Actin and myosin and cell movement

The actin content of fibroblasts

Cultures of chick skin fibroblasts were dissolved in solutions of sodium dodecyl sulphate, and their entire protein content was examined by gel electrophoresis. The most abundant species migrated in the same position as muscle actin. It gave a similar pattern of iodinated peptides after reaction with radioactive sodium iodide and digestion with proteinases, and contained comparable amounts of Nt-methylhistidine. Its amount was estimated by quantitative densitometry of stained gels with bovine serum albumin as an internal standard, and by radioactive assay of cultures that had been grown in the presence of [35S]methionine. The values obtained ranged from 7 to 14% of the total cellular protein, with an average of 8.5%. A protein band in the position of muscle myosin was also present and accounted for about 2.5% of the total protein. Both this and the actin band increased in relative amount with the age of the cultures.

Changing patterns of actin localization during cell division

Changing patterns of actin localization have been studied on a light microscopic level by means of fluorescently labeled heavy meromyosin. The cellular distribution of actin is characterized by four major patterns, each of which corresponds to a particular phase of cell division. Long actin fibers are a prominent feature of the interphase cell. They disappear as the cell rounds up for mitosis and are replaced by a diffuse distribution of actin throughout the cytoplasm. During cytokinesis, the actin is localized predominantly in the cleavage furrow. The final shift of actin occurs after the completion of cytokinesis. At this time the actin becomes concentrated in the distal poles of the cell where pseudopods form to pull the daughter cells apart. When the daughter cells have separated, they flatten on the culture dish and the fibrous pattern of actin characteristic of interphase cells returns. All of these changes take place during the 1-hr period required for cell division.

Identification of actin in situ at the ectoplasm-endoplasm interface of Nitella. Microfilament-chloroplast association

Using a glycerination procedure designed to avoid excessive plasmolysis or disruption of the ectoplasm, microfilaments in bundles at the ectoplasm-endoplasm interface of Nitella internode cell segments were found to bind rabbit heavy meromyosin (HMM) in situ. All HMM arrowheads in a bundle seem to have the same polarity and many lie in register as judged from the electron micrographs; the arrowhead periodicity is approximately 380 . The decorated microfilaments are thus similar to those seen in negatively stained cytoplasmic suspensions of internode cells. In glycerinated material, as well as in suspensions, the microfilaments are closely associated with chloroplasts. The microfilaments lie adjacent to or are attached to the chloroplast envelope. The results provide further evidence that the microfilaments thought to play a role in cytoplasmic streaming in vivo in Nitella consist of actin and suggest that they may be anchored to the chloroplasts.

Cell to substrate adhesion and spreading: inhibition by cationic anesthetics

The plasma membrane is the postulated site of action of anesthetics on nerve or muscle. The drugs may be useful in the analysis of membrane phenomena in other cells. We show here that cationic anesthetics and tranquilizers inhibit cell adhesion and spreading, metabolically dependent processes that involve membrane motility and changes in cell shape. Adhesion was measured by layering 51Cr labeled Sarcoma I (Sa I) cells on glass coverslips for 20 minutes at 34 degrees C, rinsing and estimating the glass-associated radioactivity. Spreading was evaluated microscopically. Both cell adhesion to untreated glass and the Mn2+ dependent adhesion to serum-coated coverslips were inhibited by the drugs, in the following order of increasing activity: tetracaine, promethazine, cyclomethycaine, chlorpromazine and fluphenazine. Similar ranks of drug activity have been reported for nerve blocking, inhibition of cell fusion and inhibition of induced spreading of macrophages. Microscopic observations showed the drugs also inhibited MN2+ INDUCED SPREADING OF Sa I. Drug treated cells were rounded, refractile, devoid of cell processes or ruffles visible by light microscopy. The effects of the drugs on adhesion and spreading were reversible upon washing of the cells. We postulate that the inhibition of adhesion and spreading are a consequence of the inhibition of cell surface motility by the anesthetics.

Detection and ultrastructural localization of human smooth muscle myosin-like molecules in human non-muscle cells by specific antibodies

Spectrin, a protein complex which is peripherally attached to the cytoplasmic surface of the human erythrocyte membrane, cannot be detected (by complement fixation with anti-spectrin antibodies) in homogenates of several different human non-muscle cells studied. On the other hand, a protein antigenically identical or similar to human smooth muscle myosin was detected (by complement fixation with antibodies to uterine smooth muscle myosin) in these cells. In the case of human fibroblast line WI38, this smooth muscle myosin like component was shown (by ferritin-antibody experiments in electron microscopy) to be at least partly associated with cytoplasmic surface of the plasma membrane of the cell. It is proposed that the spectrin complex of the erythrocyte membrane and the smooth muscle myosin-like component of the fibroblast membrane play similar roles in regulating the translational mobilities of integral proteins in their respective membranes.

Localization of tropomyosin in mouse embryo fibroblasts

Antiserum to chick skeletal muscle tropomyosin was used to localize tropomyosin in mouse embryo fibroblasts by the indirect fluorescein labeled antibody technique. Specific staining was observed cytoplasmic fibers, which extended out into the cell processes. The staining pattern in these cells is similar to that previously described by others for actin. This observation suggests that in fibroblasts tropomyosin, like actin, is localized in fibers in the cytoplasm.

The occurrence of actinlike filaments in association with migrating pigment granules in frog retinal pigment epithelium

In the retina of the frog and certain other animals, melanin pigment granules move in response to light so as to shield photoreceptor outer segments. The granules are contained within the cells of the pigment epithelium (PE) which lie as a continuous sheet between the neural retina and the choroid. Moderate illumination of the eye causes the melanin granules to move from a region within a PE cell body into numerous fingerlike extensions of the cell which interdigitate with the receptor outer segments. This migration takes many minutes and is reversed when the light falling on the eye increases in intensity. Several reviews are concerned with the early descriptions of this phenomenon (6,30) and with more recent experiments (1,5,19). The mechanism of the pigment granule motion is undetermined although there are studies concerning PE ultrastructure (8, 23, 31), scanning electron microscopy of the fingerlike extensions of the PE cells (27), the role of the PE in photoreceptor phagocytosis (32), the nature of the pigment granules (19), and the action spectrum of the light which induces the migration (16). This study reports the presence of a system of microfilaments associated with the pigment granules in the fingerlike extensions processes of the PE cells. We demonstrate by heavy meromyosin (HMM) labeling that the filaments are actinlike in character and suggest that these filaments could be responsible for the migration of the melanin pigment granules.

Antibody against tuberlin: the specific visualization of cytoplasmic microtubules in tissue culture cells

Cytoplasmic microtubules in tissue culture cells can be directly visualized by immunofluorescence microscopy. Antibody against tubulin from the outer doublets of sea urchin sperm flagella decorates a network of fine cytoplasmic fibers in a variety of cell lines of human, monkey, rat, mouse, and chicken origin. These fibers are separate and of uniform thickness and are seen throughout the cytoplasm. The fibers disappear either in a medium containing colchicine or after subjection of the cells to low temperature. The same treatments do not destroy the microfilamentous structures that are visualized by means of antibody against actin. When tryspin-treated enucleated cells are replated and then stained with antibody against tubulin, the fibers can be seen to traverse the entire enucleated cytoplasm.

Effects of cytochaslasin B and colcemide on myogenic cultures

Muscle cultures treated with cytochalasin B yield mono- and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arborized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mono-nucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonally-stacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous calbes are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.

Actin filaments in the acrosomal reaction of Limulus sperm. Motion generated by alterations in the packing of the filaments

When Limulus sperm are induced to undergo the acrosomal reaction, a process, 50 mum in length, is generated in a few seconds. This process rotates as it elongates; thus the acrosomal process literally screws through the jelly of the egg. Within the process is a bundle of filaments which before induction are coiled up inside the sperm. The filament bundle exists in three stable states in the sperm. One of the states can be isolated in pure form. It is composed of only three proteins whose molecular weights (mol wt) are 43,000, 55,000, and 95,000. The 43,000 mol wt protein is actin, based on its molecular weight, net charge, morphology, G-F transformation, and heavy meromyosin (HMM) binding. The 55,000 mol wt protein is in equimolar ratio to actin and is not tubulin, binds tenaciously to actin, and inhibits HMM binding. Evidence is presented that both the 55,000 mol wt protein and the 95,000 mol wt protein (possibly alpha-actinin) are also present in Limulus muscle. Presumably these proteins function in the sperm in holding the actin filaments together. Before the acrosomal reaction, the actin filaments are twisted over one another in a supercoil; when the reaction is completed, the filaments lie parallel to each other and form an actin paracrystal. This change in their packing appears to give rise to the motion of the acrosomal process and is under the control of the 55,000 mol wt protein and the 95,000 mol wt protein.

Is P-protein actin-like?-not yet

Microfilaments associated with cytoplasmic streaming in Nitella flexilis internodes can be decorated with heavy meromyosin (HMM) from rabbit, both in vitro in cytoplasmic suspensions, and in situ in glycerinated cell segments. The bound HMM consists of clearly discernible, polarized arrowheads in a regular repeat of 360-380 Å that are similar to those produced on F-actin. In contrast, similar arrowheads or decorations are not evident on P-protein filaments in sieve elements of glycerinated hypocotyl segments of Phaseolus vulgaris L. treated with HMM. Thus, these results contradict a recent claim that P-protein binds HMM and is actin-like. The mass of other evidence now available from diverse studies indicating that P-protein does not consist of actin or tubulin is discussed.

Three dimensional fine structure of cultured cells: possible implications for subcellular motility

To determine the three dimensional fine structure of whole motile cells, rat embryo cells, cultured on Formvar-coated cover-glasses, were glutaraldehyde/osmium-fixed, mounted on grids, dehydrated, critical point dried and examined by transmission electron microscopy using stereoscopic techniques. Three dimensional arrays of organelles occurred in a filament-rich cytoplasmic matrix. Here, besides microtubules and elongate filaments, inter-connected filaments formed a widespread fine-mesh space network which attached to the plasma membrane and closely surrounded all organelles. Negative staining revealed a similar newtork in unfixed cells. It is concluded that this network represents part of the force-generating mechanism for various subcellular movements.

Actin associated with membranes from 3T3 mouse fibroblast and HeLa cells

A protein component of membranes isolated from 3T3 mouse fibroblasts and HeLa cells has been identified as actin by peptide mapping. Extensive but apparently not total coincidence was found between the peptide maps of these two nonmuscle membrane-associated actins compared to chick skeletal muscle actin. Between 2 and 4 percent of the total membrane protein appears in the actin band on sodium dodecyl sulfate polyacrylamide gels of 3T3 membranes while about 4 percent of the membrane protein appears as the actin band from HeLa membranes. These values represent approximately the same proportion of actin to total protein found in the cell homogenates. Treatment of intact cells with levels of cytochalasin B sufficient to cause pronounced morphological changes did not change the amount of actin associated with the membrane in either 3T3 or HeLa cells. However, incubation of isolated membranes under conditions favoring conversion of actin from filamentous to monomeric form resulted in dissociation of approximately 80 and 60 percent of the actin from 3T3 and HeLa membranes, respectively. Thus, approximately 20 percent of 3T3 membrane actin and 40 percent of HeLa membrane actin remained associated with the membrane even under actin depolymerizing conditions.

Actin is the naturally occurring inhibitor of deoxyribonuclease I

Various tissues and cells in culture contain a specific inhibitor of DNase I (EC 3.1.4.5). In this paper evidence is presented that this inhibitor is actin, one of the major structural proteins of muscle and nonmuscle cells. (a) The inhibitor is a major cellular component constituting 5-10% of the soluble protein. (b) It migrates with actin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, having a characteristic molecular weight of 42,000. (c) It has an amino-acid composition closely similar to that of actin. (d) The peptide maps of the two proteins are nearly identical. (e) Skeletal muscle actin inhibits the enzymatic activity of DNase I. (f) DNase I-agarose affinity chromatography quantitatively retains purified skeletal muscle actin, and actin, specifically, from high-speed supernatants of whole cell extracts. (g) An antibody to purified inhibitor protein from calf thymus, used in indirect immunofluorescence on cells grown in culture, stains a two-dimensional network of fibers similar to that seen with an actin-specific antibody.The observation that actin can be isolated by DNase-agarose affinity chromatography provides a useful tool for the biochemical study of actin under different physiological conditions.

Muscle-like contractile proteins and tubulin in synaptosomes

Material in major bands with molecular weights corresponding to those of actin, brain tropomyosin, and myosin is present in purified rat synaptosomes dissolved in sodium dodecyl sulfate and subjected to electrophoresis on dodecyl sulfate-acrylamide gels. A band corresponding to tubulin appears to be the major constituent of synaptosomes, confirming the work of Feit and his coworkers. We have demonstrated by peptide mapping that the proteins in these bands have strong chemical similarities to actin, brain tropomyosin, myosin, and tubulin. We have prepared synaptic membrane, vesicle, and soluble fractions from synaptosomes. The polypeptide composition of synaptic membranes, as determined by dodecyl sulfate-acrylamide gel electrophoresis, is similar to that of synaptosomes, with tubulin, actin, and tropomyosin being major constituents. Synaptic vesicles have as their major polypeptide an unidentified protein with a molecular weight of 50,000; they also have many bands in common with synaptosomes. The soluble fraction predominantly contains actin and tubulin. The possibility that the muscle-like contractile proteins and tubulin are membrane-associated in various cell types is discussed, as is their possible role in neurotransmitter release.

Antibody to myosin: the specific visualization of myosin-containing filaments in nonmuscle cells

Myosin in human, rat, mouse, and chicken fibroblasts was localized by indirect immunofluorescence microscopy using antibodies prepared in rabbits against highly purified chicken gizzard myosin. Filaments containing myosin span the interior of the cells and are often parallel to each other. The majority of the fibers are concentrated toward the adhesive side of the cell. Most of the myosin-containing filaments show "interruptions" or "striations." From a comparison of these fibers in fluorescence and phase microscopy and from previous results on actin-containing fibers, we conclude that at least some of the cytoplasmic myosin can be found in the actin-containing fibers, which themselves have been shown to be very similar or identical to the microfilament bundles. The occurrence of both myosin and actin in the microfilament bundles provides a basis for the motility and contractility of the cell.

The use of cytochalasin B to distinguish myoblasts from fibroblasts in cultures of developing chick striated muscle

Cytochalasin B (5 mug/ml) elicits a differential effect on myoblasts and fibroblasts in culture. After 1 day in culture in the presence of the drug, two types of cells were observed, round cells and cells with elongated arms, designated "arborized" cells. Both cell types were examined in the electron microscope. The round cells contained aggregates of thin and thick filaments as well as a few intact sarcomeres. Within the arms of the arborized cells were bundles of intermediate sized filaments (100 A in diameter). The round cells could be shaken off the culture dish, washed free of cytochalasin B, and recultured to form myotubes. The remaining arborized cells lost their stellate shape when the drug was removed. The progeny of these cells gave rise to normal fibroblasts. Cytochalasin B, thus could be used to identify and isolate myoblasts prior to their fusion into developing muscle. It is suggested that this differential effect of the drug can be used to prepare pure cultures of fusible muscle cells uncontaminated by fibroblasts.

Actin antibody: the specific visualization of actin filaments in non-muscle cells

Actin purified from mouse fibroblasts by sodium dodecyl sulfate gel electrophoresis was used as antigen to obtain an antibody in rabbits. The elicited antibody was shown to be specific for actin as judged by immunodiffusion and complement fixation against partially purified mouse fibroblast actin and highly purified chicken muscle actin. The antibody was used in indirect immunofluorescence to demonstrate by fluorescence light microscopy the distribution and pattern of actin-containing filaments in a variety of cell types. Actin filaments were shown to span the cell length or to concentrate in "focal points" in patterns characteristic for each individual cell.

Heavy meromyosin complexing filaments in the phloem of Vicia faba and Xylosma congestum

Stem sections of Vicia faba L. were incubated with rabbit-muscle heavy meromyosin (HMM) and HMM complexes with phloem filaments (P-protein) were observed with the electron microscope. Treatment of sections of Vicia faba and of Xylosma congestum (Lour.) Merr. with fluorescent HMM resulted in a weak fluorescence of the phloem region. Inasmuch as HMM-binding is believed to be specific for actin-like proteins, it is proposed to classify P-protein as such.