Alpha-tropomyosin gene organization. Alternative splicing of duplicated isotype-specific exons accounts for the production of smooth and striated muscle isoforms

Does the Pachytene Checkpoint, a Feature of Meiosis, Filter Out Mistakes in Double-Strand DNA Break Repair and as a side-Effect Strongly Promote Adaptive Speciation?

This essay aims to explain two biological puzzles: why eukaryotic transcription units are composed of short segments of coding DNA interspersed with long stretches of non-coding (intron) DNA, and the near ubiquity of sexual reproduction. As is well known, alternative splicing of its coding sequences enables one transcription unit to produce multiple variants of each encoded protein. Additionally, padding transcription units with non-coding DNA (often many thousands of base pairs long) provides a readily evolvable way to set how soon in a cell cycle the various mRNAs will begin being expressed and the total amount of mRNA that each transcription unit can make during a cell cycle. This regulation complements control via the transcriptional promoter and facilitates the creation of complex eukaryotic cell types, tissues, and organisms. However, it also makes eukaryotes exceedingly vulnerable to double-strand DNA breaks, which end-joining break repair pathways can repair incorrectly. Transcription units cover such a large fraction of the genome that any mis-repair producing a reorganized chromosome has a high probability of destroying a gene. During meiosis, the synaptonemal complex aligns homologous chromosome pairs and the pachytene checkpoint detects, selectively arrests, and in many organisms actively destroys gamete-producing cells with chromosomes that cannot adequately synapse; this creates a filter favoring transmission to the next generation of chromosomes that retain the parental organization, while selectively culling those with interrupted transcription units. This same meiotic checkpoint, reacting to accidental chromosomal reorganizations inflicted by error-prone break repair, can, as a side effect, provide a mechanism for the formation of new species in sympatry. It has been a long-standing puzzle how something as seemingly maladaptive as hybrid sterility between such new species can arise. I suggest that this paradox is resolved by understanding the adaptive importance of the pachytene checkpoint, as outlined above.

cGMP interacts with tropomyosin and downregulates actin-tropomyosin-myosin complex interaction

Background

The nitric oxide-soluble guanylate cyclase-cyclic guanosine monophosphate (NO-sGC-cGMP) signaling pathway, plays a critical role in the pathogenesis of pulmonary arterial hypertension (PAH); however, its exact molecular mechanism remains undefined.

Methods

Biotin-cGMP pull-down assay was performed to search for proteins regulated by cGMP. The interaction between cGMP and tropomyosin was analyzed with antibody dependent pull-down in vivo. Tropomyosin fragments were constructed to explore the tropomyosin-cGMP binding sites. The expression level and subcellular localization of tropomyosin were detected with Real-time PCR, Western blot and immunofluorescence assay after the 8-Br-cGMP treatment. Finally, isothermal titration calorimetry (ITC) was utilized to detect the binding affinity of actin-tropomyosin-myosin in the existence of cGMP-tropomyosin interaction.

Results

cGMP interacted with tropomyosin. Isoform 4 of TPM1 gene was identified as the only isoform expressed in the human pulmonary artery smooth muscle cells (HPASMCs). The region of 68-208aa of tropomyosin was necessary for the interaction between tropomyosin and cGMP. The expression level and subcellular localization of tropomyosin showed no change after the stimulation of NO-sGC-cGMP pathway. However, cGMP-tropomyosin interaction decreased the affinity of tropomyosin to actin.

Conclusions

We elucidate the downstream signal pathway of NO-sGC-cGMP. This work will contribute to the detection of innovative targeted agents and provide novel insights into the development of new therapies for PAH.

Emerging roles for RNA-binding proteins as effectors and regulators of cardiovascular disease

The cardiovascular system comprises multiple cell types that possess the capacity to modulate their phenotype in response to acute or chronic injury. Transcriptional and post-transcriptional mechanisms play a key role in the regulation of remodelling and regenerative responses to damaged cardiovascular tissues. Simultaneously, insufficient regulation of cellular phenotype is tightly coupled with the persistence and exacerbation of cardiovascular disease. Recently, RNA-binding proteins such as Quaking, HuR, Muscleblind, and SRSF1 have emerged as pivotal regulators of these functional adaptations in the cardiovascular system by guiding a wide-ranging number of post-transcriptional events that dramatically impact RNA fate, including alternative splicing, stability, localization and translation. Moreover, homozygous disruption of RNA-binding protein genes is commonly associated with cardiac- and/or vascular complications. Here, we summarize the current knowledge on the versatile role of RNA-binding proteins in regulating the transcriptome during phenotype switching in cardiovascular health and disease. We also detail existing and potential DNA- and RNA-based therapeutic approaches that could impact the treatment of cardiovascular disease in the future.

Smooth Muscle-specific α Tropomyosin Is a Marker of Fully Differentiated Smooth Muscle in Lung

Tropomyosin (Tm) is one of the major components of smooth muscle. Currently it is impossible to easily distinguish the two major smooth muscle (sm) forms of Tm at a protein level by immunohistochemistry due to lack of specific antibodies. α-sm Tm contains a unique 2a exon not found in any other Tm. We have produced a polyclonal antibody to this exon that specifically detects α-sm Tm. We demonstrate here the utility of this antibody for the study of smooth muscle. The tissue distribution of α-sm Tm was shown to be highly specific to smooth muscle. α-sm Tm showed an identical profile and tissue colocalization with α-sm actin both by Western blotting and immunohistochemistry. Using lung as a model organ system, we examined the developmental appearance of α-sm Tm in comparison to α-sm actin in both the mouse and human. α-sm Tm is a late-onset protein, appearing much later than actin in both species. There were some differences in onset of appearance in vascular and airway smooth muscle with airway appearing earlier. α-sm Tm can therefore be used as a good marker of mature differentiated smooth muscle cells. Along with α-sm actin and sm-myosin antibodies, α-sm Tm is a valuable tool for the study of smooth muscle.

Translational control of tropomyosin expression in vertebrate hearts

The tropomyosin (TM) gene family produces a set of related TM proteins with important functions in striated and smooth muscle, and nonmuscle cells. In vertebrate striated muscle, the thin filament consists largely of actin, TM, the troponin (Tn) complex (Tn-I, Tn-C and Tn-T), and tropomodulin (Tmod) and is responsible for mediating Ca(2+) control of muscle contraction and relaxation. There are four known genes (designated as TPM1, TPM2, TPM3, and TPM4) for TM in vertebrates. The four TM genes generate a multitude of tissue- and developmental-specific isoforms through the use of different promoters, alternative mRNA splicing, different 3'-end mRNA processing and tissue-specific translational control. In this review, we have focused mainly on the regulation of TM expression in striated muscles, primarily in vertebrate hearts with special emphasis on translational control using mouse and Mexican axolotl animal models.

Periodicities designed in the tropomyosin sequence and structure define its functions

Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin, Arp2/3 and ADF-cofilin in most eukaryotic cells. Tropomyosin is the prototypical two-chained, α-helical coiled coil protein that associates end-to-end and binds to both sides of the actin filament. Each tropomyosin molecule spans four to seven actin monomers in the filament, depending on the size of the tropomyosin. Tropomyosins have a periodic heptad repeat sequence that is characteristic of coiled coil proteins as well as additional periodicities required for its interaction with the actin filament, where each periodic repeat interacts with one actin molecule. This review addresses the role of periodic features of the Tm molecule in carrying out its universal functions of binding to the actin filament and its regulation and the specific features that may determine the isoform specificity of tropomyosins.

Absence of mutation at the 5'-upstream promoter region of the TPM4 gene from cardiac mutant axolotl (Ambystoma mexicanum)

Tropomyosins are a family of actin-binding proteins that show cell-specific diversity by a combination of multiple genes and alternative RNA splicing. Of the 4 different tropomyosin genes, TPM4 plays a pivotal role in myofibrillogenesis as well as cardiac contractility in amphibians. In this study, we amplified and sequenced the upstream regulatory region of the TPM4 gene from both normal and mutant axolotl hearts. To identify the cis-elements that are essential for the expression of the TPM4, we created various deletion mutants of the TPM4 promoter DNA, inserted the deleted segments into PGL3 vector, and performed promoter-reporter assay using luciferase as the reporter gene. Comparison of sequences of the promoter region of the TPM4 gene from normal and mutant axolotl revealed no mutations in the promoter sequence of the mutant TPM4 gene. CArG box elements that are generally involved in controlling the expression of several other muscle-specific gene promoters were not found in the upstream regulatory region of the TPM4 gene. In deletion experiments, loss of activity of the reporter gene was noted upon deletion which was then restored upon further deletion suggesting the presence of both positive and negative cis-elements in the upstream regulatory region of the TPM4 gene. We believe that this is the first axolotl promoter that has ever been cloned and studied with clear evidence that it functions in mammalian cell lines. Although striated muscle-specific cis-acting elements are absent from the promoter region of TPM4 gene, our results suggest the presence of positive and negative cis-elements in the promoter region, which in conjunction with positive and negative trans-elements may be involved in regulating the expression of TPM4 gene in a tissue-specific manner.

New insights into the regulation of the actin cytoskeleton by tropomyosin

The actin cytoskeleton is regulated by a variety of actin-binding proteins including those constituting the tropomyosin family. Tropomyosins are coiled-coil dimers that bind along the length of actin filaments. In muscles, tropomyosin regulates the interaction of actin-containing thin filaments with myosin-containing thick filaments to allow contraction. In nonmuscle cells where multiple tropomyosin isoforms are expressed, tropomyosins participate in a number of cellular events involving the cytoskeleton. This chapter reviews the current state of the literature regarding tropomyosin structure and function and discusses the evidence that tropomyosins play a role in regulating actin assembly.

Echinococcus granulosus tropomyosin isoforms: from gene structure to expression analysis

Tropomyosins (Trps) constitute a family of actin filament-binding proteins found in all eukaryotic cells. In muscle cells, they play a central role in contraction by regulating calcium-sensitive interaction of actin and myosin. In non-muscle cells, tropomyosins regulate actin filament organization and dynamics. Trps genes exhibit extensive cell type-specific isoform diversity generated by alternative splicing. Here, we report the characterization of tropomyosin gene transcribed sequences from the parasitic platyhelminth Echinococcus granulosus. Using RT-PCR approach we isolated three isoforms (egtrpA, egtrpB and egtrpC), which display significant homologies to know tropomyosins of different phylogenetic origin. The corresponding gene, egtrp (5656 bp), contains eight introns and nine exons. Southern blot hybridization studies showed that egtrp is present as single copy locus in E. granulosus. We demonstrated that egtrp expresses three different transcripts which differ in alternatively spliced exon 4 and intron VI. Interestingly, intron VI suffers intron retention and contains an internal stop codon in frame. Three major bands are also detected by Western blot analysis using a specific anti-rEgTrp antiserum. Immune-localization and in situ hybridization studies showed that egtrp transcription and translation is mostly localized at the protoscoleces suckers. This is the first report of alternative splicing in this parasite.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Cooperative binding of tropomyosin to actin

Tropomyosin molecules attach to the thin filament conjointly rather than separately, in a pattern indicating very high cooperativity. The equilibrium process drawing tropomyosins together on the actin filament can be measured by application ofa linear lattice model to bindingisotherm data and hypotheses on the mechanism of cooperativity can be tested. Each end of tropomyosin overlaps and attaches to the end ofa neighboring tropomyosin, facilitating the formation of continuous tropomyosin strands, without gaps between neighboring molecules along the thin filament. Interestingly, the overlap complexes vary greatly in size and composition among tropomyosin isoforms, despite consistently cooperative binding to actin. Also, the tendency of tropomyosin to bind to actin cooperatively rather than randomly does not correlate with the strength ofend-to-end binding.By implication, tropomyosin's actin-binding cooperativity likely involves effects on the actin filament, as well as direct interactions between adjacent tropomyosins.

Gene expression profiling identifies regulatory pathways involved in the late stage of rat fetal lung development

Fetal lung development is a complex biological process that involves temporal and spatial regulations of many genes. To understand the molecular mechanisms of this process, we investigated gene expression profiles of fetal lungs on gestational days 18, 19, 20, and 21, as well as newborn and adult rat lungs. For this analysis, we used an in-house rat DNA microarray containing 6,000 known genes and 4,000 expressed sequence tags (ESTs). Of these, 1,512 genes passed the statistical significance analysis of microarray (SAM) test; an at least twofold change was shown for 583 genes (402 known genes and 181 ESTs) between at least two time points. K-means cluster analysis revealed seven major expression patterns. In one of the clusters, gene expression increased from day 18 to day 20 and then decreased. In this cluster, which contained 10 known genes and 5 ESTs, 8 genes are associated with development. These genes can be integrated into regulatory pathways, including growth factors, plasma membrane receptors, adhesion molecules, intracellular signaling molecules, and transcription factors. Real-time PCR analysis of these 10 genes showed an 88% consistency with the microarray data. The mRNA of LIM homeodomain protein 3a (Lhx3), a transcription factor, was enriched in fetal type II cells. In contrast, pleiotrophin, a growth factor, had a much higher expression in fetal lung tissues than in fetal type II cells. Immunohistochemistry revealed that Lhx3 was localized in fetal lung epithelial cells and pleiotrophin in the mesenchymal cells adjacent to the developing epithelium and blood vessel. Using GenMAPP, we identified four regulatory pathways: transforming growth factor-beta signaling, inflammatory response, cell cycle, and G protein signaling. We also identified two metabolic pathways: glycolysis-gluconeogenesis and proteasome degradation. Our results may provide new insights into the complex regulatory pathways that control fetal lung development.

Dual requirement for flexibility and specificity for binding of the coiled-coil tropomyosin to its target, actin

The coiled coil is a widespread motif involved in oligomerization and protein-protein interactions, but the structural requirements for binding to target proteins are poorly understood. To address this question, we measured binding of tropomyosin, the prototype coiled coil, to actin as a model system. Tropomyosin binds to the actin filament and cooperatively regulates its function. Our results support the hypothesis that coiled-coil domains that bind to other proteins are flexible. We made mutations that alter interface packing and stability as well as mutations in surface residues in a postulated actin binding site. Actin affinity, measured by cosedimentation, was correlated with coiled-coil stability and local instability and side chain flexibility, analyzed with circular dichroism and fluorescence spectroscopy. The flexibility from interruptions in the stable coiled-coil interface is essential for actin binding. The surface residues in a postulated actin binding site participate in actin binding when the coiled coil within it is poorly packed.

Fish fast skeletal muscle tropomyosins show species-specific thermal stability

Tropomyosin (TM) was isolated from the fast skeletal muscle of six fish species, whose amino acid sequences of this protein have already been revealed. The thermal stability of these TMs was measured by differential scanning calorimetry (DSC) and circular dichroism (CD), while the molecular weights were measured by mass spectrometry. The results showed clear differences in thermostability among these fish TMs, though the identity of amino acid sequences was more than 93.3%. Therefore, only a few amino acid substitutions could affect the overall stability of the TM molecule. Especially, several residues located on the molecular surface were considered to be responsible for such stability difference. In contrast, the molecular weights of these TMs as measured by mass spectrometry were higher than those calculated from amino acid composition, suggesting the presence of post-translational modification(s) which could also affect their thermal stability.

Cloning and nucleotide sequence of a novel 28-kDa protein from the mantle muscle of the squid Todarodes pacificus with homology to tropomyosin

In recent studies, we found autodegradation of collagen from the mantle muscle of the squid Todarodes pacificus and also that the 28- and 25-kDa proteins are closely related to this phenomenon [Connect. Tissue Res. 45 (2004) 109-121]. We obtained partial sequences of three internal portions of this protein, which suggested that 25-kDa protein is a partially degraded form of the 28-kDa protein. We determined the full cDNA sequence of this protein by the degenerate polymerase chain reaction (PCR) using the information of amino acid sequences. The deduced amino acid sequence corresponding to the 212-bp cDNA contained all of the amino acid identified from the 28-kDa protein. Rapid amplification of cDNA ends (RACE) and squid mantle muscle RNA allowed cloning of the full 522-bp sequence, corresponding to a protein of 174 amino acids. A database search indicated that this is a new protein that shares 27-34% identity with tropomyosins from various animals. Structural prediction suggested that it possesses heptad repeats that form coiled-coil structures. We expressed a recombinant protein encoded by the 212-bp cDNA in Escherichia coli and used it to generate a polyclonal antibody. Western blotting with this antibody showed that the 28-kDa protein is expressed in fin, tentacle, and mantle muscle, but not in liver.

Charged residue changes in the carboxy-terminus of alpha-tropomyosin alter mouse cardiac muscle contractility

Striated muscle tropomyosin (TM) is an essential thin filament protein that is sterically and allosterically involved in calcium-mediated cardiac contraction. We have previously shown that overexpressing the beta-TM isoform in mouse hearts leads to physiological changes in myocardial relaxation and Ca(2+) handling of myofilaments. Two important charge differences in beta-TM compared to alpha-TM are the exchange of serine and histidine at positions 229 and 276 with glutamic acid and asparagine, respectively, imparting a more negative charge to beta-TM relative to alpha-TM. Our hypothesis is that the net charge at specific sites on TM might be a major determinant of its role in modulating cardiac muscle performance and in regulating Ca(2+) sensitivity of the myofilaments. To address this, we generated transgenic (TG) double mutation mouse lines (alpha-TM DM) expressing mutated alpha-TM at the two residues that differ between alpha- and beta-TM (Ser229Glu + His276Asn). Molecular analyses show 60-88% of the native TM is replaced with alpha-TM DM in the different TG lines. Work-performing heart analyses show that alpha-TM DM mouse hearts exhibit decreased rates of pressure development and relaxation (+dP/dt and -dP/dt). Skinned myofibre preparations from the TG hearts indicate a decrease in calcium sensitivity of steady state force. Protein modelling studies show that these two charge alterations in alpha-TM cause a change in the surface charges of the molecule. Our results provide the first evidence that charge changes at the carboxy-terminal of alpha-TM alter the functional characteristics of the heart at both the whole organ and myofilament levels.

The structure of the carboxyl terminus of striated alpha-tropomyosin in solution reveals an unusual parallel arrangement of interacting alpha-helices

Coiled coils are well-known as oligomerization domains, but they are also important sites of protein-protein interactions. We determined the NMR solution structure and backbone (15)N relaxation rates of a disulfide cross-linked, two-chain, 37-residue polypeptide containing the 34 C-terminal residues of striated muscle alpha-tropomyosin, TM9a(251-284). The peptide binds to the N-terminal region of TM and to the tropomyosin-binding domain of the regulatory protein, troponin T. Comparison of the NMR solution structure of TM9a(251-284) with the X-ray structure of a related peptide [Li, Y., Mui, S., Brown, J. H., Strand, J., Reshetnikova, L., Tobacman, L. S., and Cohen, C. (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 7378-7383] reveals significant differences. In solution, residues 253-269 (like most of the tropomyosin molecule) form a canonical coiled coil. Residues 270-279, however, are parallel, linear helices, novel for tropomyosin. The packing between the parallel helices results from unusual interface residues that are atypical for coiled coils. Y267 has poor packing at the coiled-coil interface and a lower R(2) relaxation rate than neighboring residues, suggesting there is conformational flexibility around this residue. The last five residues are nonhelical and flexible. The exposed surface presented by the parallel helices, and the flexibility around Y267 and the ends, may facilitate binding to troponin T and formation of complexes with the N-terminus of tropomyosin and actin. We propose that unusual packing and flexibility are general features of coiled-coil domains in proteins that are involved in intermolecular interactions.

Functions of tropomyosin's periodic repeats

Tropomyosin binds along actin filaments and regulates actin-myosin interaction in muscle and nonmuscle cells. Seven periodic amino acid repeats are proposed to correspond to actin binding sites, and the middle periods are important for cooperative activation of actin by myosin. The functional contributions of individual periods were studied in mutants in which periods 2-6 were individually deleted from rat striated muscle alphaalpha-tropomyosin or replaced with a leucine zipper sequence. Unacetylated recombinant tropomyosins were assayed for actin binding, regulation of the actomyosin ATPase with troponin, cooperative myosin S1-induced binding to actin, and thermal stability. Tropomyosin function is relatively insensitive to deletion of period 2, but loss increases as the deletion is shifted toward the C-terminus. Retention of function upon deletion of the periodic repeats is in the order of 2 > 3 approximately 4 approximately 6 >> 5. Internal periods are important for specific functions and are not quasiequivalent. Deletion of period 5 (residues 166-207), and especially deletion or replacement of residues 166-188, a constitutively expressed region encoded by exon 5, had severe consequences on actin affinity and cooperative myosin S1-induced binding to actin. Period 6, residues 208-242, part of the troponin binding site, is required for full inhibition of the actomyosin ATPase in the absence of calcium. The effect of the deletion can depend on its context, suggesting that sequence alone is not the only factor important for function. We propose that the local structure and stability, and consequent flexibility, of the coiled coil are major determinants of actin affinity.

Structure and interactions of the carboxyl terminus of striated muscle alpha-tropomyosin: it is important to be flexible

Tropomyosin (TM) binds to and regulates the actin filament. We used circular dichroism and heteronuclear NMR to investigate the secondary structure and interactions of the C terminus of striated muscle alpha-TM, a major functional determinant, using a model peptide, TM9a(251-284). The (1)H(alpha) and (13)C(alpha) chemical shift displacements show that residues 252 to 277 are alpha-helical but residues 278 to 284 are nonhelical and mobile. The (1)H(N) and (13)C' displacements suggest that residues 257 to 269 form a coiled coil. Formation of an "overlap" binary complex with a 33-residue N-terminal chimeric peptide containing residues 1 to 14 of alpha-TM perturbs the (1)H(N) and (15)N resonances of residues 274 to 284. Addition of a fragment of troponin T, TnT(70-170), to the binary complex perturbs most of the (1)H(N)-(15)N cross-peaks. In addition, there are many new cross-peaks, showing that the binding is asymmetric. Q263, in a proposed troponin T binding site, shows two sets of side-chain (15)N-(1)H cross-peaks, indicating conformational flexibility. The conformational equilibrium of the side chain changes upon formation of the binary and ternary complexes. Replacing Q263 with leucine greatly increases the stability of TM9a(251-284) and reduces its ability to form the binary and ternary complexes, showing that conformational flexibility is crucial for the binding functions of the C terminus.

A novel striated tropomyosin incorporated into organized myofibrils of cardiomyocytes in cell and organ culture

Striated muscle tropomyosin is classically described as consisting of 10 exons, 1a, 2b, 3, 4, 5, 6b, 7, 8, and 9a/b, in both skeletal and cardiac muscle. A novel isoform found in embryonic axolotl heart maintains exon 9a/b of striated muscle but also has a smooth muscle exon 2a instead of exon 2b. Translation and subsequent incorporation into organized myofibrils, with both isoforms, was demonstrated with green fluorescent protein fusion protein construct. Mutant axolotl hearts lack sufficient tropomyosin in the ventricle and this smooth/striated chimeric tropomyosin was sufficient to replace the missing tropomyosin and form organized myofibrils.

Disease-causing mutations in cardiac troponin T: identification of a critical tropomyosin-binding region

Fifteen percent of the mutations causing familial hypertrophic cardiomyopathy are in the troponin T gene. Most mutations are clustered between residues 79 and 179, a region known to bind to tropomyosin at the C-terminus near the complex between the N- and C-termini. Nine mutations were introduced into a troponin T fragment, Gly-hcTnT(70-170), that is soluble, alpha-helical, binds to tropomyosin, promotes the binding of tropomyosin to actin, and stabilizes an overlap complex of N-terminal and C-terminal tropomyosin peptides. Mutations between residues 92 and 110 (Arg92Leu, Arg92Gln, Arg92Trp, Arg94Leu, Ala104Val, and Phe110Ile) impair tropomyosin-dependent functions of troponin T. Except for Ala104Val, these mutants bound less strongly to a tropomyosin affinity column and were less able to stabilize the TM overlap complex, effects that were correlated with increased stability of the troponin T, measured using circular dichroism. All were less effective in promoting the binding of tropomyosin to actin. Mutations within residues 92-110 may cause disease because of altered interaction with tropomyosin at the overlap region, critical for cooperative actin binding and regulatory function. A model for a five-chained coiled-coil for troponin T in the tropomyosin overlap complex is presented. Mutations outside the region (Ile79Asn, Delta 160Glu, and Glu163Lys) functioned normally and must cause disease by another mechanism.

Characterization of control and immobilized skeletal muscle: an overview from genetic engineering

To elucidate the molecular basis of muscle atrophy, we have performed the serial analysis of gene expression (SAGE) method with control and immobilized muscles of 10 rats. The genes that expressed >0.5% in muscle are involved in the following three functions: 1) contraction (troponin I, C and T; myosin light chain 1-3; actin; tropomyosin; and parvalbumin), 2) energy metabolism (cytochrome c oxidase I and III, creatine kinase, glyceraldehyde-3-phosphate-dehydrogenase, phosphoglycerate mutase, ATPase 6, and aldolase A), and 3) housekeeping (lens epithelial protein). Muscle atrophy appears to be caused by changes in mRNA levels of specific regulators of proteolysis, protein synthesis, and contractile apparatus assembling, such as polyubiquitin, elongation factor 2, and nebulin. Immobilization has produced a decrease more than threefold in gene expression of enzymes involved in energy metabolism, especially ATPase, cytochrome c oxidase, NADH dehydrogenase, and protein phosphatase 1. Differential gene expressions of selenoprotein W and uroporphyrinogen decarboxylase, which can be involved in oxidative stress, were also observed. Other genes with various functions, such as cholesterol metabolism and growth factors, were also differentially expressed. Moreover, novel genes regulated by immobilization were discovered. Thus, the current study allows a better understanding of global muscle characteristics and the molecular mechanisms of sedentarity and sarcopenia.

Importance of internal regions and the overall length of tropomyosin for actin binding and regulatory function

Tropomyosin (Tm) binds along actin filaments, one molecule spanning four to seven actin monomers, depending on the isoform. Periodic repeats in the sequence have been proposed to correspond to actin binding sites. To learn the functional importance of length and the internal periods we made a series of progressively shorter Tms, deleting from two up to six of the internal periods from rat striated alpha-TM (dAc2--3, dAc2--4, dAc3--5, dAc2--5, dAc2--6, dAc1.5--6.5). Recombinant Tms (unacetylated) were expressed in Escherichia coli. Tropomyosins that are four or more periods long (dAc2--3, dAc2--4, and dAc3--5) bound well to F-actin with troponin (Tn). dAc2--5 bound weakly (with EGTA) and binding of shorter mutants was undetectable in any condition. Myosin S1-induced binding of Tm to actin in the tight Tm-binding "open" state did not correlate with actin binding. dAc3--5 and dAc2--5 did not bind to actin even when the filament was saturated with S1. In contrast, dAc2--3 and dAc2--4 did, like wild-type-Tm, requiring about 3 mol of S1/mol of Tm for half-maximal binding. The results show the critical importance of period 5 (residues 166--207) for myosin S1-induced binding. The Tms that bound to actin (dAc2--3, dAc2--4, and dAc3--5) all fully inhibited the actomyosin ATPase (+Tn) in EGTA. In the presence of Ca(2+), relief of inhibition by these Tms was incomplete. We conclude (1) four or more actin periods are required for Tm to bind to actin with reasonable affinity and (2) that the structural requirements of Tm for the transition of the regulated filament from the blocked-to-closed/open (relief of inhibition by Ca(2+)) and the closed-to-open states (strong Tm binding to actin-S1) are different.

Alteration of tropomyosin function and folding by a nemaline myopathy-causing mutation

Mutations in the human TPM3 gene encoding gamma-tropomyosin (alpha-tropomyosin-slow) expressed in slow skeletal muscle fibers cause nemaline myopathy. Nemaline myopathy is a rare, clinically heterogeneous congenital skeletal muscle disease with associated muscle weakness, characterized by the presence of nemaline rods in muscle fibers. In one missense mutation the codon corresponding to Met-8, a highly conserved residue, is changed to Arg. Here, a rat fast alpha-tropomyosin cDNA with the Met8Arg mutation was expressed in Escherichia coli to investigate the effect of the mutation on in vitro function. The Met8Arg mutation reduces tropomyosin affinity for regulated actin 30- to 100-fold. Ca(2+)-sensitive regulatory function is retained, although activation of the actomyosin S1 ATPase in the presence of Ca(2+) is reduced. The poor activation may reflect weakened actin affinity or reduced effectiveness in switching the thin filament to the open, force-producing state. The presence of the Met8Arg mutation severely, but locally, destabilizes the tropomyosin coiled coil in a model peptide, and would be expected to impair end-to-end association between TMs on the thin filament. In muscle, the mutation may alter thin filament assembly consequent to lower actin affinity and altered binding of the N-terminus to tropomodulin at the pointed end of the filament. The mutation may also reduce force generation during activation.

Mitogen-induced up-regulation of non-smooth muscle isoform of alpha-tropomyosin in rat aortic smooth muscle cells

Correlation between the expression of the alpha-tropomyosin isoforms and cell growth was investigated in rat aortic smooth muscle cells. The levels of exon 1a, exons 1a+2a (smooth muscle type) and exons 1a+2b (non-smooth muscle type) were determined by reverse transcription-polymerase chain reaction (RT-PCR). When the cells were cultured, the level of exons 1a+2b transiently increased while reaching a maximum at 3-5 days. When the serum-deprived confluent cells were stimulated with 3-20% serum for 1.5 h, the level of exons 1a+2b increased by about twofold. The 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) but not 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X) inhibited this up-regulation. Phorbol-12, 13-dibutyrate (PDB) mimicked the effect of serum. The DNA synthesis as determined by the incorporation of 5-bromo-2'-deoxy-uridine (BrdU) was not enhanced by the 1.5 h stimulation with serum or phorbol ester. The up-regulation of non-smooth muscle isoform of alpha-tropomyosin occurred during G(0)/G(1) transition before entering S phase. Protein phosphorylation is suggested to be involved in the up-regulation. However, the responsible kinase(s) remain to be elucidated.

Independent functions for the N- and C-termini in the overlap region of tropomyosin

Tropomyosin (TM) is a coiled-coil that binds head-to-tail along the helical actin filament. The ends of 284-residue tropomyosins are believed to overlap by about nine amino acids. The present study investigates the function of the N- and C-terminal overlap regions. Recombinant tropomyosins were produced in Escherichia coli in which nine amino acids were truncated from the N-terminal, C-terminal, or both ends of striated muscle alpha-tropomyosin (TM9a) and TM2 (TM9d), a nonmuscle alpha-tropomyosin expressed in many cells. The two isoforms are identical except for the C-terminal 27 amino acids encoded by exon 9a (striated) or exon 9d (TM2). Removal of either end greatly reduces the actin affinity of both tropomyosins in all conditions and the cooperativity with which myosin promotes tropomyosin binding to actin in the open state. N-Terminal truncations generally are more deleterious than C-terminal truncations. With TM9d, truncation of the N-terminus is as deleterious as both for myosin S1-induced binding. None of the TM9d variants binds well to actin with troponin (+/-Ca(2+)). TM9a with the truncated N-terminus binds more weakly to actin with troponin (-Ca(2+)) than when the C-terminus is removed but more strongly than when both ends are removed; the actin binding of all three forms is cooperative. The results show that the ends of TM9a, though important, are not required for cooperative function and suggest they have independent functions beyond formation of an overlap complex. The nonadditivity of the TM9d truncations suggests that the ends may primarily function as a complex in this isoform. A surprising result is that all variants bound with the same affinity, and noncooperatively, to actin saturated with myosin S1. Evidently, end-to-end interactions are not required for high-affinity binding to acto-myosin S1.

The ends of tropomyosin are major determinants of actin affinity and myosin subfragment 1-induced binding to F-actin in the open state

Tropomyosin (TM) is thought to exist in equilibrium between two states on F-actin, closed and open [Geeves, M. A., and Lehrer, S. S. (1994) Biophys. J. 67, 273-282]. Myosin shifts the equilibrium to the open state in which myosin binds strongly and develops force. Tropomyosin isoforms, that primarily differ in their N- and C-terminal sequences, have different equilibria between the closed and open states. The aim of the research is to understand how the alternate ends of TM affect cooperative actin binding and the relationship between actin affinity and the cooperativity with which myosin S1 promotes binding of TM to actin in the open state. A series of rat alpha-tropomyosin variants was expressed in Escherichia coli that are identical except for the ends, which are encoded by exons 1a or 1b and exons 9a, 9c or 9d. Both the N- and C-terminal sequences, and the particular combination within a TM molecule, determine actin affinity. Compared to tropomyosins with an exon 1a-encoded N-terminus, found in long isoforms, the exon 1b-encoded sequence, expressed in 247-residue nonmuscle tropomyosins, increases actin affinity in tropomyosins expressing 9a or 9d but has little effect with 9c, a brain-specific exon. The relative actin affinities, in decreasing order, are 1b9d > 1b9a > acetylated 1a9a > 1a9d >> 1a9a > or = 1a9c congruent with 1b9c. Myosin S1 greatly increases the affinity of all tropomyosin variants for actin. In this, the actin affinity is the primary factor in the cooperativity with which myosin S1 induces TM binding to actin in the open state; generally, the higher the actin affinity, the lower the occupancy by myosin required to saturate the actin with tropomyosin: 1b9d >1a9d> 1b9a > or = acetylated 1a9a > 1a9a > 1a9c congruent with 1b9c.

Effects of tropomyosin internal deletions on thin filament function

Striated muscle tropomyosin spans seven actin monomers and contains seven quasi-repeating regions with loose sequence similarity. Each region contains a hypothesized actin binding motif. To examine the functions of these regions, full-length tropomyosin was compared with tropomyosin internal deletion mutants spanning either five or four actins. Actin-troponin-tropomyosin filaments lacking tropomyosin regions 2-3 exhibited calcium-sensitive regulation in in vitro motility and myosin S1 ATP hydrolysis experiments, similar to filaments with full-length tropomyosin. In contrast, filaments lacking tropomyosin regions 3-4 were inhibitory to these myosin functions. Deletion of regions 2-4, 3-5, or 4-6 had little effect on tropomyosin binding to actin in the presence of troponin or troponin-Ca(2+), or in the absence of troponin. However, all of these mutants inhibited myosin cycling. Deletion of the quasi-repeating regions diminished the prominent effect of myosin S1 on tropomyosin-actin binding. Interruption of this cooperative, myosin-tropomyosin interaction was least severe for the mutant lacking regions 2-3 and therefore correlated with inhibition of myosin cycling. Regions 3, 4, and 5 each contributed about 1.5 kcal/mol to this process, whereas regions 2 and 6 contributed much less. We suggest that a myosin-induced conformational change in actin facilitates the azimuthal repositioning of tropomyosin which is an essential part of regulation.

Tropomyosin isoform diversity and neuronal morphogenesis

Tropomyosins (Tm) are a large family of isoforms obtained from multiple genes and by extensive alternative splicing. They bind in the alpha-helical groove of the actin filament and are therefore core components of this extensive cytoskeletal system. In non-muscle cells the Tm isoforms have been implicated in a diversity of processes including cytokinesis, vesicle transport, motility, morphogenesis and cell transformation. Using immunohistochemical localization in cultured primary cortical neurons with an antibody that potentially identifies all non-muscle TM5 gene isoforms compared with one that specifically identifies a subset of isoforms, the possibility was raised that there were considerably more isoforms derived from this gene than the four previously described. Using polymerase chain reaction (PCR) analysis we have now shown that the rat brain generates at least 10 mRNA isoforms using multiple combinations of terminal exons and two internal exons. There is extensive developmental regulation of these isoforms in the brain and there appears to be a switch in the preferential use of the two internal exons 6a to 6b from the embryonic to the adult isoforms. Specific isoforms using alternate carboxyl-terminal exons are differentially localized within the adult rat cerebellum. It is suggested that the tightly regulated spatial and temporal expression of Tm isoforms plays an important role in the development and maintenance of specific neuronal compartments. This may be achieved by isoforms providing unique structural properties to actin-based filaments within functionally distinct neuronal domains.

Structural compartments within neurons: developmentally regulated organization of microfilament isoform mRNA and protein

The microfilament system is thought to be a crucial cytoskeletal component regulating development and mature function of neurons. The intracellular distribution of the microfilament isoform components, actin and tropomyosin (Tm), in neurons primarily in vivo, has been investigated at both the mRNA and the protein level using isoform specific riboprobes and antibodies. Our in vivo and in vitro studies have identified at least six neuronal compartments based on microfilament isoform mRNA localization: the developing soma, the mature soma, growth cone, developing axon hillock/proximal axon, mature somatodendritic and mature axonal pole soma. Protein localization patterns revealed that the isoforms were frequently distributed over a wider area than their respective mRNAs, suggesting that isoform specific patterns of mRNA targeting may influence, but do not absolutely determine, microfilament isoform location. Tm4 and Tm5 showed identical mRNA targeting in the developing neuron but distinct protein localization patterns. We suggest that in this instance mRNA location may best be viewed as a regulated site of synthesis and assembly, rather than a regulator of protein localization per se. In addition, Tm5 and beta-actin mRNA and protein locations were developmentally regulated, suggesting the possibility that environmental signals modulate targeting of specific mRNAs and their proteins. Thus, developmentally regulated mRNA localization and positional translation may act in concert with protein transport to regulate neuronal microfilament composition and consequently neuronal structure.

Splicing of two internal and four carboxyl-terminal alternative exons in nonmuscle tropomyosin 5 pre-mRNA is independently regulated during development

Four nonmuscle tropomyosin isoforms have been reported to be produced from the rat Tm5 gene by alternative splicing (Beisel, K. W., and Kennedy, J. E. (1994) Gene (Amst.) 145, 251-256). In order to detect additional isoforms that might be expressed from that gene, we used reverse transcriptase-polymerase chain reaction assays and evaluated the presence of all product combinations of two alternative internal exons (6a and 6b) and four carboxyl-terminal exons (9a, 9b, 9c, and 9d) in developing and adult rat brain. We identified five different combinations for exon 9 (9a + 9b, 9a + 9c, 9a + 9d, 9c, and 9d), and the exon combinations 9a + 9c and 9a + 9d were previously unreported. Each of these combinations existed with both exon 6a and exon 6b. Thus, the rat brain generates at least 10 different isoforms from the Tm5 gene. Northern blot hybridization with alternative exon-specific probes revealed that these isoforms were also expressed in a number of different adult rat tissues, although some exons are preferentially expressed in particular tissues. Studies of regulation of the 10 different Tm5 isoform mRNAs during rat brain development indicated that no two isoforms are coordinately accumulated. Furthermore, there is a developmental switch in the use of exon 6a to exon 6b from embryonic to adult isoforms. TM5 protein isoforms show a differential localization in the adult cerebellum.

Molecular and physiological effects of alpha-tropomyosin ablation in the mouse

Tropomyosin (TM) is an integral component of the thin filament in muscle fibers and is involved in regulating actin-myosin interactions. TM is encoded by a family of four alternatively spliced genes that display highly conserved nucleotide and amino acid sequences. To assess the functional and developmental significance of alpha-TM, the murine alpha-TM gene was disrupted by homologous recombination. Homozygous alpha-TM null mice are embryonic lethal, dying between 8 and 11.5 days post coitum. Mice that are heterozygous for alpha-TM are viable and reproduce normally. Heterozygous knockout mouse hearts show a 50% reduction in cardiac muscle alpha-TM mRNA, with no compensatory increase in transcript levels by striated muscle beta-TM or TM-30 isoforms. Surprisingly, this reduction in alpha-TM mRNA levels in heterozygous mice is not reflected at the protein level, where normal amounts of striated muscle alpha-TM protein are produced and integrated in the myofibril. Quantification of alpha-TM mRNA bound in polysomal fractions reveals that both wild-type and heterozygous knockout animals have similar levels. These data suggest that a change in steady-state level of alpha-TM mRNA does not affect the relative amount of mRNA translated and amount of protein synthesized. Physiological analyses of myocardial and myofilament function show no differences between heterozygous alpha-TM mice and control mice. The present study suggests that translational regulation plays a major role in the control of TM expression.

Targeted ablation of the murine alpha-tropomyosin gene

We created a mouse that lacks a functional alpha-tropomyosin gene using gene targeting in embryonic stem cells and blastocyst-mediated transgenesis. Homozygous alpha-tropomyosin "knockout" mice die between embryonic day 9.5 and 13.5 and lack alpha-tropomyosin mRNA. Heterozygous alpha-tropomyosin knockout mice have approximately 50% as much cardiac alpha-tropomyosin mRNA as wild-type littermates but similar alpha-tropomyosin protein levels. Cardiac gross morphology, histology, and function (assessed by working heart preparations) of heterozygous alpha-tropomyosin knockout and wild-type mice were indistinguishable. Mechanical performance of skinned papillary muscle strips derived from mutant and wild-type hearts also revealed no differences. We conclude that haploinsufficiency of the alpha-tropomyosin gene produces little or no change in cardiac function or structure, whereas total alpha-tropomyosin deficiency is incompatible with life. These findings imply that in heterozygotes there is a regulatory mechanism that maintains the level of myofibrillar tropomyosin despite the reduction in alpha-tropomyosin mRNA.

Actin-tropomyosin activation of myosin subfragment 1 ATPase and thin filament cooperativity. The role of tropomyosin flexibility and end-to-end interactions

Tropomyosin (Tm) bound to actin induces cooperative activation of actomyosin subfragment 1 (actin-S1) ATPase, observed as a sigmoid ATPase vs [S1] dependence. The activation is much steeper for gizzard muscle Tm (GTm) than for rabbit skeletal Tm (RSTm). To investigate if this greater cooperativity is due to increased communication between GTms along the thin filament, we studied effects of S1 binding on the state of actin-Tm using the fluorescence of pyrene-labeled Tm. Kinetic and equilibrium studies provided values for n, the apparent cooperative unit size [Geeves, M. A., and Lehrer, S. S. (1994) Biophys. J. 67, 273]. We report comparative studies of Tm-actin-S1 ATPase with values of n using GTm, RSTm, and 5aTm, a 1/7 shorter nonmuscle Tm from rat fibroblast cells [Pittenger, M. F., et al. (1994) Curr. Opin. Cell Biol., 6, 96]. 5aTm and GTm produce similar cooperative activation of actin-S1 ATPase and have similar n values that are 2-fold greater than RSTm, indicating a correlation between ATPase activation and n value. This appears to be due to the similarity of the C-terminal amino acid sequences of 5a and GTm which produce strong end-to-end interactions. The results are discussed in terms of a continuous flexible Tm strand on the actin filament.

The sequence of the alternatively spliced sixth exon of alpha-tropomyosin is critical for cooperative actin binding but not for interaction with troponin

Tropomyosins, a family of highly conserved coiled-coil actin binding proteins, can differ as a consequence of alternative expression of several exons (Lees-Miller, J., and Helfman, D. (1991) BioEssays 13, 429-437). Exon 6, which encodes residues 189-213 in long, 284-residue tropomyosins, has two alternative forms, exon 6a or 6b, both highly conserved throughout evolution. In alpha-tropomyosin, exon 6a or 6b is not specific to any one of the nine isoforms. Exon 6b encodes part of a putative Ca2+-sensitive troponin binding site in striated muscle tropomyosins, suggesting that the exon 6-encoded region may be specialized for certain tropomyosin functions. A series of recombinant, unacetylated tropomyosin exon 6 deletion and substitution mutants and chimeras was expressed in Escherichia coli to determine the requirements of exon 6 for tropomyosin function. Functional properties of the tropomyosins were defined by actin affinity measured by cosedimentation, troponin T affinity using a newly developed biosensor assay, and regulation of the actomyosin MgATPase. The region of tropomyosin encoded by exon 6 affects actin affinity but not thin filament assembly, troponin T binding, or regulation with troponin. The tropomyosins with exon 6a or 6b function normally whether a striated muscle exon 9a or smooth/non-muscle exon 9d is present. However, the effect of deleting 21 amino acids encoded by exon 6 or replacing it with a GCN4 leucine zipper sequence depends on the COOH-terminal sequence.

Crustacean muscle plasticity: molecular mechanisms determining mass and contractile properties

Two crustacean models for understanding molecular mechanisms of muscle plasticity are reviewed. Metabolic changes underlying muscle protein synthesis and degradation have been examined in the Bermuda land crab, Gecarcinus lateralis. During proecdysis, the claw closer muscle undergoes a programmed atrophy, which results from a highly controlled breakdown of myofibrillar proteins by Ca(2+)-dependent and, possibly, ATP/ubiquitin-dependent proteolytic enzymes. The advantage of this model is that there is neither fiber degeneration nor contractile-type switching, which often occurs in mammalian skeletal muscles. The second model uses American lobster, Homarus americanus, to understand the genetic regulation of fiber-type switching. Fibers in the claw closer muscles undergo a developmentally-regulated transformation as the isomorphic claws of larvae and juveniles differentiate into the heteromorphic cutter and crusher claws of adults. This switching occurs at the boundary between fast- and slow-fiber regions, and thus the transformation of a specific fiber is determined by its position within the muscle. The ability to predict fiber switching can be exploited to isolate and identify putative master regulatory factors that initiate and coordinate the expression of contractile proteins.

Clinical features of hypertrophic cardiomyopathy caused by mutation of a "hot spot" in the alpha-tropomyosin gene

OBJECTIVES

We studied the clinical and genetic features of familial hypertrophic cardiomyopathy (FHC) caused by an Asp175Asn mutation in the alpha-tropomyosin gene in affected subjects from three unrelated families.

BACKGROUND

Correlation of genotype and phenotype has provided important information in FHC caused by beta-cardiac myosin and cardiac troponin T mutations. Comparable analyses of hypertrophic cardiomyopathy caused by alpha-tropomyosin mutations have been hampered by the rarity of these genetic defects.

METHODS

The haplotypes of three kindreds with FHC due to an alpha-tropomyosin gene mutation, Asp175Asn, were analyzed. The cardiac histopathologic findings of this mutation are reported. Distribution of left ventricular hypertrophy in affected members was assessed by two-dimensional echocardiography, and patient survival rates were compared.

RESULTS

Genetic studies defined unique haplotypes in the three families, demonstrating that independent mutations caused the disease in each. The Asp175Asn mutation caused cardiac histopathologic findings of myocyte hypertrophy, disarray and replacement fibrosis. The severity and distribution of left ventricular hypertrophy varied considerably in affected members from the three families (mean maximal wall thickness +/- SD: 24 +/- 4.5 mm in anterior septum of Family DT; 15 +/- 2.7 mm in anterior septum and free wall of Family DB; 18 +/- 2.1 mm in posterior septum of Family MI), but survival was comparable and favorable.

CONCLUSIONS

Nucleotide residue 579 in the alpha-tropomyosin gene may have increased susceptibility to mutation. On cardiac histopathologic study, defects in this sarcomere thin filament component are indistinguishable from other genetic etiologies of hypertrophic cardiomyopathy. The Asp175Asn mutation can elicit different morphologic responses, suggesting that the hypertrophic phenotype is modulated not by genetic etiologic factors alone. In contrast, prognosis reflected genotype; near normal life expectancy is found in hypertrophic cardiomyopathy caused by the alpha-tropomyosin mutation Asp175Asn.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

Clinical implications of hypertrophic cardiomyopathy associated with mutations in the alpha-tropomyosin gene

OBJECTIVE

The disease-bearing genes for hypertrophic cardiomyopathy (HCM) in HCM families have been identified as the beta-myosin heavy chain, alpha-tropomyosin, and cardiac troponin T genes. Three HCM kindreds with three distinct point mutations in the alpha-tropomyosin gene had extensive clinical evaluations.

DESIGN AND RESULTS

Single-strand conformation polymorphism gel analysis of polymerase chain reaction amplified products was used to capture each of the nine exons from the alpha-tropomyosin gene to identify mutations in 60 familial HCM patients. Two missense mutations in exon 2 (Ala63Val and Lys70Thr) and one missense mutation in exon 5 (Asp175Asn) were found in three unrelated HCM kindreds. These kindreds were the subject of clinical, electrocardiographic and echocardiographic studies. The morphological appearance of HCM was similar in the three kindreds. All the patients had severe hypertrophy of the left ventricle with asymmetrical septal hypertrophy during the early stage of the disease, which gradually progressed to dilatation of the left ventricle. Moreover, these kindreds showed similar disease penetrance, age of onset, and incidence of premature sudden death. The disease in these kindreds was severe and resulted in frequent sudden deaths.

CONCLUSIONS

Among Japanese patients with familial HCM mutations in the alpha-tropomyosin gene are not as rare as reported, accounting for about 5% of all cases. These mutations are characterised by hypertrophy of the left ventricle which then progresses to dilatation and a high incidence of sudden or disease-related death.

Mapping the functional domains within the carboxyl terminus of alpha-tropomyosin encoded by the alternatively spliced ninth exon

Tropomyosins are highly conserved, coiled-coil actin binding proteins found in most eukaryotic cells. Striated and smooth muscle alpha-tropomyosins differ by the regions encoded by exons 2 and 9. Unacetylated smooth tropomyosin expressed in Escherichia coli binds actin with high affinity, whereas unacetylated striated tropomyosin requires troponin, found only in striated muscle, for strong actin binding. The residues encoded by exon 9 cause these differences (Cho, Y.-J., and Hitchcock-DeGregori, S. E. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 10153-10157). We mapped the functional domains encoded by the alpha-tropomyosin exon 9a (striated muscle-specific) and 9d (constitutively expressed), by measuring actin binding and regulation of the actomyosin MgATPase by tropomyosin exon 9 chimeras and truncation mutants expressed in E. coli. We have shown that: 1) the carboxyl-terminal nine residues define the actin affinity of unacetylated tropomyosin; 2) in the presence of Ca2+, the entire exon 9a is required for troponin to promote fully high affinity actin binding; 3) the first 18 residues encoded by exon 9a are critical for the interaction of troponin with tropomyosin on the thin filament, even in the absence of Ca2+. The results give new insight into the structural requirements of tropomyosin for thin filament assembly and regulatory function.

Polymorphism of the mouse E2A gene due to an intronic deletion of 536 bp

Examination of genetic polymorphism of the transcription factor-encoding gene E2A in laboratory and wild mice by Southern blotting has revealed the presence of two alleles. The most frequent allele is found in Mus musculus (Mm) musculus, as well as Mm domesticus. The less common allele is restricted to the Mm domesticus subspecies. Characterisation of these alleles has shown that the less common allele contains a deletion of approx. 500 bp located within a 1.8-kb intron immediately upstream from the E12 basic helix-loop-helix exon. DNA sequencing determined the deletion to span 536 bp including nucleotides 1045-1580 of the intron within the common allele. The deleted region includes several sequences with similarity to gene regulatory motifs; however, expression of E12 and intron splicing appeat unaltered. The occurrence of an identical deletion in mice from different geographical regions suggests that the uncommon allele may have a long evolutionary history.

Low molecular weight rat fibroblast tropomyosin 5 (TM-5): cDNA cloning, actin-binding, localization, and coiled-coil interactions

Previous studies have shown that three distinct genes encode six isoforms of tropomyosin (TM) in rat fibroblasts: the alpha gene encodes TM-2, TM-3, TM-5a, and TM-5b, the beta gene encodes TM-1, and the TM-4 gene encodes TM-4. Here we report the characterization of a cDNA clone encoding the most recent rat fibroblast TM to be identified, herein referred to as TM-5, that is the product of a fourth gene that is homologous to the human hTMnm gene, herein referred to as the rat slow-twitch alpha TM gene. The cDNA clone is approximately 1.7 kb and encodes a protein of 248 amino acids. Using two-dimensional gel electrophoresis, the TM-5 protein was found to co-migrate with fibroblast TM-5a and 5b. Comparison of the amino acid sequences of TM-5 to other fibroblast isoforms encoded by the alpha, beta, and TM-4 genes revealed a high degree of homology, although there were regions of divergence among the different isoforms. The gene encoding TM-5 is expressed in all tissues examined including skeletal muscle, stomach, heart, liver, kidney, uterus, spleen, brain, and diaphragm. However, Northern blot and RNase protection analyses revealed the presence of different mRNAs in fibroblasts, striated muscle (skeletal and diaphragm), and brain, which are expressed via alternative RNA splicing and the use of alternative promoters. The TM-5 protein was expressed in a bacterial system and tested for its ability to bind actin in vitro and in vivo. The apparent TM association constant (Ka) was taken as the free concentration at half saturation and was found to be 3 microM for TM-5 compared to 2 microM for TM-5b at an F-actin concentration of 42 microM. When fluorescently-labeled TM-5 was microinjected into living rat fibroblasts, it localized to the stress fibers and ruffles of the leading lamella. The coiled-coil interactions of TM-5 with other low and high molecular weight TM isoforms were studied. TM-5 and TM-4 were capable of dimerizing with each other as well as with other low molecular weight isoforms (TM-5a and TM-5b), but not with the HMW isoforms (TM-1, TM-2, and TM-3). In addition, TM-5a and TM-5b were unable to heterodimerize with each other. The implications of these results in understanding the role of TM diversity in cytoskeletal dynamics are discussed.

Nuclear matrix proteins as structural and functional components of the mitotic apparatus

The eukaryotic nucleus is a membrane-enclosed compartment containing the genome and associated organelles supported by a complex matrix of nonhistone proteins. Identified as the nuclear matrix, this component maintains spatial order and provides the structural framework needed for DNA replication, RNA synthesis and processing, nuclear transport, and steroid hormone action. During mitosis, the nucleoskeleton and associated chromatin is efficiently dismantled, packaged, partitioned, and subsequently reassembled into daughter nuclei. The dramatic dissolution of the nucleus is accompanied by the assembly of a mitotic apparatus required to facilitate the complex events associated with nuclear division. Until recently, little was known about the fate or disposition of nuclear matrix proteins during mitosis. The availability of specific molecular probes and imaging techniques, including confocal microscopy and improved immunoelectron microscopy using resinless sections and related procedures, has enabled investigators to identify and map the distribution of nuclear matrix proteins throughout the cell cycle. This chapter will review the structure, function, and distribution of the protein NuMA (nuclear matrix mitotic apparatus) and other nuclear matrix proteins that depart the nucleus during the interphase/mitosis transition to become structural and functional components within specific domains of the mitotic apparatus.

Molecular and physiological effects of overexpressing striated muscle beta-tropomyosin in the adult murine heart

Tropomyosins comprise a family of actin-binding proteins that are central to the control of calcium-regulated striated muscle contraction. To understand the functional role of tropomyosin isoform differences in cardiac muscle, we generated transgenic mice that overexpress striated muscle-specific beta-tropomyosin in the adult heart. Nine transgenic lines show a 150-fold increase in beta-tropomyosin mRNA expression in the heart, along with a 34-fold increase in the associated protein. This increase in beta-tropomyosin message and protein causes a concomitant decrease in the level of alpha-tropomyosin transcripts and their associated protein. There is a preferential formation of the alpha beta-heterodimer in the transgenic mouse myofibrils, and there are no detectable alterations in the expression of other contractile protein genes, including the endogenous beta-tropomyosin isoform. When expression from the beta-tropomyosin transgene is terminated, alpha-tropomyosin expression returns to normal levels. No structural changes were observed in these transgenic hearts nor in the associated sarcomeres. Interestingly, physiological analyses of these hearts using a work-performing model reveal a significant effect on diastolic function. As such, this study demonstrates that a coordinate regulatory mechanism exists between alpha- and beta-tropomyosin gene expression in the murine heart, which results in a functional correlation between alpha- and beta-tropomyosin isoform content and cardiac performance.