The contractile basis of amoeboid movement. I. The chemical control of motility in isolated cytoplasm

A calcium-sensitive preparation from Physarum polycephalum

Differential ultracentrifugation of an extract of the plasmodium of Physarum polycephalum yields a high-speed fraction which exhibits calcium-sensitive adenosine triphosphate activity at low ionic strength. The rate of inorganic phosphate production increased from 2- to 25-fold in different preparations when the calcium concentration was increased from about 10(-8) to 10(-5) M. Complement fixation using specific antibody to Physarum myosin showed the fraction to contain 3% myosin. By electron microscopy, actin-like microfilaments 50--150 nm long were present. Addition of pure rabbit F-actin or myosin to this fraction activated the ATPase measured in EGTA and so partially reversed the calcium sensitivity. If muscle myosin was added to the supernatant from which the fraction was centrifuged, a "hybrid complex" was obtained which included actin and additional protein from the plasmodium, and this hybrid was also calcium sensitive. Over 85% of the calcium-sensitive, magnesium-activated ATPase could be precipitated by sequential "hybrid" formation. The calcium sensitivity of the hybrid was maximal when formed at the lowest ratios of added myosin to Physarum proteins. It is concluded that the results do not allow a simple interpretation along the lines of either actin-linked or myosin-linked sensitivity. Evidence consistent with both a form of actin-linked and myosin-linked sensitivity is present in our results.

Contraction of isolated brush borders from the intestinal epithelium

Brush borders isolated from epithelial cells from the small intestine of neonatal rats are able to contract in the presence of ATP and Mg2+; Ca2+ is not required. Contraction is characterized by a pinching-in of the plasma membrane in the region of the zonula adherens and a subsequent rounding of the brush borders. No movement or consistent shortening of the microvilli is observed. The contraction appears to involve the 5- to 7-nm diameter microfilaments in the terminal web which associate with the zonula adherens. These filaments bind heavy meromyosin as do the actin core filaments of the microvilli. A model for contraction is presented in which, in the intact cell, terminal web filaments and core filaments interact to produce shortening of the microvilli.

Molecular analysis of the histone gene cluster of Psammechinus miliaris: I. Fractionation and identification of five individual histone mRNAs

The electrophoretic separation of labeled "9S" histone mRNAs obtained from cleaving sea urchin polysomes was found at first to be highly unreproducible. It became evident that the secondary structure of the individual mRNAs had a greater effect on their relative electrophoretic mobilities than did their molecular weight differentials. We determined the parameters affecting electrophoretic mobility by the novel method of running the labeled polysomal RNA in slab gels across polyacrylamide and urea gradients. The initially complex and species-specific electrophoretic pattern could then, by a judicious choice of denaturing conditions, be simplified to yield five well defined classes of labeled mRNAs. Using optimal conditions for the separation of the RNA components, five messengers were isolated from Psammechinus embryos by preparative disc electrophoresis, four of which, after two electrophoretic separations, exhibited a unimodal distribution. Each of the mRNAs was translated in vitro, four of the five fractions promoting the synthesis of one major protein. The in vitro products were characterized by comparison of their electrophoretic mobilities with those of known sea urchin histones. It was thus possible to correlate individual mRNAs with specific histones. We propose that the five mRNAs designated a-e in order of decreasing electrophoretic mobility code for the histones H4, H2A, H2B, H3, and H1.

The contractile basis of ameboid movement. II. Structure and contractility of motile extracts and plasmalemma-ectoplasm ghosts

The role of calcium and magnesium-ATP on the structure and contractility in motile extracts of Amoeba proteus and plasmalemma-ectoplasm "ghosts" of Chaos carolinensis has been investigated by correlating light and electron microscope observations with turbidity and birefringence measurements. The extract is nonmotile and contains very few F-actin filaments and myosin aggregates when prepared in the presence of both low calcium ion and ATP concentrations at an ionic strength of I = 0.05, pH 6.8. The addition of 1.0 mM magnesium chloride, 1.0 mM ATP, in the presence of a low calcium ion concentration (relaxation solution) induced the formation of some fibrous bundles of actin without contracting, whereas the addition of a micromolar concentration of calcium in addition to 1.0 mM magnesium-ATP (contraction solution) (Taylor, D. L., J. S. Condeelis, P. L. Moore, and R. D. Allen. 1973. J. Cell Biol. 59:378-394) initiated the formation of large arrays of F-actin filaments followed by contractions. Furthermore, plasmalemma-ectoplasm ghosts prepared in the relaxation solution exhibited very few straight F-actin filaments and myosin aggregates. In contrast, plasmalemmaectoplasm ghosts treated with the contraction solution contained many straight F-actin filaments and myosin aggregates. The increase in the structure of ameba cytoplasm at the endoplasm-ectoplasm interface can be explained by a combination of the transformation of actin from a less filamentous to a more structured filamentous state possibly involving the cross-linking of actin to form fibrillar arrays (see above-mentioned reference) followed by contractions of the actin and myosin along an undetermined distance of the endoplasm and/or ectoplasm.

Calcium control of actin-activated myosin adenosine triphosphatase from Dictyostelium discoideum

A protein fraction from the cellular slime mold Dictyostelium discoideum confers Ca2+-sensitivity on the activation of purified myosin adenosinetriphosphatase (ATP phosphohydrolase, EC 3.6.1.3) from Dictyostelium by purified Dictyostelium actin. That is, the fraction inhibits the actomyosin adenosine triphosphatase activity in the absence of Ca+ but not in the presence of Ca2+. This Ca2+-sensitizing factor affects only the actin-activated myosin adenosine triphosphatase and not the enzyme activity of myosin alone. The Ca2+-sensitivity is conserved when muscle actin replaces Dictyostelium actin, but is lost when muscle myosin replaces Dictyostelium myosin. The factor appears to be a protein since it is nondialyzable, is heat labile, and can be precipitated with ammonium sulfate. The factor can be purified 70-fold on an actin-affinity column.

Histone-gene reiteration in the genome of mouse

Histone messenger RNA labelled to high specific activity has been isolated and purified from the mouse tissue-culture cell line L929. It had an electrophoretic mobility equivalent to a mean molecular weight of 1.4 X 10(5). The synthesis of this RNA was suppressed, but apparently not completely so, by inhibition of cellular DNA synthesis to more than 95% by addition of cytosine arabinoside. The synthesis of other RNA species remained unaffected by this treatment. Poly(A)-containing mRNA, representing a 20% contaminant of the electrophoretically purified histone mRNA, was removed by poly(U)-Sepharose chromatography. Histone mRNA thus purified was hybridized to DNA fractions enriched for histone genes of the sea urchin Psammechinus miliaris by actinomycin/CsCl gradient centrifugation. The mRNA was eluted from the hybrids and challenged with mouse DNA in vast DNA excess conditions. A (cot1/2) of 360 mol s 1(-1) was obtained from the RNA trace curve, suggesting a 10-20-fold reiteration of the histone genes in the haploid genome. Thus histone genes in mouse are much less highly reiterated than in sea urchins, but nevertheless are present in considerable excess over the number theoretically necessary for histone synthesis during the S phase of the cell cycle.

The role of actin in the temperature-dependent gelation and contraction of extracts of Acanthamoeba

The temperature-dependent assembly and the interaction of Acanthamoeba contractile proteins have been studied in a crude extract. A cold extract of soluble proteins from Acanthamoeba castellanii is prepared by homogenizing the cells in a sucrose-ATP-ethyleneglycol-bis-(beta-aminoethyl ether) N,N'-tetraacetic acid buffer and centrifuging at 136,000 g for 1 h. When this supernate of soluble proteins is warmed to room temperature, it forms a solid gel. Upon standing at room temperature, the gel slowly contracts and squeezes out soluble components. The rates of gelation and contraction are both highly temperature dependent, with activation energies of about 20 kcal per mol. Gel formation is dependent upon the presence of ATP and Mg++. Low concentrations of Ca++ accelerate the contractile phase of this phenomenon. The major protein component of the gel is actin. It is associated with myosin, cofactor, a high molecular weight protein tentatively identfied as actin-binding protein, and several other unidentified proteins. Actin has been purified from these gels and was found to be capable of forming a solid gel when polymerized in the presence of ATP, MgCl3, and KCL. The rate of purified actin polymerication is very temperature dependent and is accelerated by the addition of fragments of muscle actin filaments. These data suggest that Acanthamoeba contractile proteins have a dual role in the cell; they may generate the forces for cellular movements and also act as cytoskeletal elements by controlling the consistency of the cytoplasm.

Interactions of actin, myosin, and a new actin-binding protein of rabbit pulmonary macrophages. II. Role in cytoplasmic movement and phagocytosis

Actin and myosin of rabbit pulmonary macrophages are influenced by two other proteins. A protein cofactor is required for the actin activation of macrophage myosin Mg2 ATPase activity, and a high molecular weight actin-binding protein aggregates actin filaments (Stossel T.P., and J.H. Hartwig. 1975. J. Biol. Chem. 250:5706-5711)9 When warmed in 0.34 M sucrose solution containing Mg2-ATP and dithiothreitol, these four proteins interact cooperatively. Acin-binding protein in the presence of actin causes the actin to form a gel, which liquifies when cooled. The myosin contracts the gel into an aggregate, and the rate of aggregation is accelerated by the cofactor. Therefore, we believe that these four proteins also effec the temperature-dependent gelation and aggregation of crude sucrose extracts pulmonary macrophages containing Mg2-ATP and dithiothreitol. The gelled extracts are composed of tangled filaments. Relative to homogenates of resting macrophages, the distribution of actin-binding protein in homogenates of phagocytizing macrophages is altered such that 2-6 times more actin-binding protein is soluble. Sucrose extracts of phagocytizing macrophages gel more rapidly than extracts of resting macrophages. Phagocytosis by pulmonary macrophages involves the formation of peripheral pseudopods containing filaments. The findings suggest that the actin-binding protein initiates a cooperative interaction of contractile proteins to generate cytoplasmic gelation, and that phagocytosis influences the behavior of the actin-binding protein.

Cytoskeletal functions of cytoplasmic contractile proteins

This is a review of the evidence that the cytoplasmic contractile proteins function as a cytoskeletal system in the cytoplasmic matrix. Biochemical experiments show that cycoplasmic actin filaments can form a solid gel under conditions likely to exist in living cells. The actin filaments are associated with other proteins which amy stabilize the gel and which are involved with motile force generation like myosin. Ultrastructural studies show that actin filaments are difficult to preserve, but that under stabilizing conditions networks of actin filaments are found throughtout the cytoplasmic matrix.

Stage-specific switches in histone synthesis during embryogenesis of the sea urchin

Histones H2A and H2B of the sea urchin embryo have been resolved by new methods into components that are synthesized at different stages of development. One form of H2A and one form of H2B are synthesized only during the period from fertilization to the blastula stage. Subsequently, two other types of H2A and H2B molecules are synthesized. In addition, a histonelike protein was detected which is synthesized only from fertilization until the 16-cell stage when the synthesis of still another H2A-like protein begins. None of the late-appearing forms are derived from histone polypeptide chains synthesized earlier in development. Since the early components do not disappear after their synthesis stops, these modulations of histone synthesis lead to an increase in histone multiplicity, concomitant with the beginning of cell diversification and a decrease in cell division rate.

Microtubular origin of mitotic spindle form birefringence. Demonstration of the applicability of Wiener's equation

Meiosis I metaphase spindles were isolated from oocytes of the sea-star Pisaster ochraceus by a method that produced no detectable net loss in spindle birefringence. Some of the spindles were fixed immediately and embedded and sectioned for electron microscopy. Others were laminated between gelatine pellicles in a perfusion chamber, then fixed and sequentially and reversibly imbibed with a series of media of increasing refractive indices. Electron microscopy showed little else besides microtubules in the isolates, and no other component present could account for the observed form birefringence. An Ambronn plot of the birefringent retardation measured during imbibition was a good least squares fit to a computer generated theoretical curve based on the Bragg-Pippard rederivation of the Wiener curve for form birefringence. The data were best fit by the curve for rodlet index (n1) = 1.512, rodlet volume fraction (f) = 0.0206, and coefficient of intrinsic birefringence = 4.7 X 10(-5). The value obtained for n1 is unequivocal and is virtually as good as the refractometer determinations of imbibing medium index on which it is based. The optically interactive volume of the microtubule subunit, calculated from our electron microscope determination of spindle microtubule distribution (106/mum2), 13 protofilaments per microtubules, an 8 nm repeat distance and our best value for f, is compatible with known subunit dimensions as determined by other means. We also report curves fitted to the results of Ambronn imbibition of Bouin's-fixed Lytechinus spindles and to the Noll and Weber muscle imbibition data.

[Fibrillar and tubular fine structures in the cytoplasm of Entamoeba histolytica (author's transl)]

The electron-microscopical study of conventional fixed and contrasted Entamoeba histolytica-trophozoites from Diamonds monoxenic TTY-medium revealed "thick fibrils" in the vesicular cytoplasm of the parasites. They are 9-14 nm in diameter and are therefore considered to be myosin-like filaments, that are spread mainly in the tail or the uroid of the moving ameba. No association with microfilaments (MF) or other organelles could be observed. During the same investigation microtubules (MT) of variable length are described having a diameter of 35 nm, surrounded by a 19-25 nm wide capsule. They are distributed singly or in bundles with a maximal diameter of 350 nm. Some of the MT were coiled up to a helical shape.

Leucocyte myosin and its location in the cell

The intracellular location of the binding site of antibody against purified myosin prepared from equine leucocytes was investigated in neutrophils and lymphocytes by electron microscopy using peroxidase-labelled antibody method. The myosin extracted from equine leucocytes could bind skeletal muscle F-actin and the formed complex showed the biophysical and biochemical properties and electron microscopic appearance of actomyosin. On immunodiffusion, the leucocyte myosin formed a single precipitin line with its antibody prepared in rabbits. The antibody also formed single precipitin lines with myosins from lymphocytes and thrombocytes, fusing with each other. The antibody against the leucocyte myosin did not react with myosins from skeletal or arterial smooth muscle. The specificity of the antibody was further established by determination of K+-EDTA-activated ATPase activity remained in the supernate of antigen-antibody mixture. Under electron microscope, the intracellular immunoreactive products of peroxidase labelled antibody were found in cytoplasm of neutrophils and lymphocytes incubated with antibody against leucocyte myosin, but not in neutrophils or lymphocytes treated with IgG from normal rabbits.

Actin-like filaments in bone cells of cultured mouse calvaria as demonstrated by binding to heavy meromyosin

A variety of intracellular filaments (50-70 A in diameter) found in bone cells was shown to bind specifically to HMM. Because of this property, these filaments are probably biochemically similar to muscle actin. in osteoblasts and osteocytes, these reactive filaments were oriented in bundles parallel to the plasma membrane and filling the cell processes. In the osteoclast the filaments along the cell membrane were not so highly organized. In the clear zone, the quiescent part of the cell adjacent to the motile ruffled border, organized filament bundles were oriented perpendicular to the cell membrane and terminated in short processes at the bone surface. These filaments were also reactive with HMM. The possible significance of the filaments with respect to the physiology of bone cells is discussed.

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.

Occurrence of cell junctions and microfilaments in osteoblasts

Osteoblasts in the diaphysis of the tibia during endochondral ossification in young rats are attached to one another by nexus, by "adhaerens" junctions, and by simple appositions. "Adhaerens" junctions and nexus also occur between preosteoblasts and osteoblasts. Furthermore, to osteoblasts exhibit a network of microfilament bundles in the cell periphery overlying the osteoid. From this network bundles extend into the cell processes which protrude into the unmineralized matrix. The mean diameter of individual microfilaments is 5.9 +/- 0.06 nm. A possible role of nexus and microfilaments in controlled bone growth and differentiation is discussed.

Polarizing fucoid eggs drive a calcium current through themselves

Calcium ions enter the prospective growth pole of polarizing Pelvetia eggs faster than the opposite pole and leave this antipode faster than the growth pole. The calcium current is greatest when first measured at 6 hours after fertilization and decreases as the time of final commitment to growth in a particular direction approaches.

Further evidence of transcriptional and translational control of histone messenger RNA during the HeLa S3 cycle

Using a translational assay and new analytical procedures we have found that: -Histone mRNA can be detected both associated with polyribosomes and in the postribosomal supernatant of S phase HeLa S3 cells.-Inhibition of DNA replication by cytosine arabinoside treatment causes histone mRNA to completely disappear from polyribosomes, and little histone mRNA can be detected in the postribosomal supernatant of inhibited cells. These data indicate that histone mRNA does not accumulate in the cytoplasm after the inhibition of DNA replication. -Histone mRNA species cannot be detected in the postribosomal supernatant of G1 cells synchronized by selective detachment. This observation, together with the previous finding that histone mRNA is not present on G1 polyribosomes, is consistent with the idea of a transcriptional block in histone mRNA production and transport to the cytoplasm during G1.

Endoplasmic filaments generate the motive force for rotational streaming in Nitella

The streaming endoplasm of characean cells has been shown to contain previously unreported endoplasmic filaments along which bending waves are observed under the light microscope using special techniques. The bending waves are similar to those propagated along sperm tails causing propulsion of sperm. In Nitella there is reason to believe that nearly all of the filaments are anchored in the cortex and that their beating propels the endoplasm in which they are suspended. This hypothesis is supported by calculations in which typical and average wave parameters have been inserted into the classical hydrodynamic equations derived for sperm tail bending waves. These calculations come within an order of magnitude of predicting the velocity of streaming and they show that waves of the character described, propagated along an estimated 52 m of endoplasmic filaments per cell, must generate a total motive force per cell within less than an order of magnitude of the forces measured experimentally by others. If we assume that undulating filaments produce the force driving the endoplasm, then the method described for measuring the motive force could lead to a lower than actual value for the motive force, since both centrifugation and vacuolar perfusion would reverse the orientation of some filaments. Observations of the initiation of particle translation in association with the filaments suggest that particle transport and wave propagation, which occur at the same velocity, may both be dependent on the same process. The possibility that some form of contractility provides the motive force for filament flection and particle transport is discussed.

Action of cytochalasin D on cells of established lines. II. Cortex and microfilaments

Cells in culture exposed to cytochalasin D (CD) rapidly undergo a long-sustained tonic contraction. Coincident with this contracture the thin microfilaments of the cortex become compacted into feltlike masses. The ravelled filaments of these masses remain actinlike and bind heavy meromyosin; they are not disrupted or disaggregated, but rather, appear to represent a contracted state of the microfilament apparatus of the cell cortex. On continued exposure to CD, 'myoid' bundles, containing thick, dense filaments, and larger fusiform or ribbonlike, putatively myosinoid, aggregates may appear. These appearances are interpreted as consequences of a state of hypercontraction without relaxation induced by CD. They do not occur in CD-treated cells prevented from contracting by inhibitors of energy metabolism, and are readily reversible on withdrawal of CD. Extensive ordered arrays of thin microfilaments develop in cells which are reextending during early recovery.