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PubMed2001

Circular F-actin bundles and a G-actin gradient in pollen and pollen tubes of Lilium davidii.

The distribution of and relationship between F-actin and G-actin were investigated in pollen grains and pollen tubes of Lilium davidii Duch. using a confocal laser scanning microscope after fluorescence and immunofluorescence labeling. Circular F-actin bundles were found to be the main form of microfilament cytoskeleton in pollen grains and pollen tubes. Consistent with cytoplasmic streaming in pollen tubes, there were no obvious F-actin bundles in the 10- to 20-microm tip region of long pollen tubes, only a few short F-actin fragments. Labeling with fluorescein isothiocyanate (FITC)-DNase I at first established the presence of a tip-focused gradient of intracellular G-actin concentration at the extreme apex of the tube, the concentration of G-actin being about twice as high in the 10- to 20-microm region of the tip as in other regions of the pollen tube. We also found that the distribution of G-actin was related negatively to that of the F-actin in pollen tubes of L. davidii. Caffeine treatment caused the G-actin tip-focused gradient to disappear, and F-actin to extend into the pollen tube tip. Based on these results, we speculate that the circular F-actin bundles may be the track for bidirectional cytoplasmic streaming in pollen tubes, and that in the pollen tube tip most of the F-actin is depolymerized into G-actin, leading to the absence of F-actin bundles in this region.

Actins↗
PubMed2001

Effects of Ca(2+) and calmodulin on the motile activity of characean myosin in vitro.

It is well known that the cytoplasmic streaming of characean cells is readily inhibited by Ca(2+). However, neither the actin-activated MgATPase nor the in vitro motile activity of purified characean myosin were inhibited by Ca(2+). Recently, amino acid sequence of characean myosin was determined in our laboratory and the sequence revealed that characean myosin contains six calmodulin binding sites in the neck region. We also detected calmodulin in quickly prepared characean myosin fraction. It is, therefore, possible that the insensitivity of characean myosin to Ca(2+) is due to the dissociation of some calmodulin molecules from the neck region during the course of protein purification. To determine strictly the Ca(2+) sensitivity of characean myosin, we intentionally used crude preparation of characean myosin to reduce the possibility of calmodulin dissociation and examined the motile activity of characean myosin in vitro in the presence of excess characean calmodulin. We could not observe any drastic inhibition of characean myosin activity by Ca(2+). The results suggest that the brief cessation of cytoplasmic streaming is not caused by the direct inhibition of myosin activity by Ca(2+).

Actins↗
PubMed2003

Localization and characterization of the inhibitory Ca2+-binding site of Physarum polycephalum myosin II.

A myosin II is thought to be the driving force of the fast cytoplasmic streaming in the plasmodium of Physarum polycephalum. This regulated myosin, unique among conventional myosins, is inhibited by direct Ca2+ binding. Here we report that Ca2+ binds to the first EF-hand of the essential light chain (ELC) subunit of Physarum myosin. Flow dialysis experiments of wild-type and mutant light chains and the regulatory domain revealed a single binding site that shows moderate specificity for Ca2+. The regulatory light chain, in contrast to regulatory light chains of higher eukaryotes, is unable to bind divalent cations. Although the Ca2+-binding loop of ELC has a canonical sequence, replacement of glutamic acid to alanine in the -z coordinating position only slightly decreased the Ca2+ affinity of the site, suggesting that the Ca2+ coordination is different from classical EF-hands; namely, the specific "closed-to-open" conformational transition does not occur in the ELC in response to Ca2+. Ca2+- and Mg2+-dependent conformational changes in the microenvironment of the binding site were detected by fluorescence experiments. Transient kinetic experiments showed that the displacement of Mg2+ by Ca2+ is faster than the change in direction of cytoplasmic streaming; therefore, we conclude that Ca2+ inhibition could operate in physiological conditions. By comparing the Physarum Ca2+ site with the well studied Ca2+ switch of scallop myosin, we surmise that despite the opposite effect of Ca2+ binding on the motor activity, the two conventional myosins could have a common structural basis for Ca2+ regulation.

Amino Acid Sequence↗
PubMed2003

Plant 115-kDa actin-filament bundling protein, P-115-ABP, is a homologue of plant villin and is widely distributed in cells.

In many cases, actin filaments are arranged into bundles and serve as tracks for cytoplasmic streaming in plant cells. We have isolated an actin-filament bundling protein, which is composed of 115-kDa polypeptide (P-115-ABP), from the germinating pollen of lily, Lilium longiflorum [Nakayasu et al. (1998) BIOCHEM: Biophys. Res. Commun. 249: 61]. P-115-ABP shared similar antigenicity with a plant 135-kDa actin-filament bundling protein (P-135-ABP), a plant homologue of villin. A full-length cDNA clone (ABP115; accession no. AB097407) was isolated from an expression cDNA library of lily pollen by immuno-screening using antisera against P-115-ABP and P-135-ABP. The amino acid sequence of P-115-ABP deduced from this clone showed high homology with those of P-135-ABP and four villin isoforms of Arabidopsis thaliana (AtVLN1, AtVLN2, AtVLN3 and AtVLN4), especially AtVLN4, indicating that P-115-ABP can also be classified as a plant villin. The P-115-ABP isolated biochemically from the germinating lily pollen was able to arrange F-actin filaments with uniform polarity into bundles and this bundling activity was suppressed by Ca2+-calmodulin (CaM), similar to the actin-filament bundling properties of P-135-ABP. The P-115-ABP type of plant villin was widely distributed in plant cells, from algae to land plants. In root hair cells of Hydrocharis dubia, this type of plant villin was co-localized with actin-filament bundles in the transvacuolar strands and the sub-cortical regions. Microinjection of the antiserum against P-115-ABP into living root hair cells caused the disappearance of transvaculor strands and alteration of the route of cytoplasmic streaming. In internodal cells of Chara corallina in which the P-135-ABP type of plant villin is lacking, the P-115-ABP type showed co-localization with actin-filament cables anchored on the intracellular surface of chloroplasts. These results indicated that plant villins are widely distributed and involved in the organization of actin filaments into bundles throughout the plant kingdom.

Actin Cytoskeleton↗
PubMed1999

Maternal mRNA localization of zebrafish DAZ-like gene.

Members of the DAZ gene family encode RNA-binding proteins and have been shown to play a pivotal role in gametogenesis. In Xenopus, a DAZ-like gene encodes an RNA component of the germ plasm. We have identified a zebrafish DAZ homologue, zDazl. zDazl mRNA was expressed in gonads of both sexes. In ovary, it was localized in the cortex of oocytes. At the onset of embryogenesis, maternal zDazl mRNA was detected at the vegetal pole. It migrated toward blastomeres through cytoplasmic streams as early embryogenesis proceeded. This is the first report showing maternal mRNA localization at the vegetal pole in fish and the existence of mRNA streams in the yolk cytoplasm.

Animals↗
PubMed1965

THE CHANGING PATTERN OF BIREFRINGENCE IN PLASMODIA OF THE SLIME MOLD, PHYSARUM POLYCEPHALUM.

Plasmodia of the acellular slime mold, Physarum polycephalum, reveal a complex and changing pattern of birefringence when examined with a sensitive polarizing microscope. Positively birefringent fibrils are found throughout the ectoplasmic region of the plasmodium. In the larger strands they may be oriented parallel to the strand axis, or arranged circularly or spirally along the periphery of endoplasmic channels. Some fibrils exist for only a few minutes, others for a longer period. Some, particularly the circular fibrils, undergo changes in birefringence as they undergo cyclic deformations. In the ramifying strand region and the advancing margin there is a tendency for fibrils of various sizes to become organized into mutually orthogonal arrays. In some plasmodia the channel wall material immediately adjacent to the endoplasm has been found to be birefringent. The sign of endoplasmic birefringence is negative, and its magnitude is apparently constant over the streaming cycle. The pattern of plasmodial birefringence and its changes during the shuttle streaming cycle of Physarum are considered in the light of several models designed to explain either cytoplasmic streaming alone or the entire gamut of plasmodial motions. The results of this and other recent physical studies suggest that both streaming and the various other motions of the plasmodium may very likely be explained in terms of coordinated contractions taking place in the fibrils which are rendered visible in polarized light.

Animals↗
PubMed1983

Correlation between torsion and streaming in Physarum.

Experiments are described which show that the torsional oscillations arising in a strand of the slime mould, Physarum polycephalum, under certain conditions remain closely correlated to oscillations in cytoplasmic streaming. When this is the case, the direction of rotation is related to the direction of streaming by a left-hand rule.

Cytoplasmic Streaming↗
PubMed1991

Gravity sensing mechanisms in plant cells.

Sensing of gravity is essential for the survival of plant seedlings. Therefore it is understandable that gravistimulation of only 0.5 sec-duration causes a graviresponse. The earliest graviresponses could be measured within seconds as alterations in membrane potentials of the statocytes in the root cap. Root statocytes are polarly organized. From a 6-day microgravity (10(-3) - 10(-4) g) experiment in the Spacelab D1 Mission it has been concluded that the observed polar differentiation is a result of a genetically prepatterned developmental program. Statoliths, the sedimentable organelles of statocytes, are surrounded by actin filaments which partly keep them in position. Under 6 min of microgravity during parabolic flights of rockets it could be demonstrated that the statoliths moved in the opposite direction to the initial gravity vector. It is concluded that shearing forces are exerted by microfilaments. It is supposed that the change of the position of statoliths is transmitted to gravisensitive structures of the statocytes (ER, plasma membrane) via microfilaments. As graviperception is influenced by calcium ions, it is suggested that these interactions regulate the activity of ion channels and/or pumps in the membranes thus initiating the graviresponse chain. In the case of cytoplasmic streaming in Chara rhizoids, the endogenous difference between the opposing streaming directions is diminished under microgravity during the flights of rockets. Possibly, shear stresses are affected by gravity, thus inducing gravity-related differences in the streaming velocities via actin filaments.

Actin Cytoskeleton↗
PubMed1980

Organization and spatial arrangement of fluorescein-labeled native actin microinjected into normal locomoting and experimentally influenced Amoeba proteus.

Fully polymerization-competent fluorescein-labeled actin from skeletal muscle was microinjected into both normal moving and experimentally treated Amoeba proteus. Its intracellular distribution was followed by integral image intensification of the fluorescence on a television screen and compared with controls injected with rhodamine-labeled serum albumin. The labeled actin was incorporated into the endogenous actin pool and exhibited a characteristic redistribution depending on the cellular morphology. Increased amounts of labeled actin could be detected within a thin layer separating the hyalo- and granuloplasm or running immediately beneath the plasma membrane when hyaloplasmic regions were absent. The topography of the fluorescent layer demonstrated in living cells is in agreement with the cortical microfilament layer described ultrastructurally recently in corresponding cells. The combined results emphasize the important role of the cortical filament layer in both morphogenetic processes (e.g., hyalo-granuloplasm separation or changes in cell shape) and motive force generation for cytoplasmic streaming and amoeboid movement.

Actins↗
PubMed1967

Structural aspects of saltatory particle movement.

A variety of cells possess particles which show movements statistically different from Brownian movements. They are characterized by discontinuous jumps of 2-30 micro at velocities of 0.5-5 micro/sec or more. A detailed analysis of these saltatory, jumplike movements makes it most likely that they are caused by transmission of force to the particles by a fiber system in the cell outside of the particle itself. Work with isolated droplets of cytoplasm from algae demonstrates a set of fibers involved in both cytoplasmic streaming and saltatory motion, suggesting that both phenomena are related to the same force-generating set of fibers. Analysis of a variety of systems in which streaming and/or saltatory movement occurs reveals two types of fiber systems spatially correlated with the movement, microtubules and 50 A microfilaments. The fibers in Nitella (alga) are of the microfilament type. In other systems (melanocyte processes, mitotic apparatus, nerve axons) microtubules occur. A suggestion is made, based on work on cilia, that a microtubule-microfilament complex may be present in those cases in which only microtubules appear to be present, with the microfilament closely associated with or buried in the microtubule wall. If so, then microfilaments, structurally similar to smooth muscle filaments, may be a force-generating element in a very wide variety of saltatory and streaming phenomena.

Animals↗
PubMed1986

Amyloplast sedimentation and organelle saltation in living corn columella cells.

Amyloplast sedimentation during gravistimulation and organelle movements was studied in living central rootcap cells of Zea mays L. cv. Merit. Cells from sectioned roots were viewed with a horizontally-mounted videomicroscope. The kinetics of gravity-induced amyloplast sedimentation were comparable to those calculated from experiments using fixed material. Individual amyloplasts fell at an average velocity of 5.5 micrometers min-1; the maximal velocity of fall measured was 18.0 micrometers min-1. Amyloplasts often rotated, sometimes rose in the cytoplasm, and occasionally underwent sudden rapid movements as fast as 58 micrometers min-1. Saltations of other organelles were frequently observed. This appears to be the first report of cytoplasmic streaming in the presumptive statocytes of roots.

Cytoplasmic Streaming↗
PubMed1980

The influence of an actin-modulating protein (AM-protein) from Physarum polycephalum on the cell motility of Amoeba proteus.

An actin-modulating protein (AM-protein) isolated from the acellular slime mold Physarum polycephalum and microinjected into living Amoeba proteus causes characteristic changes in cell shape, locomotory behaviour, and organization of the microfilament system of the amebae. The peptide chain weight of the AM-protein, which binds to one actin molecule with high affinity thus forming a heterodimer, is 43 000. The heterodimer is a powerful inhibitor of actin polymerization, when added to G-actin. A 50% inhibition is already obtained at a weight ratio of 0.05 AM-protein/actin. The AM-protein heterodimer has only little effect on F-actin, which can be neglected under the experimental conditions used. The microinjected AM-protein transforms normal locomoting polytactic amebae into orthotactic cell models. The orthotactic forms are characterized by a polarized organization, i.e. a flattened advancing cell pole consisting of one large pseudopod showing clear suppression of further pseudopod formation, and a contracting uroid undergoing cellular budding processes by the formation and vesiculation of numerous small evaginations. The observed changes in morphology can be interpreted as a relaxation of the advancing cell region due to a significant reduction of the polymerizable actin pool. As the AM-protein heterodimer does not attack F-actin filaments or the interaction of actin and myosin the middle region and the uroid of the microinjected cells are able to perform contractions which result in an orthotactic moving pattern. In conjunction with control experiments using double injection of AM-protein and phalloidin our observations prove the important role of controlled actin polymerization and depolymerization processes for cytoplasmic streaming activity and cell locomotion in ameboid cells.

Amoeba↗
PubMed2004

Rheology of active-particle suspensions.

We study the interplay of activity, order, and flow through a set of coarse-grained equations governing the hydrodynamic velocity, concentration, and stress fields in a suspension of active, energy-dissipating particles. We make several predictions for the rheology of such systems, which can be tested on bacterial suspensions, cell extracts with motors and filaments, or artificial machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis, and active mechanosensing find natural explanations within our model.

Cytoplasmic Streaming↗
PubMed1994

How an actin network might cause fountain streaming and nuclear migration in the syncytial Drosophila embryo.

We show here using time-lapse video tapes that cytoplasmic streaming causes nuclear migration along the anterior-posterior axis (axial expansion) in the early syncytial embryo of Drosophila melanogaster. Using confocal microscopy and labeled phalloidin we explore the distribution of F-actin during axial expansion. We find that a network of F-actin fibers fills the cytoplasm in the embryo. This actin network partially disassembles around the nuclei during axial expansion. Our observations of normal development, fixed embryos, and drug injection experiments indicate that disassembly of the actin network generates cytoplasmic movements. We suggest that the cell cycle regulates disassembly of the actin network, and that this process may be mediated directly or indirectly by the microtubules. The cytoplasmic movements we observe during axial expansion are very similar to fountain streaming in the pseudopod of amoebae, and by analogy with the pseudopod we propose a working hypothesis for axial expansion based on solation-contraction coupling within the actin network.

Actin Cytoskeleton↗
PubMed1999

Cytoplasm dynamics and cell motion: two-phase flow models.

The motion of amoeboid cells is characterized by cytoplasmic streaming and by membrane protrusions and retractions which occur even in the absence of interactions with a substratum. Cell translocation requires, in addition, a transmission mechanism wherein the power produced by the cytoplasmic engine is applied to the substratum in a highly controlled fashion through specific adhesion proteins. Here we present a simple mechano-chemical model that tries to capture the physical essence of these complex biomolecular processes. Our model is based on the continuum equations for a viscous and reactive two-phase fluid model with moving boundaries, and on force balance equations that average the stochastic interactions between actin polymers and membrane proteins. In this paper we present a new derivation and analysis of these equations based on minimization of a power functional. This derivation also leads to a clear formulation and classification of the kinds of boundary conditions that should be specified at free surfaces and at the sites of interaction of the cell and the substratum. Numerical simulations of a one-dimensional lamella reveal that even this extremely simplified model is capable of producing several typical features of cell motility. These include periodic 'ruffle' formation, protrusion-retraction cycles, centripetal flow and cell-substratum traction forces.

Actins↗
PubMed1976

Localization of actin filaments in internodal cells of characean algae. A scanning and transmission electron microscope study.

New methods of visualizing subcortical actin filament bundles, or fibrils, in Characean internodes confirm that they are associated with chloroplasts at the surface facing the streaming endoplasm, and reveal that they are continuous over long distances. With the scanning electron microscope, an average of four to six fibrils are seen bridging a file of chloroplasts. The same configuration appears in negatively stained preparations of large blocks of chloroplast files connected by actin fibrils. Few branches of the subcortical fibrils are evident. These findings are discussed with respect to the mechanism of cytoplasmic streaming in Characeae.

Actins↗
PubMed1970

Actomyosin from Physarum polycephalum: electron microscopy of myosin-enriched preparations.

Negatively stained slime mold actomyosin examined by electron microscopy consists mainly of actin-like filaments with occasional angular projections. If some of the actin is removed, the myosin-enriched actomyosin appears as continuous arrowhead structures similar to those of vertebrate striated muscle actomyosin. Together with other evidence, the findings suggest that cytoplasmic streaming in Physarum may involve a contractile process operating at a relatively low myosin-actin ratio.

Cytoplasmic Streaming↗