Electron microscope observations on actomyosin and actin preparations from Physarum polycephalum, and on their interaction with heavy meromyosin subfragment I from muscle myosin

Interaction of phalloidin with actin

Phalloidin, a toxic bicyclic peptide of rapid action from the toadstool, Amanita phalloides, gives rise to polymerization of G-actin to filamentous structures (Ph-actin) in a medium of low ionic strength. Ph-actin closely resembles the microfilaments found in liver membrane fractions (Ph-filaments) after in vivo or in vitro poisoning. Both phalloidin induced filaments are resistant to 0.6 M KI in contrast to F-actin, and become decorated by heavy meromyosin. After preincubation with cytochalasin B significantly fewer actin filaments are observed.

Actin in the green alga, Nitella

Bundles of microfilaments very similar in appearance to actin are present in cytoplasmic suspensions obtained from Nitella flexilis. The microfilaments bind rabbit heavy meromyosin in arrowhead arrays similar to those produced on muscle actin. The arrowheads are removed with ATP. The results provide evidence that actin is present in green plants, probably in the form of microfilaments thought to be involved in cytoplasmic streaming.

The biosynthesis of plasmodial myosin during starvation of Physarum polycephalum

The actomyosin protein complex of Physarum polycephalum was prepared from vegetative and starved plasmodia. The yield of actomyosin per unit wet wt. was the same from both types of plasmodia. Myosin was resolved from the complex by gel filtration and purified by ion-exchange chromatography. The Ca(2+)-stimulated adenosine triphosphatase activities of myosin preparations from vegetative and starved plasmodia were not appreciably different. Synthesis of myosin de novo was shown to occur during the starvation phase of the life-cycle by the isolation of labelled myosin preparations from plasmodia starved in the presence of [2-(14)C]glycine. Fractionation of polyacrylamide gels after gel filtration of labelled myosin confirmed the presence of label in the adenosine triphosphatase-active myosin band. It is concluded that during starvation myosin synthesis continues although there is a net loss of approx. 50% of the total protein. Sodium dodecyl sulphate-polyacrylamide-gel electrophoresis of Physarum myosin showed the presence of low-molecular-weight components of the molecule, similar to those of muscle myosins. The content and composition of the free amino acid pool of Physarum was measured at various time-intervals during the vegetative and starvation phases of the life-cycle.

Ultrastructural aspects of a mutant of Schizophyllum commune with continuous nuclear migration

Study of a mutant of the basidomycete Schizophyllum commune with continuous migration of nuclei revealed that the mutant is characterized by vacuolization, bundles of fibrillar-like material, and microtubules. The bundles of fibrillar-like material and microtubules extend through degraded septa to adjacent cells and are found in proximity to nuclei.

The polymerization of actin: its role in the generation of the acrosomal process of certain echinoderm sperm

When Asterias or Thyone sperm come in contact with egg jelly, a long process which in Thyone measures up to 90 microm in length is formed from the acrosomal region. This process can be generated in less than 30 s. Within this process is a bundle of microfilaments. Water extracts prepared from acetone powders of Asterias sperm contain a protein which binds rabbit skeletal muscle myosin forming a complex whose viscosity is reduced by ATP. Within this extract is a protein with the same molecular weight as muscle actin. It can be purified either by collecting the pellet produced after the addition of Mg(++) or by reextracting an acetone powder of actomyosin prepared by the addition of highly purified muscle myosin to the extract. The sperm actin can be polymerized and by electron microscopy the polymer is indistinguishable from muscle F-actin. The sperm actin was shown to be localized in the microfilaments in the acrosomal processes by: (a) heavy meromyosin binding in situ, (b) sodium dodecyl sulfate (SDS) gel electrophoresis of the isolated acrosomal processes and a comparison to gels of flagella which contain no band corresponding to the molecular weight of actin, and (c) SDS gel electrophoresis of the extract from isolated acrosomal caps. Since the precursor for the microfilaments in the unreacted sperm appears amorphous, we suspected that the force for the generation of the acrosomal process is brought about by the polymerization of the sperm actin. This supposition was confirmed, for when unreacted sperm were lysed with the detergent Triton X-100 and the state of the actin in the sperm extract was analyzed by centrifugation, we determined that at least 80% of the actin in the unreacted sperm was in the monomeric state.

Vasopressin: possible role of microtubules and microfilaments in its action

Colchicine, vinblastine, podophyllotoxin, and cytochalasin B inhibit the action of vasopressin and cyclic adenosine monophosphate on osmotic water movement across the toad bladder. The findings suggest that microtubules, and possibly microfilaments, play a role in the action of vasopressin, perhaps through involvement in the mechanism of release of secretory material from the bladder epithelial cells.

Actin in dividing cells: contractile ring filaments bind heavy meromyosin

Many microfilaments and microtubules are well preserved after glycerol-extraction of HeLa cells at room temperature (22 degrees ). Incubation in heavy meromyosin from rabbit skeletal muscle results in conspicuous and characteristic "decoration" of microfilaments of the contractile ring. Decoration is completely prevented by 10 mM ATP or 2 mM pyrophosphate, and fails to occur if heavy meromyosin is either omitted or replaced by egg albumin, a nonspecific protein. Decorated microfilaments have a substructure consisting of polarized, repeating arrowheads 27-35 nm apart. The specificity of these results strongly suggests that microfilaments of the contractile ring in HeLa cells are closely related to muscle actin. Very thin undecorated strands among the microfilaments of the contractile ring possibly represent a myosin component. These findings are discussed in terms of: the actomyosin-like properties of the contractile ring as a mechanochemical organelle that causes cell cleavage; the probable universal occurrence of actin-like protein in all dividing animal cells; and the contractile ring's combined sensitivity to cytochalasin B and its affinity for heavy meromyosin, a combination unique among microfilamentous organelles.

Contact-inhibited revertant cell lines isolated from SV 40-transformed cells. IV. Microfilament distribution and cell shape in untransformed, transformed, and revertant Balb-c 3T3 cells

A COMPARISON IS MADE OF THE ULTRASTRUCTURE OF THE CELL PERIPHERY IN THREE CLONED CELL LINES: untransformed Balb/c 3T3 cells, SV40-transformed Balb/c 3T3 cells, and revertant cells obtained from the transformed cell line by a selection technique utilizing concanavalin A. Both thin-section and surface replication techniques are used for in situ examination of the cell lines. Microfilaments, 70 A in diameter (called alpha filaments), are abundant in untransformed and revertant cell lines, particularly in the anterior expansions of the cells, which tend to have many microvilli and small pseudopodia. Alpha filaments are diminished in the anterior expansions of transformed cells, which contain large blunt pseudopodia and relatively few microvilli. Surface replicas confirm the impression gained from thin sections that transformed cells have a greater proportion of their cell surface involved in bulging pseudopodia than either untransformed or revertant cells. Since alpha filaments are shown to bind heavy meromyosin and are similar to F-actin, these filaments are thought to be important in cell motility. These observations suggest that a close relationship exists between decreased alpha filaments, bulging pseudopodia, and loss of contact inhibition of movement in transformed cells.

Filament formation by purified Physarum myosin

Physarum myosin can be separated from actomyosin by ultracentrifugation, and purified by gel filtration. Unlike actomyosin, myosin is soluble in 0.05 M KCl in the pH range of 6-7. However, in the absence of actin, the slime mold myosin can be precipitated in 0.05 M KCl by the addition of millimolar concentrations of CaCl(2). The precipitates consist of aggregated, short bipolar filaments. Magnesium has a similar effect, but results in the precipitation of more loosely packed aggregates. The length of the compact filaments is 0.45 mum; thus, predominantly tail-to-tail, but also some head-to-tail, interactions occur under these conditions. Since the size and shape of these thick filaments are close to those seen in fixed and sectioned ameboid cells and in platelets, all of these filaments are probably composed of myosins.

The effects of cytochalasin B on the microfilaments of baby hamster kidney (BHK-21) cells

Attempts were made to test the motile functions of bundles of microfilaments found in baby hamster kidney (BHK-21) cells, by using cytochalasin B (CB). It was found that individual cells respond differently to the drug. These differential effects are quite obvious in both light and electron microscope preparations. Some cells contain normal bundles of microfilaments even after 24 hr in CB, and other cells form muscle-like configurations which also contain arrays of microfilaments. These varied effects suggest the existence of several types of microfilaments in BHK-21 cells, and make the interpretation of the motile role of microfilaments difficult to evaluate at the present time.

The sensitivity of developing cardiac myofibrils to cytochalasin-B (electron microscopy-polarized light-Z-bands-heartbeat)

Developing cardiac muscle cells of 11- to 13-somite chick embryos are sensitive to cytochalasin-B. In cultured chick embryos, ranging in development from 11 to 13 somites, hearts stop beating in the presence of this agent. Both polarized light and electron microscopic examination show that cytochalasin-B disrupts existing myofibrils and inhibits the formation of new ones. Discrete Z-bands are not present in treated heart cells and thick, presumably myosin, filaments are found in disarray. These effects are reversible; after cytochalasin-B is removed from the medium, heartbeat recovers and myofibrils with discrete Z-bands reappear. Fibrillar sensitivity appears to be a function of age since fibrils in hearts of embryos having from 22 to 28 pairs of somites are more resistant.

The actin and myosin filaments of human and bovine blood platelets

The contractility of platelets has been attributed to an actomyosin-like protein which has been well defined on a physicochemical basis. Moreover, platelets contain +/-80 A filaments which resemble actin filaments in smooth muscle. Studies were undertaken on human and bovine platelets to better define the morphologic structures which may subserve this contractile function. In order to identify actin, the ability of the filaments to react with heavy meromyosin (HMM) was tested. Accordingly, platelets were glycerinated and treated with HMM. In addition, platelet actin was extracted, reacted with HMM, and examined by negative staining. In both instances typical arrowhead structures with clearly defined polarity and a periodicity of +/-360 A formed. As is the case with purified muscle actin, the complexes were dissociable with Mg-ATP. The formation of myosin-like filaments was observed when osmotically shocked platelets were incubated with MgCl(2) and excess ATP. These "thick" filaments measured 250-300 A in width, tapered at both ends and often occurred in clumps. They resembled aggregates of thick filaments described in contracted smooth muscle. Extraction of platelets by methods suitable for the demonstration of myosin showed filaments with an average length of 0.3 mu, a smooth shaft, and frayed or bulbous ends. These appeared identical to those seen in synthetically prepared myosin of striated muscle. It is suggested that the filaments described here represent the actin and myosin of platelets.

Cytochalasin B, its interaction with actin and actomyosin from muscle (cell movement-microfilaments-rabbit striated muscle)

Cytochalasin B, an alkaloid that inhibits a wide variety of cellular movements, interacts with actomyosin, the contractile protein complex of striated muscle. This interaction causes a decrease in viscosity of the actomyosin complex and an inhibition of acto-heavy meromyosin ATPase activity of at least 60%. Cytochalasin B does not affect the viscosity of myosin nor the ATPase activity of heavy meromyosin, suggesting that the drug might interact directly with the actin moiety of the actomyosin complex. Indeed, as judged by viscometry, there is a strong interaction of cytochalasin B with actin, at nearly stoichiometric concentrations. Myosin appears to compete with cytochalasin for binding to actin.

Cytochalasin B: effects on cell morphology, cell adhesion, and mucopolysaccharide synthesis (cultured cells-contractile microfilaments-glycoproteins-embryonic cells-sorting-out)

Cytochalasin B reversibly causes extensive branching of myoblasts, fibroblasts, and nonencapsulated chondroblasts; it does not induce the formation of similar processes in myotubes, erythrocytes, amnion cells, encapsulated chondroblasts, or HeLa cells. The drug has no effect on the spontaneous contractions of isolated skeletal, cardiac, or smooth-muscle cells. Within 60 min, it depresses the incorporation of [(14)C]glucosamine into total mucopolysaccharide and glycoproteins by over 50%. The drug interferes with adhesion and sorting-out of dissociated embryonic cells. Cytochalasin B is likely to produce changes in components of the cell surface whose function is not readily or solely related to a system of "primitive contractile microfilaments."

The role of three cytoplasmic fibers in BHK-21 cell motility. I. Microtubules and the effects of colchicine

Microtubule breakdown in the presence of 5 or 40 microg/ml of colchicine is observed in BHK-21/C13 fibroblast-like cells. Several morphological and physiological effects are noted in the absence of microtubules: (a) the cells transform from fibroblast-like to epithelial-like cells; (b) the normal pattern of intracellular birefringence changes and a juxtanuclear cap of birefringent filaments is formed; (c) time-lapse cinematography demonstrates that cell locomotion is inhibited in colchicine-treated cells, even though membrane ruffling persists. The results are discussed in terms of the specific roles of microtubules in cultured cell motility and possible functional relationships of the three types of cytoplasmic fibers seen in BHK-21 cells.

Actin in the brush-border of epithelial cells of the chicken intestine

The major soluble protein of the isolated brush-border of the intestinal epithelium has a molecular weight and net charge indistinguishable from those of skeletal-muscle actin, as determined by polyacrylamide gel electrophoresis. Furthermore, this protein, isolated from acetone powders of the purified brush-border, undergoes a G to F transformation in the presence of Mg(++). The filaments have a substructure indistinguishable from muscle actin, as seen by the negative-staining technique, and bind heavy meromyosin with the arrowhead configuration characteristic of actin. The filaments in the microvilli of the intact bruch-border also bind heavy meromyosin. Thus, actin seems to be present in intestinal epithelial cells.

Local supply of energy to the fast axoplasmic transport mechanism

Local anoxia of a small region of cat sciatic nerves causes a block of fast axoplasmic transport in vitro. Activity becomes dammed up at the face of the anoxic region, and falls off rapidly distally just inside the anoxic region. Such blocks were reversible if they were removed within 1 hr. Earlier studies had given evidence that fast axoplasmic transport is closely dependent on ATP derived from oxidative metabolism. The present results indicate that ATP is required all along the length of the nerve fiber to supply a hypothesized "transporting filament" mechanism of fast transport.