Rat embryonic fibroblast tropomyosin 1. cDNA and complete primary amino acid sequence

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.

Tropomyosin isoforms in rat neurons: the different developmental profiles and distributions of TM-4 and TMBr-3 are consistent with different functions

Antipeptide antisera specific for TM-4 and TMBr-3, the two tropomyosin isoforms in neurons, were used to investigate the concentrations and distributions of these F-actin-binding proteins in neurons in vitro and in vivo. TM-4 and TMBr-3 tropomyosins had different developmental profiles. TM-4 was found mainly in immature stages, while the concentration of TMBr-3 increased with maturation. The two isoforms also had different subcellular distributions. TM-4 was concentrated in the growth cones of cultured neurons and, in vivo, in areas where neurites were growing. Later, when development was complete, TM-4 was restricted to postsynaptic sites in the cerebellar cortex, whereas TMBr-3 was found in the presynaptic terminals. These data suggest that the tropomyosin isoforms have different functions, through their interaction with the actin cytoskeleton. TM-4 may be involved in the motile events of neurite growth and synaptic plasticity, while TMBr-3 could play a role in stabilizing neuronal networks and synaptic functioning.

The expression of tropomyosin genes in pure cultures of rat neurons, astrocytes and oligodendrocytes is highly cell-type specific and strongly regulated during development

Transcripts from the alpha-, beta- and delta-tropomyosin genes were studied during development of pure cultures of rat neurons, astrocytes and oligodendrocytes. The three cell types contained five alpha-tropomyosin messengers, produced using both alternative promoters and splicing; one was specific for mature neurons. The beta-tropomyosin gene is expressed only in astrocytes and the delta-tropomyosin gene in all three cell types, especially in immature cells. Most of the tropomyosin isoforms are highly cell-specific. Their developmental regulation involves either differential expression of genes, in neurons and oligodendrocytes, and/or changes in alternative splicing, in astrocytes, delta-Tropomyosin (TM-4) may be important during the growth of neuronal and glial cell processes, while specialized isoforms such as the neuron-specific alpha-tropomyosin TMBr-3 may be involved in the function or plasticity of the mature cells.

Human fibroblast tropomyosin isoforms: characterization of cDNA clones and analysis of tropomyosin isoform expression in human tissues and in normal and transformed cells

A tropomyosin-specific oligonucleotide probe (REN29) designed to hybridize to all known human tropomyosin isoforms was used to study tropomyosin mRNA levels in normal and transformed human cells. At least four different sizes of RNAs were detected in normal human fibroblast KD cells by Northern blot analysis. The major bands of 1.1 kb RNA for hTM1 and 3.0 kb RNA for hTM4 were decreased substantially in various transformed cell lines. One of the minor RNA bands (2.0 kb for hTM2 and hTM3) appeared to be absent in a human pancreatic carcinoma cell line. The level of the other minor RNA band (2.5 kb for hTM5) was found to be unchanged or slightly decreased in transformed cells. This differential expression of tropomyosin isoforms at the RNA level was not totally in agreement with the difference in the protein amounts found in normal and transformed cells, suggesting that translational control may also play an important role in the expression of some tropomyosin isoforms. The REN29 probe was further used to screen lambda gt10 and lambda gt11 cDNA libraries, which were constructed from poly(A)+ RNAs of human fibroblast cell lines HuT-14 and WI-38, respectively. In addition to cDNA clones encoding known isoforms, we obtained three classes of new cDNA clones that encode two low M(r) isoforms (hTM5a and hTM5b), and a high M(r) isoform (hTMsm alpha). Sequence comparison revealed that hTM5a and hTM5b are alternatively spliced products derived from the same gene that encodes hTM2 and hTM3. Northern blot analysis and amino acid sequence comparison suggested that the hTMsm alpha represents a smooth muscle tropomyosin which is also expressed in human fibroblasts. The exon specific for, and common to, hTM5a and hTM5b was found to be highly expressed in small intestine. However, there was no detectable expression of this exon in stomach and skeletal muscle. The difference in tissue-specific expression suggests that different isoforms may perform distinct functions in different tissues.

Platelet-derived growth factor-BB-induced suppression of smooth muscle cell differentiation

Previously, we demonstrated that treatment of postconfluent quiescent rat aortic smooth muscle cells (SMCs) with platelet-derived growth factor (PDGF)-BB dramatically reduced smooth muscle (SM) alpha-actin synthesis. In the present studies, we focused on the expression of two other SM-specific proteins, SM myosin heavy chain (SM-MHC) and SM alpha-tropomyosin (SM-alpha TM), to determine whether the actions of PDGF-BB were specific to SM alpha-actin or represented a global ability of PDGF-BB to inhibit expression of cell-specific proteins characteristic of differentiated SMCs. SM-MHC and SM-alpha TM expression were assessed by one- or two-dimensional gel electrophoretic analysis of proteins from cells labeled with [35S]methionine, as well as by Northern analysis of mRNA levels. Synthesis of both SM-specific proteins was decreased by 50-70% in PDGF-BB--treated cells as compared with cells treated with PDGF vehicle. Treatment of cells with 10% fetal bovine serum, which produced a mitogenic effect equivalent to that of PDGF-BB, decreased SM-MHC synthesis by 40% but increased SM-alpha TM synthesis. SM-MHC and SM-alpha TM mRNA expression was decreased by 80% at 24 hours in PDGF-BB--treated postconfluent SMCs, whereas treatment with 10% fetal bovine serum did not decrease the expression of SM-alpha TM mRNA but did inhibit SM-MHC mRNA expression by 36%. Consistent with the absence of detectable PDGF alpha-receptors on these cells, PDGF-AA had no effect on either mitogenesis or expression of SM-MHC or SM-alpha TM.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of tropomyosin expression in transformed granulosa cell lines with steroidogenic ability

The expression of the different tropomyosin isoforms was analyzed in primary granulosa cell cultures and in established granulosa cell lines cotransfected with SV40 and Ha-ras DNA which retain a high steroidogenic response to cAMP stimulation. In contrast to normal cells which greatly reduce the expression of all tropomyosin isoforms during development of steroidogenic ability, in the doubly transformed cells only the synthesis of the high molecular weight isoforms nos 2 and 3 was decreased. The expression of isoforms 1 and 5 was elevated in the cotransfected lines and that of tropomyosin 1 was further enhanced by cAMP stimulation. The increased synthesis of tropomyosins 1 and 5 is unique to SV40 transformation, since it was observed also in cells transfected with SV40 DNA alone. These cells displayed a well organized microfilament system, but have lost the ability to differentiate. The reduced expression of tropomyosins 2 and 3 and a poorly organized microfilament system appear to be a dominant feature of both the highly differentiated normal- and transformed-granulosa cells. It is suggested that the switches in tropomyosin isoform expression during development of the steroidogenic phenotype and in cell transformation may account for necessary changes in microfilament organization which accompany these cellular processes.

Structure and complete nucleotide sequence of the gene encoding rat fibroblast tropomyosin 4

We have isolated and determined the complete nucleotide sequence of the gene that encodes the 248 amino acid residue fibroblast tropomyosin, TM-4. The TM-4 sequence is encoded by eight exons, which span approximately 16,000 bases. The position of the intron-exon splice junctions relative to the final transcript are identical to those present in other vertebrate tropomyosin genes and the Drosophila melanogaster TMII gene. We have found no evidence that the rat TM-4 gene is alternatively spliced, unlike all the other tropomyosin genes from multicellular organisms that have been described. Typical vertebrate tropomyosin genes contain some, or all, of alternatively spliced exons 1a and 1b, 2a and 2b, 6a and 6b, and 9a, 9b, 9c and 9d in addition to common exons 3, 4, 5, 7 and 8. The rat fibroblast TM-4 mRNA is encoded by sequences most similar to exons 1b, 3, 4, 5, 6b, 7, 8 and 9d. Two exon-like sequences that are highly similar to alternatively spliced exons 2b and 9a of the rat beta-tropomyosin gene and the human TMnm gene have been located in the appropriate region of the gene encoding rat fibroblast TM-4. However, several mutations in these sequences render them non-functional as tropomyosin coding exons. We have termed these exon-like sequences, vestigial exons. The evolutionary relationship of the rat TM-4 gene relative to other vertebrate tropomyosin genes is discussed.

Generation of skeletal, smooth and low molecular weight non-muscle tropomyosin isoforms from the chicken tropomyosin 1 gene

We have determined the organization of the chicken tropomyosin 1 gene by sequencing the cloned genomic DNA. The single-copy gene spans approximately 11,000 bases and includes 12 exons. Comparison of cDNA and genomic sequences demonstrates that three tissue-specific tropomyosins are encoded by the gene: a 284 amino acid skeletal muscle beta-tropomyosin, a 284 amino acid smooth muscle tropomyosin, and a 248 amino acid non-muscle (fibroblast) beta-tropomyosin. Skeletal and smooth muscle transcripts use the same putative promoter and transcription initiation site. However, they are alternatively spliced to generate mRNAs that differ in the region giving rise to amino acids 188 to 213 and 258 through the poly(A) site. The fibroblast transcript uses a promoter, initiation site and first exon that is distinct from that used for both the smooth and the skeletal muscle transcripts. However, beyond the first exon the fibroblast transcript undergoes splicing and polyadenylation that is identical with the smooth muscle transcript.

Identification of two distinct intron elements involved in alternative splicing of beta-tropomyosin pre-mRNA

The rat beta-tropomyosin gene encodes two isoforms, termed skeletal muscle beta-tropomyosin and fibroblast last tropomyosim 1 (TM-1), via an alternative RNA processing mechanism. The gene contains 11 exons. Exons 1-5 and exons 8 and 9 are common to all mRNAs expressed from the gene. Exons 6 and 11 are used in fibroblasts, as well as smooth muscle, whereas exons 7 and 10 are used only in skeletal muscle. In the present studies we focused on the mutually exclusive internal alternative splice choice involving exon 6 (fibroblast-type splice) and exon 7 (skeletal muscle-type splice). We have identified two distinct elements in the intron, upstream of exon 7, involved in splice site selection. The first element is comprised of a polypyrimidine tract located 89-143 nucleotides upstream of the 3' splice site, which specifies the location of the lariat branchpoints used, 144-153 nucleotides upstream of exon 7. The 3' splice site AG dinucleotide has no role in selection of these branchpoints. The second element is comprised of intron sequences located between the polypyrimidine tract and the 3' splice site of exon 7. It contains an important determinant in alternative splice site selection, because deletion of these sequences results in the use of the skeletal muscle-specific exon in nonmuscle cells. We propose that the use of lariat branchpoints located far upstream from a 3' splice site may be a general feature of some alternatively excised introns, reflecting the presence of regulatory sequences located between the lariat branch site and the 3' splice site. The data also indicate that alternative splicing of the rat beta-tropomyosin gene is regulated by a somewhat different mechanism from that described for rat alpha-tropomyosin gene and the transformer-2 gene of Drosophila melanogaster.

Schistosoma mansoni tropomyosin: cDNA characterization, sequence, expression, and gene product localization

We have defined the polypeptide pattern of 3-hr Schistosoma mansoni schistosomula on nonequilibrium two-dimensional gels (NEPHGE). An acidic group of polypeptides with a molecular weight of about 40 kDa and a pI value of around 5.0 (numbered 48/59/53) were identified as antigens on Western blots probed with chronic human infection sera or vaccinated mouse sera. Polypeptides 48/49/53 from silver-stained NEPHGE gels produced antisera that were specific as demonstrated by Western blot analysis and immunoprecipitations of in vitro translation products. A cDNA clone (clone 1) from a S. mansoni adult worm pBR322 library was isolated by using cDNA probes made from size-fractionated mRNA and defined as encoding polypeptide 49 by hybridization selection of the mRNA which was in vitro translated and immunoprecipitated with specific mouse antiserum. A lambda gt 11 expression clone which contained an insert close to the full length mRNA was isolated from a S. mansoni cercariae library. The complete sequence of the mRNA was determined by sequencing the insert of this clone as well as primer extension of total RNA. The only open reading frame coding for 284 amino acids in the 1316 nucleotide sequence showed a 44.76 to 55.44% homology with the amino acid sequences of 18 different tropomyosins from various species. Computer-predicted secondary structure of schistosome tropomyosin was mainly alpha-helix which was very similar to other tropomyosins. Northern analysis showed the mRNA to be about 1.5 kb in size and detectable at much higher levels in the adult worm stage as compared to the cercariae and the egg stages. Western blot analysis likewise showed that greater amounts of tropomyosin were detected in extracts from adult worm stage as compared to extracts from cercariae and egg stages. Immunocytochemical analysis shows that tropomyosin is strongly associated with the tegument of adult worms. The restriction digestion pattern given by genomic Southern analysis suggests the existence of introns and/or multiple gene copies. Thus polypeptide 49, an immunodominant antigen, represents schistosome tropomyosin.

Regulation of tropomyosin expression in the maturing ovary and in primary granulosa cell cultures

Granulosa cell differentiation in vitro in response to gonadotropins is characterized by major changes in cell shape, cell aggregation, and the organization of microfilaments. These changes are associated with enhanced steroidogenesis in maturing granulosa-lutein cells. Since nonmuscle tropomyosin isoforms were implicated in stabilizing actin filaments, we studied the organization and expression of tropomyosin in differentiating primary cultures of rat granulosa cells and during ovarian folliculogenesis and luteinization. In unstimulated primary granulosa cell cultures tropomyosin was found mainly along stress fibers. In differentiating cells tropomyosin staining was diffuse with sometimes a subcortical organization. The changes in tropomyosin organization were accompanied by a pronounced decrease in the synthesis, translation in vitro, and mRNA levels of all the rat nonmuscle tropomyosin isoforms, with a greater reduction in the higher molecular weight isoforms than in the smaller isoforms. Similar results were obtained whether cells were stimulated to differentiate with gonadotropins, with cAMP, by culturing cells on an extracellular matrix, or by treatment with cytochalasin B. The effect of cytochalasin B was reversible; upon removal of the drug tropomyosin synthesis increased to near control levels, while that of proteins associated with luteinization decreased drastically. RNA isolated from ovaries with follicles at the preantral, preovulatory stage and from corpora lutea contained decreased tropomyosin mRNA levels during ovarian luteinization when the level of RNA for a key steroidogenic enzyme, cytochrome P-450 cholesterol side chain cleavage (P-450 scc), increased. The results suggest a physiological relevance for the low level of tropomyosin expression in the mechanisms which bring about the morphological and biochemical development and maturation of granulosa cells.

Disruption of the single tropomyosin gene in yeast results in the disappearance of actin cables from the cytoskeleton

The yeast tropomyosin gene, designated TPM1, is present in a single copy per haploid genome and encodes a protein with a predicted molecular weight of 23.5 kd. The protein sequence is homologous to higher cell tropomyosins, including the characteristic hydrophobic-hydrophilic pseudoheptapeptide repeats. Indirect immunofluorescence microscopy reveals that tropomyosin is localized with actin cables in wild-type cells. Disruption of TPM1 is not lethal, but results in a reduced growth rate and disappearance of actin cables. Strains carrying the conditional actin mutation act1-2 also lack actin cables; overexpression of tropomyosin in these strains partially restores actin cables. These results strongly suggest that tropomyosin interacts with F actin in vivo and may play an important role in assembling or stabilizing actin cables in yeast.

Organization of the hTMnm gene. Implications for the evolution of muscle and non-muscle tropomyosins

We have isolated clones of human genomic DNA which contain the structural elements of the hTMnm gene. In non-muscle tissue this gene produces a 2.5 kb (1 kb = 10(3) bases or base-pairs) mRNA encoding TM30nm, a 248 amino acid cytoskeletal tropomyosin. In muscle, alternative splicing of this gene results in the expression of a 1.3 kb mRNA encoding a 285 amino acid skeletal muscle alpha-tropomyosin. The hTMnm gene spans at least 42 kb of DNA and consists of 13 exons, only five of which are common to both the 2.5 kb and 1.3 kb transcripts. The boundaries of the exons giving rise to the muscle-specific isoform are identical to the base to those of other genes encoding muscle tropomyosins. A comparison of the structures of exons encoding the amino-terminal sequences of the muscle and non-muscle isoforms suggests that the hTMnm gene has evolved by a specific pattern of exon duplication with alternative splicing.