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

Restriction fragment length polymorphism among Israeli Holstein-Friesian dairy bulls

Israeli Holstein-Friesian dairy bulls were screened for restriction fragment length polymorphisms by hybridizing cloned DNA probes for bovine growth hormone, for chymosin, and for rat muscle beta-actin to restriction endonuclease-digested DNA immobilized on nitrocellulose filters. The population proved to be polymorphic at the growth hormone locus, with evidence consistent with the phenotypes being inherited in allelic fashion. A low level of polymorphism was also observed at one of the beta-actin gene family loci. The chymosin locus was monomorphic with the restriction enzymes utilized. The results illustrate the power of restriction fragment length polymorphism methodology in visualizing genetic variability in dairy cattle populations.

Anti-actin antibodies. Chemical modification allows the selective production of antibodies to the N-terminal region

The specific properties of sera elicited by various native, unfolded or chemically modified actins were compared to provide a means of obtaining high titres of antibodies directed against the N-terminal (1-39) or the central regions of the actin sequence. The antigenic structure of the N-terminal region of actin was analyzed. It has at least 2 discrete epitopes, one of which appears to be species-specific and is composed of the hydrophilic N-terminal heptapeptide sequence.

Structural variations in actins. A study of the immunological reactivity of the N-terminal region

The antigenicity of the N-terminal region of skeletal-muscle actin was analysed. Two epitopes, corresponding to the 1-7 and 18-28 sequences, were determined. The antibodies specific for the first epitope discriminate skeletal-muscle actin from cardiac-muscle and smooth-muscle actins. The antibodies specific for the second epitope interact with all the actins tested, ranging from invertebrate to higher-vertebrate actins.

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.

Identification of amino acid substitutions differentiating actin isoforms in their interaction with myosin

Various aspects of actin--myosin interaction were studied with actin preparations from two types of smooth muscle: bovine aorta and chicken gizzard, and from two types of sarcomeric muscle: bovine cardiac and rabbit skeletal. All four preparations activated the Mg2+-ATPase activity of skeletal muscle myosin to the same Vmax, but the Kapp for the smooth muscle preparations was higher. At low KCl, pH 8.0 and millimolar substrate concentrations the Kapp values differed by a factor of 2.5. This differential behaviour of the four actin preparations correlates with amino acid substitutions at positions 17 and 89 of actin polypeptide chain, differentiating the smooth-muscle-specific gamma and alpha isomers from cardiac and skeletal-muscle-specific alpha isomers. This correlation provides evidence for involvement of the NH2-terminal portion of the actin polypeptide chain in the interaction with myosin. The differences in the activation of myosin ATPase by various actins were sensitive to changes in the substrate and KCl concentration and pH of the assay medium. Addition of myosin subfragment-1 or heavy meromyosin in the absence of nucleotide produced similar changes in the fluorescence of a fluorescent reagent N-(1-pyrenyl)-iodoacetamide, attached at Cys-374, or 1,N6-ethenoadenosine 5'-diphosphate substituted for the bound ADP in actin protomers in gizzard and skeletal muscle F-actin. The results are consistent with an influence of the amino acid substitutions on ionic interactions leading to complex formation between actin and myosin intermediates in the ATPase cycle but not on the associated states.

The thin filaments of smooth muscles

Contraction in vertebrate smooth and striated muscles results from the interaction of the actin filaments with crossbridges arising from the myosin filaments. The functions of the actin based thin filaments are (1) interaction with myosin to produce force; (2) regulation of force generation in response to Ca2+ concentration; and (3) transmission of the force to the ends of the cell. The major protein components of smooth muscle thin filaments are actin, tropomyosin and caldesmon, present in molar ratios of 28:4:1 respectively. Other smooth muscle proteins which may be associated with the thin filaments in the cell are filamin, vinculin, alpha-actinin, myosin light chain kinase and calmodulin. We have reviewed the structural and functional properties of these proteins and where possible we have suggested what their function and mechanism of action may be. We propose that actin and tropomyosin are involved in the force producing interaction with myosin, and that this interaction is controlled by a Ca2+-dependent mechanism involving caldesmon, tropomyosin and calmodulin. Vinculin, alpha-actinin and filamin appear to be involved in the attachment of the thin filaments to the cell membrane and their spatial organization within the cell. We conclude that the filaments of smooth muscles share many common properties with those from skeletal muscle, but that they are also quite distinct in terms of both their caldesmon based regulatory mechanism and their mode of organization into a contractile apparatus.

Stage and sex specific differences in actin gene expression in Schistosoma mansoni

We have characterized actin gene expression in Schistosoma mansoni at the RNA and protein levels. Northern blot analyses showed two size classes of actin mRNA in eggs, cercariae and adult worms of both sexes, approximately 1 900 and 1 400 bases in length. A higher abundance of actin mRNA of both size classes was demonstrated in male worms than in eggs, cercariae, and females. Using a phalloidin-rhodamine conjugate, male worms were observed to contain more actin protein than females. Southern blot-hybridization indicated that the sexual differences in actin mRNA and protein levels were not related to some S. mansoni actin genes being sex linked. In addition, two other trematodes, Schistosoma japonicum and Fasciola hepatica and a cestode, Taenia pisiformis contained two classes of actin mRNA similar in size as those in S. mansoni. In contrast, a turbellarian, Dugesia tigrina contained only a single short actin message size class approximately 1 400 bases in length.

The phalloidin binding site of F-actin

Tritium-containing affinity-labelling derivatives of phalloidin, an alkylating iodoacetyl compound (EAL) and a photolabile, carbene generating diazirine (PAL), have been reacted with rabbit muscle actin, the former after protection of thiol groups with N-ethylmaleimide. Labelled peptides generated by tryptic and/or thermolysin digestion were isolated by paper peptide mapping and characterized by determination of their amino acid sequences. EAL binds to methionine-119 and methionine-355; PAL binds to glutamic acid-117. These residues are located in regions with extremely conserved amino acid sequences. The cleft between the two domains of the actin monomer is suggested as the possible binding site for phalloidin.

Structural variations in actins. Biochemical and immunological tools for probing the structure of rabbit skeletal-muscle and bovine aortic actins

Structural differences between skeletal-muscle and aortic actins were studied by using biochemical and immunological approaches. By using proteinase digestion we found that three regions of actin show structural differences: (a) in the C-terminal part, (b) the region around residue 227 and (c) the region around residue 167. By using antibodies specific to particular actin conformations we can discriminate between monomeric and filamentous forms of the two actins. Our results show that the minor sequence variations of the N- and C-terminal regions induce structural change in these regions, but also some long-range structural variations in other regions.

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.

Remodeling of the aortic smooth muscle cell cytoskeleton during developmental and pathological conditions

The remodeling of aortic smooth muscle cell cytoskeleton has been investigated qualitatively and quantitatively during rat aorta development and experimental or human atheromatosis, using immunofluorescent and biochemical techniques. The cytoskeleton of smooth muscle cells in the intimal thickening 15 days after endothelial removal and in human atheromatous plaque is very similar to that of poorly differentiated aortic smooth muscle cells of foetal and newborn rats. Our studies suggest that cytoskeletal changes (a switch in the synthesis of actin isoforms in particular) are reliable markers of proliferative aortic smooth muscle cells, and of atheromatous smooth muscle cells.

Protein-blotting on Polybrene-coated glass-fiber sheets. A basis for acid hydrolysis and gas-phase sequencing of picomole quantities of protein previously separated on sodium dodecyl sulfate/polyacrylamide gel

A procedure has been developed which allows the immobilization on glass-fiber sheets coated with the polyquaternary amine, Polybrene, of proteins and protein fragments previously separated on sodium-dodecylsulfate-containing polyacrylamide gels. The transfer is carried out essentially as has been used for protein blotting on nitrocellulose membranes [Towbin, H., Staehelin, T. and Gordon, J. (1979) Proc. Natl Acad. Sci. USA 76, 4350-4354], but is now used to determine the amino acid composition and partial sequence of the immobilized proteins. Protein transfer could be carried out after staining the proteins in the gels with Coomassie blue, by which immobilized proteins are visible as blue spots, or without previous staining, after which transferred proteins are detected as fluorescent spots following reaction with fluorescamine. The latter procedure was found to be more efficient and yielded binding capacities of +/- 20 micrograms/cm2. Fluorescamine detection was of equal or higher sensitivity than the classical Coomassie staining of proteins in the gel. Immobilized proteins could be hydrolyzed when still present on the glass fiber and reliable amino acid compositions were obtained for various reference proteins immobilized in less than 100 pmol quantities. In addition, and more importantly, glass-fiber-bound proteins could be subjected to the Edman degradation procedure by simply cutting out the area of the sheet carrying the immobilized protein and mounting the disc in the reaction chamber of the gas-phase sequenator. Results of this immobilization-sequencing technique are shown for immobilized myoglobin (1 nmol) and two proteolytic fragments of actin (+/- 80 pmol each) previously separated on a sodium-dodecylsulfate-containing gel.

Mouse testes contain two size classes of actin mRNA that are differentially expressed during spermatogenesis

Using several actin isotype-specific cDNA probes, we found actin mRNA of two size classes, 2.1 and 1.5 kilobases (kb), in extracts of polyadenylated and nonpolyadenylated RNA from sexually mature CD-1 mouse testes. Although the 2.1-kb sequence was present in both meiotic and postmeiotic testicular cell types, it decreased manyfold in late haploid cells. The 1.5-kb actin sequence was not detectable in meiotic pachytene spermatocytes (or in liver or kidney cells), but was present in round and elongating spermatids and residual bodies. To differentiate between the beta- and gamma-actin mRNAs, we isolated a cDNA, pMGA, containing the 3' untranslated region of a mouse cytoplasmic actin that has homology to the 3' untranslated region of a human gamma-actin cDNA but not to the 3' untranslated regions of human alpha-, beta-, or cardiac actins. Dot blot hybridizations with pMGA detected high levels of presumptive gamma-actin mRNA in pachytene spermatocytes and round spermatids, with lower amounts found in elongating spermatids. Hybridization with the 3' untranslated region of a rat beta-actin probe revealed that round spermatids contained higher levels of beta-actin mRNA than did pachytene spermatocytes or residual bodies. Both probes hybridized to the 2.1-kb actin mRNA but failed to hybridize to the 1.5-kb mRNA.

Molecular structure of the human cytoplasmic beta-actin gene: interspecies homology of sequences in the introns

A recombinant phage that carries the cytoplasmic beta-actin gene was isolated from a human DNA library. The nucleotide sequence of this gene was determined. The amino acid sequence deduced from the nucleotide sequence matches perfectly that of beta-actin from human fibroblasts. The gene contains five introns. A large intron was found in the 5' untranslated region six nucleotides upstream from the ATG initiation codon. Four introns were found within the coding region at codons specifying amino acids 41/42, 121/122, 267, and 327/328. In contrast to the human cardiac muscle actin gene, the aorta-type smooth muscle actin gene, and the stomach-type smooth muscle actin gene, the beta-actin gene lacks the codon for cysteine between the ATG initiation codon and the codon for the NH2-terminal amino acid of the mature protein. Hybridization of genomic DNA with DNA fragments derived from intron I in the 5' untranslated region and from intron III strongly suggests the presence of a single beta-actin gene in the human genome. The DNA sequences of the coding region, of the 3' untranslated region, and of the sequence block between the "CCAAT" box and "TATA" box in the 5' flanking DNA of the human beta-actin gene are highly homologous to the corresponding sequences of the rat and chicken beta-actin genes. Unexpectedly, the sequence of intron III of the human beta-actin gene shows considerable homology to that of the rat beta-actin gene.

The isoforms of C protein and their distribution in mammalian skeletal muscle

A monoclonal antibody that is specific for the slow skeletal muscle isoform of C protein of rabbit muscle has been prepared by immunizing mice with a crude preparation of human myosin. It reacted with the X protein fraction of rabbit skeletal muscle and stained all type I cells in this tissue. It also stained a fraction of the type II cells with varying intensities. The type II cells staining with antibody to slow C protein also stained with a polyclonal antibody prepared against rabbit fast muscle C protein. The type II cells not staining with antibody to slow C protein stained strongly with antibody to fast C protein. In the human skeletal muscle antibody to slow C protein stained all cells whereas antibody to fast C stained only type II cells. It is concluded that the distribution of the isoforms of C protein in adult vertebrate skeletal muscle is more complex than is the case with proteins such as components of the troponin complex.

Synthesis of cytoskeletal and contractile proteins by cultured IMR-90 fibroblasts

Models of the assembly of cytoskeletal and contractile proteins of eukaryotic cells require quantitative information about the rates of synthesis of individual component proteins. We applied the dual isotope technique of Clark and Zak (1981, J. Biol. Chem., 256:4863-4870) to measure the synthesis rates of cytoskeletal and contractile proteins in stationary and growing cultures of IMR-90 fibroblasts. Fibroblast proteins were labeled to equilibrium with [14C]leucine over several days, at the end of which there was a 4-h pulse with [3H]leucine. Fractional synthesis rates (percent per hour) were calculated from the 3H/14C ratio of cell protein extracts or protein purified by one- or two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of medium-free leucine. The average fractional synthesis rate for total, SDS- or urea-soluble; Triton-soluble; and cytoskeletal protein extracts in stationary cells each was approximately 4.0%/h. The range of values for the synthesis of individual proteins from total cell extracts or cytoskeletal extracts sliced from one-dimensional gels was similar, though this range was greater than that for major proteins of Triton-soluble protein extracts. Three specific cytoskeletal proteins--actin, vimentin, and tubulin--were synthesized at similar rates that were significantly slower than the average fractional synthesis rate for total protein. Myosin, on the other hand, was synthesized faster than average. Synthesis rates were the same for beta-and gamma-actin and polymerized (cytoskeletal extract) vs. Triton-soluble actin. The same was true for alpha- and beta-tubulin and two different forms of vimentin. Synthesis rates were uniformly higher in growing cells, though the same pattern of differential rates was observed as for stationary cells. Synthesis rates in growing cells were higher than the rate necessary to maintain the growth rate, even for those cytoskeletal proteins being synthesized slowly. Therefore, there appears to be some turnover of these cytoskeletal elements even during growth. We conclude that proteins in cytoskeletal extracts may have nonuniform rates of synthesis, but at least one important subclass of cytoskeletal proteins that comprise filament subunits have the same synthesis rates.

Novel chicken actin gene: third cytoplasmic isoform

We identified a novel chicken actin gene. The actin protein deduced from its nucleotide sequence very closely resembles the vertebrate cytoplasmic actins; accordingly, we classified this gene as a nonmuscle type. We adopted the convention for indicating the nonmuscle actins of the class Amphibia (Vandekerckhove et al., J. Mol. Biol. 152:413-426) and denoted this gene as type 5. RNA blot analysis demonstrated that the type 5 actin mRNA transcripts accumulate in adult tissues in a pattern indicative of a nonmuscle actin gene. Genomic DNA blots indicated that the type 5 actin is a single copy gene and a distinct member of the chicken actin multigene family. Inspection of the nucleotide sequence revealed many features that distinguished the type 5 gene from all other vertebrate actin genes examined to date. These unique characteristics include: (i) an initiation Met codon preceding an Ala codon, a feature previously known only in plant actins, (ii) a single intron within the 5' untranslated region, with no interruptions in the coding portion of the gene, and (iii) an atypical Goldberg-Hogness box (ATAGAA) preceding the mRNA initiation terminus. These unusual features have interesting implications for actin gene diversification during evolution.

Mouse testes contain two size classes of actin mRNA that are differentially expressed during spermatogenesis

Using several actin isotype-specific cDNA probes, we found actin mRNA of two size classes, 2.1 and 1.5 kilobases (kb), in extracts of polyadenylated and nonpolyadenylated RNA from sexually mature CD-1 mouse testes. Although the 2.1-kb sequence was present in both meiotic and postmeiotic testicular cell types, it decreased manyfold in late haploid cells. The 1.5-kb actin sequence was not detectable in meiotic pachytene spermatocytes (or in liver or kidney cells), but was present in round and elongating spermatids and residual bodies. To differentiate between the beta- and gamma-actin mRNAs, we isolated a cDNA, pMGA, containing the 3' untranslated region of a mouse cytoplasmic actin that has homology to the 3' untranslated region of a human gamma-actin cDNA but not to the 3' untranslated regions of human alpha-, beta-, or cardiac actins. Dot blot hybridizations with pMGA detected high levels of presumptive gamma-actin mRNA in pachytene spermatocytes and round spermatids, with lower amounts found in elongating spermatids. Hybridization with the 3' untranslated region of a rat beta-actin probe revealed that round spermatids contained higher levels of beta-actin mRNA than did pachytene spermatocytes or residual bodies. Both probes hybridized to the 2.1-kb actin mRNA but failed to hybridize to the 1.5-kb mRNA.

Preparation and characterization of bovine aortic actin

A functional vascular smooth-muscle actin from bovine aorta was purified to homogeneity by an original method and was able to polymerize. Aortic actin is composed of two major isoforms and at least two minor ones. This actin was not phosphorylated by either cyclic AMP-dependent protein kinase or C kinase. The physical properties of aortic actin were found to be very similar to those of skeletal-muscle actin, except for amino acid composition (three tryptophan residues instead of four). The aortic actin and skeletal-muscle actin differ in the extent of activation of the Mg-dependent ATPase of skeletal-muscle myosin.

Evidence for the existence of an actin-derived protein in ataxia-telangiectasia lymphoblastoid cell lines

In a number of lymphoblastoid cell lines from individuals with the genetic disease ataxia-telangiectasia (A-T), a decrease in the levels of actin and a concomitant increase in the levels of a protein species of 37K has been observed to occur following high cell density. In this paper we describe ataxia-telangiectasia lymphoblastoid lines that show this response constitutively, and demonstrate that this 37K protein is most likely an actin-derived peptide. This is because of extensive similarity between the two proteins in the spectrum of peptides produced by proteolytic fingerprinting. It appears likely that this 37K protein is similar to the well documented protease-resistant actin fragments produced by protease digestion of actin in vitro. Examination of a number of possible initiators found no clear signal that promoted this response, although the response is clearly very dependent on cell culture conditions. The possible involvement of a cytoskeletal anomaly in ataxia-telangiectasia is discussed.

Novel chicken actin gene: third cytoplasmic isoform

We identified a novel chicken actin gene. The actin protein deduced from its nucleotide sequence very closely resembles the vertebrate cytoplasmic actins; accordingly, we classified this gene as a nonmuscle type. We adopted the convention for indicating the nonmuscle actins of the class Amphibia (Vandekerckhove et al., J. Mol. Biol. 152:413-426) and denoted this gene as type 5. RNA blot analysis demonstrated that the type 5 actin mRNA transcripts accumulate in adult tissues in a pattern indicative of a nonmuscle actin gene. Genomic DNA blots indicated that the type 5 actin is a single copy gene and a distinct member of the chicken actin multigene family. Inspection of the nucleotide sequence revealed many features that distinguished the type 5 gene from all other vertebrate actin genes examined to date. These unique characteristics include: (i) an initiation Met codon preceding an Ala codon, a feature previously known only in plant actins, (ii) a single intron within the 5' untranslated region, with no interruptions in the coding portion of the gene, and (iii) an atypical Goldberg-Hogness box (ATAGAA) preceding the mRNA initiation terminus. These unusual features have interesting implications for actin gene diversification during evolution.

Characterization of genomic poly(dT-dG).poly(dC-dA) sequences: structure, organization, and conformation

Hybridization studies suggest the abundant presence of poly(dT-dG).poly(dC-dA) (TG-element), a potential Z-DNA sequence, in eucaryotic genomes. We have isolated and characterized TG-elements from different locations in the human genome: from randomly isolated clones, associated with the actin gene family, and linked to another repeated element. The results indicate that the following features are typical of these TG-elements: the elements consist of 20 to 60 base pairs of (dT-dG)n.(dC-dA)n, the sequences characterized in our study were not flanked by direct or inverted repeats, the sequences are interspersed rather than in satellite blocks, the elements are not usually associated with other repeated elements, and some of the elements are found near coding sequences or in introns. Studies on the conformation of a genomic TG-element in a supercoiled plasmid indicate several distinct properties of the TG-element: it is in the Z-form only at low ionic strength, S1 nuclease recognizes its Z-form with a marked preference for one of the B-Z junctions, and the sensitive region extends for 20 base pairs near the B-Z junction. In contrast to the result with the supercoiled plasmid, S1 nuclease failed to recognize the TG-element in minichromosomes.

Bovine aorta actin. Development of an improved purification procedure and comparison of polymerization properties with actins from other types of muscle

Crude actin extracts from acetone-dried powder of the muscle layer of bovine aorta contain an actin-modulating protein which promotes nucleation of actin monomers and decreases the average length of actin filaments in a Ca2+-dependent manner. This observation has allowed the development of an improved purification procedure for aorta actin which increases the yield 2- to 3-times. The actin obtained with this procedure consists of 77% alpha- and 23% gamma-isoelectric species. Pure aorta actin is indistinguishable from actins from skeletal, cardiac and chicken-gizzard smooth muscle in its polymerization rate, critical concentration, and reduced viscosity when polymerized with KCl at 25 degrees C. It differs from sarcomeric actins, but not from chicken-gizzard smooth muscle actin, in the temperature dependence of polymerization equilibria in KCl. This difference correlates with the amino acid replacements Val-17----Cys-17 and Thr-89----Ser-89, supporting a conclusion drawn from other studies that the N-terminal portion of actin polypeptide chain contains sites important for polymerization.

Effects of various amino acid replacements on the conformational stability of G-actin

Circular dichroic spectra of native, EDTA-treated and heat-denatured G-actin from chicken gizzard smooth muscle are virtually the same as those of rabbit skeletal muscle actin. The rates of changes produced by EDTA or heat in the secondary structure are, however, higher in the case of gizzard actin. Similar differences were found in the rates of inactivation as measured by loss of polymerizability during incubation with EDTA or Dowex 50. The results are explicable in terms of local differences in the conformation at specific site(s) important for maintaining the native state of actin monomer. Involvement of the ATP binding site was shown by measuring the equilibrium constant for the binding of ATP to the two actins. Difference in the conformation of some additional site(s) is indicated by a higher rate constant of inactivation of nucleotide-free actin observed for gizzard actin. No significant difference was found in the equilibrium constant for the binding of Ca2+ at the single high-affinity site in gizzard and skeletal muscle actin. Comparison of inactivation kinetics of actin from chicken gizzard, rabbit skeletal, bovine aorta, and bovine cardiac muscle suggests that the amino acid replacements Val-17----Cys-17 and/or Thr-89----Ser-89 have a destabilizing effect on the native conformation of G-actin. The results indicate that deletion of the acidic residue at position 1 of the amino acid sequence has no effect on the conformation of the ATP binding site and the high-affinity site for divalent cation as well.

A smooth muscle-specific monoclonal antibody recognizes smooth muscle actin isozymes

Injection of chicken gizzard actin into BALB/c mice resulted in the isolation of a smooth muscle-specific monoclonal antibody designated CGA7. When assayed on methanol-Carnoy's fixed, paraffin-embedded tissue, it bound to smooth muscle cells and myoepithelial cells, but failed to decorate striated muscle, endothelium, connective tissue, epithelium, or nerve. CGA7 recognized microfilament bundles in early passage cultures of rat aortic smooth muscle cells and human leiomyosarcoma cells but did not react with human fibroblasts. In Western blot experiments, CGA7 detected actin from chicken gizzard and monkey ileum, but not skeletal muscle or fibroblast actin. Immunoblots performed on two-dimensional gels demonstrated that CGA7 recognizes gamma-actin from chicken gizzard and alpha- and gamma-actin from rat colon muscularis. This antibody was an excellent tissue-specific smooth muscle marker.

The complete sequence of the chicken alpha-cardiac actin gene: a highly conserved vertebrate gene

We sequenced the entire chicken alpha-cardiac actin gene. A single intron was positioned 20 bp upstream from the initiation ATG codon in the 5' non-coding region while the coding region was interrupted by 5 introns at amino acid positions 41/42, 150, 204, 267, and 327/328. Sequencing allowed the first comparison of the alpha-cardiac and alpha-skeletal actin transcriptional promoters. These highly G+C rich promoters share two regions of homology which are found at position -134 (10 bp) and -296 (12 bp) in the alpha-cardiac actin promoter. A smaller 9 bp motif (CCGCGCCGG) homologous to the -134 sequence was detected before, between and after the TATA and CAAT boxes of the alpha-cardiac actin gene. The polyadenylation signal (AATAAA) was located 156 bp downstream from the translation termination codon. The complete length of the alpha-cardiac actin mRNA excluding the poly A tail is 1370 nucleotides. The 3' noncoding transcribed portion of the chicken alpha-cardiac actin gene was found to be extraordinarily conserved when compared to the human and rat alpha-cardiac actin mRNA sequences.

Isolation and characterization of six different chicken actin genes

Genes representing six different actin isoforms were isolated from a chicken genomic library. Cloned actin cDNAs as well as tissue-specific mRNAs enriched in different actin species were used as hybridization probes to group individual actin genomic clones by their relative thermal stability. Restriction maps showed that these actin genes were derived from separate and nonoverlapping regions of genomic DNA. Of the six isolated genes, five included sequences from both the 5' and 3' ends of the actin-coding area. Amino acid sequence analysis from both the NH2- and COOH-terminal regions provided for the unequivocal identification of these genes. The striated isoforms were represented by the isolated alpha-skeletal, alpha-cardiac, and alpha-smooth muscle actin genes. The nonmuscle isoforms included the beta-cytoplasmic actin gene and an actin gene fragment which lacked the 5' coding and flanking sequence; presumably, this region of DNA was removed from this gene during construction of the genomic library. Unexpectedly, a third nonmuscle chicken actin gene was found which resembled the amphibian type 5 actin isoform (J. Vandekerckhove, W. W. Franke, and K. Weber, J. Mol. Biol., 152:413-426). This nonmuscle actin type has not been previously detected in warm-blooded vertebrates. We showed that interspersed, repeated DNA sequences closely flanked the alpha-skeletal, alpha-cardiac, beta-, and type 5-like actin genes. The repeated DNA sequences which surround the alpha-skeletal actin-coding regions were not related to repetitious DNA located on the other actin genes. Analysis of genomic DNA blots showed that the chicken actin multigene family was represented by 8 to 10 separate coding loci. The six isolated actin genes corresponded to 7 of 11 genomic EcoRI fragments. Only the alpha-smooth muscle actin gene was shown to be split by an EcoRI site. Thus, in the chicken genome each actin isoform appeared to be encoded by a single gene.

Nucleotide sequence of the chicken cardiac alpha actin gene: absence of strong homologies in the promoter and 3'-untranslated regions with the skeletal alpha actin sequence

The entire nucleotide sequence of the chicken cardiac alpha-actin (CC alpha A) gene has been determined. This is the first complete sequence of a cardiac actin gene that includes the promoter region, cap site, all the introns, and the polyadenylation site. The gene contains six introns, five of which interrupt the coding region at amino acids (aa) 41, 150, 204, 267, and 327. The first intron is in the 5'-noncoding region and is 438 bp in length. The CC alpha A gene encodes an mRNA of approx. 1400 bp with 5'- and 3'-untranslated region of 59 and 184 nucleotides (nt), respectively. Like the chicken skeletal alpha-actin gene, the CC alpha A gene has the codon for the aa cysteine between the initiator ATG and the codon for the N-terminal aspartic acid residue of the mature protein. There are no strong homologies (less than 13 consecutive nt) in the promoter or 3'-untranslated regions between the CC alpha A and chicken skeletal alpha-actin genes even though both are expressed in skeletal muscle during development. However, the 3'-untranslated region of the CC alpha A gene demonstrates significant sequence homology (76% over a 200-nt region) with the same region in the partial sequence of the human cardiac gene. The conservation of these sequence homologies between identical isoforms rather than the different alpha actin genes suggests these conserved regions may have a role in regulation rather than tissue-specific expression, as previously proposed.

The structure of a cloned mouse gamma-actin processed pseudogene

The complete nucleotide (nt) sequence of a gamma-actin-like pseudogene (M gamma A-psi 1), isolated from a mouse genomic library in phage lambda, was determined. The pseudogene was shown to be of the processed type by the fact that it lacked introns, ended in a poly(dA) region, and was flanked by direct repeats. There were ten differences in the predicted amino acid (aa) sequence from that of the authentic nonmuscle gamma-actin. An unusual feature of M gamma A-psi 1 was the complete absence of DNA corresponding to the 5' end of the mRNA up to the nt preceding the Ala codon at aa position 7. This suggests that M gamma A-psi 1 originated from a truncated mRNA or from an incomplete reverse transcript.

Identical distribution of fluorescently labeled brain and muscle actins in living cardiac fibroblasts and myocytes

We have investigated whether living muscle and nonmuscle cells can discriminate between microinjected muscle and nonmuscle actins. Muscle actin purified from rabbit back and leg muscles and labeled with fluorescein isothiocyanate, and nonmuscle actin purified from lamb brain and labeled with lissamine rhodamine B sulfonyl chloride, were co-injected into chick embryonic cardiac myocytes and fibroblasts. When fluorescence images of the two actins were compared using filter sets selective for either fluorescein isothiocyanate or lissamine rhodamine B sulfonyl chloride, essentially identical patterns of distribution were detected in both muscle and nonmuscle cells. In particular, we found no structure that, at this level of resolution, shows preferential binding of muscle or nonmuscle actin. In fibroblasts, both actins are associated primarily with stress fibers and ruffles. In myocytes, both actins are localized in sarcomeres. In addition, the distribution of structures containing microinjected actins is similar to that of structure containing endogenous F-actin, as revealed by staining with fluorescent phalloidin or phallacidin. Our results suggest that, at least under these experimental conditions, actin-binding sites in muscle and nonmuscle cells do not discriminate among different forms of actins.

Actin mRNAs in HeLa cells. Stabilization after adenovirus infection

We have carried out a comparative analysis of the expression of the actin genes in HeLa and adenovirus-infected HeLa cells. The rate of actin gene transcription was examined in these cells by pulse-labeling of the newly synthesized RNA and/or by in vitro transcription in nuclei isolated from uninfected or infected HeLa cells. In addition, accumulation of actin-specific heterogeneous nuclear RNA, and rate of appearance of the actin mRNAs in the cytoplasm were examined by dot and Northern blot analysis. The rate of actin gene transcription remained constant after infection of HeLa cells with adenovirus serotype 2, while the level of the actin precursor in the nuclei was slightly reduced. In the infected cells, newly synthesized actin mRNA enters the cytoplasm at a very reduced rate. The deficiency of transport does not affect the steady-state level of the messages in the cytoplasm. The half-life of cytoplasmic actin mRNAs was analyzed by traditional pulse-chase experiments and by a novel procedure using 5-6-diCl-1-beta-d-ribofuranosylbenzimidazole, which does not rely on labeled RNA. Both procedures gave identical results. Uninfected HeLa cells have actin mRNAs with relatively short half-lives, from less than six to 12 hours. In contrast, the half-lives of the actin-specific mRNAs, in the cytoplasm of adenovirus-infected cells, is greater than 14 to 24 hours. These observations suggest that, although the rate of transport of actin mRNAs to the cytoplasm is reduced upon infection with adenovirus, increased half-lives result in accumulation of actin mRNAs to normal levels in the cytoplasm.

Amino acid sequence diversity between bovine epidermal cytokeratin polypeptides of the basic (type II) subfamily as determined from cDNA clones

The nucleotide sequences of four cDNA clones, each representing the carboxyterminal portion of a bovine epidermal cytokeratin of the "basic" (type II) subfamily, were determined, i.e., components Ia (Mr 68,000), Ib (Mr 68,000), III (Mr 60,000), and IV (Mr 59,000). The comparison of the sequences with each other and with the human type-II cytokeratin of Mr 56,000 reported by Hanukoglu and Fuchs [24] allows the following conclusions: The four major epidermal keratins of the basic (type II) subfamily, which are co-expressed in keratinocytes of the bovine muzzle, exhibit a high homology (greater than 90%) in the alpha-helical portion, but differ considerably in their nonhelical carboxy-terminal regions. The nonhelical carboxyterminal regions of all four cytokeratins are exceptionally rich in glycine and serine. Within the extrahelical tail, three different domains can be distinguished. The consensus sequence TYR(X)LLEGE which demarcates the end of the alpha-helical rod in all intermediate filaments is followed by a relatively short (22-27 amino acids) intercept rich in hydroxy amino acids and valine (carboxyterminal tail domain C1). This is followed by a long region that is variable in size and sequence, rich in glycine di-, tri-, and tetrapeptides, and contains diverse repeated sequences (domain C2). This is followed by another short (20 residues) hydroxy-amino-acid-rich intercept (domain C3) that ends with a conspicuously basic sequence of approximately four to six carboxyterminal amino acids. The first half of domain C1 is also homologous in all four keratins, suggesting that this region also assumes a common conformation and/or serves a special common function.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of genomic poly(dT-dG).poly(dC-dA) sequences: structure, organization, and conformation

Hybridization studies suggest the abundant presence of poly(dT-dG).poly(dC-dA) (TG-element), a potential Z-DNA sequence, in eucaryotic genomes. We have isolated and characterized TG-elements from different locations in the human genome: from randomly isolated clones, associated with the actin gene family, and linked to another repeated element. The results indicate that the following features are typical of these TG-elements: the elements consist of 20 to 60 base pairs of (dT-dG)n.(dC-dA)n, the sequences characterized in our study were not flanked by direct or inverted repeats, the sequences are interspersed rather than in satellite blocks, the elements are not usually associated with other repeated elements, and some of the elements are found near coding sequences or in introns. Studies on the conformation of a genomic TG-element in a supercoiled plasmid indicate several distinct properties of the TG-element: it is in the Z-form only at low ionic strength, S1 nuclease recognizes its Z-form with a marked preference for one of the B-Z junctions, and the sensitive region extends for 20 base pairs near the B-Z junction. In contrast to the result with the supercoiled plasmid, S1 nuclease failed to recognize the TG-element in minichromosomes.

Cardiac actin is the major actin gene product in skeletal muscle cell differentiation in vitro

We examined the expression of alpha-skeletal, alpha-cardiac, and beta- and gamma-cytoskeletal actin genes in a mouse skeletal muscle cell line (C2C12) during differentiation in vitro. Using isotype-specific cDNA probes, we showed that the alpha-skeletal actin mRNA pool reached only 15% of the level reached in adult skeletal muscle and required several days to attain this peak, which was then stably maintained. However, these cells accumulated a pool of alpha-cardiac actin six times higher than the alpha-skeletal actin mRNA peak within 24 h of the initiation of differentiation. After cells had been cultured for an additional 3 days, this pool declined to 10% of its peak level. In contrast, over 95% of the actin mRNA in adult skeletal muscle coded for alpha-actin. This suggests that C2C12 cells express a pattern of sarcomeric actin genes typical of either muscle development or regeneration and distinct from that seen in mature, adult tissue. Concurrently in the course of differentiation the beta- and gamma-cytoskeletal actin mRNA pools decreased to less than 10% of their levels in proliferating cells. The decreases in beta- and gamma-cytoskeletal actin mRNAs are apparently not coordinately regulated.

Cell type-specific activation of actin genes in the early amphibian embryo

Muscle actin genes are the earliest yet described to show cell type-specific activation in amphibian embryos. Gene-specific probes show that alpha-skeletal and alpha-cardiac actin genes start to be transcribed simultaneously at the end of gastrulation, but only in those regions of the mesoderm that subsequently form embryonic muscle. Their expression provides a molecular marker for early cell determination.

A single troponin T gene regulated by different programs in cardiac and skeletal muscle development

A cloned complementary DNA derived from a messenger RNA transiently present at low abundance levels in early chick embryonic skeletal muscle hybridizes to a messenger RNA present at high abundance levels in cardiac muscle. Genomic DNA hybridization and nucleotide sequence identity of complementary DNA's from both heart and skeletal muscle demonstrate that the messenger RNA's from both sources are encoded by the same gene. The encoded polypeptide is a troponin T sequence which is probably a cardiac isoform. This single copy troponin T isogene is governed by different regulatory programs in heart and skeletal muscle differentiation.

Chordate muscle actins differ distinctly from invertebrate muscle actins. The evolution of the different vertebrate muscle actins

A total of 30 actins from various chordate and invertebrate muscle sources were either characterized by full amino acid sequence data or typed by those partial sequences in the NH2-terminal tryptic peptide which are known to be specific markers for different actin isoforms. The results show that most, if not all, invertebrate muscle actins are homologous to each other and to the isoforms recognized as vertebrate cytoplasmic actins. In contrast the actin forms typically found in muscle cells of warm-blooded vertebrates are noticeably different from invertebrate muscle actins and seem to have appeared in evolution already with the origin of chordates. During subsequent vertebrate evolution there has been a high degree of sequence conservation similar or stronger than that seen in histone H4. Urochordates, Cephalochordates and probably also Agnathes express only one type of muscle actin. Two types, a striated muscle-specific form and a smooth muscle form, are already observed in Chondrichthyes and Osteichthyes. Later in evolution, with the origin of reptiles, both muscle actins seem to have duplicated again; the striated muscle type branched into a skeletal- and cardiac-specific form, while the smooth muscle form duplicated into a vascular- and stomach-specific type. These findings support the hypothesis that each of the four muscle actins of warm-blooded vertebrates are coded for by a small number and possibly only one functional gene.

Isolation and characterization of six different chicken actin genes

Genes representing six different actin isoforms were isolated from a chicken genomic library. Cloned actin cDNAs as well as tissue-specific mRNAs enriched in different actin species were used as hybridization probes to group individual actin genomic clones by their relative thermal stability. Restriction maps showed that these actin genes were derived from separate and nonoverlapping regions of genomic DNA. Of the six isolated genes, five included sequences from both the 5' and 3' ends of the actin-coding area. Amino acid sequence analysis from both the NH2- and COOH-terminal regions provided for the unequivocal identification of these genes. The striated isoforms were represented by the isolated alpha-skeletal, alpha-cardiac, and alpha-smooth muscle actin genes. The nonmuscle isoforms included the beta-cytoplasmic actin gene and an actin gene fragment which lacked the 5' coding and flanking sequence; presumably, this region of DNA was removed from this gene during construction of the genomic library. Unexpectedly, a third nonmuscle chicken actin gene was found which resembled the amphibian type 5 actin isoform (J. Vandekerckhove, W. W. Franke, and K. Weber, J. Mol. Biol., 152:413-426). This nonmuscle actin type has not been previously detected in warm-blooded vertebrates. We showed that interspersed, repeated DNA sequences closely flanked the alpha-skeletal, alpha-cardiac, beta-, and type 5-like actin genes. The repeated DNA sequences which surround the alpha-skeletal actin-coding regions were not related to repetitious DNA located on the other actin genes. Analysis of genomic DNA blots showed that the chicken actin multigene family was represented by 8 to 10 separate coding loci. The six isolated actin genes corresponded to 7 of 11 genomic EcoRI fragments. Only the alpha-smooth muscle actin gene was shown to be split by an EcoRI site. Thus, in the chicken genome each actin isoform appeared to be encoded by a single gene.

Purification and kinetic characterisation of human erythrocyte actin

Human erythrocyte actin can be extracted from membrane ghosts by low ionic strength treatment in the presence of protective amounts of calcium and ATP. Purification then involves a single chromatographic step. The erythrocyte actin can be labelled with N-(1-prenyl)iodoacetamide. The fluorescence enhancement which accompanies polymerisation can be used to determine the critical concentration for assembly and to follow the polymerisation reaction time-course. The polymerisation kinetics of erythrocyte actin are compared with those of rabbit skeletal muscle actin. The two are shown to be markedly different.

Transcription of muscle-specific actin genes in early Xenopus development: nuclear transplantation and cell dissociation

Cloned cDNA probes that recognize muscle-specific alpha-actin gene transcripts have been used to analyze two kinds of experimental embryos in Xenopus. In one, genetically marked nuclei of larval muscle cells were transplanted to wild-type enucleated eggs; alpha-actin genes became transcriptionally inactive in the resulting blastulae but were reactivated when these embryos reached the normal stage of alpha-actin expression (late gastrula). In the other, blastula embryos reared from fertilized eggs were separated into animal, vegetal, and equatorial regions, and their cells dissociated and reaggregated. alpha-Actin RNA was synthesized at the normal time in development, but only by equatorial cells. We conclude that alpha-actin gene transcription is normally regulated in nuclear-transplant embryos and is undisturbed by the absence of cell contacts during cleavage.

Structure of a human smooth muscle actin gene (aortic type) with a unique intron site

A recombinant phage containing an actin gene (lambda Ha201) was isolated from a human DNA library and the structure of the actin gene was determined. The amino acid sequences deduced from the nucleotide sequences of lambda Ha201 were compared with those of six actin isoforms; they matched those of bovine aortic smooth muscle actin, except for codon 309, which was valine (GTC) in lambda Ha201 and alanine (GCN) in bovine aortic smooth muscle actin. Southern blot hybridization experiments showed that the gene of normal human cells did not have the TaqI-sensitive site around position 309, whereas half of the genes of HUT14 cells did. These results indicate that one allele of the aortic smooth muscle actin gene in HUT14 cells has a transition point mutation (C----T) at codon 309 and that the amino acid sequences of normal human aorta and bovine smooth muscle actins are probably identical. In addition to the five introns interrupting exons at codons 150, 204, and 267, and between codons 41 and 42 and 327 and 328, which are common to skeletal muscle and cardiac muscle actin genes, the smooth muscle actin gene has two more intron sites between codons 84 and 85 and 121 and 122. The previously unreported intron site between codons 84 and 85 is unique to the smooth muscle actin gene. The intron site between codons 121 and 122 is common to beta-actin genes but is not found in other muscle actin genes. A hypothesis is proposed for the evolutionary pathway of the actin gene family.

Cardiac actin is the major actin gene product in skeletal muscle cell differentiation in vitro

We examined the expression of alpha-skeletal, alpha-cardiac, and beta- and gamma-cytoskeletal actin genes in a mouse skeletal muscle cell line (C2C12) during differentiation in vitro. Using isotype-specific cDNA probes, we showed that the alpha-skeletal actin mRNA pool reached only 15% of the level reached in adult skeletal muscle and required several days to attain this peak, which was then stably maintained. However, these cells accumulated a pool of alpha-cardiac actin six times higher than the alpha-skeletal actin mRNA peak within 24 h of the initiation of differentiation. After cells had been cultured for an additional 3 days, this pool declined to 10% of its peak level. In contrast, over 95% of the actin mRNA in adult skeletal muscle coded for alpha-actin. This suggests that C2C12 cells express a pattern of sarcomeric actin genes typical of either muscle development or regeneration and distinct from that seen in mature, adult tissue. Concurrently in the course of differentiation the beta- and gamma-cytoskeletal actin mRNA pools decreased to less than 10% of their levels in proliferating cells. The decreases in beta- and gamma-cytoskeletal actin mRNAs are apparently not coordinately regulated.

The 3'-untranslated sequence of human skeletal muscle alpha-actin mRNA

A human actin cDNA clone pGF3 isolated from a fetal skeletal muscle cDNA library is described. The insert cDNA is homologous to skeletal muscle alpha-actin as judged by restriction mapping and nucleotide sequencing. The recombinant contains a substantial portion of the coding and the complete 3'-untranslated region. Comparison of the 3' ends of human and rat skeletal muscle and human cardiac alpha-actins reveals little homology between different types of actin genes in man but marked conservation of this region in the skeletal muscle actins of man and rat.

Evolution of the human sarcomeric-actin genes: evidence for units of selection within the 3' untranslated regions of the mRNAs

The complete 3' untranslated region (3'UTR) sequence of the human skeletal-actin gene has been compared with the corresponding regions of the rat and chicken skeletal-actin genes. This comparison reveals that the skeletal-actin 3'UTR is composed of conserved and nonconserved segments. By using genomic Southern transfer blots and thermal stability (Tm) measurements, we found that the cardiac-actin gene 3'UTR also consists of conserved and nonconserved segments. Comparison of human and Xenopus laevis cardiac-actin mRNA sequences confirms the presence of a region of high similarity in the 3'UTR. We conclude that subsegments of the 3'UTRs of both skeletal- and cardiac-actin genes of birds and mammals are under considerable selective pressure. This suggests that these conserved sequences may have functional roles in actin-gene expression or regulation, and that these roles might be different for each actin isoform.