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

A Conversation with Jacob Nachmias

We are sad to report that Professor Jacob (Jack) Nachmias passed away on March 2, 2019. Nachmias was born in Athens, Greece, on June 9, 1928. To escape the Nazis, he and his family came to the United States in 1939. He received his undergraduate degree from Cornell University and then an MA from Swarthmore College, where he worked with Hans Wallach and Wolfgang Kohler; his PhD in Psychology was from Harvard University. Nachmias spent the majority of his career as a Professor of Psychology at the University of Pennsylvania. He made fundamental contributions to our understanding of vision, most notably through the study of eye movements, the development of signal detection theory and forced-choice psychophysical methods, and the psychophysical characterization of spatial-frequency-selective visual channels. Nachmias' work was recognized by his election to the National Academy of Sciences and receipt of the Optical Society's Tillyer Award.

A bioenergetic mechanism for amoeboid-like cell motility profiles tested in a microfluidic electrotaxis assay

The amoeboid-like cell motility is known to be driven by the acidic enzymatic hydrolysis of ATP in the actin-myosin system. However, the electro-mechano-chemical coupling, whereby the free energy of ATP hydrolysis is transformed into the power of electrically polarized cell movement, is poorly understood. Previous experimental studies showed that actin filaments motion, cytoplasmic streaming, and muscle contraction can be reconstituted under actin-activated ATP hydrolysis by soluble non-filamentous myosin fragments. Thus, biological motility was demonstrated in the absence of a continuous protein network. These results lead to an integrative conceptual model for cell motility, which advocates an active role played by intracellular proton currents and cytoplasmic streaming (iPC-CS). In this model, we propose that protons and fluid currents develop intracellular electric polarization and pressure gradients, which generate an electro-hydrodynamic mode of amoeboid motion. Such energetic proton currents and active streaming are considered to be mainly driven by stereospecific ATP hydrolysis through myosin heads along oriented actin filaments. Key predictions of this model are supported by microscopy visualization and in-depth sub-population analysis of purified human neutrophils using a microfluidic electrotaxis assay. Three distinct phases in cell motility profiles, morphology, and cytoplasmic streaming in response to physiological ranges of chemoattractant stimulation and electric field application are revealed. Our results support an intrinsic electric dipole formation linked to different patterns of cytoplasmic streaming, which can be explained by the iPC-CS model. Collectively, this alternative biophysical mechanism of cell motility provides new insights into bioenergetics with relevance to potential new biomedical applications.

Hugh E. Huxley: the compleat biophysicist

The sliding filament model of muscle contraction, put forward by Hugh Huxley and Jean Hanson in 1954, is 60 years old in 2014. Formulation of the model and subsequent proof was driven by the pioneering work of Hugh Huxley (1924-2013). We celebrate Huxley's integrative approach to the study of muscle contraction; how he persevered throughout his career, to the end of his life at 89 years, to understand at the molecular level how muscle contracts and develops force. Here we show how his life and work, with its focus on a single scientific problem, had impact far beyond the field of muscle contraction to the benefit of multiple fields of cellular and structural biology. Huxley introduced the use of x-ray diffraction to study the contraction in living striated muscle, taking advantage of the paracrystalline lattice that would ultimately allow understanding contraction in terms of single molecules. Progress required design of instrumentation with ever-increasing spatial and temporal resolution, providing the impetus for the development of synchrotron facilities used for most protein crystallography and muscle studies today. From the time of his early work, Huxley combined electron microscopy and biochemistry to understand and interpret the changes in x-ray patterns. He developed improved electron-microscopy techniques, thin sections and negative staining, that enabled answering major questions relating to the structure and organization of thick and thin filaments in muscle and the interaction of myosin with actin and its regulation. Huxley established that the ATPase domain of myosin forms the crossbridges of thick filaments that bind actin, and introduced the idea that myosin makes discrete steps on actin. These concepts form the underpinning of cellular motility, in particular the study of how myosin, kinesin, and dynein motors move on their actin and tubulin tracks, making Huxley a founder of the field of cellular motility.

Remembrance of Hugh E. Huxley, a founder of our field

Hugh E. Huxley (1924-2013) carried out structural studies by X-ray fiber diffraction and electron microscopy that established how muscle contracts. Huxley's sliding filament mechanism with an ATPase motor protein taking steps along an actin filament, established the paradigm not only for muscle contraction but also for other motile systems using actin and unconventional myosins, microtubules and dynein and microtubules and kinesin.

A next-generation sequencing approach to study the transcriptomic changes during the differentiation of physarum at the single-cell level

Physarum polycephalum is a unicellular eukaryote belonging to the amoebozoa group of organisms. The complex life cycle involves various cell types that differ in morphology, function, and biochemical composition. Sporulation, one step in the life cycle, is a stimulus-controlled differentiation response of macroscopic plasmodial cells that develop into fruiting bodies. Well-established Mendelian genetics and the occurrence of macroscopic cells with a naturally synchronous population of nuclei as source of homogeneous cell material for biochemical analyses make Physarum an attractive model organism for studying the regulatory control of cell differentiation. Here, we develop an approach using RNA-sequencing (RNA-seq), without needing to rely on a genome sequence as a reference, for studying the transcriptomic changes during stimulus-triggered commitment to sporulation in individual plasmodial cells. The approach is validated through the obtained expression patterns and annotations, and particularly the results from up- and downregulated genes, which correlate well with previous studies.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

Small molecules, big impact: a history of chemical inhibitors and the cytoskeleton

Chemical inhibitors, whether natural products or synthetic, have had an enormous impact on the study of the eukaryotic cytoskeleton. Here we review the history of some of the most widely used cytoskeletal poisons and their influence on our understanding of cytoskeletal functions. We then highlight several new inhibitors and the targeted screens used to identify them and discuss why this approach has been successful.

Actin in the ciliated protozoan Climacostomum virens: purification by DNAse I affinity chromatography, electrophoretic characterization, and immunological analysis

The anti-actin monoclonal antibody (mab) JLA20 (Lin: Proc. Natl. Acad. Sci. U.S.A. 78:2335-2339, 1981) labels a 43 kD protein on Western blots of Climacostomum cell extracts; this protein does not react with an anti-alpha-smooth muscle actin mab (Skalli et al.: J. Cell Biol. 103:2787-2796, 1986) nor with an anti-alpha-sarcomeric actin mab (Skalli et al.: Am. J. Pathol. 130:515-531, 1988). This protein binds to DNAse I and can be purified by DNAse I affinity chromatography. The affinity-purified actin also reacts with mab JLA20. Two-dimensional gel analysis reveals that Climacostomum actin focuses as three spots which are more basic than the mammalian actin isoforms. After addition of KCl, the affinity-purified actin polymerizes into filaments as shown by electron microscopy after negative staining.

Kinetic heterogeneity of F-actin polymers. Further evidence that the elongation reaction may occur through condensation of the actin filaments with small aggregates

We have shown that F-actin, polymerized in 50 mM-KCl at 20 degrees C and pH 8.0, can be resolved by centrifugation into two polymer populations, which differ morphologically as well as kinetically. The first population represents about 10% of the overall polymer and is composed of small amorphous aggregates. It rapidly exchanges the bound nucleotide with free ATP in the medium, either directly or through the monomers. The second population is composed of long actin filaments. These are labelled by free ATP in the medium only through condensation with labelled small amorphous aggregates.

Influence of colchicine on the distribution of horseradish peroxidase in the secretory ameloblast layer in vitro

The distribution of exogenous horseradish peroxidase (HRP) in the secretory ameloblast layer of developing rat molar tooth germs was examined in a culture system with and without colchicine. The secretory ameloblast of cultured tooth germs engulfed HRP added to the medium regardless of the presence of colchicine. The reaction product was localized in various vesicles and granules. Without colchicine in the medium, many small vesicles containing HRP were located in the Tomes' processes, whereas only a few were present with colchicine at concentrations above 5 microM. An intense reaction of HRP also appeared in the distal extracellular spaces beyond the distal junctional complexes of ameloblasts cultured without colchicine, whereas it became almost indiscernible in the tooth germs cultured with colchicine. The lack of HRP in the Tomes' processes and distal extracellular spaces of ameloblasts treated with colchicine might be attributed to the disruption of microtubules. The present study suggests that the secretory ameloblast is able to transport tracer molecules through the intracellular pathway from the proximal and lateral extracellular spaces to the distal extracellular space.

Actin from Saccharomyces cerevisiae

Inhibition of DNase I activity has been used as an assay to purify actin from Saccharomyces cerevisiae (yeast actin). The final fraction, obtained after a 300-fold purification, is approximately 97% pure as judged by sodium dodecyl sulfate-gel electrophoresis. Like rabbit skeletal muscle actin, yeast actin has a molecular weight of about 43,000, forms 7-nm-diameter filaments when polymerization is induced by KCl or Mg2+, and can be decorated with a proteolytic fragment of muscle myosin (heavy meromyosin). Although heavy meromyosin ATPase activity is stimulated by rabbit muscle and yeast actins to approximately the same Vmax (2 mmol of Pi per min per mumol of heavy meromyosin), half-maximal activation (Kapp) is obtained with 14 micro M muscle actin, but requires approximately 135 micro M yeast actin. This difference suggests a low affinity of yeast actin for muscle myosin. Yeast and muscle filamentous actin respond similarly to cytochalasin and phalloidin, although the drugs have no effect on S. cerevisiae cell growth.

Actin from Saccharomyces cerevisiae

Inhibition of DNase I activity has been used as an assay to purify actin from Saccharomyces cerevisiae (yeast actin). The final fraction, obtained after a 300-fold purification, is approximately 97% pure as judged by sodium dodecyl sulfate-gel electrophoresis. Like rabbit skeletal muscle actin, yeast actin has a molecular weight of about 43,000, forms 7-nm-diameter filaments when polymerization is induced by KCl or Mg2+, and can be decorated with a proteolytic fragment of muscle myosin (heavy meromyosin). Although heavy meromyosin ATPase activity is stimulated by rabbit muscle and yeast actins to approximately the same Vmax (2 mmol of Pi per min per mumol of heavy meromyosin), half-maximal activation (Kapp) is obtained with 14 micro M muscle actin, but requires approximately 135 micro M yeast actin. This difference suggests a low affinity of yeast actin for muscle myosin. Yeast and muscle filamentous actin respond similarly to cytochalasin and phalloidin, although the drugs have no effect on S. cerevisiae cell growth.

Cytoplasmic actomyosin fibrils after preservation with high pressure freezing

The fine structure of the actomyosin system of Physarum polycephalum was investigated in vitrified specimens after applying a pressure of greater than 2.1 kbar and freezing rates of 500 to 5,000 degrees C/s. The frozen specimens were either freeze-substituted or freeze-fractured and compared with material processed according to conventional methods of freeze-etching preparation. Artifactual alterations, as seen in the form of destroyed areas of the cytoplasm after chemical fixation, were not observed after freeze-substitution. However, small ice crystals formed by recrystallization within most of the cytoplasmic actomyosin fibrils prevented a fine structural analysis. Such a destruction of the fibrillar fine structure was not found after freeze-etching. In replicas of deep-etched objects 10 nm-thick filaments were localized, which could be conclusively identified as F-actin. The actin filaments are located randomly in the peripheral cytoplasm forming the cell cortex. By the process of parallel aggregation, the filaments can be differentiated to fibrils. Thick myosin filaments were not observed. However, structures resembling cross bridges between single actin filaments suggest the existence of oligomeric myosin. The present investigation shows that, in addition to biomembranes, other cytoplasmic differentiations such as components of the groundplasm can be successfully demonstrated employing the deep-etching technique when the freezing methods are improved by avoiding freeze-protection pretreatments.

Physarum myosin light chain binds calcium

Myosin from the slime mold Physarum polycephalum contains three sizes of polypeptides: a heavy chain and two light chains, LC-1 and LC-2. Using a simple qualitative test for calcium binding by comparing electrophoretic migration of the polypeptides in sodium dodecyl sulfate (SDS) acrylamide gels in the presence and absence of calcium, we have found that Physarum myosin light chain LC-2 migrates with an apparent molecular weight of 16,900 daltons in the presence of the metal ion chelator ethylene glycol bis (B-aminoethyl ether) N,N'-tetraacetic acid (EGTA). However, if calcium chloride is added to the sample prior to electrophoresis, the apparent molecular weight decreases to 16,100. Lanthanide and cadmium ions, but not magnesium, can substitute for calcium. Because the ionic radii of Ca2+, La3+, and Cd2+ are almost identical, we conclude that Physarum myosin LC-2 possesses a very size-specific binding site for calcium. Physarum myosin LC-1 and the heavy chain give no evidence for binding calcium by this test. Since cytoplasmic streaming in the plasmodium of Physarum requires calcium, our evidence indicates that the calcium-binding property of Physarum myosin LC-2 may be important in regulating the production of force by actomyosin in the ectoplasm. Unexpectedly, the myosin light chain in Physarum capable of binding calcium, LC-2, is the essential light chain, while LC-1 is a member of the regulatory class of myosin light chains [V. T. Nachmias, personal communication]. Until now, essential myosin light chains have not been shown to have high affinity divalent cation binding sites. This means a new version of the myosin-based model for actomyosin regulation by calcium may be required to explain cytoplasmic movement in Physarum, and perhaps in other motile systems involving cytoplasmic myosins as well.

The effect of colcemid on the structure and secretory activity of ameloblasts in the rat incisor as shown by radioautography after injection of 3H-proline

Enamel secretion by ameloblasts was investigated in the incisors of 100 gm normal and colcemid-injected male rats. Morphological studies were done on rats given a single intraperitoneal injection of 0.1 mg (1.25 mM) of colcemid and sacrified 1 to 4 hours after injection. Protein synthesis and secretion were investigated with radioautography in normal and colcemid-treated rats injected with 3H-proline and sacrificed at intervals between 0.5 and 3.5 hours after injection. Colcemid was injected 0.5 hours prior to 3H-proline in each experimental rat. Electron microscopic examination revealed several morphological alterations between 1 and 4 hours after injection of colcemid. These changes included fragmentation of the normally elongated rough endoplasmic reticulum into shorter profiles; a disorganization of the normally tubular configuration of the Golgi apparatus into a number of seples and profiles of smooth endoplasmic reticulum from Tomes' processes; and the accumulation of secretion granules at the mature face of the Golgi stacks, as well as in the infranuclear cytoplasm where thye are normally not found. Radioautography revealed that protein synthesis by the rough endoplasmic reticulum had continued in colcemid-altered ameloblasts. Labeled secretion granules were found at the mature surface of the Golgi stacks and in the infranuclear cytoplasm, however they did not migrate into Tomes' processes. Consequently, labeled enamel matrix did not appear extracellularly at the same time as in normal controls. Quantitative radioautography in the light microscope revealed that the effect of colcemid, although reversed within 4 hours, had temporarily inhibited normal migration, and exocytosis of secretion granules.

Effects of colchicine and vinblastine on in vitro gastric secretion

The effects of colchicine and vinblastine on in vitro bullfrog gastric mucosal preparations were studied with respect to H+ and pepsinogen secretion. In the concentration range of 1--50 mM, an initial but transient colchicine-mediated stimulation of H+ secretion is followed by a dose-dependent inhibition. The transient stimulation of H+ secretion can be confirmed in resting preparations in the absence of added secretagogues. In the same concentration range, colchicine inhibits pepsinogen secretion to a greater degree than H+ secretion. Vinblastine (10(-5)--5 X 10(-4) M) was more effective than colchicine in inhibiting both H+ and pepsinogen secretion. The kinetics of inhibition of secretion by both colchicine and vinblastine were slow. Cytochalasin B had no effect on either secretion.

Presence of an actin-like protein in mycelium of Neurospora crassa

Addition of ATP, CaCl2, and KCl to supernatants prepared from mycelia of Snowflake (strain 507), a morphological mutant of Neurospora crassa, results in the formation of filaments 70 nm in diameter. The "decorated" appearance of these filaments after incubation with heavy meromyosin from rabbits suggests they are actin-like.

Actin- and myosin-like filaments in rat brain pericytes

Heavy meromyosin (HMM) labeling was used to identify the nature of the filaments which form bundles in the cytoplasm of the pericytes in brain tissue. Rat brain tissue pieces were incubated in glycerol solutions at 4 degrees and then transferred into buffer (pH 7.0), (1) without HMM, (2) with HMM, (3) with HMM + 5 mM ATP, and (4) with HMM + 2.5 mM Na+ pyrophosphate. In pericytes from untreated tissue, smooth-surfaced microfilaments, averaging 6 nm in diameter, appear to branch and anastomose and to anchor on the plasma membrane. After exposure to HMM, the number and the density of the microfilaments are strikingly increased. These tightly-packed microfilaments are now heavily coated with exogeneous HMM thus increasing in width to 18-20 mm. They intertwine in closely-woven networks. After incubation in HMM solutions containing ATP or Na+ phosphate, they are no longer coated with thick sidearms. It can thus be concluded that these microfilaments are of actin-like nature. In addition, after incubation in ATP, they are intermingled with, and converge onto the surfaces of, thick, tapered filaments, which we have tentatively identified as of myosin-like nature. Thus, it appears that certain of the major elements necessary for contraction are present in brain pericytes.

Actomyosin content of Physarum plasmodia and detection of immunological cross-reactions with myosins from related species

The content of myosin in plasmodia of the myxomycete Physarum polycephalum was measured by an immunological technique, quantitative microcomplement (C') fixation. Migrating plasmodia (starved after growth on rolled oats) contained 0.60 +/- 0.08 (SD) mg myosin per g fresh plasmodia. Myosin comprised 0.77% +/- 0.05 (SD) of the total plasmodial protein. When total plasmodial proteins were separated by electrophoresis on SDS-polyacrylamide gels, a large amount of protein appeared in a band comigrating with muscle actin. Densitometry performed after Coomassie blue staining indicated that as much as 15-25% of the total protein in the plasmodium could be actin. This gives an actin/myosin ratio by weight in the myxomycete plasmodium as high as 19-33, a very "actin-rich" actomyosin compared with rabbit skeletal muscle actomyosin with an actin/myosin ratio of 0.6. Starvation stimulates rapid migration and is correlated with a higher percent of both myosin and actin in the total protein of the plasmodium compared with normally growing cultures. Immunological cross-reaction of myosins from a variety of species was measured by C' fixation using an antiserum produced against purified native myosin from P. polycephalum. Although myxomycete and vertebrate striated muscle myosins have very similar morphological and biochemical properties, and apparently possess similar binding properties to F-actin, only myosins from myxomycetes in the order Physarales, rather closely related to P. polycephalum, gave detectable cross-reactions. This finding suggests that many amino acid sequences in myosin have been variable during evolution.

Studies of muscle proteins in embryonic myocardial cells of cardiac lethal mutant mexican axolotls (Ambystoma mexicanum) by use of heavy meromyosin binding and sodium dodecyl sulfate polyacrylamide gel electrophoresis

In the Mexican axolotl Ambystoma mexicanum recessive mutant gene c, by way of abnormal inductive processes from surrounding tissues, results in an absence of embryonic heart function. The lack of contractions in mutant heart cells apparently results from their inability to form normally organized myofibrils, even though a few actin-like (60-A) and myosin-like (150-A) filaments are present. Amorphous "proteinaceous" collections are often visible. In the present study, heavy meromyosin (HMM) treatment of mutant heart tissue greatly increases the number of thin filaments and decorates them in the usual fashion, confirming that they are actin. The amorphous collections disappear with the addition of HMM. In addition, an analysis of the constituent proteins of normal and mutant embryonic hearts and other tissues is made by sodium dodecyl sulfate (SDS) gel electrophoresis. These experiments are in full agreement with the morphological and HMM binding studies. The gels show distinct 42,000-dalton bands for both normal and mutant hearts, supporting the presence of normal actin. During early developmental stages (Harrison's stage 34) the cardiac tissues in normal and mutant siblings have indistinguishable banding patterns, but with increasing development several differences appear. Myosin heavy chain (200,000 daltons) increases substantially in normal hearts during development but very little in mutants. Even so the quantity of 200,000-dalton protein in mutant hearts is significantly more than in any of the nonmuscle tissues studied (i.e. gut, liver, brain). Unlike normal hearts, the mutant hearts lack a prominent 34,000-dalton band, indicating that if mutants contain muscle tropomyosin at all, it is present in drastically reduced amounts. Also, mutant hearts retain large amounts of yolk proteins at stages when the platelets have virtually disappeared from normal hearts. The morphologies and electrophoresis patterns of skeletal muscle from normal and mutant siblings are identical, confirming that gene c affects only heart muscle differentiation and not skeletal muscle. The results of the study suggest that the precardiac mesoderm in cardiac lethal mutant axolotl embryos initiates but then fails to complete its differentiation into functional muscle tissue. It appears that this single gene mutation, by way of abnormal inductive processes, affects the accumulation and organization of several different muscle proteins, including actin, myosin, and tropomyosin.

Microfilaments in human epithelial cancer cells

The occurence, distribution, and ultrastructural morphology of microfilaments in malignant epithelial cells of invasive squamous cell carcinoma of human oral cavity were studied by electron microscopy. The findings are compared with those in malignant oral epithelial cells of carcinoma-in-situ. In the malignant cells of invasive carcinoma, microfilaments 50-70 A in diameter are prominent in the cortical cytoplasm of the lateral and basal cell surfaces, adjacent and parallel to the plasma membrane, and extending into cell processes and microvillous extensions. Additional microfilaments are found to run from the peripheral cytoplasm to the perinuclear region. The microfilaments are aggregated into bundles aligned parallel to the long axis of the cell and display foci of increased electron density. They also tend to be aggregated into complex polygonal arrays. These microfilaments are similar in organization, concentration and ultrastructural architecture to those of various other nonmuscle cells, where they are thought to be capable of contraction and associated with cell motility. The presence of a microfilament system believed to be associated with contractile and motile cell processes may be an important characteristic of malignant cells of invasive tumors. The lack of abundant organized microfilaments in malignant cells in the absence of tumor invasion, and the presence of a prominent microfilament system in cells of invasive tumors, suggest that the microfilaments are related to the invasive properties of malignant tumor cells.

Cytoplasmic filaments and cellular wound healing in Amoeba proteus

The flexibility and self-healing properties of animal cell surface membranes are well known. These properties have been best exploited in various micrurgical studies on living cells (2, 3), especially in amoebae (7, 20). During nuclear transplantation in amoebae, the hole in the membrane through which a nucleus passes can have a diameter of 20-30 mum, and yet such holes are quickly sealed, although some cytoplasm usually escapes during the transfer. While enucleating amoebae in previous studies, we found that if a very small portion of a nucleus was pushed through the membrane and exposed to the external medium, the amoeba expelled such a nucleus on its own accord. When this happened, a new membrane appeared to form around the embedded portion of the nucleus and no visible loss of cytoplasm occurred during nuclear extrusion. In the present study, we examined amoebae that were at different stages of expelling partially exposed nuclei, to follow the sequence of events during the apparent new membrane formation. Unexpectedly, we found that a new membrane is not formed around the nucleus from inside but a hole is sealed primarily by a constriction of the existing membrane, and that cytoplasmic filaments are responsible for the prevention of the loss of cytoplasm.

Oriented thick and thin filaments in Amoeba proteus

Actin and myosin filaments as a foundation of contractile systems are well established from ameba to man (3). Wolpert et al. (19) isolated by differential centrifugation from Amoeba proteus a motile fraction composed of filaments which moved upon the addition of ATP. Actin filaments are found in amebas (1, 12, 13) which react with vertebrate heavy meromyosin (HMM), forming arrowhead complexes as vertebrate actin (3, 9), and are prominent within the ectoplasmic tube where some of them are attached to the plasmalemma (1, 12). Thick and thin filaments possessing the morphological characteristics of myosin and actin have been obtained from isolated ameba cytoplasm (18, 19). In addition, there are filaments exhibiting ATPase activity in amebas which react with actin (12, 16, 17). However, giant ameba (Chaos-proteus) shapes are difficult to preserve, and the excellent contributions referred to above are limited by visible distortions occurring in the amebas (rounding up, pseudopods disappearing, and cellular organelles swelling) upon fixation. Achievement of normal ameboid shape in recent glycerination work (15) led us to attempt other electron microscope fixation techniques, resulting in a surprising preservation of A. proteus with a unique orientation of thick and thin filaments in the ectoplasmic region.

Motility in Echinosphaerium nucleofilum. II. Cytoplasmic contractility and its molecular basis

Echinosphaerium nucleofilum exhibits at least three kinds of movement: locomotion by the bending and shortening of its many axopodia, feeding by means of food-cup pseudopodia formed from its cortical cytoplasm, and saltatory motion of cytoplasmic particles, especially in the cortex and axopodia. Since previously presented evidence indicated that the microtubular axoneme is not essential for particle motion, the cytoplasm was investigated for the possible existence of contractile behavior and for the possible presence of linear elements other than microtubules. Cytoplasm can be isolated in physiological media in which rigor, relaxation, and contraction can be induced, as in muscle, by manipulating the concentrations of calcium ions and magnesium-adenosine triphosphate. Contraction is initiated by calcium ions at concentrations above 2.4 times 10-minus 7 M. The rigor-to-relaxation transition occurs at subthreshold calcium concentrations on the addition of 10-minus 3 M ATP. Negatively stained preparations of isolated cytoplasm show two types of filaments: thin filaments identified as cytoplasmic actin by virtue of their binding heavy meromyosin from striated muscle in characteristic arrowhead arrays, and thicker filaments which do not strictly resemble myosin aggregates from muscle or amoeba but could conceivably by myosin aggregated in an unfamiliar form.

Inducation of antibody against actin from myxomycete plasmodium and its properties

Plasmodium actin was highly purified by gel filtration of crude G-actin on Sephadex G-100 followed by ultracentrifugation after polymerization in the presence of 1 M urea and 1 mM ATP. Purified actin showed a single band in the sodium dodecyl sulfate gel electrophoretic pattern. Antibody against this purified actin was induced in rabbits. The antibody obtained was immunologically monospecific for plasmodium actin, judging from the following results. (1) The addition of the antibody to a plasmodium F-action solution increased the turbidity of the mixed solution, showing the formation of the antibody-action complex. (2) In immunodiffusion and immunoelectrophoresis, the antibody formed single preciptin lines with the purified actin preparation and with the crude actin extract from the acetone-dried powder of plasmodium. (3) The antibody inhibited polymerization of plasmodium G-actin. (4) Plasmodium F-actin filaments were decorated with antibody in electron micrographs. The antibody reacted not only with plasmodium F- and G-actin, but also reacted with sea urchin egg actin, but it did not react with actin from rabbit striated muscle.

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.

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.

Studies on the possible occurrence of actomyosin-like proteins in phloem

Extracts of large quantities of petioles and of isolated vascular tissues of Heracleum mantegazzianum have been analysed for actomyosin-like contractile proteins. Concentrated preparations, subjected to a standard isolation and purification procedure for actomyosin, failed to demonstrate either superprecipitation or viscosity changes in response to the addition of ATP and divalent cations. In addition, ATPase activities in phloem and xylem extracts have been fractionated by gel electrophoresis and characterised with regard to their substrate specificity, pH optima and ion requirements. Phloem extracts provide two phosphatases: one is non-specific in its substrate requirements; the other is a nucleoside triphosphatase but is stimulated only by monovalent cations and is also present in xylem extracts. All the enzymes are strongly inhibited by divalent cations and do not possess any of the characteristics of ATPases associated with contractile systems. The results are discussed in relation to the postulated involvement of contractile proteins in the translocation of sugars in phloem.

Properties of Physarum myosin purified by a potassium iodide procedure

Myosin has been purified free of actin from Physarum actomyosin by a two step adaptation of the classical potassium iodide method for depolymerizing actin. On 12% sodium dodecyl sulfate (SDS) gels, the single major slowly moving protein band present in the calcium activated adenosine triphosphatase peak (90% pure) is associated with two fast moving bands of molecular weights of approximately 17,000 and 21,000 daltons, respectively. Densitometry shows the molar ratio of heavy chains to the 21,000 and 17,000 dalton chains on the gels to be 1:2:1. The highly purified myosin forms filaments up to 2.5 microm long in the presence of 5 mM magnesium and 0.05 M KCl. Calcium ions were not required for the formation of long filaments from this highly purified myosin. At low ionic strength (0.05 M KCl) the magnesium ATPase of the highly purified myosin is activated four- to tenfold by muscle actin. The extent of activation is a function of the actin concentration and levels off at high levels of actin. In 0.1 mM calcium salts the ATPase activity is approximately 60% of that in 1 mM EGTA. In summary, Physarum myosin is similar to a number of muscle myosins as well as to platelet and fibroblast myosin, which all possess light chains of two different molecular weights associated with the heavy chains. Under ionic conditions close to those in vivo, highly purified Physarum myosin aggregates into long filaments.