Sequences of complete cDNAs encoding four variants of chicken skeletal muscle troponin T

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Monoclonal Antibodies as Probes to Study Ligand-Induced Conformations of Troponin Subunits

Striated muscle contraction and relaxation is regulated by Ca2+ at the myofilament level via conformational modulations of the troponin complex. To understand the structure-function relationship of troponin in normal muscle and in myopathies, it is necessary to study the functional effects of troponin isoforms and mutations at the level of allosteric conformations of troponin subunits. Traditional methodologies assessing such conformational studies are laborious and require significant amounts of purified protein, while many current methodologies require non-physiological conditions or labeling of the protein, which may affect their physiological conformation and function. To address these issues, we developed a novel approach using site-specific monoclonal antibodies (mAb) as molecular probes to detect and monitor conformational changes of proteins. Here, we present examples for its application in studies of two subunits of troponin: the Ca2+-binding subunit, TnC, and the tropomyosin-binding/thin filament-anchoring subunit, TnT. Studies using a high-throughput microplate assay are compared with that using localized surface plasmon resonance (LSPR) to demonstrate the effectiveness of using mAb probes to assess ligand-induced conformations of troponin subunits in physiological conditions. The assays utilize relatively small amounts of protein and are free of protein modification, which may bias results. Detailed methodologies using various monoclonal antibodies (mAbs) are discussed with considerations for the optimization of assay conditions and the broader application in studies of other proteins as well as in screening of therapeutic reagents that bind a specific target site with conformational and functional effects.

Protein Structure-Function Relationship at Work: Learning from Myopathy Mutations of the Slow Skeletal Muscle Isoform of Troponin T

Troponin T (TnT) is the sarcomeric thin filament anchoring subunit of the troponin complex in striated muscles. A nonsense mutation in exon 11 of the slow skeletal muscle isoform of TnT (ssTnT) gene (TNNT1) was found in the Amish populations in Pennsylvania and Ohio. This single nucleotide substitution causes a truncation of the ssTnT protein at Glu¹⁸⁰ and the loss of the C-terminal tropomyosin (Tm)-binding site 2. As a consequence, it abolishes the myofilament integration of ssTnT and the loss of function causes an autosomal recessive nemaline myopathy (NM). More TNNT1 mutations have recently been reported in non-Amish ethnic groups with similar recessive NM phenotypes. A nonsense mutation in exon 9 truncates ssTnT at Ser¹⁰⁸, deleting Tm-binding site 2 and a part of the middle region Tm-binding site 1. Two splicing site mutations result in truncation of ssTnT at Leu²⁰³ or deletion of the exon 14-encoded C-terminal end segment. Another splicing mutation causes an internal deletion of the 39 amino acids encoded by exon 8, partially damaging Tm-binding site 1. The three splicing mutations of TNNT1 all preserve the high affinity Tm-binding site 2 but still present recessive NM phenotypes. The molecular mechanisms for these mutations to cause myopathy provide interesting models to study and understand the structure-function relationship of TnT. This focused review summarizes the current knowledge of TnT isoform regulation, structure-function relationship of TnT and how various ssTnT mutations cause recessive NM, in order to promote in depth studies for further understanding the pathogenesis and pathophysiology of TNNT1 myopathies toward the development of effective treatments.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Troponin T isoforms and posttranscriptional modifications: Evolution, regulation and function

Troponin-mediated Ca²(+)-regulation governs the actin-activated myosin motor function which powers striated (skeletal and cardiac) muscle contraction. This review focuses on the structure-function relationship of troponin T, one of the three protein subunits of the troponin complex. Molecular evolution, gene regulation, alternative RNA splicing, and posttranslational modifications of troponin T isoforms in skeletal and cardiac muscles are summarized with emphases on recent research progresses. The physiological and pathophysiological significances of the structural diversity and regulation of troponin T are discussed for impacts on striated muscle function and adaptation in health and diseases.

Troponin T core structure and the regulatory NH2-terminal variable region

The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.

Troponin T expression in trout red muscle correlates with muscle activation

Red or aerobic muscle from the anterior of rainbow trout Oncorhynchus mykiss activates (generates force) more quickly than that from the posterior. TnT is a component of the troponin complex that modulates muscle activation once Ca2+ is bound. Since trout express at least two forms of TnT in their red muscle (S1 and S2), the differential expression of these two forms was predicted to explain variations in contractile properties. TnT isoforms from trout muscle were identified through hydroxy-apatite chromatography of purified myofibrillar proteins followed by SDS-PAGE. Western blots employing a mammalian anti-troponin T monoclonal antibody were used to identify TnT isoforms. The relative expression of the two isoforms of TnT was then examined at seven longitudinal positions from each of three fish using SDS-PAGE and densitometry on the silver-stained TnT bands. A significant shift in expression was detected from anterior to posterior in all three fish with TnT S1 becoming more dominant in the posterior red muscle. As predicted, a shift in TnT expression was associated with the decrease in activation rate along the length of the fish. This study was then extended to include a different species of salmonid, brook trout Salvelinus fontinalis, to explore the generality of TnT modulation of muscle activation. Muscle contractile properties were determined from anterior and posterior muscle, and relative expression of S1 and S2 was determined. Unlike rainbow trout, there is no consistent longitudinal pattern of muscle activation in brook trout:some fish have kinetically faster muscle in the anterior, some in the posterior. Similarly, there is no consistent pattern of TnT expression. Individual analysis of muscle activation and TnT expression in brook trout provides insight into the role of TnT in modulating muscle activation in slow fish muscle.

Genomic structure of the chicken slow skeletal muscle troponin T gene

Troponin T (TnT) is a key protein for Ca(2+)-sensitive molecular switching of muscle contraction. In vertebrates, three TnT genes have been identified, which produce isoforms characteristic of cardiac, fast skeletal, and slow skeletal muscles through alternative splicing in a tissue-specific and developmentally regulated manner. The diversification of myofibers into forms with specific metabolic and contractile characteristics is thought to be closely associated with the differential expression of these TnT isoforms. Herein, we determined the nucleotide sequence of the chicken slow skeletal muscle TnT gene and its upstream region. The gene was simpler in structure than the two other chicken genes. The transcription initiation site was positioned 183 bp upstream of the 3' end of exon 1. Alternative splicing of exon 5 using an internal acceptor site generated two distinct slow skeletal muscle troponin T (sTnT) transcripts. We identified possible regulatory elements, M-CAT-like, CACC-box, and E-box (E-box1 to E-box3) motifs in the upstream region and an E-box motif (E-box4) in exon 1.

Binding of Calcium Ions to an Avian Flight Muscle Troponin T†

Numerous troponin T (TnT) isoforms are generated by alternative RNA splicing primarily in its N-terminal hypervariable region, but the functions of these isoforms are not completely understood. Here for the first time, we discovered that a chicken fast TnT isoform with a unique Tx motif (HEEAH)(n) binds calcium. The metal binding behavior of this TnT isoform was first investigated using terbium as a calcium analogue due to its more readily detectable fluorescence variation upon TnT binding. Both intact TnT and TnT N-terminal fragment (TnT N47) bound terbium with high affinity indicating that the N-terminal sequence was the site of binding. Since terbium often substitutes at calcium-binding sites, radioactive calcium was tested and found to bind both intact TnT and TnT N47. Fluorescence measurements using the calcium-sensitive fluorescent dye, calcium green 5N, confirmed that calcium bound to the tertiary complex of TnT and the tropomyosin dimer with a fast on-rate (10(6)-10(7) M(-1) s(-1)) as detected in stopped-flow analysis. Consistent with these observations, computational predictions suggest that TnT N47 might fold into an elongated structure with at least one high-affinity metal ion binding pocket comprised primarily of the Tx motif sequence and several lower affinity binding sites. These results suggest that TnT may play a role in modulating the calcium-mediated regulatory process of striated muscle contraction.

Temperature and the expression of seven muscle-specific protein genes during embryogenesis in the Atlantic cod Gadus morhua L

Seven cDNA clones coding for different muscle-specific proteins (MSPs) were isolated from the fast muscle tissue of Atlantic cod Gadus morhua L. In situ hybridization using cRNA probes was used to characterize the temporal and spatial patterns of gene expression with respect to somite stage in embryos incubated at 4 degrees C, 7 degrees C and 10 degrees C. MyoD transcripts were first observed in the presomitic mesoderm prior to somite formation, and in the lateral compartment of the forming somites. MyoD expression was not observed in the adaxial cells that give rise to the slow muscle layer, and expression was undetectable by in situ hybridization in the lateral somitic mesoderm after the 35-somite stage, during development of the final approximately 15 somites. RT-PCR analysis, however, confirmed the presence of low levels of the transcript during these later stages. A phylogenetic comparison of the deduced aminoacid sequences of the full-length MyoD cDNA clone and those from other teleosts, and inference from the in situ expression pattern suggested homology with a second paralogue (MyoD2) recently isolated from the gilthead seabream Sparus aurata. Following MyoD expression, alpha-actin was the first structural gene to be switched on at the 16-somite stage, followed by myosin heavy chain, troponin T, troponin I and muscle creatine kinase. The final mRNA in the series to be expressed was troponin C. All genes were switched on prior to myofibril assembly. The troponin C sequence was unusual in that it showed the greatest sequence identity with the rainbow trout Oncorhynchus mykiss cardiac/slow form, but was expressed in the fast myotomal muscle and not in the heart. In addition, the third TnC calcium binding site showed a lower level of sequence conservation than the rest of the sequence. No differences were seen in the timing of appearance or rate of posterior progression (relative to somite stage) of any MSP transcripts between embryos raised at the different temperatures. It was concluded that myofibrillar genes are activated asynchronously in a distinct temporal order prior to myofibrillar assembly and that this process was highly canalized over the temperature range studied.

The calcium-induced switch in the troponin complex probed by fluorescent mutants of troponin I

The Ca2+-induced transition in the troponin complex (Tn) regulates vertebrate striated muscle contraction. Tn was reconstituted with recombinant forms of troponin I (TnI) containing a single intrinsic 5-hydroxytryptophan (5HW). Fluorescence analysis of these mutants of TnI demonstrate that the regions in TnI that respond to Ca2+ binding to the regulatory N-domain of TnC are the inhibitory region (residues 96-116) and a neighboring region that includes position 121. Our data confirms the role of TnI as a modulator of the Ca2+ affinity of TnC; we show that point mutations and incorporation of 5HW in TnI can affect both the affinity and the cooperativity of Ca2+ binding to TnC. We also discuss the possibility that the regulatory sites in the N-terminal domain of TnC might be the high affinity Ca2+-binding sites in the troponin complex.

Amino acid sequences of multiple fast and slow troponin T isoforms expressed in adult bovine skeletal muscles

Multiple nucleotide sequences of complementary DNA (cDNA) of bovine troponin T (TnT) isoforms expressed in the adult skeletal muscles were determined to facilitate the elucidation of the TnT degradation progress during postmortem aging of muscles. Fresh muscle samples were excised from the lingual, masseter, pectoralis, diaphragm, psoas major, longissimus thoracis, spinnalis, semitendinosus, semimembranosus, and biceps femoris muscles of three Holstein cows within 1 h of slaughter. Complementary DNA fragments of fast and slow TnT isoforms expressed in each muscle were amplified by reverse-transcribed PCR. Consequently, four major fragments of fast TnT and two fragments of slow TnT, all of which contained the complete coding region, were obtained. The sequence determination of these fragments revealed that at least eight and two isoforms were generated by the alternative splicing from bovine fast and slow TnT messenger RNA, respectively. In the fast TnT isoforms, five small variable exons were observed; three of these five exons were in the amino (N)-terminal region. The calculated molecular weight of fast and slow TnT isoforms ranged from 29,816 to 32,125 and from 30,166 to 31,284, respectively. The deduced amino acid sequences revealed that the N-terminal region of all the TnT isoforms was extremely glutamic acid-rich. Reverse-transcribed PCR analysis revealed that expression of each of these isoforms was distributed in a fast or slow muscle-specific manner. Given that TnT degradation has been reported to accompany a decrease in glutamic acid content in the conventional 30-kDa degradation product, the sequence data suggested that the 30-kDa fragment seem to be generated by the proteolytic removal of the glutamic acid-rich N-terminal ends. The multiplicity of TnT isoforms may result in a complicated pattern of TnT degradation on SDS-PAGE gel during beef aging.

Equilibrium unfolding and conformational plasticity of troponin I and T

The structures and stabilities of recombinant chicken muscle troponin I (TnI) and T (TnT) were investigated by a combination of bis-ANS binding and equilibrium unfolding studies. Unlike most folded proteins, isolated TnI and TnT bind the hydrophobic fluorescent probe bis-ANS, indicating the existence of solvent-exposed hydrophobic domains in their structures. Bis-ANS binding to binary or ternary mixtures of TnI, TnT and troponin C (TnC) in solution is significantly lower than binding to the isolated subunits, which can be explained by burial of previously exposed hydrophobic domains upon association of the subunits to form the native troponin complex. Equilibrium unfolding studies of TnT and TnI by guanidine hydrochloride and urea monitored by changes in far-UV CD and bis-ANS fluorescence revealed noncooperative folding transitions for both proteins and the existence of partially folded intermediate states. Taken together, these results indicate that isolated TnI and TnT are partially unstructured proteins, and suggest that conformational plasticity of the isolated subunits may play an important role in macromolecular recognition for the assembly of the troponin complex.

A modulatory role for the troponin T tail domain in thin filament regulation

In striated muscle the force generating acto-myosin interaction is sterically regulated by the thin filament proteins tropomyosin and troponin (Tn), with the position of tropomyosin modulated by calcium binding to troponin. Troponin itself consists of three subunits, TnI, TnC, and TnT, widely characterized as being responsible for separate aspects of the regulatory process. TnI, the inhibitory unit is released from actin upon calcium binding to TnC, while TnT performs a structural role forming a globular head region with the regulatory TnI- TnC complex with a tail anchoring it within the thin filament. We have examined the properties of TnT and the TnT(1) tail fragment (residues 1-158) upon reconstituted actin-tropomyosin filaments. Their regulatory effects have been characterized in both myosin S1 ATPase and S1 kinetic and equilibrium binding experiments. We show that both inhibit the actin-tropomyosin-activated S1 ATPase with TnT(1) producing a greater inhibitory effect. The S1 binding data show that this inhibition is not caused by the formation of the blocked B-state but by significant stabilization of the closed C-state with a 10-fold reduction in the C- to M-state equilibrium, K(T), for TnT(1). This suggests TnT has a modulatory as well as structural role, providing an explanation for its large number of alternative isoforms.

Expression of multiple troponin T isoforms in chicken breast muscle regeneration induced by sub-serous implantation

Chicken fast-muscle type (F-type) troponin T (TnT) isoforms are classified into two types, leg-muscle type (L-type) and breast-muscle type (B-type), which are generated by exclusion and inclusion of exon x series-derived sequences in mRNAs, respectively. The B-type isoforms are further classified into neonatal breast-muscle (BN), young chicken breast-muscle (BC), and adult chicken breast-muscle (BA) subtypes. It is known that the multiple F-type TnT isoforms are transiently expressed in the breast muscle tissue during normal development. To examine whether the transition of the isoforms was fixed in muscle cell lineage, breast muscle pieces (pectoralis major) of 1-day old chicks were cultured under gizzard serous membrane of the same chicks for 60 days at the longest. TnT isoform expression of the implants was monitored by immunoblotting and immunostaining using anti-F-type TnT against both L-type and B-type isoforms, anti-exon x3 against only B-type isoforms, and anti-S-type TnT against slow-muscle-type (S-type) isoforms. Muscle fibers in the implant degenerated first, and then new myotubes expressing L-type isoforms were formed by the fusion of myoblasts from surviving satellite cells. When the maturation of the myotubes into myofibers proceeded, BN-, BC-, and BA-subtype isoforms were expressed in the order of developmental stage specific-manner, indicating that the order of appearance of these isoforms was fixed in muscle cell lineage. In immunostaining of the implants recovered on the 60th day after implantation, at least three kinds of the regenerated myofibers were observed, expressing mainly B-type, both B-type and L-type, and only L-type isoforms. The immunohistochemical results suggested that the regulation of alternative splicing of F-type TnT pre-mRNAs was different among individual myofibers, and that the regulation was programmed in myogenic cells, probably satellite cells, which were the primary source of the fibers.

Developmental changes of cardiac and slow skeletal muscle troponin T expression in chicken cardiac and skeletal muscles

Numerous troponin T (TnT) isoforms are produced by alternative splicing from three genes characteristic of cardiac, fast skeletal, and slow skeletal muscles. Apart from the developmental transition of fast skeletal muscle TnT isoforms, switching of TnT expression during muscle development is poorly understood. In this study, we investigated precisely and comprehensively developmental changes in chicken cardiac and slow skeletal muscle TnT isoforms by two-dimensional gel electrophoresis and immunoblotting with specific antisera. Four major isoforms composed of two each of higher and lower molecular weights were found in cardiac TnT (cTnT). Expression of cTnT changed from high- to low-molecular-weight isoforms during cardiac muscle development. On the other hand, such a transition was not found and only high-molecular-weight isoforms were expressed in the early stages of chicken skeletal muscle development. Two major and three minor isoforms of slow skeletal muscle TnT (sTnT), three of which were newly found in this study, were expressed in chicken skeletal muscles. The major sTnT isoforms were commonly detected throughout development in slow and mixed skeletal muscles, and at developmental stages until hatching-out in fast skeletal muscles. The expression of minor sTnT isoforms varied from muscle to muscle and during development.

Conformational modulation of slow skeletal muscle troponin T by an NH(2)-terminal metal-binding extension

Troponin T (TnT) is an essential element in the thin filament Ca(2+)-regulatory system controlling striated muscle contraction. Alternative RNA splicing generates developmental and muscle type-specific TnT isoforms differing in the hypervariable NH(2)-terminal region. Using avian fast skeletal muscle TnT containing a metal-binding segment, we have demonstrated a role of the NH(2)-terminal domain in modulating the conformation of TnT (Wang J and Jin JP. Biochemistry 37: 14519-14528, 1998). To further investigate the structure-function relationship of TnT, the present study constructed and characterized a recombinant protein in which the metal-binding peptide present in avian fast skeletal muscle TnT was fused to the NH(2) terminus of mouse slow skeletal muscle TnT. Metal ion or monoclonal antibody binding to the NH(2)-terminal extension induced conformational changes in other domains of the model TnT molecule. This was shown by the altered affinity to a monoclonal antibody against the COOH-terminal region and a polyclonal antiserum recognizing multiple epitopes. Protein binding assays showed that metal binding to the NH(2)-terminal extension had effects on the interaction of TnT with troponin I, troponin C, and most significantly, tropomyosin. The data indicate that the NH(2)-terminal Tx [4-7 repeats of a sequence motif His-(Glu/Ala)-Glu-Ala-His] extension confers a specific conformational modulation in the slow skeletal muscle TnT.

Modulation of troponin T molecular conformation and flexibility by metal ion binding to the NH2-terminal variable region

Troponin T (TnT) plays an allosteric signal transduction role in the actin thin-filament-based Ca(2+)-regulation of striated muscle contraction. Developmentally regulated alternative RNA splicing produces TnT isoforms differing in their NH(2)-terminal structure. Physical property variations of the NH(2)-terminal hypervariable region of TnT may have a role in tuning the Ca(2+)-sensitivity and overall cooperativity of the muscle. We have previously demonstrated that metal ion or monoclonal antibody binding to the NH(2)-terminal region can modulate the epitopic conformation and troponin I and tropomyosin binding affinity of TnT. To further establish the molecular basis of this conformational and functional modulation, we have characterized the NH(2)-terminal variable region-originated secondary conformational effect in TnT using fluorescence spectral analysis. The chicken fast skeletal muscle TnT isoform, TnT8e16, containing a cluster of transition-metal ion binding sites (Tx) in the NH(2)-terminal variable region was used in this study. TnT8e16 was titrated for Cu(II) binding-induced changes in fluorescence intensity and anisotropy of the COOH-domain Trp residues (W234, W236, and W285), which demonstrated considerable environmental sensitivity in TnT denaturation studies. Nonlinear Stern-Volmer plots of Trp quenching indicated a metal ion binding-induced conformational change in TnT. Fluorescence anisotropy changes upon metal ion binding indicated a decrease in the mobility of the Trp residues and an increase in the flexibility of fluorescein-labeled Cys263 in the COOH domain. These data support a model that the alternatively spliced NH(2)-terminal variable region of TnT modulates conformation and flexibility of other domains of the protein.

Mapping the domain of troponin T responsible for the activation of actomyosin ATPase activity. Identification of residues involved in binding to actin

The in vitro Ca(2+) regulation of the actomyosin Mg(2+)-ATPase at physiological ratios of actin, tropomyosin, and troponin occurs only in the presence of troponin T. We have previously demonstrated that a polypeptide corresponding to the first 191 amino acids of troponin T (TnT-(1-191)) activates the actomyosin Mg(2+)-ATPase in the presence of tropomyosin. In order to further characterize this activation domain, we constructed troponin T fragments corresponding to residues 1-157 (TnT-(1-157)), 1-76 (TnT-(1-76)), 77-157 (TnT-(77-157)), 77-191 (TnT-(77-191)), and 158-191 (TnT-(158-191)). Assays using these fragments demonstrated the following: (a) residues 1-76 do not bind to tropomyosin or actin; (b) residues 158-191 bind to actin cooperatively but not to tropomyosin; (c) the sequence 77-157 is necessary for troponin interaction with residue 263 of tropomyosin; (d) TnT-(77-191) on its own activates the actomyosin ATPase activity as described previously for TnT-(1-191). TnT-(1-157), TnT-(1-76), TnT-(77-157), TnT-(158-191), and combinations of TnT-(158-191) with TnT-(1-157) or TnT-(77-157) showed no effect on the ATPase activity. We conclude that the activation of actomyosin ATPase activity is mediated by a direct interaction between amino acids 77 and 191 of troponin T, tropomyosin, and actin.

Cooperative interaction between developmentally regulated troponin T and tropomyosin isoforms in the absence of F-actin

Troponin T (TnT) is the tropomyosin (Tm) binding subunit of the troponin complex that mediates the Ca(2+) regulation of actomyosin interaction in striated muscles. Troponin T isoform diversity is marked by a developmentally regulated acidic to basic switch that may modulate muscle contractility. We previously reported that transgenic expression of fast skeletal muscle TnT altered the cooperativity of cardiac muscle. In the present study, we have demonstrated that the binding of acidic TnT to troponin I is weaker than that of basic TnT. However, affinity chromatography experiments showed that Tm bound to acidic TnT with a greater affinity than to basic TnT, consistent with the significantly higher maximal binding of acidic TnT to Tm in solid phase binding assays. Competition and co-immunoprecipitation experiments demonstrated that the binding of TnT to Tm was cooperative in the absence of F-actin. The cooperativity between TnT molecules for Tm binding can be initiated by the conserved COOH-terminal T2 fragment of TnT. This indicates that the interaction of TnT with Tm induces a conformational change in Tm, promoting interaction of TnT with adjacent Tm dimers. This finding suggests a role for TnT and its acidic and basic isoforms in the cooperative release of the inhibition of striated muscle actomyosin interaction.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Fast skeletal muscle troponin T increases the cooperativity of transgenic mouse cardiac muscle contraction

1. To investigate the functional significance of different troponin T (TnT) isoforms in the Ca2+ activation of muscle contraction, transgenic mice have been constructed with a chicken fast skeletal muscle TnT transgene driven by a cardiac alpha-myosin heavy chain gene promoter. 2. Cardiac muscle-specific expression of the fast skeletal muscle TnT has been obtained with significant myofibril incorporation. Expression of the endogenous cardiac muscle thin filament regulatory proteins, such as troponin I and tropomyosin, was not altered in the transgenic mouse heart, providing an authentic system for the functional characterization of TnT isoforms. 3. Cardiac muscle contractility was analysed for the force vs. Ca2+ relationship in skinned ventricular trabeculae of transgenic mice in comparison with wild-type litter-mates. The results showed unchanged pCa50 values (5.1 +/- 0.04 and 5.1 +/- 0.1, respectively) but significantly steeper slopes (the Hill coefficient was 2.0 +/- 0.2 vs. 1.0 +/- 0.2, P < 0.05). 4. The results demonstrate that the structural and functional variation of different TnT isoforms may contribute to the difference in responsiveness and overall cooperativity of the thin filament-based Ca2+ regulation between cardiac and skeletal muscles.

Structure and evolution of the alternatively spliced fast troponin T isoform gene

The vertebrate fast skeletal muscle troponin T gene, TnTf, produces a complexity of isoforms through differential mRNA splicing. The mechanisms that regulate splicing and the physiological significance of TnTf isoforms are poorly understood. To investigate these questions, we have determined the complete sequence structure of the quail TnTf gene, and we have characterized the developmental expression of alternatively spliced TnTf mRNAs in quail embryonic muscles. We report the following: 1) the quail TnTf gene is significantly larger than the rat TnTf gene and has 8 non-homologous exons, including a pectoral muscle-specific set of alternatively spliced exons; 2) specific sequences are implicated in regulated exon splicing; 3) a 900-base pair sequence element, composed primarily of intron sequence flanking the pectoral muscle-specific exons, is tandemly repeated 4 times and once partially, providing direct evidence that the pectoral-specific TnT exon domain arose by intragenic duplications; 4) a chicken repeat 1 retrotransposon element resides upstream of this repeated intronic/pectoral exon sequence domain and is implicated in transposition of this element into an ancestral genome; and 5) a large set of novel isoforms, produced by regulated exon splicing, is expressed in quail muscles, providing insights into the developmental regulation, physiological function, and evolution of the vertebrate TnTf isoforms.

Acidic and basic troponin T isoforms in mature fast-twitch skeletal muscle and effect on contractility

Developmentally regulated alternative RNA splicing generates distinct classes of acidic and basic troponin T (TnT) isoforms. In fast-twitch skeletal muscles, an acidic-to-basic TnT isoform switch ensures basic isoform expression in the adult. As an exception, an acidic segment in the NH2-terminal variable region of adult chicken breast muscle TnT isoforms is responsible for the unique exclusive expression of acidic TnTs in this muscle (O. Ogut and J.-P. Jin. J. Biol. Chem. 273: 27858-27866, 1998). To understand the relationship between acidic vs. basic TnT isoform expression and muscle contraction, the contractile properties of fibers from adult chicken breast muscle were compared with those of the levator coccygeus muscle, which expresses solely basic TnT isoforms. With use of Triton X-100-skinned muscle fibers, the force and stiffness responses to Ca2+ were measured. Relative to the levator coccygeus muscle, the breast muscle fibers showed significantly increased sensitivity to Ca2+ of force and stiffness with a shift of approximately 0.15 in the pCa at which force or stiffness was 50% of maximal. The expression of tropomyosin, troponin I, and troponin C isoforms was also determined to delineate their contribution to thin-filament regulation. The data indicate that TnT isoforms differing in their NH2-terminal charge are able to alter the sensitivity of the myofibrillar contractile apparatus to Ca2+. These results provide evidence linking the regulated expression of distinct acidic and basic TnT isoform classes to the contractility of striated muscle.

Tissue-specific distribution of breast-muscle-type and leg-muscle-type troponin T isoforms in birds

In order to show the tissue-specific distribution of troponin T (TnT) isoforms in avian skeletal muscles, their expression was examined by electrophoresis of the breast and leg muscles of seven avian species and immunoblotting with the antiserum against fast skeletal muscle TnT. It has been reported in the chicken that breast-muscle-type (B-type) and leg-muscle-type (L-type) TnT isoforms are expressed specifically in the adult breast and leg muscles, respectively. Their differential expression patterns were confirmed in all birds examined in this study. The expression of a segment encoded by the exon x series of TnT was also examined by immunoblotting with the antiserum against a synthetic peptide derived from the exon x3 sequence, because the segment has been shown to be included exclusively in the B-type, but not in the L-type TnT. The expression of the segment was found only in the breast muscle, but not in the leg muscle of all birds examined. TnT cDNA sequences from the duck breast and leg muscles were determined and showed that only B-type TnT had an exon x-related sequence, suggesting that the expression of B-type TnT containing the exon x-derived segment is conserved consistently in the birds.

Developmentally regulated, alternative RNA splicing-generated pectoral muscle-specific troponin T isoforms and role of the NH2-terminal hypervariable region in the tolerance to acidosis

The structure-function relationship of the alternative RNA splicing-generated NH2-terminal variable region of troponin T (TnT) is essential for understanding the physiological significance of developmental or muscle-specific TnT isoforms. Representing the hypervariable nature of the NH2-terminal region, a repeating transition metal-binding sequence (H(E/A)EAH)4-7 (Tx) has been found in chicken fast skeletal muscle TnT. In the present study, the developmentally regulated pectoral muscle-specific expression of this novel TnT isoform has been characterized. It was found that the variable amino terminus determined the isoelectric points of the TnT isoforms expressed, and the adult muscle-specific inclusion of the Tx sequence resulted in pectoralis TnTs, which were significantly more acidic in their NH2-terminal segment versus gastrocnemius TnTs. Experiments testing the effect of pH on TnT interaction with troponin I and tropomyosin indicated that although the interaction of acidic TnT isoforms with troponin I was less sensitive to the decrease of pH than the basic TnTs, the binding affinity of acidic TnT isoforms with tropomyosin was minimally affected by the decreased pH in contrast to basic TnT isoforms. Given that the majority of adult skeletal muscles express basic fast TnT isoforms, the switching between acidic and basic TnT isoforms may play a role in the functional adaptation of muscle to acidosis.

Conformational modulation of troponin T by configuration of the NH2-terminal variable region and functional effects

Troponin T (TnT) is an essential element in the thin filament-based regulatory system of striated muscle. Alternative mRNA splicing generates multiple TnT isoforms with primary structural differences in the NH2-terminal region. The functional significance of this hypervariable NH2-terminal domain and the developmental or muscle type-specific TnT isoforms is not fully understood. We have analyzed chicken breast muscle TnT containing a metal-binding cluster [H(E/A)EAH]4-7 (Tx) in the NH2-terminal region to demonstrate potential effects of the NH2-terminal structure on the conformation of TnT [Ogut, O., and Jin, J.-P. (1996) Biochemistry 35, 16581-16590]. Using specific antibody epitope analysis on this metal-binding TnT model, this study revealed that the binding of Zn2+ to the NH2-terminal region of chicken breast muscle TnT induces extensive conformational changes in the whole protein as demonstrated by a significant decrease in binding avidity of a polyclonal anti-TnT serum which recognizes multiple epitopes on the TnT molecule. This NH2-terminal configuration-based effect is not restricted to the metal ion interaction, whereas the binding of anti-NH2 terminus monoclonal antibodies to TnT induced similar changes. Protein-binding assays have shown that the NH2-terminal variability-induced conformational changes can alter TnT's binding affinity for tropomyosin and troponin I. The results suggest a functional modulation of TnT through the configuration of the NH2-terminal domain, and this novel mechanism may mediate the physiological significance of the TnT isoform regulation.

Three alternatively spliced mouse slow skeletal muscle troponin T isoforms: conserved primary structure and regulated expression during postnatal development

We have cloned and sequenced full-length cDNAs encoding mouse slow skeletal muscle troponin T (sTnT). Alternative mRNA splicing-generated two high Mr isoforms and one low Mr sTnT isoform differing in the NH2-terminal primary structure have been identified by Western blotting, reverse transcription-polymerase chain reaction and cDNA cloning/expression analyses. Together with a 5'-alternative exon that was also found in human sTnT encoding an 11-amino-acid acidic segment, the results revealed a novel alternative splicing pathway to include or exclude a three-base segment to generate additional sTnT isoforms with NH2-terminal charge variations. Overriding the phylogenetic divergence, primary structure of sTnT is better conserved between mammalian and avian species than that of cardiac, fast and skeletal muscle TnTs from one species. Western blots demonstrate four expression patterns of sTnT during postnatal skeletal muscle development: (1) a decrease to a non-detectable level in mouse masseter, (2) an increase to become the sole TnT in sheep masseter, (3) an increase of the total level as well as the proportion of the low Mr isoform in sheep diaphragm and, (4) no significant change in total level or high/low Mr isoform ratio in sheep gastrocnemius. The highly conserved primary structure and fiber type-specific and developmentally regulated expression of sTnT indicate a physiological importance of this under-studied member of the TnT gene family.

Regulatory properties of the NH2- and COOH-terminal domains of troponin T. ATPase activation and binding to troponin I and troponin C

The contraction of skeletal muscle is regulated by Ca2+ binding to troponin C, which results in an internal reorganization of the interactions within the troponin-tropomyosin complex. Troponin T is necessary for Ca2+-dependent inhibition and activation of actomyosin. Troponin T consists of an extended NH2-terminal domain that interacts with tropomyosin and a globular COOH-terminal domain that interacts with tropomyosin, troponin I, and troponin C. In this study we used recombinant troponin T and troponin I fragments to delimit further the structural and regulatory interactions with the thin filament. Our results show the following: (i) the NH2-terminal region of troponin T activates the actomyosin ATPase in the presence of tropomyosin; (ii) the interaction of the globular domain of troponin T with the thin filament blocks ATPase activation in the absence of Ca2+; and (iii) the COOH-terminal region of the globular domain anchors the troponin C-troponin I binary complex to troponin T through a direct Ca2+-independent interaction with the NH2-terminal region of troponin I. This interaction is required for Ca2+-dependent activation of the actomyosin ATPase activity. Based on these results we propose a refined model for the troponin complex and its interaction with the thin filament.

Developmentally regulated muscle type-specific alternative splicing of the COOH-terminal variable region of fast skeletal muscle troponin T and an aberrant splicing pathway to encode a mutant COOH-terminus

Distinct from the cardiac and slow skeletal muscle troponin Ts, an alternative RNA splicing-generated COOH-terminal variable region exists in the fast skeletal muscle troponin T. Mutually exclusive splicing of exon 16 and 17 encoded sequence into the mature mRNA produces the alpha- and beta-isoform, respectively. By cloning and sequence analysis of large numbers of fast troponin T cDNAs, we have quantitatively demonstrated that expression of the exon 16-encoded structure is mature fast muscle-specific (its utilization ranges from null in neonatal mouse muscles to 97% in adult chicken pectoralis), indicating a functional adaptation to the contractile feature of muscle. An aberrant splicing of this variable region to exclude both exons 16 and 17 from the mRNA was found in neonatal mouse skeletal muscle by cloning and sequencing characterization of a full length fTnT cDNA. The unusual splicing of exon 18 and exon 15 in the mRNA sequence results in not only a deletion of the exon 16/17 segment but also a shift of the downstream translation reading frame to produce a troponin T polypeptide with mutant COOH-terminus. Similar to an abnormal splicing of cardiac troponin T caused by cis-mutation and a dominant allele causing human familial hypertrophic cardiomyopathy, this trans-factor-determined aberrant mRNA splicing pathway generates a truncated troponin T molecule lacking the developmentally regulated fast muscle-specific COOH-terminal domain, indicating potential etiopathological significance.

Primary structure and developmental acidic to basic transition of 13 alternatively spliced mouse fast skeletal muscle troponin T isoforms

Large samples of original cDNAs encoding neonatal and adult mouse fast skeletal muscle troponin T (fTnT) have been isolated and characterized. The results demonstrate expression relationships of 8 alternatively spliced exons of the fTnT gene and reveal the primary structure of as many as 13 fTnT isoforms that diverge into acidic and basic classes due to differential mRNA splicing in the N-terminal variable region. In the C-terminal variable region encoded by the mutually exclusive exons 16 and 17, the splicing pathway and structure of exon 16 appears to be adult fTnT-specific, suggesting an adaptation to the functional demands of mature fast skeletal muscle. The cloned cDNAs were expressed in E. coli as standards to identify a high M(r) to low M(r), acidic to basic fTnT isoform transition in postnatal developing skeletal muscles. Different from the developmental cardiac TnT switch generated by alternative splicing of a single exon, the fTnT isoform transition is an additive effect of alternative splicing of multiple N-terminal-coding exons, especially exons 4, 8 and fetal that are expressed at higher frequencies in the neonatal than in the adult muscle. The developmental fTnT isoform primary structure transition in both N- and C-terminal variable regions suggest a physiological importance of the apparently complex TnT isoform expression.

Production, crystallization, and preliminary X-ray analysis of rabbit skeletal muscle troponin complex consisting of troponin C and fragment (1-47) of troponin I

Troponin is a ternary protein complex consisting of subunits TnC. TnI, and TnT, and plays a key role in calcium regulation of the skeletal and cardiac muscle contraction. In the present study, a partial complex (CI47) was prepared from Escherichia coli-expressed rabbit skeletal muscle TnC and fragment 1-47 of TnI, which is obtained by chemical cleavage of an E. coli-expressed mutant of rabbit skeletal muscle TnI. Within the ternary troponin complex, CI47 is thought to form a core that is resistant to proteolytic digestion, and the interaction within CI47 likely maintains the integrity of the troponin complex. Complex CI47 was crystallized in the presence of sodium citrate. The addition of trehalose improved the diffraction pattern of the crystals substantially. The crystal lattice belongs to the space group P3(1)(2)21, with unit cell dimensions a = b = 48.2 A, c = 162 A. The asymmetric unit presumably contains one CI47 complex. Soaking with p-chloromercuribenzenesulfonate (PCMBS) resulted in loss of isomorphism, but enhanced the quality of the crystals. The crystals diffracted up to 2.3 A resolution, with completeness of 91% and R(merge) = 6.4%. The crystals of PCMBS-derivative should be suitable for X-ray studies using the multiple-wavelength anomalous diffraction technique. This is the first step for elucidating the structure of the full troponin complex.

Stability of chicken troponin T expression in cultured muscle cells

Cells prepared from chicken skeletal muscles of early developmental stages were cultured to study their troponin T isoform expression, using antisera specific to fast- and slow-muscle-type isoforms, and compared with the cells from later stages described in the previous study (Mashima at al., 1996). We found that cultured myogenic cells from chickens and chick embryos could be classified, as in the previous study, into two types, fast type and fast/slow type in which fast- and slow-muscle-type isoforms were coexpressed. Ratios of these two types of muscle cells varied depending on their origins and developmental stages, and fast/slow type cells were in the majority at early stages. Since two distinct populations of cells committed to myogenic cell lineages were supposed to give rise to the two types of myotubes, we investigated the intrinsic stability of troponin T expression of the cultured myogenic cells using the serial subcloning method. The results of clonal analysis suggested that the expression pattern of troponin T isoform in cultured muscle cells is stable and that myogenic cell lineages play an important role in giving rise to different muscle types.

Expression, zinc-affinity purification, and characterization of a novel metal-binding cluster in troponin T: metal-stabilized alpha-helical structure and effects of the NH2-terminal variable region on the conformation of intact troponin T and its association with tropomyosin

A repeating metal-binding (Cu2+ > Ni2+ > Zn2+ approximately Co2+) sequence [HE/AEAH]4 (Tx) has been recently identified in the NH2-terminal variable region of troponin T (TnT) isoforms specifically expressed in the breast but not leg muscles of the avian orders of Galliformes and Craciformes [Jin, J.-P., & Smillie, L. B. (1994) FEBS Lett. 341, 135-140]. In the present study, two expression plasmids were constructed to produce chicken TnT1 NH2-terminal fragments of 47 (N47) or 165 (N165) amino acids containing the Tx metal-binding cluster. The recombinant protein/peptide was expressed in Escherichia coli BL21(DE3)pLysS and purified by a highly effective Zn(2+)-affinity chromatography method. Amino acid analyses, NH2-terminal peptide sequencing, mass spectrometry and immunological identification confirmed the authenticity of the genetically engineered TnT fragments. In the presence of 2,2,2-trifluoroethanol, transition metals had significant effects on the secondary structure of TnT fragment N47, as shown by circular dichroism. N165 in non-denaturing buffer demonstrated alpha-helical content comparable to previous data from rabbit fast skeletal TnT fragment T1. Zn(2+)-binding avidity of the metal-binding TnT and its fragments demonstrated tertiary relationships between the NH2-terminal variable region and the COOH-terminal segment of the intact TnT protein. Solid-phase protein-binding assays established that Zn(2+)-binding to the Tx cluster induces epitopic structure changes in this NH2-terminal segment, further affecting other epitopic structures of intact TnT as well as the function of TnT's tropomyosin binding-sites. The results demonstrate that metal ion-binding to the Tx cluster reconfigures the overall conformation of TnT through structural relationships between the NH2-terminal variable region and other domains of the intact TnT molecule. Accordingly, the developmental and/or muscle type specific NH2-terminal structure of TnT isoforms may modulate the Ca(2+)-activation of muscle contraction.

Distinct troponin T genes are expressed in embryonic/larval tail striated muscle and adult body wall smooth muscle of ascidian

During development of the ascidian Halocynthia roretzi, the tadpole larva hatched from the tailbud embryo metamorphoses to the sessile adult with a body wall muscle. Although the adult body wall muscle is morphologically nonsarcomeric smooth muscle, it contains troponin complex consisting of three subunits (T, I, and C) as do vertebrate striated muscles. Different from vertebrate troponins, however, the smooth muscle troponin promotes actomyosin Mg2+-ATPase activity in the presence of high concentration of Ca2+, and this promoting property is attributable to troponin T. To address whether the embryonic/larval tail striated muscle and the adult smooth muscle utilize identical or different regulatory machinery, we cloned troponin T cDNAs from each cDNA library. The embryonic and the adult troponin Ts were encoded by distinct genes and shared only <60% identity with each other. Northern blotting and whole mount in situ hybridization revealed that these isoforms were specifically expressed in the embryonic/larval tail striated muscle and the adult smooth muscle, respectively. These results may imply that these isoforms regulate actin-myosin interaction in different manners. The adult troponin T under forced expression in mouse fibroblasts was unexpectedly located in the nuclei. However, a truncated protein with a deletion including a cluster of basic amino acids colocalized with tropomyosin on actin filaments. Thus, complex formation with troponin I and C immediately after the synthesis is likely to be essential for the protein to properly localize on the thin filaments.

Expression of chicken troponin T isoforms in cultured muscle cells

Cells prepared from chicken skeletal muscles of different developmental stages were cultured to study their troponin T isoform expression, using antisera specific to the fast- and slow-muscle-type isoforms. We found that the cultured myogenic cells from chickens and chick embryos were classified into two types, fast type and fast/slow type in which fast- and slow-muscle-type isoforms were coexpressed. Cells expressing only slow-muscle-type troponin T isoforms could not be found. Most cells prepared from pectoralis major (fast muscle) and gastrocnemius (mixed muscle) of 11-day old embryos belonged to the latter, with only a small fraction belonging to the former. The percentage of fast type cells in those cells prepared from pectoralis major increased along development to over 90% by the 17th day of incubation, while, in the cells prepared from gastrocnemius, it reached a plateau of 30-40% by the 13th day of incubation. All the cells from anterior latissimus dorsi (slow muscle) belonged to the fast/slow type. Ratios of these two types of muscle cells varied depending on their origins and stages. The in vitro expression of troponin T isoforms was different from the in vivo expression, and each muscle seems to be determined differently in the composition of cell types during the developmental course.

Expression of cDNAs encoding mouse cardiac troponin T isoforms: characterization of a large sample of independent clones

We have isolated 52 mouse cardiac troponin-T-encoding cDNA clones (TnT) by specific antibody screening of a lambda ZAPII expression library. Sequencing data from the large sample of independent cDNAs demonstrated relationships among the expression of four alternatively-spliced exons of the cardiac TnT gene, producing seven classes of cDNAs encoding four protein isoforms differing in two variable regions. In the N-terminal variable region and next to the embryonic-specific exon 4, an alternatively spliced exon 3a was identified in 20% of the adult isoforms. The alternatively spliced exon 12, corresponding to a central variable region between the two functional domains of TnT, was found in approx. 79% of the 52 mouse cardiac TnT cDNAs with a single base mutation completely abolishing the splicing at an internal acceptor site. Three novel alternative splicing acceptor sites in the 5'-untranslated portion of exon 2 have been identified with different frequencies.

Expression and epitopic conservation of calponin in different smooth muscles and during development

Calponin is a thin filament associated protein found in smooth muscle as a potential modulator of contraction. Five mouse monoclonal antibodies (mAbs CP1, CP3, CP4, CP7, and CP8) were prepared against chicken gizzard alpha-calponin. The CP1 epitopic structure is conserved in smooth muscles across vertebrate phyla and is highly sensitive to CNBr cleavage in contrast with the chicken-specific CP4 and the avian-mammalian-specific CP8 epitopes that are resistant to CNBr fragmentation. Using this panel of mAbs against multiple epitopes, only alpha-calponin was detected in adult chicken smooth muscles and throughout development of the gizzard. Western blotting showed that the calponin content varied among different smooth muscle tissues and correlated with that of h-caldesmon. In contrast with the constitutive expression of calponin in phasic smooth muscle of the digestive tract, very low levels of calponin were detected in adult avian tracheas and no calponin expression was detected in embryonic and young chick tracheas. These results provide information on the structural conservation of calponins and suggest a relationship between calponin expression and smooth muscle functional states.

Human cardiac troponin T: cloning and expression of new isoforms in the normal and failing heart

Troponin T, like many myofibrillar proteins, exists as multiple isoforms encoded by distinct genes or generated by splicing of the same primary RNA transcript. We have previously cloned the first human cardiac troponin T (cTnT) cDNA and showed the differential expression of cTnT in cardiac and skeletal muscle during ontogenic development. In this work we located the human cTnT gene by means of fluorescent in situ hybridization to 1q32 and, by sequencing thirteen cDNAs isolated from a human fetal heart cDNA library, identified three new isoforms resulting from specific combinations of three variable regions in human cTnT cDNA. The first variable region is a 30-bp box located at the 5' end of the cDNA, which can be excised either totally or only from the first 3 bp onwards; the second is a codon which can be completely excised; and the third is a 9-bp box in the 3' half of the cDNA, which can also be excised either totally or only from the first 3 bp. The existence of the corresponding RNAs in fetal and adult ventricles was confirmed by RNase protection studies. No accumulation of the fetal isoforms was found in failing ventricles compared with controls.

Alpha-tropomyosin and cardiac troponin T mutations cause familial hypertrophic cardiomyopathy: a disease of the sarcomere

We demonstrate that missense mutations (Asp175Asn; Glu180Gly) in the alpha-tropomyosin gene cause familial hypertrophic cardiomyopathy (FHC) linked to chromosome 15q2. These findings implicated components of the troponin complex as candidate genes at other FHC loci, particularly cardiac troponin T, which was mapped in this study to chromosome 1q. Missense mutations (Ile79Asn; Arg92Gln) and a mutation in the splice donor sequence of intron 15 of the cardiac troponin T gene are also shown to cause FHC. Because alpha-tropomyosin and cardiac troponin T as well as beta myosin heavy chain mutations cause the same phenotype, we conclude that FHC is a disease of the sarcomere. Further, because the splice site mutation is predicted to function as a null allele, we suggest that abnormal stoichiometry of sarcomeric proteins can cause cardiac hypertrophy.

Assembly of functional skeletal muscle troponin complex in Escherichia coli

The production of multi-subunit proteins of eukaryotic origin in Escherichia coli usually relies on the different subunits being expressed individually and the protein being reassembled in vitro. Here we describe the construction and characterization of plasmids capable of coexpressing the three subunits of chicken skeletal muscle troponin complex in E. coli. We demonstrate that the troponin subunits assembled in the cytoplasm of E. coli cell are fully functional. The troponin complex was purified to homogeneity in high yields. When reconstituted into actin filaments, the complex assembled in vivo was capable of regulating the myosin ATPase with a calcium dependence that was identical to the complex reconstituted in vitro. These results demonstrate that the coexpression of the subunits of a protein complex can prevent the accumulation of denatured proteins in inclusion granules.

Investigation of the effects of phosphorylation of rabbit striated muscle alpha alpha-tropomyosin and rabbit skeletal muscle troponin-T

FPLC has been employed to prepare the phosphorylated and unphosphorylated forms of rabbit striated muscle alpha alpha-tropomyosin (TM), and the major isoform of rabbit fast-skeletal-muscle troponin-T (Tn-T2f) and corresponding chymotryptic fragment T1 (residues 1-158), in order to investigate the effects which these in vivo modifications have on thin filament function. In all instances, no significance could be attributed to the presence of a phosphate moiety on acetyl serine 1 of Tn-T (or fragment T1). As expected, fragment T1 increased the relative viscosities of solutions of unphosphorylated alpha alpha-TM, but this induction was noticeably lower for phosphorylated alpha alpha-TM. In affinity chromatography experiments, fragment T1 bound equally well to either form of alpha alpha-TM, but the interaction between fragment T2 (residues 159-259) and phosphorylated alpha alpha-TM was strengthened relative to the control. In the presence of alpha alpha-TM (unphosphorylated), fragment T1 was found to down regulate the actin-activated myosin-S1 MgATPase activity, indicating that this portion of Tn-T possesses modulatory properties. Under the same conditions, less inhibition was observed with phosphorylated alpha alpha-TM. When the two different forms of alpha alpha-TM were reconstituted into a complete regulatory system, the activation of myosin-S1 was double for those thin filaments containing the phosphorylated molecule. Dephosphorylation of the phospho alpha alpha-TM reduced the rates to control values. In ATPase Ca2+ titrations, these systems exhibited no difference in the co-operativity of activation and little or no difference in the pCa2+ 1/2 value. Developmentally linked changes in the steady-state phosphorylation of alpha alpha-TM could be a mechanism to increase the activating propensity of thin filaments, by modifying the functional properties of the T1 section of Tn-T.

An unusual metal-binding cluster found exclusively in the avian breast muscle troponin T of Galliformes and Craciformes

A repeating metal-binding (Cu2+ > Ni2+ > Zn2+ approximately Co2+) sequence (HE/AEAH)4 has been identified in troponin T isoforms specifically expressed in the breast but not leg muscles of all Galliformes and Craciformes. It is absent in the skeletal and cardiac muscles of mammals and all other avian species investigated. Concentration of the metal-binding sites is adequate to affect free metal levels in the muscle cell and we suggest a possible link between its presence in breast muscle of Galliformes and the high ratio of breast muscle to total body muscle mass and explosive but short-lived flight pattern of these birds. This sequence can be used for a highly selective metal-affinity chromatographic purification of muscle or engineered TnTs even in high salt and/or urea.

Isolation and characterization of human fast skeletal beta troponin T cDNA: comparative sequence analysis of isoforms and insight into the evolution of members of a multigene family

A cDNA encoding human fast skeletal beta troponin T (beta TnTf) has been isolated and characterized from a fetal skeletal muscle library. The cDNA insert is 1,000 bp in length and contains the entire coding region of 777 bp and 5' and 3' untranslated (UT) segments of 12 and 211 bp, respectively. The 3' UT segment shows the predicted stem-loop structure typical of eukaryotic mRNAs. The cDNA-derived amino acid sequence is the first available sequence for human beta TnTf protein. It is encoded by a single-copy gene that is expressed in a tissue-specific manner in fetal and adult fast skeletal muscles. Although the human beta TnTf represents the major fetal isoform, the sequence information indicates that this cDNA and the coded protein are quite distinct from the fetal and neonatal TnTf isoforms reported in other mammalian fetal muscles. The hydropathy plot indicates that human beta TnTf is highly hydrophilic along its entire length. The protein has an extremely high degree of predicted alpha-helical content involving the entire molecule except the carboxy-terminal 30 residues. Comparative sequence analysis reveals that the human beta TnTf shares a high level of sequence similarity in the coding region with other vertebrate TnTf and considerably reduced similarity with slow skeletal and cardiac TnT cDNAs. The TnT isoforms have a large central region consisting of amino acid residues 46-204 which shows a high sequence conservation both at the nucleotide and amino acid levels. This conserved region is flanked by the variable carboxy-terminal and an extremely variable amino-terminal segment. The tropomyosin-binding peptide of TnT, which is represented by amino acid residues 47-151 and also includes a part of troponin I binding region, is an important domain of this central segment. It is suggested that this conserved segment is encoded by an ancestral gene. The variable regions of vertebrate striated TnT isoforms reflect the subsequent addition and modification of genomic sequences to give rise to members of the TnT multigene family.

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.

Complete nucleotide sequence and structural organization of rat cardiac troponin T gene. A single gene generates embryonic and adult isoforms via developmentally regulated alternative splicing

We have previously demonstrated that rat cardiac troponin T (TnT) is expressed as two different isoforms during development, the larger, more acidic embryonic isoform and the smaller, more basic adult isoform, which appear to be generated from a common transcript of the cardiac TnT gene by alternative RNA splicing. In this study, Southern blot analysis confirmed the existence of a single copy of cardiac TnT gene in the rat genome. For investigation of the molecular mechanism of isoform switch and the control of this gene expression in myocardial development, several overlapping genomic clones were isolated from a rat genomic library. Complete nucleotide sequences were determined from these genomic clones and revealed a 19,186 base-pair DNA fragment containing 16 exons of rat cardiac TnT gene. Its DNA sequence and exon organization appeared to differ from that of the rat fast skeletal muscle TnT gene or chicken cardiac TnT gene. Comparison of genomic and cDNA clones also confirmed that the cardiac TnT isoform switching was due to the inclusion or exclusion of exon 4 during RNA processing. Sequence analysis allowed us to further identify the other alternatively spliced exon containing only nine nucleotides in size (exon 12). The inclusion and complete or partial exclusion of this exon may be responsible for generating three classes of mRNAs detected by our cDNA clones. The functional significance of this variation in TnT isoforms remained unknown, but its splicing pattern did not appear to link to the developmental changes. The 5' upstream structure was very similar to that in chicken cardiac TnT gene but differed from that in the rat fast skeletal muscle TnT gene, suggesting a similar regulatory mechanism for mammalian and avian cardiac TnT expression.

Expression pattern of skeletal muscle troponin T isoforms is fixed in cell lineage

The expression of fast-muscle-type troponin T isoforms in chicken skeletal muscles was studied by two-dimensional SDS-polyacrylamide gel electrophoresis and immunoblotting. According to the pattern of troponin T isoform expression, chicken fast muscle was classified into two groups: One group expressed breast-fast-muscle-type troponin T in addition to leg-fast-muscle-type troponin T, the other expressed only leg-fast-muscle-type troponin T. To the former group belong breast and wing fast muscles and some of the back fast muscles, and to the latter group belong the fast muscles in leg, abdomen, and neck. Transplantation of breast muscle into leg was performed in order to change the physical environment and to investigate the mechanism of isoform expression. Histological observation of the transplant revealed severe degeneration of muscle cells, followed by differentiation of myoblasts in which breast-muscle-type troponin T was eventually expressed. The results showed that the pattern of troponin T isoform expression is primarily fixed in the cell lineage, although nerves modulate it.