At least six different actins are expressed in a higher mammal: an analysis based on the amino acid sequence of the amino-terminal tryptic peptide

[From the ameba to the pulsating heart: evolution and fine structure of the intracellular movement apparatus (author's transl)]

Different kinds of cell motility are reviewed in this paper with special regard to development and ultrastructure. The variety of animal cell motility types can be reduced to three principles : ciliary and ameboid movements and muscle contraction. The ultrastructure of all kinds of cilia is very similar from single cell organisms to highly specialized cells of the human body e.g., ciliary respiratory epithelium. As a rule, ciliary movement is caused by minimal sliding of the nine double tubules consisting of tubulin, a protein differing from myosin and actin. Ameboid movement and muscle cell contraction are based on the sliding filament mechanism of actin and myosin. Although the principles of this mechanism have not changed during evolution some differences in the structure and arrangement of actin and myosin filaments occurred. Obviously, the high degree of order of the myofibrils of vertebrate heart and skeletal muscle cells has developed from loose and rapid changing arrangement of contractile filaments in ameboid cells. There are some changes of residues in the actin and myosin molecules during the development of the intracellular contractile system. Finally, some peculiarities of the myocardium, its special arrangement of muscle cells and some disturbances of the contractile filaments under pathologic conditions are discussed.

Isolation and characterization of human actin genes

We have utilized cloned actin genes from Drosophila melanogaster and from chicken to isolate 12 actin gene fragments from a human DNA library. Each of these 12 clones was shown to contain actin coding regions by its ability to selectively hybridize to human actin mRNA as assayed by in vitro translation. The translation product was judged to be actin on the basis of its comigration with authentic actins when electrophoresed on one- and two-dimensional NaDodSO4/polyacrylamide gels and on the basis of its partial proteolysis products. Determination of the sizes and order of the fragments generated by restriction endonuclease digestion of each of these recombinant phages allows us to conclude that they are nonallelic and are from nonoverlapping regions of the genome. We have used these cloned human actin genes and the Drosophila and chicken actin gene clones to show that the human genome contains 25-30 EcoRI fragments homologous to actin genes and that, among three nonconsanguineous individuals tested, none of these fragments exhibit length or restriction-site polymorphism.

Coexistence of three major isoactins in a single sarcoma 180 cell

Actin is transformed sarcoma 180 cells is composed of the nonmuscle beta and gamma species and of a third, more acidic stable variant termed zeta. Two-dimensional peptide analysis shows that zeta is similar to beta actin, differing in the mobility of only one tryptic peptide. Several lines of evidence indicate that zeta is not a modified beta-actin species. This third actin species comprises 20% of the total labeled actin, has the same molecular weight as the beta and gamma actins and has a different mobility in isoelectric focusing gels from that of the known alpha actins from skeletal, cardiac and vascular smooth muscle. Like beta and gamma actin, zeta can be extracted with the actin depolymerizing factor from slime mold. Two-dimensional gel electrophoresis (isoelectric focusing) of the 35S-methionine-labeled polypeptides synthesized by a single sarcoma 180 cell showed that all three major actin species coexist within the same cell. This analysis also showed for the first time the coexistence and alpha and beta tubulin, vimentin, alpha actinin and three other polypeptides present in intermediate-filament-enriched cytoplast cytoskeletons (spots 12, 24 and 31). Determination of the ratio of gamma plus beta to zeta actin in different cytoskeletal preparations of intact and enucleated sarcoma 180 cells indicated that this actin species is not localized specifically to any of the major actin-containing structures preserved in the cytoskeletons.

Organization and expression of multiple actin genes in the sea urchin

A set of at least 11 actin genes has been isolated from genomic recombinant deoxyribonucleic acid libraries of the sea urchin Strongylocentrotus purpuratus. Most of the isolates derive from a library which represents the genome of a single animal. There are at least five distinct types of sea urchin actin gene, some of which are represented by multiple copies in the genome. The actin gene types are distinguished by nonhomologous flanking sequences and intervening sequences, though the protein coding sequences appear in most cases to be quite similar. Eight of the 11 genes isolated have been recovered in lambda recombinants that contain two actin genes, linked at 5- to 9-kilobase distances. Restriction map overlaps suggest that the genome contains an array of at least three of these genes spaced over about 30 kilobases of deoxyribonucleic acid. In the linkage patterns observed, actin genes of diverse types were linked to each other. In early embryos, actin messenger ribonucleic acid (RNA) transcripts of 1.8 and 2.2 kilobases were found, and the longer of these transcripts was more prevalent in the maternal RNA of the egg. From RNA gel blot experiments, we conclude that the two transcripts derive from different actin gene types. Different repetitive sequences were located to either side of most of the actin genes, and in most observed cases the repeat sequences which were adjacent to actin genes of a given type were similar. The repeat sequences flanking the actin genes belonged to families which were transcribed, but those repeats in the neighborhood of the actin genes which have been investigated were not themselves represented in the stable RNAs of eggs or early embryos.

Differential expression and 5' end mapping of actin genes in Dictyostelium

Using a modification of the Berk and Sharp S1 nuclease mapping procedure and by analyzing actin cDNA clones, we have examined the expression of several members of the 17-member multigene family encoding actin in Dictyostelium. The mapping procedure, which takes advantage of the fact that the actin genes are homologous in the protein-coding region but are very divergent in the proposed 5' untranslated region has enabled us to quantitate the relative expression of several genes during the Dictyostelium life cycle. We have shown that at least six of the 17 potential actin-coding sequences are expressed. One is not expressed at levels of more than 0.5--1% of total actin mRNA at the developmental times examined and appears to be a pseudogene. By quantitating the amount of actin mRNA in mRNA populations isolated from cells at various times in development, we have shown that four of the actin genes show different patterns of expression. Interestingly, three of the four genes appear to encode the same protein. We have also taken advantage of the S1 mapping procedure to identify the 5' ends of the actin mRNAs from four genes and have compared the sequences outside the 5' ends on these genes with the nucleotide sequences of seven other actin genes. We have identified homologous sequences in most of these genes that may be involved in initiation of transcription.

Brain actin synthesized in vitro undergoes two different and sequential posttranslational modifications

The have studied the posttranslational processing of actin molecules synthesized in a cell-free system. The results of these experiments indicate that during the in vitro synthesis of the actins from rat brain the primary translational products undergo two different and sequential posttranslational modifications. These modifications are accompanied by slight changes in the isoelectric points of the proteins and can be detected by isoelectric focusing analysis. The same posttranslational modifications can be detected during the in vitro synthesis of chick embryo skeletal muscle actin. The evidence presented suggest that the first posttranslational modification may correspond to the methylation of a histidine residue, and the second modification most likely corresponds to the acetylation of the NH(2)-terminal amino acid residues of actin molecules.

The first step in the polymerisation of actin

In the presence of certain cations (e.g. K+ or Mg2+) actin polymerizes. Below a certain concentration (the critical concentration) the monomer G-actin does not polymerize on the addition of K+ or Mg2+. However, the proteolysis experiments of Rich and Estes [J. Mol. Biol. 104, 777--792 (1976)] strongly suggest that cations induce a change in conformation of G-actin leading to a novel form of actin, G*-actin. This conformational change may be the first step in the polymerization of actin. We have studied G*-actin induced by K+, by difference spectroscopy. We show that G*-actin is a monomer and we confirm that the bound ATP is not cleaved. We also studied the G-actin in equilibrium with G*-actin equilibrium at 4 degrees C as a function of K+ or Mg2+ concentration. With KCl, the transformation can be accounted for as a screening effect. The effect of Mg2+ is more specific and the change in conformation of the G-actin could result from the binding of two or three Mg2+ ions/molecule. We suggest that the G-actin in equilibrium with G*-actin transformation results from the neutralization of a polyanionic region on the actin surface and that this region could be the highly negatively charged N terminus.

Actin acetylation in Drosophila tissue culture cells

In a permanent cell line derived from Drosophila embryos, cytoplasmic actin is produced as an unstable precursor, which is subsequently converted to a stable form. This conversion results in a reduction in isoelectric point, with no apparent change in molecular weight. The conversion involves an enzymatic acetylation, and results in an insensitivity to aminopeptidase digestion, suggesting N-terminal blockage. Both the acetylated and unacetylated actins can participate in the assembly of F-actin, but with different efficiencies.

Actin from pig and rat uterus

Smooth-muscle actin was isolated from pig uterus and from pregnant-rat uterus. Methods involving acetone-dried powders were unsuccessful, and a column-chromatographic procedure was developed, with proteinase inhibitors and avoiding polymerization as a purification step. The yield of pure actin was 0.8--1.5 mg/g wet wt. of uterus, which should be compared with an expected yield of actin from skeletal muscle of 2--4 mg/g wet wt. The actin was pure as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, and exhibited alpha-, beta-, and gamma-forms on isoelectric focusing. It possessed a blocked N-terminal amino acid residue, and its amino acid analysis conformed to those of other actins. The rat uterine actin was available only in small amounts (5--10 mg) and did not polymerize. The pig uterine actin could be obtained in amounts up to 30 mg, polymerized reversibly, and activated a skeletal myosin Mg2+-dependent ATPase.

Skeletal muscle actin mRNA. Characterization of the 3' untranslated region

Plasmids p749, p106, and p150 contain cDNA inserts complementary to rat skeletal muscle actin mRNA. Nucleotide sequence analysis indicates the following sequence relationships: p749 specifies codons 171 to 360; p150 specifies codons 357 to 374 together with 120 nucleotides of the 3'-non-translated region; p106 specifies the last actin amino acid codon, the termination codon and the entire 3' non-translated region. Plasmid p749 hybridized with RNA extracted from rat skeletal muscle, cardiac muscle, smooth (stomach) muscle, and from brain. It also hybridizes well with RNA extracted from skeletal muscle and brain of dog and chick. Plasmid p106 hybridized specifically with rat striated muscles (skeletal and cardiac muscle) mRNA but not with mRNA from rat stomach and from rat brain. It also hybridized to RNA extracted from skeletal muscle of rabbit and dog but not from chick. Thermal stability of the hybrids and sensitivity to S1 digestion also indicated substantial divergence between the 3' untranslated end of rat and dog skeletal muscle actins. The investigation shows that the coding regions of actin genes are highly conserved, whereas the 3' non-coding regions diverged considerably during evolution. Probes constructed from the 3' non-coding regions of actin mRNAs can be used to identify the various actin mRNA and actin genes.

The chromosomal arrangement of two linked actin genes in the sea urchin S. purpuratus

Four distinct actin genes of the sea urchin Strongylocentrotus purpuratus have been isolated from a recombinant Charon 4 phage library of genomic DNA. The four genes differ considerably from each other in many of their restriction sites. Two of the four genes are closely linked; they are present in the same fragment of cloned DNA. This fragment has been extensively mapped, and some parts of the DNA have been sequenced. The two linked genes are oriented in the same direction, separated by 7.5 kb of DNA. One has an intron following the CAG that codes for the glutamine residue at position 121 in the amino acid sequence of actin. This represents the fifth distinct site at which introns have been found in actin genes, suggesting that the primordial actin gene had at least 6 exons and 5 introns. The actin genes from a distinctive family in which most introns have apparently been precisely excised from the genes.

Vascular smooth muscle cells differ from other smooth muscle cells: predominance of vimentin filaments and a specific alpha-type actin

Smooth muscle cells of the digestive, respiratory, and urogenital tracts contain desmin as their major, if not exclusive, intermediate-size filament constituent and also show a predominance of gamma-type smooth muscle actin. We have now examined smooth muscle tissue of different blood vessels (e.g., aorta, small arteries, arterioles, venules, and vena cava) from various mammals (man, cow, pig, rabbit, rat) by one- and two-dimensional gel electrophoresis of cell proteins and by immunofluorescence microscopy using antibodies to different intermediate-sized filament proteins. Intermediate-sized filaments of vascular smooth muscle cells contain abundant amounts of vimentin and little, if any, desmin. On gel electrophoresis, vascular smooth muscle vimentin appears as two isoelectric variants of apparent pI values of 5.30 and 5.29, shows the characteristic series of proteolytic fragments, and is one of the major cell proteins. Thus vimentin has been demonstrated in a smooth muscle cell present in the body. Vascular smooth muscle cells are also distinguished by the predominance of a smooth muscle-specific alpha-type actin, whereas gamma-type smooth muscle actin is present only as a minor component. It is proposed that the intermediate filament and actin composition of vascular smooth muscle cells reflects a differentiation pathway separate from that of other smooth muscle cells and may be related to special functions and pathological disorders of blood vessels.

Actin typing on total cellular extracts: a highly sensitive protein-chemical procedure able to distinguish different actins

Based on the finding that the amino-terminal tryptic peptide of actin is a reliable marker for actin divergence, we describe in detail a highly sensitive protein-chemical procedure for actin typing. The method is performed on non-radioactivity labeled cells and tissues and six actins can be identified unambiguously in warm-blooded vertebrates. The method is quantitative and gives directly the ratio of the different actions in the specimens. It does not require previous purification of actin and can be used on total cellular extracts without any prior fractionation. The procedure can be extended to actins not previously characterized by amino acid sequence analysis and makes certain predictions possible about the partial amino acid sequences of the amino-terminal tryptic peptides, mostly sufficient for a correlation with DNA sequences derived from cloned actin genes. This is done as an example for the cytoplasmic action present in Schneider L-2 Drosophila melanogaster cells. Although the method is currently used routinely on 10(5) cells, modifications are discussed, which should allow the analysis to be performed with even higher sensitivity.

Multigene family of actin-related sequences isolated from a soybean genomic library

We have investigated the actin-related sequences in soybean using heterologous actin DNA probes from Dictyostelium, Drosophila, and yeast. Southern blot analysis of restriction digests of soybean DNA indicates that actin is encoded in a small multigene family. In order to isolate individual members of this gene family, we have constructed a soybean genomic library in the lambda vehicle Charon 4A. A partial characterization of this library shows it to be nearly complete. We have isolated from this library a number of recombinant clones that hybridize to actin-coding sequences from all three heterologous probes. We have identified the fragments containing the actin-related sequences on the physical maps of two of these clones lambda SAc1 and lambda SAc3. These fragments were subcloned in the plasmid vehicle pBR322. Using electron microscope heteroduplex mapping we show that the subclones, pSAc1 and pSAc3, share homology with the entire actin-coding sequence (1.1 kb) of Drosophila and Dictyostelium. Furthermore, pSAc1 and pSAc3 have additional homology of approximately 0.22 kb at the 5' ends of their coding sequences. No homology is detected in the 3' flanking regions of these clones. The actin sequence in pSAc1 contains an interruption of approximately 0.30 kb located 0.39 kb from the 5' end of the actin polypeptide coding region.

Permanently proliferating rat vascular smooth muscle cell with maintained expression of smooth muscle characteristics, including actin of the vascular smooth muscle type

Cells of an established clonal line (RVF-SMC) derived from rat vena cava are described by light and electron microscope methods and biochemical analysis of the major proteins. The cells are flat, and they moderately elongate and form monolayers. They are characterized by prominent cables of microfilaments bundles decoratable with antibodies to actin and alpha-actinin. These bundles contain numerous densely stained bodies and are often flanked by typical rows of surface caveolae and vesicles. The cells are rich in intermediate-sized filaments of the vimentin type but do not show detectable amounts of desmin and cytokeratin filaments. Isoelectric focusing and protein chemical studies have revealed actin heterogeneity. In addition to the two cytoplasmic actins, beta and gamma, common to proliferating cells, two smooth muscle-type actins (an acidic alpha-like and a gamma-like) are found. The major (alpha-type) vascular smooth muscle actin accounts for 28% of the total cellular actin. No skeletal muscle or cardiac muscle actin has been detected. The synthesis of large amounts of actin and vimentin and the presence of at least three actins, including alpha-like actin, have also been demonstrated by in vitro translation of isolated poly(A)+ mRNAs. This is, to our knowledge, the first case of expression of smooth muscle-type actin in a permanently growing cell. We conclude that permanent cell growth and proliferation is compatible with the maintained expression of several characteristic cell features of the differentiated vascular smooth muscle cell including the formation of smooth muscle-type actin.

Coexpression of a mutant beta-actin and the two normal beta- and gamma-cytoplasmic actins in a stably transformed human cell line

HUT 14 is a cloned transformed cell line derived from normal diploid human KD fibroblasts. HUT 14 cells have an altered actin phenotype. In addition to the two nonmuscle actins beta and gamma, also present in the parent KD cells, they show the stable expression of a novel actin species (Ax-actin). Amino acid sequence analysis has been used to identify the three actins of HUT 14 cells. beta- and gamma-actins are identified as normal mammalian nonmuscle actins whereas Ax-actin is characterized as a beta-actin mutant revealing a single amino acid substitution at position 244. The results obtained are compatible with a simple mutational event involving a point mutation in one of the two beta-nonmuscle actin genes assumed to be present in proliferating human diploid fibroblasts. Certain emerging principles of nonmuscle actin gene expression in higher vertebrates are discussed.

Structure and developmental expression of the chick alpha-actin gene

Recombinant DNA clones containing chick alpha-actin mRNA sequence have been isolated and used as probes to analyze the structure and developmental expression of the chick alpha-actin gene. The full length, 2000 nucleotide alpha-actin mRNA is detected in poly(A) RNA at early and late stages of in vivo leg muscle development. As expected, the alpha-actin mRNA is present at very low levels at early myogenic stages but is a high abundance species in terminally differentiated muscle. However, most of the alpha-actin mRNA from fused leg muscle is shorter than 2000 nucleotides, and occurs in relatively discrete size classes. An alpha-actin-like mRNA can be detected in poly(A) RNA from early embryonic brain, indicating that transcription of the alpha-actin gene may not be strictly muscle-specific at all stages of development. We have identified at least 3, very short (< 100 base pairs) intervening sequences in the alpha-actin gene which was isolated from a chick genomic library. The structure of the chick alpha-actin gene differs, therefore, from the structures of actin genes from yeast and Drosophila, both of which contain a single, relatively long, intervening sequence.

A family of structural genes for human lymphoblastoid (leukocyte-type) interferon

Amino acid sequences of tryptic and chymotryptic peptides from human lymphoblastoid interferon (IFN-alpha) have been determined. The results show that IFN-alpha consists of a family of proteins with at least five different, but homologous, primary structures. There appears to be little, if any, glycosylation of the major components of IFN-alpha.

Structure of a split yeast gene: complete nucleotide sequence of the actin gene in Saccharomyces cerevisiae

The complete nucleotide sequence of the actin gene from Saccharomyces cerevisiae has been determined. The coding region is interrupted by a 304-base-pair intervening sequence that is located within the triplet coding for amino acid 4. DNA sequences of the intron-exon junctions are similar to those found in higher eukaryotes and can be aligned such that the intron starts with the dinucleotide 5'-G-T-3' and ends with 5'-A-G-3'. Regions fo homology within the sequences upstream from the initiation codon and those following the termination codon have been detected between the yeast iso-1-cytochrome c gene and the actin gene. As deduced from the nucleotide sequence, yeast actin has 374 amino acid residues. Its primary structure, especially the NH2-terminal third of the protein, is highly conserved during evolution.

Vegetative Dictyostelium cells containing 17 actin genes express a single major actin

Although actin is highly conserved between different eukaryotic species, six tissue-specific actins have been characterised in higher vertebrates by complete amino acid sequence analysis (two cytoplasmic actins, two smooth muscle actins and two sarcomeric actins). Their tissue specificity suggests they may differ in some important although unknown physiological property. Actin expression in lower eukaryotes seems to be a simpler process than in higher eukaryotes since biochemical experiments have indicated only one major type in purified preparations from various species. However, Firtel et al. have isolated several recombinant plasmids containing sequences of Dictyostelium discoldeum DNA complementary to actin messenger RNA and have suggested that this unicellular slime mould may have 17 actin genes potentially giving rise to several different actins. We have, therefore, determined the complete amino acid sequence of actin from vegetative Dictyostelium cells. This sequence is unique and agrees with the DNA sequences of four actin genes for that region of the DNA, which is currently known. The protein sequence does not agree with the three other 'genes' and we discuss the possible expression of minor actin species.

Immunological differences between actins from cardiac muscle, skeletal muscle, and brain

The antigenic similarities and differences between various actins were explored by use of antisera against purified bovine cardiac actin and chicken embryo brain actin. In double-antibody coprecipitation tests, purified iodinated actins from bovine cardiac muscle, rabbit skeletal muscle, chicken embryo brain, and bovine brain all bound to antiserum against chicken embryo brain actin. This result demonstrates the presence of shared antigenic determinants among these actins. Cardiac actin antiserum, on the other hand, bound cardiac and skeletal actin, but failed to bind significantly either brain actin. In radioimmunoassay, all four unlabeled actins were capable of some degree of inhibition of binding of (125)I-labeled chicken embryo brain actin to homologous antiserum. The results confirm the existence of shared or similar antigenic determinants, but also show that the molecules are not antigenically identical. In the cardiac actin radioimmunoassay, unlabeled cardiac and skeletal muscle actins inhibited the binding of (125)I-labeled cardiac actin to homologous antiserum, but neither brain actin inhibited the binding. Thus, the muscle actins possess at least one antigenic determinant not expressed by the brain actins, in addition to the shared determinants. Furthermore, cardiac actin and skeletal actin generated different inhibition curves in the cardiac actin radioimmunoassay, demonstrating that, although antigenically related, they are not identical. Correlations with existing sequence data imply that substitutions in only a few residues alter the antigenic properties of actin.

Isolation and characterization of profilactin and profilin from calf thymus and brain

1. Profilactin and profilin have been purified from calf thymus and calf brain. Thymus profilactin could be crystallized with a similar technique as described earlier for the spleen protein. 2. Preparations of profilactin from the three calf tissues spleen, thymus and brain contain a mixture of beta actin and gamma actin. The ratio beta/gamma differs between the tissues, but is constant throughout the purification steps. 3. The properties of profilin isolated from three sources (spleen, thymus and brain) indicate that it is a highly conserved protein.

In vitro synthesis of beta- and gamma-isoactins by messenger RNA of rat ascites hepatoma

Messenger RNA was extracted from polysomes of rat ascites hepatoma, AH 7974, which contains both beta- and gamma-actins, and was translated in nuclease-treated reticulocyte lysate. The isoactins, beta and gamma, synthesized in vitro were characterized by (1) a high affinity to DNAase I-agarose; (2) polymerization with actin purified from bovine brain; (3) coelectrophoresis with bovine brain actin on two-dimensional gel, and (4) peptide mapping of each isoactin by partial digestion with papain (EC 3.4.22.2) followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The optimum conditions with respect to the concentration of RNA, Mg2+, and K+ for the synthesis of beta- and gamma-isoactins were identical: 300 micrograms/ml, 1.5 mM, and 100 mM, respectively. Aurintricarboxylic acid and 7-methyl-GMP (7MeGMP) inhibited the synthesis of beta- and gamma-actins to the same extent. These results strongly suggest that there is very little possibility of differential translational control of each isoactin gene. When polysomal RNA was separated by sucrose gradient centrifugation, both beta- and gamma-actin mRNAs appeared as a sharp peak at the region slightly heavier than 18 S RNA.

Molecular cloning of the actin gene from yeast Saccharomyces cerevisiae

Two overlapping DNA fragments from yeast Saccharomyces cerevisiae containing the actin gene have been inserted into pBR322 and cloned in E.coli. Clones were identified by hybridization to complementary RNA from a plasmid containing a copy of Dictyostelium actin mRNA. One recombinant plasmid obtained (pYA102) contains a 3.93-kb Hindlll fragment, the other (pYA208) a 5.1-kb Pstl fragment, both share a common 2.2-kb fragment harboring part of the actin gene. Cloned yeast actin DNA was identified by R-loop formation and translation of the hybridized actin mRNA and by DNA sequence analysis. Cytoplasmic actin mRNA has been estimated to be about 1250 nucleotides long. There is only one type of the actin gene in S.cerevisiae.

The actin genes of Drosophila: a dispersed multigene family

We have initiated a study of the organization and expression of the actin genes of D. melanogaster. Using actin gene-specific probes from both chicken and Dictyostelium sources, a clone--denoted lambda DmA2--containing a Drosophila actin gene has been isolated from a representative library of Drosophila genomic DNA cloned in the lambda bacteriophage vector, Charon 4. Southern blotting experiments reveal that there is only one actin structural gene contained in the 17.5 kb Drosophila insert of lambda DmA2 and that the sequences immediately flanking the structural gene are single copy. Observations by electron microscopy of the R loop structures formed by hybridizing total cytoplasmic poly(A)+ RNA from Drosophila embryos to an appropriate subcloned segment of lambda DmA2 indicate that the gene consists of an approximately 70-170 nucleotide leader sequence encoding the 5' portion of the mature mRNA, a 1.65 kb intervening sequence not present in the mRNA and a 1.55 kb sequence containing the major portion of the gene. Using genomic blots with actin-specific probes derived from lambda DmA2, we show that there are six actin genes per haploid Drosophilia genome. They direct the synthesis of three major size classes of mRNA. Using in situ hybridization, the six genes have been localized to six widely dispersed sites on the polytene chromosomes; the locus for lambda DmA2 is 5C on the X chromosome. In vitro translation of mRNA selected hybridization by a DNA segment specific to lambda DmA2 suggests that this particular gene codes for one of the cytoplasmic actin polypeptides.

Intermediate filaments of bovine pulmonary artery smooth muscle distribution, isolation and polypeptide composition

1. The distribution of intermediate filaments in bovine pulmonary artery smooth muscle was studied. 2. These filaments contained at least two polypeptides of 43,000 and 55,000 daltons. 3. The 55,000 dalton polypeptide was purified by ion-exchange chromatography. 4. The amino acid compositions of the polypeptides were determined. 5. The 55,000 dalton polypeptide and a mixture of the 43,000 and 55,000 dalton polypeptide reassembled into structures similar to native intermediate filaments.

Interaction of alpha-actinin, filamin and tropomyosin with F-actin

The abilities of alpha-actinin, filamin and tropomyosin to bind F-actin were examined by cosedimentation experiments. Results indicated that smooth muscle alpha-actinin and filamin can bind to actin filaments simultaneously with little evidence of competition. In contrast, tropomyosin exhibits marked competition with either filamin or alpha-actinin for sites on actin filaments.

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

Complete amino acid sequences for four mammalian muscle actins are reported: bovine skeletal muscle actin, bovine cardiac actin, the major component of bovine aorta actin, and rabbit slow skeletal muscle actin. The number of different actins in a higher mammal for which full amino acid sequences are now available is therefore increased from two to five. Screening of different smooth muscle tissues revealed in addition to the aorta type actin a second smooth muscle actin, which appears very similar if not identical to chicken gizzard actin. Since the sequence of chicken gizzard actin is known, six different actins are presently characterized in a higher mammal. The two smooth muscle actins--bovine aorta actin and chicken gizzard actin--differ by only three amino acid substitutions, all located in the amino-terminal end. In the rest of their sequences both smooth muscle actins share the same four amino acid substitutions, which distinguish them from skeletal muscle actin. Cardiac muscle actin differs from skeletal muscle actin by only four amino acid exchanges. No amino acid substitutions were found when actins from rabbit fast and slow skeletal muscle were compared. In addition we summarize the amino acid substitution patterns of the six different mammalian actins and discuss their tissue specificity. The results show a very close relationship between the four muscle actins in comparison to the nonmuscle actins. The amino substitution patterns indicate that skeletal muscle actin is the highest differentiated actin form, whereas smooth muscle actins show a noticeably cloer relation to nonmuscle actins. By these criteria cardiac muscle actin lies between skeletal muscle actin and smooth muscle actins.