Fluorescence studies on modes of cytochalasin B and phallotoxin action on cytoplasmic streaming in Chara

Various investigations have suggested that cytoplasmic streaming in characean algae is driven by interaction between subcortical actin bundles and endoplasmic myosin. To further test this hypothesis, we have perfused cytotoxic actin-binding drugs and fluorescent actin labels into the cytoplasm of streaming Chara cells. Confirming earlier work, we find that cytochalasin B (CB) reversibly inhibits streaming. In direct contrast to earlier investigators, who have found phalloidin to be a potent inhibitor of movement in amoeba, slime mold, and fibroblastic cells, we find that phalloidin does not inhibit streaming in Chara but does modify the inhibitory effect of CB. Use of two fluorescent actin probes, fluorescein, isothiocyanate-heavy meromyosin (FITC-HMM) and nitrobenzoxadiazole-phallacidin (NBD-Ph), has permitted visualization of the effects of CB and phalloidin on the actin bundles. FITC-HMM labeling in perfused but nonstreaming cells has revealed a previously unobserved alteration of the actin bundles by CB. Phalloidin alone does not perceptibly alter the actin bundles but does block the alteration by CB if applied as a pretreatment, NBD-Ph perfused into the cytoplasm of streaming cells stains actin bundles without inhibiting streaming. NBD-Ph staining of actin bundles is not initially observed in cells inhibited by CB but does appear simultaneously with the recovery of streaming as CB leaks from the cells. The observations reported here are consistent with the established effects of phallotoxins and CB on actin in vitro and support the hypothesis that streaming is generated by actin-myosin interactions.     

Myosin and Ca2+-sensitive streaming in the alga Chara: detection of two polypeptides reacting with a monoclonal anti-myosin and their localization in the streaming endoplasm

A monoclonal antibody to the heavy chain of myosin from mouse 3T3 cells was used to detect and localize related proteins in the green alga Chara. Proteins of 200,000 and 110,000 Mr reacted on immunoblots of proteins precipitated rapidly with trichloroacetic acid to minimize proteolysis. Immunofluorescence of whole cells localized these proteins to organelles of the streaming endoplasm, to a system of endoplasmic strands and to the subcortical actin bundles. Except that fewer endoplasmic strands and organelles were found and the strands were tangled, the localization pattern was similar in cells rapidly perfused to remove the bulk of the streaming endoplasm. Actin was confined almost entirely to the system of subcortical actin bundles in both whole and perfused cells. Myosin that was associated with the tangled endoplasmic strands but not that associated with the organelles or actin bundles was removed by concentrations of Ca2+ inhibiting ATP-dependent streaming in perfused cells. ATP extracted both organelles and endoplasmic strands but left a continuous pattern of myosin immunostaining along the actin bundles. The findings are discussed in relation to the possible existence of two forms of myosin and of separate mechanisms moving the bulk endoplasm and individual organelles.     

Membranous sacs associated with cilia of Paramecium

The properties of the sub-cortical actin bundles in the perfused Chara cell model are altered by concentrations of cytochalasin B (CB) which inhibit streaming. This is demonstrated by treating the bundles with intracellularly introduced CB after first using ATP to strip away their associated motile organelles. Such CB-treated bundles are resistant to a solution of low ionic strength which, as judged by light and electron microscopy, extracts untreated bundles. It is therefore concluded that a site of action for CB lies within the actin bundles and that, while the alteration detected may not itself be of direct physiological significance, it may nonetheless indicate the site at which CB inhibits streaming in both the cell model and the intact cell.     

Chara myosin and the energy of cytoplasmic streaming

Recently, it was found that myosin generating very fast cytoplasmic streaming in Chara corallina has very high ATPase activity. To estimate the energy consumed by this myosin, its concentration in the internodal cells of C. corallina was determined by quantitative immunoblot. It was found that the concentration of Chara myosin was considerably high (200 nM) and the amount of ATP consumed by this myosin would exceed that supplied by dark respiration if all myosin molecules were fully activated by the interaction with actin. These results and model calculations suggested that the energy required to generate cytoplasmic streaming is very small and only one-hundredth of the existing myosin is enough to maintain the force for the streaming in the Chara cell.     

Binding of chara Myosin globular tail domain to phospholipid vesicles

Binding of Chara myosin globular tail domain to phospholipid vesicles was investigated quantitatively. It was found that the globular tail domain binds to vesicles made from acidic phospholipids but not to those made from neutral phospholipids. This binding was weakened at high KCl concentration, suggesting that the binding is electrostatic by nature. The dissociation constant for the binding of the globular tail domain to 20% phosphatidylserine vesicles (similar to endoplasmic reticulum in acidic phospholipid contents) at 150 mM KCl was 273 nM. The free energy change due to this binding calculated from the dissociation constant was -37.3 kJ mol(-1). Thus the bond between the globular tail domain and membrane phospholipids would not be broken when the motor domain of Chara myosin moves along the actin filament using the energy of ATP hydrolysis (DeltaG degrees ' = -30.5 kJ mol(-1)). Our results suggested that direct binding of Chara myosin to the endoplasmic reticulum membrane through the globular tail domain could work satisfactorily in Chara cytoplasmic streaming. We also suggest a possible regulatory mechanism of cytoplasmic streaming including phosphorylation-dependent dissociation of the globular tail domain from the endoplasmic reticulum membrane.     

A kinetic mechanism for the fast movement of Chara myosin

Endoplasmic streaming of characean cells of Nitella or Chara is known to be in the range 30-100 microm/second. The Chara myosin extracted from the cells and fixed onto a glass surface was found to move muscle actin filaments at a velocity of 60 microm/second. This is ten times faster than that of skeletal muscle myosin (myosin II). In this study, the displacement caused by single Chara myosin molecules was measured using optical trapping nanometry. The step size of Chara myosin was approximately 19nm. This step size is longer than that of skeletal muscle myosin but shorter than that of myosin V. The dwell time of the steps was relatively long, and this most likely resulted from two rate-limiting steps, the dissociation of ADP and the binding of ATP. The rate of ADP release from Chara myosin after the completion of the force-generation step was similar to that of myosin V, but was considerably slower than that of skeletal muscle myosin. The 19nm step size and the dwell time obtained could not explain the fast movement. The fast movement could be explained by the load-dependent release of ADP. As the load imposed on the myosin decreased, the rate of ADP release increased. We propose that the interaction of Chara myosin with an actin filament resulted in a negative load being imposed on other myosin molecules interacting with the same actin filament. This resulted in an accelerated release of ADP and the fast sliding movement.     

The effects of halothane on the DNase I activity in an isolated enzyme preparation and in the DNase I-G actin complex

The effects of halothane on the DNase I activity in an isolated enzyme preparation and in a DNase I-globular (G) actin complex was investigated. DNase I, DNase I-G actin complexes and G actin were exposed to various (0.2–4.0 vol./%) halothane concentrations for 3 h. Thereafter, DNase I was mixed with a DNA solution and the extinction of the acid soluble supernatant of the DNase I assay was determined as a measure of DNase I activity. After 10 min of halothane exposure the DNase I activity is inhibited in direct proportion to halothane concentrations between 0.6 and 4.0 vol/%. After 10 min halothane activates inactive DNase I by inhibiting G actin, an inhibitor of DNase I. G actin, exposed to halothane, does not inhibit the activity of DNase I. The results suggest a mechanism by which halothane may contribute to chromosomal defects and disturbances of DNA metabolism in cells.     

Hydrodynamic models of viscous coupling between motile myosin and endoplasm in characean algae

Cytoplasmic streaming in characean algae is thought to be driven by interaction between stationary subcortical actin bundles and motile endoplasmic myosin. Implicit in this mechanism is a requirement for some form of coupling to transfer motive force from the moving myosin to the endoplasm. Three models of viscous coupling between myosin and endoplasm are presented here, and the hydrodynamic feasibility of each model is analyzed. The results show that individual myosinlike molecules moving along the actin bundles at reasonable velocities cannot exert enough viscous pull on the endoplasm to account for the observed streaming. Attachment of myosin to small spherical organelles improves viscous coupling to the endoplasm, but results for this model show that streaming can be generated only if the myosin-spheres move along the actin bundles in a virtual solid line at about twice the streaming velocity. In the third model, myosin is incorporated into a fibrous or membranous network or gel extending into the endoplasm. This network is pulled forward as the attached myosin slides along the actin bundles. Using network dimensions estimated from published micrographs of characean endoplasm, the results show that this system can easily generate the observed cytoplasmic streaming.     

Protein phosphorylation regulates actomyosin-driven vesicle movement in cell extracts isolated from the green algae,Chara corallina

In Characean cells endoplasmic streaming stops upon membrane depolarization accompanied by Ca(2+) entry. We investigated the mechanism of this cessation of endoplasmic streaming by reconstituting the vesicle movement in vitro. In a living cell of Chara corallina, there are a number of vesicles moving along actin cables. Vesicles in the endoplasm squeezed out of the cell into a medium containing Mg-ATP showed directional movements under a dark field microscope. When the extracted endoplasm was treated with 20 nM okadaic acid, vesicles showed only movements like the Brownian motion. When it was treated with 50 nM staurosporine, directional movements of vesicles were activated. These movements were analyzed by image processing of videomicroscopic records. Vesicle movements along F-actin filaments were also observed by merging both images of the same field by dark field microscopy and fluorescence microscopy, indicating that myosin on the vesicle surface was responsible for vesicle movements. We also examined the effects of okadaic acid and staurosporine on in vitro sliding of F-actin on Chara myosin. When Chara myosin was treated with 20 nM okadaic acid in the cell extract, the number of sliding F-actin filaments was greatly reduced. In contrast, it increased when Chara myosin was treated with 50 nM staurosporine. In addition, Chara myosin treated with protein kinase C greatly diminished its motility. These results suggest that inactivation of Chara myosin via its phosphorylation is responsible for cessation of endoplasmic streaming.     

Immunolocalization of myosin in rhizoids ofChara globularis Thuill

Summary Myosin-related proteins have been localized immunocytochemically in gravity-sensing rhizoids of the green algaChara globularis using a monoclonal antibody against the heavy chain of myosin from mouse 3T3 cells and a polyclonal antibody to bovine skeletal and smooth muscle myosin. In the basal zone of the rhizoids which contain a large vacuole, streaming endoplasm and stationary cortical cytoplasm, the monoclonal antibody stained myosin-related proteins as diffusely fluorescing endoplasmic strands. This pattern is similar to the arrangement of subcortical actin filament bundles. In the apical zone which contains an aggregation of ER membranes and secretory vesicles for tip growth, diffuse immunofluorescence was detected; the intensity of the signal increasing towards the apical cell wall. The most prominent myosin-staining was associated with the surface of statoliths in the apical zone. The polyclonal antibody produced a punctate staining pattern in the basal zone, caused by myosin-related proteins associated with the surface of drganelles in the streaming endoplasm and the periphery of the nucleus. In the apical zone, this antibody revealed myosin-immunofluorescence on the surface of statoliths in methacrylate-embedded rhizoids. Neither antibody revealed myosin-immunofluorescence on the surface of organelles and vesicles in the relatively stationary cytoplasm of the subapical zone. These results indicate (i) that different classes of myosin are involved in the various transport processes inChara rhizoids; (ii) that cytoplasmic streaming in rhizoids is driven by actomyosin, corresponding to the findings onChara internodal cells; (iii) that actindependent control of statolith position and active movement is mediated by myosin-related proteins associated with the statolith surfaces; and (iv) that myosin-related proteins are involved in the process of tip growth.     

Filaments associated with the endoplasmic reticulum in the streaming cytoplasm of Chara corallina

Perfused Chara cells capable of resuming ATP-dependent cytoplasmic streaming in low free Ca++ solutions have been examined by electron microscopy for myosin-like filaments. Filaments 44 nm in diameter and up to 3 micron in length have been found associated with the endoplasmic reticulum that along with mitochondria, microbodies and dictyosomes from the endoplasm becomes immobilised around the sub-cortical actin bundles when ATP is depleted. Such endoplasmic filaments have not been detected in association with mitochondria or microbodies and they have not been found in the stationary cortex. These filaments are extracted from the perfused cell by ATP unless motility-inhibiting levels of cytochalasin B are present. The filaments are not detectable in cells inactivated in solutions containing high (10(-4) M) Ca++ concentrations even when the Ca++ level is subsequently lowered. Consistent with their being required for motility, cytoplasmic streaming cannot be effeiciently reactivated by ATP in such filament-depleted cells. The possibility is discussed that the filaments contain myosin and that the endoplasmic reticulum with which they are associated has a major role in generating and transmitting the motive force for streaming.     

Actin-microtubule interactions in the algaNitella: analysis of the mechanism by which microtubule depolymerization potentiates cytochalasin's effects on streaming

Summary In the characean algaNitella, depolymerization of microtubules potentiates the inhibitory effects of cytochalasins on cytoplasmic streaming. Microtubule depolymerization lowers the cytochalasin B and D concentrations required to inhibit streaming, accelerates inhibition and delays streaming recovery. Because microtubule depolymerization does not significantly alter³H-cytochalasin B uptake and release, elevated intracellular cytochalasin concentrations are not the basis for potentiation. Instead, microtubule depolymerization causes actin to become more sensitive to cytochalasin. This increased sensitivity of actin is unlikely to be due to direct stabilization of actin by microtubules, however, because very few microtubules colocalize with the subcortical actin bundles that generate streaming. Furthermore, microtubule reassembly, but not recovery of former transverse alignment, is sufficient for restoring the normal cellular responses to cytochalasin D. We hypothesize that either tubulin or microtubule-associated proteins, released when microtubules depolymerize, interact with the actin cytoskeleton and sensitize it to cytochalasin.Abbreviations APW artificial pond water - Ca_c cytoplasraic free calcium concentration - DMSO dimethyl sulfoxide - MT microtubule-minus - MT+ microtubule-plus.     

Effects of isoflurane on the DNase I activity in an isolated enzyme preparation and on the DNase I-G actin complex

Effects of isoflurane on the DNase I activity in an isolated enzyme preparation and in the DNase I-globular (G) actin complex were investigated. DNase I, DNase I-G actin complex, and G actin were exposed to various (0.2-4.0 vol%) isoflurane concentrations for 180 min. Thereafter, DNase I activity was determined. DNase I activity was inhibited in relation to time and concentration of isoflurane exposure. At concentrations ranging from 0.2 to 1.0 vol% of isoflurane inactive DNase I was activated in the DNase I-G actin complex. The DNase I inhibitor G actin showed a reduced capability to inhibit DNase I following isoflurane exposure. Albumin can inhibit the DNase I inactivation possibly by competition in the reactions between DNase I/albumin and isoflurane. After exposure to isoflurane the absorption maximum of DNase I was identical with the absorption maximum of heat-denatured DNase I. The results suggest a mechanism by which isoflurane may affect DNA in an indirect way at concentrations to which the patient is exposed during clinical anesthesia.     

The motility of Chara corallina myosin was inhibited reversibly by 2,3-butanedione monoxime (BDM)

We studied the effects of 2,3-butanedione monoxime (BDM) on the cytoplasmic streaming of Chara corallina and on the motility of myosin prepared from the same plant to examine whether this reagent really affects the plant class XI myosin. It was found that BDM inhibited both cytoplasmic streaming and the motility of myosin at a very similar concentration range (10-100 mM). BDM introduced directly into tonoplast-free cells also inhibited cytoplasmic streaming. These results suggested that effect of BDM on cytoplasmic streaming was exerted through myosin and not through ion channels at least in Chara corallina, though a very high concentration of BDM was required.     

Concanavalin A induces interactions between surface glycoproteins and the platelet cytoskeleton

We have measured the association of platelet surface membrane proteins with Triton X-100 (Triton)-insoluble residues in platelets surface labeled with 125I. In both concanavalin A (Con A)-stimulated and resting platelets, this fraction is composed largely of polypeptides with apparent molecular weights of 45,000, 200,000, and 250,000 which comigrate with authentic actin, myosin heavy chain, and actin binding protein, respectively, as judged by PAGE in SDS. Less than 10% of the two major 125I-labeled surface glycoproteins, GPiib and GPIII, were associated with the Triton residue in resting platelets. Within 45 s after Con A addition, 80-95% of these two glycoproteins became associated with the Triton residue and the amount of sedimentable actin doubled. No cosedimentation of GPIIb and III with the cytoskeletal protein-containing Triton residue was seen when Con A was added to a Triton extract of resting cells, indicating that the sedimentation of GPIIb and III seen in Con A-stimulated platelets was not due to precipitation of the glycoproteins by Con A after detergent lysis. Treatment of Triton extracts of Con A-stimulated platelets with DNase I (deoxyribonucleate 5'-oligonucleotidido-hydrolase [EC 3.1.4.5]) inhibited the sedimentation of actin and the two surface glycoproteins in a dose-dependent manner. This inhibition of cosedimentation was not due to an effect of DNase I on Con A-glycoprotein interactions since these two glycoproteins could be quantitatively recovered by Con A- Sepharose affinity absorption in the presence of DNase I. When the Con A bound to the Triton residue was localized ultrastructurally, it was associated with cell-sized structures containing filamentous material. In intact cells, there was simultaneous immunofluorescent coredistribution of surface-bound Con A and myosin under conditions which induced a redistribution of platelet myosin. These data suggest that Con A can, in the intact platelet, induce physical interactions between certain surface glycoproteins and the internal cytoskeleton.     

Accelerated sliding of pollen tube organelles along Characeae actin bundles regulated by Ca2+

Pollen tubes show active cytoplasmic streaming. We isolated organelles from pollen tubes and tested their ability to slide along actin bundles in characean cell models. Here, we show that sliding of organelles was ATP-dependent and that motility was lost after N-ethylmaleimide or heat treatment of organelles. On the other hand, cytoplasmic streaming in pollen tube was inhibited by either N-ethylmaleimide or heat treatment. These results strongly indicate that cytoplasmic streaming in pollen tubes is supported by the "actomyosin"-ATP system. The velocity of organelle movement along characean actin bundles was much higher than that of the native streaming in pollen tubes. We suggested that pollen tube "myosin" has a capacity to move at a velocity of the same order of magnitude as that of characean myosin. Moreover, the motility was high at Ca2+ concentrations lower than 0.18 microM (pCa 6.8) but was inhibited at concentration higher than 4.5 microM (pCa 5.4). In conclusion, cytoplasmic streaming in pollen tubes is suggested to be regulated by Ca2+ through "myosin" inactivation.     

Characean actin bundles as a tool for stydying actomyosin-based motility

At the inner surface of the stagnant chloroplasts of Characeae cells, bundles of actin filaments having uniform polarity are anchored. These bundles are responsible for generating the motive force of cytoplasmic streaming. It is now possible to induce movement of either beads coated with foreign myosin or organelles associated with myosin along the characean actin bundles. The Ca²⁺ sensitivities of the reconstitued movements are consistent with those of the actin-activated myosin ATPases. The use of reconstituted systems is finding wide application in the detection of various myosins in materials from which myosin is not significantly purified. Furthermore, sliding velocities and the Ca²⁺ regulation of myosins bound to organelles are now being determined. Recipient of the Botanical Society Award for Young Scientists, 1987.     

Cytoplasmic streaming in giant algal cells: A historical survey of experimental approaches

Various methods have been used to study cytoplasmic streaming in giant algal cells during the past three decades. Simple techniques can be used with characean internodal cells to modify the cell constitution in various ways to gain insight into the mechanism of cytoplasmic streaming. Another method involves isolatingin vitro a huge drop of uninjured endoplasm, to examine its physical and dynamic properties. The motive force responsible for streaming has been measured by three different techniques with similar results. Subcortical fibrils consisting of bundles of F-actin with the same polarity are indispensable for streaming. Differential treatment of the endoplasm and ectoplasm has shown that putative characean myosin is localized in the endoplasm. Studies of the roles of ATP, Mg²⁺, Ca²⁺, H⁺ etc. in the streaming have been conducted by cellular perfusion, which allows removal of the tonoplast, or by techniques permeabilizing the protoplasmic membrane. A slow version of the movement can even be artificially reproduced by combining characean actinin situ and exogenous myosin in the presence of Mg-ATP. The findings thus far obtained support the hypothesis that cytoplasmic streaming in characean cells is caused by an active shearing force produced by interaction of the actin filament bundles on the cortex with myosin in the endoplasm.     

Regeneration of actin bundles in Chara: polarized growth and orientation by endoplasmic flow

Strong irradiation of localized areas of the alga Chara produces chloroplast damage and extensive loss of the actin bundles responsible for cytoplasmic streaming. Immunofluorescence using a monoclonal antibody binding to the actin bundles has been used to follow their regrowth. Bundle regeneration is polarized so that new bundles develop from the ends of the actin bundles delivering endoplasm to the damaged area and not from bundles removing endoplasm. According to the previously established polarity of the actin filaments this growth is occurring from the "barbed" but not the "pointed" ends of the component filaments. The frequently irregular orientation of the regenerated bundles contrasts with the straight, parallel arrangement of the bundles before destruction. The arrangement of the regenerated bundles is suggested to depend on orientation by passive endoplasmic flow rather than a cortical template. As a result, bundles follow sweeping curves and can form a U-turn connecting oppositely polarized bundles normally separated by the neutral line. In addition to development in continuity with the free ends of pre-existing bundles, visualization of small, discrete fluorescent structures suggests that bundles can begin to form in isolation within the damaged areas. The results are discussed in terms of the polarized actin polymerisation seen in vitro, additional controls which may operate on bundle growth in vivo, and the ability of flow to orient F-actin. The relevance of the findings to normal cell ontogeny is assessed.     

Mechanism of inhibition of cytoplasmic streaming by a myosin inhibitor, 2,3-butanedione monoxime

Summary On the basis of the inhibition of myosin by 2,3-butanedione monoxime (BDM), the protein's involvement in various cell activities is discussed. However, it has not been established whether BDM inhibits plant myosin. In the present study, the effect of BDM on isolated plant myosin was analyzed in vitro. The sliding between myosin from lily (Lilium longiflorum) pollen tubes and actin filaments from skeletal muscle was inhibited to 25% at a concentration of 60 mM, indicating that BDM can be used as a myosin inhibitor for plant materials. Cytoplasmic streaming was completely inhibited by BDM at 30 mM in lily pollen tubes and at 70 mM in short root hair cells, and at 100 mM in long root hair cells ofHydrocharis dubia. However, BDM at high concentrations induced the disorganization of actin filament bundles in lily pollen tubes and short root hair cells. In addition, cortical microtubules were also fragmented in short root hair cells treated with BDM, suggesting a possible side effect of BDM.Abbreviations AF actin filament - BDM 2,3-butanedione monoxime - MT microtubule