractile function during early embryogenesis. In the adult heart, MLC-2v function is regulated by phosphorylation, which displays a specific 1`expression pattern (high in epicardium and low in endocardium) across the heart. These data along with new data from computational models, genetic mouse models, and human studies have revealed a direct role for MLC-2v phosphorylation in cross-bridge cycling kinetics, calcium-dependent cardiac muscle contraction, cardiac torsion, cardiac function and various cardiac diseases. This review focuses on the regulatory functions of MLC-2 in the embryonic and adult heart, with an emphasis on phosphorylation-driven actions of MLC-2v in adult cardiac muscle, which provide new insights into mechanisms regulating myosin cycling kinetics and human cardiac diseases.

Expression characterization and promoter activity analysis of the tilapia (Oreochromis niloticus) myosin light chain 3 promoter in skeletal muscle of fish

A tilapia (Oreochromis niloticus) myosin light chain 3 (Mlc3) promoter region (~4.3 kb) was isolated and characterized. Sequence analysis of the clone revealed high similarity with a tilapia gene encoding the Mlc3 promoter region, exon 1, and intron 1. The clone contained several putative binding sequences for transcription factors, including MEF-2, MYOG, MyoD, PKNOX1, and AREB6. Deletion of a region of the tilapia Mlc3 promoter (801 to -3,881 bp) enhanced promoter activity, as determined by direct injection of a luciferase reporter construct into skeletal muscle of Archocentrus nigrofasciatus. These findings suggest that the region between -801 and -3,881 bp may contain negative regulatory elements. Stable germline transgenic strains of the ornamental fish species A. nigrofasciatus var. carrying the Taiwan coral red fluorescent protein (TcRFP) driven by the Mlc3 promoter were established. F1 adult transgenic A. nigrofasciatus var. exhibited brilliant pink fluorescence in skeletal muscles in the daylight. Therefore, our current study demonstrates the feasibility of using the tilapia Mlc3 promoter to drive fluorescence in new fish species, such as Perciformes.

Myosin regulatory light chain (RLC) phosphorylation change as a modulator of cardiac muscle contraction in disease

Understanding how cardiac myosin regulatory light chain (RLC) phosphorylation alters cardiac muscle mechanics is important because it is often altered in cardiac disease. The effect this protein phosphorylation has on muscle mechanics during a physiological range of shortening velocities, during which the heart generates power and performs work, has not been addressed. We have expressed and phosphorylated recombinant Rattus norvegicus left ventricular RLC. In vitro we have phosphorylated these recombinant species with cardiac myosin light chain kinase and zipper-interacting protein kinase. We compare rat permeabilized cardiac trabeculae, which have undergone exchange with differently phosphorylated RLC species. We were able to enrich trabecular RLC phosphorylation by 40% compared with controls and, in a separate series, lower RLC phosphorylation to 60% of control values. Compared with the trabeculae with a low level of RLC phosphorylation, RLC phosphorylation enrichment increased isometric force by more than 3-fold and peak power output by more than 7-fold and approximately doubled both maximum shortening speed and the shortening velocity that generated peak power. We augmented these measurements by observing increased RLC phosphorylation of human and rat HF samples from endocardial left ventricular homogenate. These results demonstrate the importance of increased RLC phosphorylation in the up-regulation of myocardial performance and suggest that reduced RLC phosphorylation is a key aspect of impaired contractile function in the diseased myocardium.

Peroxisome proliferator-activated receptor alpha (PPAR alpha) agonist, WY-14,643, increased transcription of myosin light chain-2 in cardiomyocytes

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that can be activated by xenobiotics and natural fatty acids. To assess the potential physiological activity of PPAR ligands on cardiac muscular cells, the effects of PPAR alpha agonist, WY-14,643, on both rat hearts and a rat cardiomyocyte cell line (H9c2 cells) were investigated. Male F344 rats were fed a diet containing WY-14,643 at a concentration of 100 ppm for 26 weeks. Cardiac muscular hypertrophy was revealed by morphometric analysis in which the diameter of the muscular fibers in WY-14,643-treated rats was larger than those of control rats. Using H9c2 cells in vitro, the protein content per cell was increased in a dose-dependent manner with the treatment of WY-14,643. The transcription of myosin light chain-2 (MLC-2), a parameter of myocardial hypertrophy, was increased in H9c2 cells transfected with the rat MLC-2/luciferase fusion gene by WY-14,643 as well as other peroxisome proliferators, clofibrate and di(2-ethylhexyl) phthalate. In addition, accumulation of myosin light chain protein was confirmed in H9c2 cells treated with WY-14,643 at 10 micrograms/ml for 7 days or more by immunohistochemistry. These results suggest that PPAR alpha ligands have a potential to regulate MLC-2, which is a contractile protein in cardiomyocytes and may play a part of role in the pathogenesis of cardiac hypertrophy.

Fast skeletal muscle-specific expression of a zebrafish myosin light chain 2 gene and characterization of its promoter by direct injection into skeletal muscle

A zebrafish myosin light chain 2 cDNA clone was isolated and characterized. Sequence analysis of the clone revealed a high homology with the mammalian and avian genes encoding the fast skeletal muscle isoform, MLC2f. In situ hybridization and Northern blot hybridization analyses indicated that the zebrafish MLC2f mRNA is expressed exclusively in the fast skeletal muscle. Ontogenetically, the MLC2f mRNA appears around 16 hours postfertilization (hpf) in the first few well-formed anterior somites. At later stages, the MLC2f mRNA can also be detected in fin buds, eye muscles, and jaw muscles. To develop a useful model system for analyzing muscle gene regulation, the promoter of the zebrafish MLC2f gene was isolated and linked to the chloramphenicol acetyltransferase (CAT) reporter gene. The MLC2f/CAT chimeric constructs were analyzed by direct injection into the zebrafish skeletal muscle, and significant CAT activity was observed; in contrast, little or no CAT activity was generated from a similarly injected prolactin gene promoter/CAT gene construct. Within the 1 kb of the MLC2f promoter region, several MEF2-binding sites and E-boxes were identified, suggesting that MLC2f can be regulated by muscle transcription factors MEF2 and myogenic bHLH proteins. A 5' deletion analysis indicated that the proximal 79 nucleotides from the transcription start site, which contains a single MEF2-binding site, is sufficient to drive a high level of CAT activity in injected muscle. Internal deletion of the MEF2 element in the -79-bp construct caused an 80% decrease in CAT activity, whereas internal deletion of the same MEF2 element in a -1044-bp construct had no effect on induced CAT activity. These observations suggest that an MEF2 element is important to activate the MLC2f gene in muscle cells, and the effect of loss of the proximal MEF2 element can be compensated for by the presence of the upstream MEF2 elements. This study also demonstrated that direct injection of DNA into skeletal muscle is a valid and valuable approach to analyze muscle gene promoters in the zebrafish.

Stress and strain as regulators of myocardial growth

The response of the heart to altered hemodynamic loading is growth or remodeling of myocytes and the extracellular matrix. In order to describe and mathematically model this dynamic and complex system of growing and resorbing tissue, the stimulating factor for tissue growth must be found, and up to now is not known. Most evidence, both in tissue and at the cellular level, points to a mechanical factor as the stimulus, and most likely a deformation signal is transduced to initiate protein synthesis. At the cellular level mechanotransduction likely takes place at the cellular membrane, although multiple biochemical and mechanical pathways have been proposed which induce transcription in the nucleus and eventual protein upregulation. The results of a recent mathematical analysis based on experimental data suggest that end-diastolic fiber strain at the tissue level may be the stimulus to one mode of tissue growth: volume-overload hypertrophy. This is the only mechanical factor that we found to be normalized after volume overload hypertrophy. But other studies do not agree with this result, and other modes of hypertrophy may be regulated by different factors or combinations of factors.

Regulation of very-low-density lipoprotein receptor in hypertrophic rat heart

To elucidate the regulation of very-low density-lipoprotein (VLDL) receptor, we have studied its gene expression in the heart of spontaneously hypertensive rats-stroke prone (SHR-SP, an animal model for hypertension-induced cardiac hypertrophy) compared with Wistar-Kyoto rats. RNase protection assay showed that ventricular VLDL receptor mRNA falls to 41% of normal levels at 4 weeks when hypertension is not yet fully developed, and drops further to 14% at 13 weeks, when cardiac hypertrophy is established. Lipoprotein lipase mRNA decreases in parallel with VLDL receptor mRNA. In cultured neonatal rat ventricular cardiomyocytes, VLDL receptor mRNA decreases in parallel with the process of cardiocyte hypertrophy during the 24 hours after treatment with 10-8 mol/L endothelin-1, falling to 40% of the initial value. These results demonstrate that there is downregulation of VLDL receptor gene expression in cardiac hypertrophy both in vivo and in vitro and suggest that the regulation of the VLDL receptor is possibly linked with the switch in energy substrate from lipid to glucose known to occur in cardiac hypertrophy.

Functional significance of alterations in cardiac contractile protein isoforms

Multiple closely related, yet distinct, isoforms exist for each of the cardiac contractile proteins. The isoform composition of the heart changes in response to developmental and physiologic cues. This paper reviews the molecular basis for cardiac contractile protein isoform diversity and the functional consequences of isoform shifts.

Adaptational response in transcription factors during development of myocardial hypertrophy

Cardiac hypertrophy is characterized, among others, by the molecular events which selectively activate the expression of genes for contractile proteins within individual myocardial cells. As such, myosin light chain 2 (MLC-2), which is upregulated in the hypertrophic state in both rat and human, serves as a marker for hypertrophy. In an attempt to investigate the gene regulatory mechanisms of this phenomenon, we tested the hypothesis that certain transcription factors are directly involved in the development of cardiac hypertrophy by demonstrating the presence of cardiac tissue-specific regulatory elements in the 5'-flanking region of the MLC-2 promoter and testing them in the gel mobility shift assay for their binding activity to nuclear proteins from hypertrophied and normal cardiac tissue. In nuclear extracts from the ventricular tissues of the spontaneously hypertensive rat (SHR), distinctive changes in two families of activator proteins, the A/T-rich DNA-binding transcription factors, myocyte enhancer factor (MEF-2) and CArG-binding factor, manifested in a developmentally dictated manner paralleling the evolution of cardiac hypertrophy in these animals. Extracts isolated from brains and skeletal muscle tissues from the same animals did not exhibit the changes in binding activity. Also, the changes were not apparent when a distal negative regulatory element (CSS), which confers cardiac-specific expression, was tested in gel mobility shift assays. The ubiquitous TATA-binding proteins remained unchanged in comparing SHR with the control strain WKY in the same assay. These data support the notion that the expression of specific transcription factors is modulated in response to hypertrophy related signals which execute changes at the gene level effecting the enrichment of certain contractile proteins in an effort discrete and estranged from the basal transcription machinery.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Localization of the gene coding for ventricular myosin regulatory light chain (MYL2) to human chromosome 12q23-q24.3

Human myosin light chain-2 (MYL2) is an important protein involved in the regulation of myosin ATPase activity in smooth muscle. In cardiac muscle, the precise role of MYL2 is not well understood; however, an increase in ventricular MYL2 is observed during myocardial hypertrophy in cardiac patients with valve stenosis. The chromosomal location of the gene coding for MYL2 was identified using a cloned cDNA for human MYL2. Southern blot analysis of DNA from a human/rodent somatic cell hybrid mapping panel showed that the BamHI fragment that hybridized with this cDNA probe was concordant with chromosome 12. The 768-bp cDNA was hybridized to human metaphase chromosomes. The results revealed a significant clustering of silver grains over chromosome 12 bands q23-q24.3, indicating that the gene coding for MYL2 is located in this region.

Probing myosin light chain 1 structure with monoclonal antibodies

Five monoclonal antibodies that react with different regions of myosin light chain 1 from human ventricular myocardial muscle were used to obtain information on interactions between the light chain 1 and heavy chains and generally on the tertiary structure of the light chain 1 within the myosin head. We performed Western blot assays of the five antibodies with myosins from different cardiac and skeletal muscles, with different proteolytic fragments of bovine ventricular myosin light chain 1 (LC1) and to different recombinant fragments of human ventricular LC1 and rat fast skeletal light chain LC1/LC3. The five antibodies were mapped in three different regions of the light chain 1: two antibodies mapped within the first eight amino-terminal residues, two between residues 71 and 74, and one between residues 129 and 134. The apparent dissociation constants of the last three antibodies, determined by antibody-antigen equilibria in solution, were lower than when isolated light chains were used as antigens. It is probable that the corresponding amino acids involved in the antibody epitopes were either involved in interactions between the light and heavy myosin subunits, or somehow hindered by the myosin heavy chain bulk. In contrast, the apparent dissociation constants measured for both other antibodies were higher when myosin, rather than isolated light chains, was used as antigen. Thus LC1 fixation to heavy chains within the myosin molecule induced conformation changes at the amino-terminal end of the light chain 1. No difference in the accessibility of this mobile LC1 segment was detected in the presence of actin. Finally, observed differences in epitope accessibility on the light chain LC1 in myosin, as compared with chymotryptic subfragment 1 (SF1), indicated conformational differences between native myosin and extensively studied SF1 molecules.

Structure, organization and regulation of a rat cysteine proteinase inhibitor-encoding gene

During postnatal development, submandibular glands of rats produce the secretory protein, cystatin S (CysS), which belongs to family 2 of the mammalian cysteine proteinase inhibitor superfamily. While the rat CysS gene is not expressed in the salivary glands of adult rats, it can be induced by isoproterenol (IPR), which acts via beta-adrenergic receptor/adenylate cyclase/cyclic AMP (cAMP) mechanisms. In addition, IPR-induction of CysS mRNA in submandibular glands is more pronounced in females than in males, at both prepuberal and mature ages. These results suggest that sex hormones may participate in the regulation of the rat CysS gene via estrogen-responsive elements (ERE), and IPR induction of this gene supports the hypothesis that cAMP-responsive elements (CRE) may also play a role in regulating CysS gene expression. We have isolated, sequenced and characterized the complete gene. The CysS gene contains three exons interrupted by two intervening sequences, with consensus splice junctions. The transcription start point is 73 nucleotides upstream from the start codon which is surrounded by a typical Kozak sequence. CCAAT and TATA boxes are present in the 5'-flanking region of the CysS gene. This region also contains several possible regulatory elements that resemble those of other eukaryotic genes, i.e., ERE, CRE, and glucocorticoid-responsive elements. The first intron sequence contains other potential CRE highly homologous to those found in the IPR-inducible mouse and hamster proline-rich-protein-encoding genes.

cDNA sequence, tissue-specific expression, and chromosomal mapping of the human slow-twitch skeletal muscle isoform of troponin I

Troponin I (TnI) is a myofibrillar protein involved in the calcium-mediated regulation of striated muscle contraction. Three isoforms of TnI are known and each is expressed in a muscle fiber-type-specific manner. TnI-fast and TnI-slow are expressed exclusively in fast-twitch and slow-twitch skeletal muscle myofibers, respectively, while a third isoform, TnI-card, is expressed in both the atrium and the ventricle of the heart. An explanation of the myofiber-type-restricted expression of the troponin I multigene family will further aid in understanding how various types of striated muscle fibers are established. To initiate the study of TnI isoform gene expression, we have isolated a full-length cDNA representing the human slow-twitch skeletal muscle isoform of troponin I. Sequence comparisons demonstrate that the TnI-slow protein is highly conserved between species. Therefore, the cDNA was used as a probe to investigate the tissue-specific and developmental regulation of the TnI-slow gene in both rodent and human myogenic cells. TnI-slow message appears to be restricted to muscle tissue containing slow-twitch skeletal muscle myofibers. TnI-slow gene expression is induced in differentiated cultures of primary human muscle cells and several (but not all) myogenic cell lines. In addition, a human-specific probe prepared from the 3' untranslated region of the cDNA has been used to probe a panel of human/mouse somatic cell hybrid lines, resulting in the assignment of the human TnI-slow gene to the q12----qter region of chromosome 1. The locus is designated TNNI1.

Evolution of EF-hand calcium-modulated proteins. I. Relationships based on amino acid sequences

The relationships among 153 EF-hand (calcium-modulated) proteins of known amino acid sequence were determined using the method of maximum parsimony. These proteins can be ordered into 12 distinct subfamilies--calmodulin, troponin C, essential light chain of myosin, regulatory light chain, sarcoplasmic calcium binding protein, calpain, aequorin, Stronglyocentrotus purpuratus ectodermal protein, calbindin 28 kd, parvalbumin, alpha-actinin, and S100/intestinal calcium-binding protein. Eight individual proteins--calcineurin B from Bos, troponin C from Astacus, calcium vector protein from Branchiostoma, caltractin from Chlamydomonas, cell-division-cycle 31 gene product from Saccharomyces, 10-kd calcium-binding protein from Tetrahymena, LPS1 eight-domain protein from Lytechinus, and calcium-binding protein from Streptomyces--are tentatively identified as unique; that is, each may be the sole representative of another subfamily. We present dendrograms showing the relationships among the subfamilies and uniques as well as dendrograms showing relationships within each subfamily. The EF-hand proteins have been characterized from a broad range of organismal sources, and they have an enormous range of function. This is reflected in the complexity of the dendrograms. At this time we urge caution in assigning a simple scheme of gene duplications to account for the evolution of the 600 EF-hand domains of known sequence.

Prediction of the secondary structure of myosin light chains from comparison of homologous sequences. Implications for the interaction between myosin heavy and light chains

The primary sequences of seventeen essential and seventeen regulatory myosin light chains were analyzed and compared, using algorithms based on the different structural properties of their amino acid residues. This process allowed estimation of the structural homology between the proteins studied, and improved the prediction of their mean secondary structure and functionally important segments or residues. On the basis of the crystal structure of troponin C, a model of the myosin essential light chain with a fairly compact form is proposed. The possible sites of interaction between myosin light and heavy chains from rabbit skeletal muscle were also investigated by a complementarity method adapted to helix-rich proteins. Segments 139-149 and 65-75 in the essential light chain and segments 27-37, 67-77 and 97-107 in the regulatory light chain are suggested to constitute some of these sites, as most of them were found to have the features of surface-seeking helices.

Myosin P-light chain isoenzymes in the human heart: evidence for diphosphorylation of the atrial P-LC form

We studied myosin light chains (LC) of human atrium and ventricle of normal and diseased individuals by a high-resolution 2-dimensional polyacrylamide gel electrophoresis (2D-PAGE) technique. Atrial LCs (ALC-1, ALC-2 (= P-LC)) revealed both higher molecular weights and lower isoelectric points (IEP) than their ventricular counterparts (VLC-1, VLC-2 (= P-LC)). Different P-LC forms with their distinct myosin isoenzymes have been designated as P-LC-polymorphism and myosin P-LC isoenzymes, respectively. In the dephosphorylated state two VLC-2 forms (VLC-2 and VLC-2*) with the same MW and different IEP, but only one ALC-2 form, were found. In the partially phosphorylated state ALC-2 appeared to be single- and double-phosphorylated (three spots in the 2D-PAGE), whereas the two VLC-2 forms appeared to be single-phosphorylated each (four spots in the 2D-PAGE). Phosphoryl-transfer from ATP to the P-LC forms was studied using skinned fibers incubated with MLCK (myosin light chain kinase) and (gamma-32P)ATP. Ventricular myosin P-LC isoenzyme pattern was usually the same in normal and diseased patients: the VLC-2 to VLC-2* ratio was approx. 70/30, but in one patient with valvular heart disease (VHD) the relation was 55/45 (shift to the VLC-2* form). In hypertrophied atria of VHD patients a shift of the myosin P-LC isoenzyme pattern to the VLC-2* form occurred, too.

Expression of ventricular-type myosin light chain messenger RNA in spontaneously hypertensive rat atria

Using cloned DNA probes specific for two isoforms of cardiac myosin light chains (MLCs), nonphosphorylatable MLC1 and phosphorylatable, regulatory MLC2, we have observed that the MLC1 messenger RNA of ventricular type does not appear in detectable amounts in atrial cells of either normotensive Wistar-Kyoto rat strain (WKY) or spontaneously hypertensive rat strain (SHR). The messenger RNA of regulatory isoform of ventricular MLC2, on the other hand, is found in threefold excess in atria of SHR relative to that of age-matched WKY. The increased level of MLC2 messenger RNA is present even in 6-week-old SHR atria where there is no established overloading of the heart. Thus, it appears that the increased expression of the regulatory MLC2 gene in SHR atrial cells is a predetermined event, which, most likely, participates in functional adaptation of the myocardium in response to pressure overload and subsequent hypertrophy.