The complete amino acid sequence of actins from bovine aorta, bovine heart, bovine fast skeletal muscle, and rabbit slow skeletal muscle. A protein-chemical analysis of muscle actin differentiation

Nucleotide sequence and deduced amino acid sequence of a Plasmodium falciparum actin gene

The nucleotide sequence of a Plasmodium falciparum actin gene has been established. The gene codes for a protein of 376 amino acids and is not interrupted by introns. The nucleotide sequence reveals an extreme bias in codon usage. Not less than 85% of the codons possess an A or T at the third position. As has been found for the actins in other unicellular eukaryotes, P. falciparum actin is related both to vertebrate cytoplasmic and vertebrate muscle specific actins. However, the malarial actin is one of the most alpha-like actins hitherto found in lower eukaryotes.

ADP-ribosylation of skeletal muscle and non-muscle actin by Clostridium perfringens iota toxin

The enzymatically active component ia of Clostridium perfringens iota toxin ADP-ribosylated actin in human platelet cytosol and purified platelet beta/gamma-actin, in a similar way to that been reported for component I of botulinum C2 toxin. ADP-ribosylation of cytosolic and purified actin by either toxin was inhibited by 0.1 mM phalloidin indicating that monomeric G-actin but not polymerized F-actin was the toxin substrate. Perfringens iota toxin and botulinum C2 toxin were not additive in ADP-ribosylation of platelet actin. Treatment of intact chicken embryo cells with botulinum C2 toxin decreased subsequent ADP-ribosylation of actin in cell lysates by perfringens iota or botulinum C2 toxin. In contrast to botulinum C2 toxin, perfringens iota toxin ADP-ribosylated skeletal muscle alpha-actin with a potency and efficiency similar to non-muscle actin. ADP-ribosylation of purified skeletal muscle and non-muscle actin by perfringens iota toxin led to a dose-dependent impairment of the ability of actin to polymerize.

Unique isoactins in the brush border of rat intestinal epithelial cells

The mammalian genome contains 20-30 genes encoding a family of actins. To date, however, only six proteins (four muscle and two nonmuscle isoforms) encoded by this multigene complex have been identified. We have isolated two actins from the brush border of rat intestinal epithelial cells that have isoelectric points and N-terminal peptides characteristic of the cytoplasmic beta- and gamma-actins. However, using a panel of actin-specific monoclonal antibodies, we show that these actins contain a set of epitopes that distinguishes them from any of the known cytoplasmic or muscle isoforms. These unique actins share features of both the nonmuscle and muscle isoforms, suggesting that they represent an intermediate in the evolution of the specialized muscle actins.

The actin content of different muscles from beef and pork

In the literature 3-methylhistidine (3-MeHis) is mentioned as a parameter for the determination of meat content. Because of the variable molar ratio of 3-MeHis in the myofibrillar protein myosin, this indicator cannot be used without some restriction. However, the content of 3-MeHis in the myofibrillar protein actin is constant. Moreover, actin is relatively heat-stable in comparison with other muscle proteins. These facts made actin an interesting parameter for determination of the meat (protein) content of heated and raw meat products. Fast protein liquid chromatography (FPLC) was used to separate from a meat extract an actin-containing fraction without myosin. The actin content of the meat was calculated from its content in the fraction. The latter was determined by means of a 3-MeHis method. Several muscles of the beef and pig carcass were examined for their actin contents. Generally, the actin values were comparable with the data reported in the literature. The influence of heat treatment was also studied on one type of beef muscle. The determined actin content was not affected up to a temperature of 85°C.

Two monoclonal antibodies to actin: one muscle selective and one generally reactive

Two IgG1, kappa monoclonal antibodies (Mab) against actin have been obtained from a fusion in which chicken gizzard actin was used as the immunogen. One Mab, designated B4, shows a preferential reactivity toward enteric smooth muscle actin but also cross-reacts with skeletal, cardiac, and aorta actins on the basis of immunoblots, ELISA assays, and indirect immunofluorescence. However, this antibody does not react with either cytoplasmic actin in any of these assay systems. A second Mab, designated C4, reacts with all six known vertebrate isoactins as well as Dictyostelium discoideum and Physarum polycephalum actins. Thus B4 Mab appears to react with an epitope that is at least partially shared among the muscle actins but not found in cytoplasmic actins, while C4 Mab binds to an antigenic determinant that has been highly conserved among the actins. The binding sites of both Mabs on skeletal actin overlap that of pancreatic DNase I. Both antibodies bind a SV8 proteolytic product comprising the amino-terminal two-thirds of the actin molecule, and their epitopes appear to overlap since C4 can compete for the binding of B4 to skeletal actin. Neither antibody is able to prevent actin polymerization.

Development of a muscle actin specified by maternal and zygotic mRNA in ascidian embryos

In this investigation, we characterize the embryonic and adult actins and describe the embryonic expression of a muscle actin in the ascidian Styela. Two-dimensional polyacrylamide gel electrophoresis showed that embryos, tadpole larvae, and adult organs contain three major and two minor isoforms of actin. Two of the major isoforms, which are present in the mantle, branchial sac, alimentary tract, and gonads of adults and in eggs, embryos, and heads and tails of tadpoles, are likely to be cytoplasmic actins. The third major isoform, which was enriched in the mantle and branchial sac of adults and localized primarily in the tails of tadpoles, is a muscle actin. The muscle actin isoform was not detected in eggs and early embryos. Radioactivity incorporation studies showed that the cytoplasmic actins were synthesized throughout early development, but muscle actin synthesis was first detected between the 16- and 64-cell stages, 2-3 hr after fertilization. Two lines of evidence indicate that embryonic muscle actin synthesis is directed in part by maternal mRNA. First, poly(A)+ RNA isolated from unfertilized eggs directed the synthesis of muscle actin in an mRNA-dependent reticulocyte lysate. Second, muscle actin was synthesized in anucleate egg fragments. Arguments are also presented that muscle actin synthesis is not directed exclusively by maternal mRNA. It is concluded that embryonic and adult Styela exhibit actin heterogeneity, that one of the actin isoforms is a muscle actin, and that the muscle actin is synthesized during embryogenesis under the direction of maternal and zygotic mRNA.

Fetal expression of muscle-specific isoactins in multiple organs of the Wistar-Kyoto rat

Actin, a cytoskeletal and contractile protein, is expressed in six different isoforms that exhibit striking specificity. No studies have considered the muscle-specific actin expression in multiple organ systems in the intact fetus. Using a monoclonal antibody (B4) which reacts specifically with the isoactins of the smooth and skeletal muscle our immunohistochemical study examined whole fetal body sections to follow the development of actin expression throughout the last third of gestation in the Wistar-Kyoto rat. B4 staining was exclusively localized to muscle, confirming its high specificity and its usefulness for studying the ontogeny of muscle-specific isoactins. At 15 days of gestation, B4 staining was detected in the heart, the thoracic aorta and the skeletal muscle of the chest wall. The distribution and intensity of staining in the heart were initially higher than in the aorta or skeletal muscle and remained unchanged throughout the remainder of gestation, suggesting that the maturation of cardiac actin expression is well developed, although not fully completed before birth. Expression of muscle-specific actins in skeletal muscle was age-dependent and correlated with the maturational changes of muscle cell precursors. B4 staining in the fetal kidney was not apparent until day 20 of gestation and was localized to the inner cortical vessels. in association with the most mature nephrons, suggesting a centrifugal maturation of the intrarenal vasculature. The intensity of B4 staining in most tissues including bronchi, bowel, diaphragm, chest wall muscle and peripheral and pulmonary arteries increased by the end of gestation.

A single intronless action gene in the fission yeast Schizosaccharomyces pombe: nucleotide sequence and transcripts formed in homologous and heterologous yeast

The actin gene of the fission yeast Schizosaccharomyces pombe has been isolated by using as a hybridization probe cloned actin DNA from the budding yeast Saccharomyces cerevisiae. In contrast to most actin genes studied from diverse eukaryotic species, the S. pombe gene is not interrupted by introns. The protein sequence deduced from the nucleotide sequence of the gene shows that the S. pombe actin is more closely related to the mammalian gamma-actin than to the actin of S. cerevisiae. Three transcripts of 1240, 1650 and 1850 nucleotides having the same 5' end but differing in the length of their 3' untranslated region are generated in the fission yeast. Only one messenger RNA of 1330 nucleotides is formed from the S. pombe actin gene in S. cerevisiae. Contrary to the observation made with other S. pombe genes transcribed in the budding yeast, the heterologous actin gene transcript is initiated 39 nucleotides upstream of the initiation start site used in the homologous yeast. The mRNA termination (or 3' processing) mechanism in the two ascomycetes also differs as the 3'end of the S. pombe actin gene transcript in S. cerevisiae does not coincide with either of the three 3'ends mapped in the fission yeast.

Analysis of alpha-smooth-muscle actin mRNA expression in rat aortic smooth-muscle cells using a specific cDNA probe

We constructed two cDNA probes, the first of which hybridizes with all rat actin mRNAs while the second is specific for alpha-smooth muscle (SM) actin mRNA. Northern hybridization using these probes showed that, in normal rat aortic media, the proportion of alpha-SM actin mRNA expression increases during development, reaching about 90% of the total actin mRNA level in adult animals. As compared to the situation in normal aortic media, the proportion of alpha-SM actin mRNA was found to decrease significantly in intimal thickening 15 days after endothelial injury, i.e. when SM cells (SMCs) are actively replicating. At 60 days after injury, the SMCs were observed to have stopped dividing and to have recovered a normal content of alpha-SM actin mRNA. The content of alpha-SM actin mRNA was also selectively decreased (as compared to controls) in the hypotensive abdominal aortic media located below an aortic ligature, while it was not modified in the thoracic hypertensive segment above the same ligature. Primary cultures of rat aortic SMCs synthesize and contain low amounts of alpha-SM actin, but their alpha-SM actin mRNA content is similar to that of SMCs in vivo. As compared to primary cultures, the proportion of alpha-SM actin mRNA was found to be significantly decreased in SMCs at the fifth passage, at which stage it became comparable to the level of synthesized alpha-SM actin. Thus, the synthesis and expression of alpha-SM actin in SMCs appear to be regulated predominantly at the level of gene transcription in certain situations (e.g. aortic ligature in vivo and culture at the fifth passage), and predominantly at a post-transcriptional level in other situations (e.g. primary culture).

Identification and quantification of actin isoforms in vertebrate cells and tissues

The cytoskeletal protein actin exists in vertebrates as six different isoforms, which are difficult to identify conclusively because of a high degree (greater than 90%) of overall sequence homology. We have used IEF immunoblotting in combination with a panel of isoform-specific and -selective antibodies to analyze the actin isoform composition of nine tissues from adult rat. In three nonmuscle tissues (lung, spleen, and testis), we detected a previously unreported isoform that we identified as smooth muscle alpha. The IEF immunoblot technique was also used to quantify the proportions of the isoforms expressed in these nine rat tissues.

Gamma-actin: unusual mRNA 3'-untranslated sequence conservation and amino acid substitutions that may be cancer related

beta-Actin mutations in chemically transformed human cell lines have been associated with tumorigenicity, an association consistent with other evidence suggesting that altered cytoskeletal proteins may have an important role in cancer initiation or progression. From a human promyelocytic leukemia cell line, we have isolated a gamma-actin cDNA clone with amino acid substitutions in a region highly conserved in the many actins analyzed. To our knowledge, this is the first example of a variant gamma-actin in a human neoplasm. A separate finding from the analysis of this clone is that the gamma-actin 3'-untranslated region is among the most highly conserved of all 3'-untranslated sequences so far reported, but is entirely different from the beta-actin 3'-untranslated region. The high degree of evolutionary conservation suggests that the 3'-untranslated regions of these two mRNAs have important and distinct functional roles that were already fully differentiated more than 100 million years ago. Mutations affecting four major cytoskeletal components have now been identified in human neoplastic cells. These findings suggest that mutated cytoskeletal genes may be members of a class of oncogenes, fundamentally different from both the nuclear-acting (e.g., myc and simian virus 40 large tumor antigen) and growth factor/receptor/protein kinase-related (e.g., sis, erbB, and ras) types of oncogenes.

Fluorescence resonance energy transfer measurements of distances in actin and myosin. A critical evaluation

The contractile proteins actin and myosin are of considerable biological interest. They are essential for muscle contraction and in eukaryotic cells they play a crucial role in most contractile phenomena. Over the years since the first fluorescence resonance energy transfer (FRET) paper appeared, an extensive body of literature has accumulated on this technique using actin, myosin and the actomyosin complex. These papers are reviewed with several aims in mind: we assess the reliability and consistency of intra- and inter-molecular distances measured between the fluorescent probes attached to specific sites on these proteins; we determine whether the measurements can be assembled into an internally consistent model which can be fitted to the known dimensions of the actomyosin complex; several of the FRET distances are consistent with the available structural data from crystallographic and electron microscopic dimensions; the modelled FRET distances suggest that the assumed value of the orientation factor (k2 = 2/3) is reasonable; we conclude that the model has a predictive value, i.e. it suggests that a small number of the published dimensions may be incorrect and predicts the magnitude of a larger number of measurements which have not yet been reported; and finally (vi) we discuss the contribution of FRET determinations to the current debate on the molecular mechanism of contraction.

Structure and organization of the CyIII actin gene subfamily of the sea urchin, Strongylocentrotus purpuratus

We describe here the organization of the CyIII subfamily of cytoskeletal actin genes in the sea urchin Strongylocentrotus purpuratus. The functional genes CyIIIa and CyIIIb are linked at a 6 X 10(3)-base distance. Gene CyIIIc appears to be a pseudogene that lacks 5' exons and displays unselected mutational changes. Gene CyIIIa codes for a protein that differs at only nine out of 376 residues from that coded by another cytoskeletal actin gene, CyI. However, five of these nine changes occur within an 11-amino acid region that could represent a functional specialization of the CyIIIa actin protein. The CyIIIa gene possesses three introns, located, respectively, 25 nucleotides upstream from the translation start site, between the codons for amino acids 121 and 122, and within the codon for amino acid 204. These intron positions have also been observed in other cytoskeletal sea urchin actin genes. Comparison of both intron and 3'-terminal sequences shows that the CyIIIa and CyIIIb genes are closely related, while no homology in these untranslated sequences is observed between the CyIII genes and the other cytoskeletal actin genes of the S. purpuratus genome. The CyIII genes probably arose by duplication events at least 40 X 10(6) years ago, prior to radiation of the genus Strongylocentrotus. Consideration of the biological role of the embryo and larval aboral ectoderm cells to which CyIIIa and CyIIIb transcripts are confined suggests that these actins might contribute to cytoskeletal elements that endow the larval body wall with its rigid structure.

Actin antibodies. Preparation and characterization of antibodies specific for smooth-muscle actin isoforms

We have determined the specificity of sera elicited by glutaraldehyde-stabilized bovine aortic actin. This modification induces a high titre of antibodies directed against the N-terminal (residues 1-39) and the C-terminal region of smooth-muscle actins. The crude antisera were purified on peptide (corresponding to the 1-9 or 1-8 N-terminal sequences of smooth-muscle isoactins)-polyacrylic-resin columns. By fractionating the antisera we obtained oligoclonal antibody populations specific for each isoactin.

Relation of streptococcal M protein with human and rabbit tropomyosin: the complete amino acid sequence of human cardiac alpha tropomyosin, a highly conserved contractile protein

Partial sequences of group A streptococcal M proteins exhibit up to 50% sequence identity with segments of rabbit skeletal tropomyosin. It is well recognized that rheumatic fever and rheumatic heart disease in humans are sequelae of group A streptococcal infection. To examine whether the human cardiac tropomyosin would exhibit greater homology with the streptococcal M proteins, we have now determined its complete amino acid sequence. The amino acid sequence of human cardiac tropomyosin was established from sequence analyses of its peptides derived by enzymic and chemical cleavages, and comparison of these sequences to the reported sequence of rabbit skeletal tropomyosin. These studies have revealed that the amino acid sequence of human cardiac alpha tropomyosin is identical to that of the rabbit skeletal alpha tropomyosin, but for a single conservative substitution of Arg/Lys at position 220. This observation increases the significance of the previously observed sequence homology between streptococcal M protein and rabbit skeletal tropomyosin and may have relevance to the pathogenesis of rheumatic fever. Furthermore, these results rank tropomyosin as one of the most highly conserved contractile proteins between vertebrate species reported thus far.

Localization of the phalloidin and nucleotide-binding sites on actin

Phalloidin was found to block nucleotide exchange in F-actin, without interfering with nucleotide hydrolysis. This inhibition of nucleotide exchange occurs under conditions in which monomers are able to exchange. The distance separating a fluorescent chromophore attached to phalloidin from the nucleotide on actin was determined using fluorescence resonance energy-transfer spectroscopy. They are separated by less than 1.0 nm. Added confirmation of the close proximity of phalloidin to nucleotide was obtained by extracting a small peptide-ATP complex from an actin digest. The peptide comprises residues 114-118, which are from the same region as the residues that others have shown to crosslink to phalloidin [Vandekerckhove et al. (1985) EMBO J. 4, 2815-2818]. The results suggest that phalloidin has two major effects. It traps actin monomers in a conformation which appears to be distinct from G-actin and it stabilizes the structure of F-actin, an event accompanied by the trapping of ADP.

Actin-isoform pattern as a marker of normal or pathological smooth-muscle and fibroblastic tissues

The relative proportions of actin isoforms present in smooth-muscle (SM) and fibroblastic human and non-human tissue extracts were examined by densitometric evaluation of Coomassie-Blue-stained spots in two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) as well as by quantification of radiolabeled actin NH2-terminal peptide spots separated by two-dimensional paper electrophoresis. SM tissues contained alpha- and gamma-SM as well as beta- and gamma-cytoplasmic (CY) actins in different proportions in different organs. Species differences with respect to the ratios of the isoactins were also observed. Moreover, during pregnancy, both human and rat myometrium exhibited a changed actin-isoform pattern, there being an increased proportion of gamma-actin. Analysis of the NH2-terminal peptides showed that, in human myometrium, this was essentially due to an increase in the amount of the gamma-SM isoform. Fibroblastic tissues were found to contain only the beta- and gamma-CY isoforms, the ratio being approximately 2.6:1. Thus, the presence or absence of alpha-actin provides a reliable biochemical criterion for distinguishing between fibroblastic and SM cell populations and/or tissues. This distinction and the evaluation of changes in isoactin ratios may be useful in the study of differentiation as well as physiological and pathological phenomena, and for determining the origin of certain soft-tissue tumours.

Tropomyosin isoform switching in tumorigenic human fibroblasts

We identified six tropomyosin (Tm) isoforms in diploid human fibroblasts. We used computerized microdensitometry of 2-dimensional protein profiles to measure the relative rates of synthesis and abundance of the individual Tm isoforms and actin, the two major structural constituents of microfilaments. In carcinogen-transformed human fibroblasts (HuT-14), the rates of synthesis of three Tm isoforms (Tm1, Tm2, and Tm6) were greatly decreased relative to normal diploid parental fibroblasts and to actin. In contrast, related nontumorigenic HuT fibroblasts which are "immortalized" and anchorage independent exhibited both slight down-regulation of Tm1 and Tm6 and 3.5-fold up-regulation of Tm3. Thus, Tm isoform switching from the predominance of the larger more avid Tm isoforms (Tm1, Tm2, Tm3, and Tm6) to the smaller, less avid Tm isoforms (Tm4 and Tm5) in microfilaments was a transformation-induced change correlated with tumorigenicity in human fibroblasts.

The complete sequence of the mouse skeletal alpha-actin gene reveals several conserved and inverted repeat sequences outside of the protein-coding region

The complete nucleotide sequence of a genomic clone encoding the mouse skeletal alpha-actin gene has been determined. This single-copy gene codes for a protein identical in primary sequence to the rabbit skeletal alpha-actin. It has a large intron in the 5'-untranslated region 12 nucleotides upstream from the initiator ATG and five small introns in the coding region at codons specifying amino acids 41/42, 150, 204, 267, and 327/328. These intron positions are identical to those for the corresponding genes of chickens and rats. Similar to other skeletal alpha-actin genes, the nucleotide sequence codes for two amino acids, Met-Cys, preceding the known N-terminal Asp of the mature protein. Comparison of the nucleotide sequences of rat, mouse, chicken, and human skeletal muscle alpha-actin genes reveals conserved sequences (some not previously noted) outside of the protein-coding region. Furthermore, several inverted repeat sequences, partially within these conserved regions, have been identified. These sequences are not present in the vertebrate cytoskeletal beta-actin genes. The strong conservation of the inverted repeat sequences suggests that they may have a role in the tissue-specific expression of skeletal alpha-actin genes.

Upstream regions of the human cardiac actin gene that modulate its transcription in muscle cells: presence of an evolutionarily conserved repeated motif

Transfection into cultured cell lines was used to investigate the transcriptional regulation of the human cardiac actin gene. We first demonstrated that in both human heart and human skeletal muscle, cardiac actin mRNAs initiate at the identical site and contain the same first exon, which is separated from the first coding exon by an intron of 700 base pairs. A region of 485 base pairs upstream from the transcription initiation site of the human cardiac actin gene directs high-level transient expression of the bacterial chloramphenicol acetyltransferase gene in differentiated myotubes of the mouse C2C12 muscle cell line, but not in mouse L fibroblast or rat PC-G2 pheochromocytoma cells. Deletion analysis of this region showed that at least two physically separated sequence elements are involved, a distal one starting between -443 and -395 and a proximal one starting between -177 and -118, and suggested that these sequences interact with positively acting transcriptional factors in muscle cells. When these two sequence elements are inserted separately upstream of a heterologous (simian virus 40) promoter, they do not affect transcription but do give a small (four- to fivefold) stimulation when tested together. Overall, these regulatory regions upstream of the cap site of the human cardiac actin gene show remarkably high sequence conservation with the equivalent regions of the mouse and chick genes. Furthermore, there is an evolutionarily conserved repeated motif that may be important in the transcriptional regulation of actin and other contractile protein genes.

A monoclonal antibody against alpha-smooth muscle actin: a new probe for smooth muscle differentiation

A monoclonal antibody (anti-alpha sm-1) recognizing exclusively alpha-smooth muscle actin was selected and characterized after immunization of BALB/c mice with the NH2-terminal synthetic decapeptide of alpha-smooth muscle actin coupled to keyhole limpet hemocyanin. Anti-alpha sm-1 helped in distinguishing smooth muscle cells from fibroblasts in mixed cultures such as rat dermal fibroblasts and chicken embryo fibroblasts. In the aortic media, it recognized a hitherto unknown population of cells negative for alpha-smooth muscle actin and for desmin. In 5-d-old rats, this population is about half of the medial cells and becomes only 8 +/- 5% in 6-wk-old animals. In cultures of rat aortic media SMCs, there is a progressive increase of this cell population together with a progressive decrease in the number of alpha-smooth muscle actin-containing stress fibers per cell. Double immunofluorescent studies carried out with anti-alpha sm-1 and anti-desmin antibodies in several organs revealed a heterogeneity of stromal cells. Desmin-negative, alpha-smooth muscle actin-positive cells were found in the rat intestinal muscularis mucosae and in the dermis around hair follicles. Moreover, desmin-positive, alpha-smooth muscle actin-negative cells were identified in the intestinal submucosa, rat testis interstitium, and uterine stroma. alpha-Smooth muscle actin was also found in myoepithelial cells of mammary and salivary glands, which are known to express cytokeratins. Finally, alpha-smooth muscle actin is present in stromal cells of mammary carcinomas, previously considered fibroblastic in nature. Thus, anti-alpha sm-1 antibody appears to be a powerful probe in the study of smooth muscle differentiation in normal and pathological conditions.

Gene reorganization during serial divisions of normal human cells

We have followed during serial divisions of human fibroblasts the presence in chromosomal and extrachromosomal DNA, of two genes that are expressed in fibroblasts, actin and interferon, and of one that is not expressed, globin. The intensity of the blot hybridization of the actin and globin probes with chromosomal DNA diminished during serial divisions of diploid fibroblasts. The interferon gene remained constant throughout the human fibroblast life span. Chromosomal DNA sequences were present in extrachromosomal circular DNA which appeared at the end of the fibroblast life span. The results could explain some functional changes that occur in these cell populations when their division potential declines.

Extraction of myosin light chains and actin from bovine cardiac muscle acetone powder

A joint preparation of the two myosin light chains and actin from bovine cardiac muscle acetone powder is described. There is a significant improvement in the ease of purification, while the yield of the myosin light chains equals the best yields obtained from the use of established techniques. The actin yield greatly exceeds that obtained in an earlier published report.

Alpha-skeletal muscle actin mRNA's accumulate in hypertrophied adult rat hearts

Cardiac hypertrophy due to a chronic hemodynamic overload is accompanied by isoformic changes of two proteins of the thick filament of the sarcomere, myosin, and creatine phosphokinase. We have looked for isoactin changes, using deoxyribonucleic acid probes complementary to alpha-skeletal and alpha-cardiac actin messenger ribonucleic acids. Three groups of rats were studied at various days after application of a pressure overload (2-4 days, n = 13, 8-15 days, n = 5, and 30-40 days, n = 7) and were compared to control animals (n = 11). Whereas alpha-skeletal actin messenger ribonucleic acids were hardly detectable in the normal hearts (0.6 +/- 0.16%), they accumulated significantly in the first 4 days after the aortic stenosis (4.6 +/- 3.1%, p less than 0.001 vs. controls) and then slowly declined (8-15 days, 3.2 +/- 1.7% and 30-40 days, 1.6 +/- 0.6%, p less than 0.05 and NS vs. controls). This figure is similar to that observed in 8-day-old rats (2.27 +/- 0.3%, p less than 0.01 vs. controls). We conclude that, in rat myocardium, the expression of messenger ribonucleic acids encoding the sarcomeric actins is altered at the onset of a pressure overload hypertrophy. Although the physiological significance of isoactin changes is unknown, our results show that the thin filament participates as well as the thick filament in the response of cardiac muscle to new functional requirements.

Evolution of the functional human beta-actin gene and its multi-pseudogene family: conservation of noncoding regions and chromosomal dispersion of pseudogenes

We have assigned six members of the human beta-actin multigene family to specific human chromosomes. The functional gene, ACTB, is located on human chromosome 7, and the other assigned beta-actin-related sequences are dispersed over at least four different chromosomes including one locus assigned to the X chromosome. Using intervening sequence probes, we showed that the functional gene is single copy and that all of the other beta-actin related sequences are recently generated in evolution and are probably processed pseudogenes. The entire nucleotide sequence of the functional gene has been determined and is identical to cDNA clones in the coding and 5' untranslated regions. We have previously reported that the 3' untranslated region is well conserved between humans and rats (Ponte et al., Nucleic Acids Res. 12:1687-1696, 1984). Now we report that four additional noncoding regions are evolutionarily conserved, including segments of the 5' flanking region, 5' untranslated region, and, surprisingly, intervening sequences I and III. These conserved sequences, especially those found in the introns, suggest a role for internal sequences in the regulation of beta-actin gene expression.

The structure of a cDNA clone corresponding to mouse cardiac muscle actin mRNA

A cDNA library was constructed from mouse cardiac muscle mRNA, and a clone corresponding to part of the mRNA for the cardiac muscle isoform of actin was isolated from this library. The nucleotide sequence of the cloned insert was determined and was found to contain almost the complete amino acid coding region for actin (only codons for the first two amino acids, absent from the mature protein, were lacking) and a substantial portion derived from the 3' untranslated region of the mRNA. Comparison of the latter with the corresponding region in cardiac actin mRNA from man and rat showed that this 3' untranslated region has been subject to conservational pressure during evolution. However a comparison with the corresponding region in skeletal muscle actin mRNAs indicated that the pattern of conservation is quite different in the two striated muscle actin isoforms.

Fluorescence resonance energy transfer between the nucleotide binding site and Cys-10 in G-actin and F-actin

Intramonomer fluorescence resonance energy transfer between the donor epsilon-ATP bound to the nucleotide site and the acceptor N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (DDPM) or 4-dimethylaminophenyl-azophenyl-4'-maleimide bound to Cys-10 in G-actin was measured. The donor-acceptor distance was calculated to be about 40 A. The intermonomer energy transfer in F-actin occurring between epsilon-ADP and DABMI was also measured. The radial coordinate of Cys-10 was calculated to be 25 A based on the helical symmetry of F-actin and the recently calculated radial coordinate of the nucleotide binding site in F-actin i.e. 25 A (Miki, M., Hambly, B. and dos Remedios, C.G. (1986) Biochim. Biophys. Acta 871, 137-141). (The assumption has been made in calculating these distances that the energy donor and acceptor rotate rapidly relative to the fluorescence lifetime.) Corresponding distances separating the donor nucleotide in one monomer from acceptors on Cys-10 in the first and second nearest neighbours in F-actin are 39-40 A and 41-43 A.

Tropomyosin isoform switching in tumorigenic human fibroblasts

We identified six tropomyosin (Tm) isoforms in diploid human fibroblasts. We used computerized microdensitometry of 2-dimensional protein profiles to measure the relative rates of synthesis and abundance of the individual Tm isoforms and actin, the two major structural constituents of microfilaments. In carcinogen-transformed human fibroblasts (HuT-14), the rates of synthesis of three Tm isoforms (Tm1, Tm2, and Tm6) were greatly decreased relative to normal diploid parental fibroblasts and to actin. In contrast, related nontumorigenic HuT fibroblasts which are "immortalized" and anchorage independent exhibited both slight down-regulation of Tm1 and Tm6 and 3.5-fold up-regulation of Tm3. Thus, Tm isoform switching from the predominance of the larger more avid Tm isoforms (Tm1, Tm2, Tm3, and Tm6) to the smaller, less avid Tm isoforms (Tm4 and Tm5) in microfilaments was a transformation-induced change correlated with tumorigenicity in human fibroblasts.

Sequence homologies of glucose-dehydrogenases of Bacillus megaterium and Bacillus subtilis

The sequence homologies of the glucose dehydrogenase subunits of B. megaterium and B. subtilis are compared. From the known B. megaterium aminoacid sequence and the base sequence of the cloned B. subtilis structural gene we predict the B. megaterium structural glucose dehydrogenase gene. Assuming the minimal mutational changes to convert one gene into the other 23 transitions, 30 transversions, 1 inversion, 3 insertion-deletions, but no frameshifts are postulated necessary to interconvert the structural genes. The homology of both enzyme subunits of 85% reflects the close evolutionary distance between B. subtilis and B. megaterium.

Isolation and characterization of cDNA clones from mouse skeletal muscle actin mRNA

The sequence corresponding to approximately 98% of mouse skeletal muscle actin mRNA was determined from cDNA clones isolated from a library of recombinants in pBR322. One of these clones contains DNA corresponding to the complete amino acid coding region and a large part of the 5' and 3' untranslated regions of the mRNA. Comparison of the mouse coding region (conserved at the amino acid level) and noncoding regions with the corresponding regions of the rat skeletal muscle actin gene indicates that the noncoding regions have also been under selective pressure during evolution.

Upstream regions of the human cardiac actin gene that modulate its transcription in muscle cells: presence of an evolutionarily conserved repeated motif

Transfection into cultured cell lines was used to investigate the transcriptional regulation of the human cardiac actin gene. We first demonstrated that in both human heart and human skeletal muscle, cardiac actin mRNAs initiate at the identical site and contain the same first exon, which is separated from the first coding exon by an intron of 700 base pairs. A region of 485 base pairs upstream from the transcription initiation site of the human cardiac actin gene directs high-level transient expression of the bacterial chloramphenicol acetyltransferase gene in differentiated myotubes of the mouse C2C12 muscle cell line, but not in mouse L fibroblast or rat PC-G2 pheochromocytoma cells. Deletion analysis of this region showed that at least two physically separated sequence elements are involved, a distal one starting between -443 and -395 and a proximal one starting between -177 and -118, and suggested that these sequences interact with positively acting transcriptional factors in muscle cells. When these two sequence elements are inserted separately upstream of a heterologous (simian virus 40) promoter, they do not affect transcription but do give a small (four- to fivefold) stimulation when tested together. Overall, these regulatory regions upstream of the cap site of the human cardiac actin gene show remarkably high sequence conservation with the equivalent regions of the mouse and chick genes. Furthermore, there is an evolutionarily conserved repeated motif that may be important in the transcriptional regulation of actin and other contractile protein genes.

Sequential expression of chicken actin genes during myogenesis

Embryonic muscle development permits the study of contractile protein gene regulation during cellular differentiation. To distinguish the appearance of particular actin mRNAs during chicken myogenesis, we have constructed DNA probes from the transcribed 3' noncoding region of the single-copy alpha-skeletal, alpha-cardiac, and beta-cytoplasmic actin genes. Hybridization experiments showed that at day 10 in ovo (stage 36), embryonic hindlimbs contain low levels of actin mRNA, predominantly consisting of the alpha-cardiac and beta-actin isotypes. However, by day 17 in ovo (stage 43), the amount of alpha-skeletal actin mRNA/microgram total RNA increased more than 30-fold and represented approximately 90% of the assayed actin mRNA. Concomitantly, alpha-cardiac and beta-actin mRNAs decreased by 30% and 70%, respectively, from the levels observed at day 10. In primary myoblast cultures, beta-actin mRNA increased sharply during the proliferative phase before fusion and steadily declined thereafter. alpha-Cardiac actin mRNA increased to levels 15-fold greater than alpha-skeletal actin mRNA in prefusion myoblasts (36 h), and remained at elevated levels. In contrast, the alpha-skeletal actin mRNA remained low until fusion had begun (48 h), increased 25-fold over the prefusion level by the completion of fusion, and then decreased at later times in culture. Thus, the sequential accumulation of sarcomeric alpha-actin mRNAs in culture mimics some of the events observed in embryonic limb development. However, maintenance of high levels of alpha-cardiac actin mRNA as well as the transient accumulation of appreciable alpha-skeletal actin mRNA suggests that myoblast cultures lack one or more essential components for phenotypic maturation.

Comparison of three actin-coding sequences in the mouse; evolutionary relationships between the actin genes of warm-blooded vertebrates

We have determined the sequences of three recombinant cDNAs complementary to different mouse actin mRNAs that contain more than 90% of the coding sequences and complete or partial 3' untranslated regions (3'UTRs): pAM 91, complementary to the actin mRNA expressed in adult skeletal muscle (alpha sk actin); pAF 81, complementary to an actin mRNA that is accumulated in fetal skeletal muscle and is the major transcript in adult cardiac muscle (alpha c actin); and pAL 41, identified as complementary to a beta nonmuscle actin mRNA on the basis of its 3'UTR sequence. As in other species, the protein sequences of these isoforms are highly (greater than 93%) conserved, but the three mRNAs show significant divergence (13.8-16.5%) at silent nucleotide positions in their coding regions. A nucleotide region located toward the 5' end shows significantly less divergence (5.6-8.7%) among the three mouse actin mRNAs; a second region, near the 3' end, also shows less divergence (6.9%), in this case between the mouse beta and alpha sk actin mRNAs. We propose that recombinational events between actin sequences may have homogenized these regions. Such events distort the calculated evolutionary distances between sequences within a species. Codon usage in the three actin mRNAs is clearly different, and indicates that there is no strict relation between the tissue type, and hence the tRNA precursor pool, and codon usage in these and other muscle mRNAs examined. Analysis of codon usage in these coding sequences in different vertebrate species indicates two tendencies: increases in bias toward the use of G and C in the third codon position in paralogous comparisons (in the order alpha c less than beta less than alpha sk), and in orthologous comparisons (in the order chicken less than rodent less than man). Comparison of actin-coding sequences between species was carried out using the Perler method of analysis. As one moves backward in time, changes at silent sites first accumulate rapidly, then begin to saturate after -(30-40) million years (MY), and actually decrease between -400 and -500 MY. Replacements or silent substitutions therefore cannot be used as evolutionary clocks for these sequences over long periods. Other phenomena, such as gene conversion or isochore compartmentalization, probably distort the estimated divergence time.

Monoclonal antibodies distinguish titins from heart and skeletal muscle

Murine monoclonal antibodies specific for titin have been elicited using a chicken heart muscle residue as antigen. The three antibodies T1, T3, and T4 recognize both bands of the titin doublet in immunoblot analysis on polypeptides from chicken breast muscle. In contrast, on chicken cardiac myofibrils two of the antibodies (T1, T4) react only with the upper band of the doublet indicating immunological differences between heart and skeletal muscle titin. This difference is even more pronounced for rat and mouse. Although all three antibodies react with skeletal muscle titin, T1 and T4 did not detect heart titin, whereas T3 reacts with this titin both in immunofluorescence microscopy and in immunoblots. Immunofluorescence microscopy of myofibrils and frozen tissues from a variety of vertebrates extends these results and shows that the three antibodies recognize different epitopes. All three titin antibodies decorate at the A-I junction of the myofibrils freshly prepared from chicken skeletal muscle and immunoelectron microscopy using native myosin filaments demonstrates that titin is present at the ends of the thick filaments. In chicken heart, however, antibodies T1 and T4 stain within the I-band rather than at the A-I junction. The three antibodies did not react with any of the nonmuscle tissues or permanent cell lines tested and do not decorate smooth muscle. In primary cultures of embryonic chicken skeletal muscle cells titin first appears as longitudinal striations in mononucleated myoblasts and later at the myofibrillar A-I junction of the myotubes.

Isolation and characterization of sarcomeric actin genes expressed in Xenopus laevis embryos

A Xenopus laevis complementary DNA (cDNA) library prepared from messenger RNAs extracted from embryos has been screened for actin-coding sequences. Two cDNA clones corresponding to an alpha cardiac and an alpha skeletal muscle actin mRNA have been identified and characterized. From a genomic library, we have furthermore isolated the genes that correspond to the characterized cDNAs. In addition we have identified an actin processed gene which seems to be derived from a second type of skeletal muscle actin gene. Southern blot analysis of X. laevis DNA reveals that each of the three genes is present in at least two copies. In Xenopus tropicalis, a similar Southern blot analysis demonstrates that the three alpha actin genes exist as single copy. This result correlates with the genome duplication that has been proposed to have occurred recently in a X. laevis ancestor. A sequence comparison of the X. laevis cardiac and skeletal muscle actin cDNAs shows that the encoded peptides are highly conserved. Nevertheless, the numerous nucleotide changes at silent mutation sites suggest that the genes originated before the amphibia/reptile-bird divergence, more than 350 million years ago. Comparison of the promoters of the cardiac and skeletal actin genes, which are co-expressed in embryos, reveals a few common structural sequence elements.

Expression of smooth muscle-specific alpha-isoactin in cultured vascular smooth muscle cells: relationship between growth and cytodifferentiation

The relationship between growth and cytodifferentiation was studied in cultured rat aortic smooth muscle cells (SMCs) using expression of the smooth muscle (SM)-specific isoactins (Vanderkerckhove, J., and K. Weber, 1979, Differentiation, 14:123-133) as a marker for differentiation in these cells. Isoactin expression was evaluated by: (a) measurements of fractional isoactin content and synthesis ([35S]methionine incorporation) by densitometric evaluation of two-dimensional isoelectric focusing sodium dodecyl sulfate gels, and (b) immunocytological examination using SM-specific isoactin antibodies. Results showed the following: (a) Loss of alpha-SM isoactin was not a prerequisite for initiation of cellular proliferation in primary cultures of rat aortic SMCs. (b) alpha-SM isoactin synthesis and content were low in subconfluent log phase growth cells but increased nearly threefold in density-arrested postconfluent cells. Conversely, beta-nonmuscle actin synthesis and content were higher in rapidly dividing subconfluent cultures than in quiescent postconfluent cultures. These changes were observed in primary and subpassaged cultures. (c) alpha-SM actin synthesis was increased by growth arrest of sparse cultures in serum-free medium (SFM; Libby, P., and K. V. O'Brien, 1983, J. Cell. Physiol., 115:217-223) but reached levels equivalent to density-arrested cells only after extended periods in SFM (i.e., greater than 5 d). (d) SFM did not further augment alpha-SM actin synthesis in postconfluent SMC cultures. (e) Serum stimulation of cells that had been growth-arrested in SFM resulted in a dramatic decrease in alpha-SM actin synthesis that preceded the onset of cellular proliferation. These findings demonstrate that cultured vascular SMCs undergo differential expression of isoactins in relation to their growth state and indicate that growth arrest promotes cytodifferentiation in these cells.