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.     

Polar auxin transport and auxin-induced elongation in the absence of cytoplasmic streaming

Summary When cytoplasmie streaming in oat and maize coleoptile cells is completely inhibited by cytochalasin B (CB), polar transport of auxin (indole-3-acetic acid) continues at a slightly reduced rate. Therefore, cytoplasmic streaming is not required for polar transport. Auxin induces elongation in CB-inhibited coleoptile and pea stem segments, but elongation rate is reduced about 40% by CB. Therefore, stimulation of cytoplasmic streaming cannot be the means by which auxin promotes cell elongation, but streaming may be beneficial to elongation growth although not essential to it. A more severe inhibition of elongation develops after several hours in CB. With coleoptiles this could be due to inhibition of sugar uptake; in pea tissue it may be due to permeability changes and cytoplasmic degeneration. CB does not disorganize or disorient microfilament bundles when it inhibits streaming in maize, but appears instead to cause hypercondensation of microfilament material.     

Microtubule anchoring by cortical actin bundles prevents streaming of the oocyte cytoplasm

The localisation of the determinants of the body axis during Drosophila oogenesis is dependent on the microtubule (MT) cytoskeleton. Mutations in the actin binding proteins Profilin, Cappuccino (Capu) and Spire result in premature streaming of the cytoplasm and a reorganisation of the oocyte MT network. As a consequence, the localisation of axis determinants is abolished in these mutants. It is unclear how actin regulates the organisation of the MTs, or what the spatial relationship between these two cytoskeletal elements is. Here, we report a careful analysis of the oocyte cytoskeleton. We identify thick actin bundles at the oocyte cortex, in which the minus ends of the MTs are embedded. Disruption of these bundles results in cortical release of the MT minus ends, and premature onset of cytoplasmic streaming. Thus, our data indicate that the actin bundles anchor the MTs minus ends at the oocyte cortex, and thereby prevent streaming of the cytoplasm. We further show that actin bundle formation requires Profilin but not Capu and Spire. Thus, our results support a model in which Profilin acts in actin bundle nucleation, while Capu and Spire link the bundles to MTs. Finally, our data indicate how cytoplasmic streaming contributes to the reorganisation of the MT cytoskeleton. We show that the release of the MT minus ends from the cortex occurs independently of streaming, while the formation of MT bundles is streaming dependent.     

Effects of exogenous proteins on cytoplasmic streaming in perfused Chara cells

Cytoplasmic streaming in characean algae is thought to be generated by interaction between subcortical actin bundles and endoplasmic myosin. Most of the existing evidence supporting this hypothesis is of a structural rather than functional nature. To obtain evidence bearing on the possible function of actin and myosin in streaming, we used perfusion techniques to introduce a number of contractile and related proteins into the cytoplasm of streaming Chara cells. Exogenous actin added at concentrations as low as 0.1 mg/ml is a potent inhibitor of streaming. Deoxyribonuclease I (DNase I), an inhibitor of amoeboid movement and fast axonal transport, does not inhibit streaming in Chara. Fluorescein-DNase I stains stress cables and microfilaments in mammalian cells but does not bind to Chara actin bundles, thus suggesting that the lack of effect on streaming is due to a surprising lack of DNase I affinity for Chara actin bundles. Heavy meromyosin (HMM) does not inhibit streaming, but fluorescein-HMM (FL-HMM), having a partially disabled EDTA ATPase, does. Quantitative fluorescence micrography provides evidence that inhibition of streaming by FL-HMM may be due to a tendency for FL-HMM to remain bound to Chara actin bundles even in the presence of MgATP. Perfusion with various control proteins, including tubulin, ovalbumin, bovine serum albumin, and irrelevant antibodies, does not inhibit streaming. These results support the hypothesis that actin and myosin function to generate cytoplasmic streaming in Chara.     

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.     

The role of actin in root hair morphogenesis: studies with lipochito-oligosaccharide as a growth stimulator and cytochalasin as an actin perturbing drug

Root hairs develop from bulges on root epidermal cells and elongate by tip growth, in which Golgi vesicles are targeted, released and inserted into the plasma membrane on one side of the cell. We studied the role of actin in vesicle delivery and retention by comparing the actin filament configuration during bulge formation, root hair initiation, sustained tip growth, growth termination, and in full-grown hairs. Lipochito-oligosaccharides (LCOs) were used to interfere with growth ( De Ruijter et al . 1998 , Plant J. 13, 341–350), and cytochalasin D (CD) was used to interfere with actin function. Actin filament bundles lie net-axially in cytoplasmic strands in the root hair tube. In the subapex of growing hairs, these bundles flare out into fine bundles. The apex is devoid of actin filament bundles. This subapical actin filament configuration is not present in full-grown hairs; instead, actin filament bundles loop through the tip. After LCO application, the tips of hairs that are terminating growth swell, and a new outgrowth appears from a site in the swelling. At the start of this outgrowth, net-axial fine bundles of actin filaments reappear, and the tip region of the outgrowth is devoid of actin filament bundles. CD at 1.0 μ m , which does not affect cytoplasmic streaming, does not inhibit bulge formation and LCO-induced swelling, but inhibits initiation of polar growth from bulges, elongation of root hairs and LCO-induced outgrowth from swellings. We conclude that elongating net-axial fine bundles of actin filaments, which we call FB-actin, function in polar growth by targeting and releasing Golgi vesicles to the vesicle-rich region, while actin filament bundles looping through the tip impede vesicle retention.     

Wide-ranging effects of eight cytochalasins and latrunculin A and B on intracellular motility and actin filament reorganization in characean internodal cells

Numerous forms of cytochalasins have been identified and, although they share common biological activity, they may differ considerably in potency. We investigated the effects of cytochalasins A, B, C, D, E, H and J and dihydrocytochalasin B in an ideal experimental system for cell motility, the giant internodal cells of the characean alga Nitella pseudoflabellata. Cytochalasins D (60 microM) and H (30 microM) were found to be most suited for fast and reversible inhibition of actin-based motility, while cytochalasins A and E arrested streaming at lower concentrations but irreversibly. We observed no clear correlation between the ability of cytochalasins to inhibit motility and the actual disruption of the subcortical actin bundle tracks on which myosin-dependent motility occurs. Indeed, the actin bundles remained intact at the time of streaming cessation and disassembled only after one to several days' treatment. Even when applied at concentrations lower than that required to inhibit cytoplasmic streaming, all of the cytochalasins induced reorganization of the more labile cortical actin filaments into actin patches, swirling clusters or short rods. Latrunculins A and B arrested streaming only after disrupting the subcortical actin bundles, a process requiring relatively high concentrations (200 microM) and very long treatment periods of >1 d. Latrunculins, however, worked synergistically with cytochalasins. A 1 h treatment with 15 nM latrunculin A and 4 microM cytochalasin D induced reversible fragmentation of subcortical actin bundles and arrested cytoplasmic streaming. Our findings provide insights into the mechanisms by which cytochalasins and latrunculins interfere with characean actin to inhibit motility.     

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.     

Distribution of actin and tubulin in cells and in glycerinated cell models after treatment with cytochalasin B (CB)

The organization of microfilaments and microtubules in cultured cells before and after the addition of cytochalasin B (CB) was studied both by electron microscopy and immunofluorescence microscopy using antibodies specific for actin, tubulin and tropomyosin. CB induces a rapid disorganization of normal microfilament bundles. Star-like patches of actin and tropomyosin are visualized in immunofluorescence microscopy and dense aggregates of condensed microfilaments are seen in electron microscopy. The integrity of the microtubules is not changed by CB treatment. Addition of CB to glycerinated cells, in contrast to normal cells, does not result in the disorganization of microfilament bundles. CB-treated glycerinated models can still contract upon addition of ATP. Thus the CB-induced rearrangement of microfilament bundles occurs only in vivo and not in glycerinated cell contractility models.     

Microtubule disassembly enhances reversible cytochalasin-dependent disruption of actin bundles in characean internodes

Summary Parallel bundles of actin filaments at the cortex-endoplasm interface provide tracks for myosin-generated cytoplasmic streaming in characean internodes. These bundles resist disassembly or structural modification when exposed to 10 μM cytochalasin D (CD) even though this concentration of CD rapidly (within minutes) but reversibly arrests streaming. Unexpectedly, we discovered that prolonged treatment with lower concentrations of CD could partially disassemble the subcortical actin bundles. Actin bundles became discontinuous following one- to several-day treatment with concentrations (6 μM) that reduced but did not arrest streaming, and the residual fragments mostly remained parallel to the chloroplast files. When microtubules were concurrently disassembled with tubulin-specific drugs, however, low CD concentrations (2.5–3 μM) completely arrested bulk streaming, disrupted the largely 2-dimensional actin bundle array and caused the formation of a coarse, thick-meshed actin network that extended from the cortex to the endoplasm. Despite such massive reconstruction, drug removal enabled cells to recover continuous parallel bundles and streaming. Recovery was possible if both or just one of the drugs were removed. In recovered cells, the streaming pattern frequently redeveloped in new directions that did not follow the chloroplast files, and later, chloroplast files readjusted to the new polarity established by the actin bundles. This first report on the complete and reversible disassembly of characean actin bundles provides new insights into the mechanism of actin bundle assembly and organization and supports the idea of indirect interactions between actin filaments and microtubules.     

Roles of actin filaments in cytoplasmic streaming and organization of transvacuolar strands in root hair cells ofHydrocharis

Summary Effects of cytochalasin B and mycalolide-B on cytoplasmic streaming, organizations of actin filaments and the transvacuolar strand were studied in root hair cells ofHydrocharis, which shows reverse fountain streaming. Both toxins inhibited cytoplasmic streaming and destroyed the organizations of actin filaments and transvacuolar strands. However, we found a great difference between these toxins with respect to reversibility. The effects of cytochalasin B were reversible but not those of mycalolide B. The present results suggest that actin filaments work as a track of cytoplasmic streaming and as a cytoskeleton to maintain the transvacuolar strand. The usefulness of root hair cells ofHydrocharis in studying the dynamic organization of actin filaments of plant is discussed.Abbreviations CB cytochalasin B - DMSO dimethylsulfoxide - ML-B mycalolide B     

Possible involvement of protein phosphorylation in the regulation of cytoplasmic organization and streaming in root hair cells as revealed by a protein phosphatase inhibitor, calyculin A

Summary The effects of a protein phosphatase inhibitor, calyculin A (CA), on cytoplasmic streaming and cytoplasmic organization were examined in root hair cells ofLimnobium stoloniferum. CA at concentrations higher than 50 nM inhibited cytoplasmic streaming and also induced remarkable morphological changes in the cytoplasm. The transvacuolar strands, in which actin filament bundles were oriented parallel to the long axis, disappeared and spherical cytoplasmic bodies emerged in the CA-treated cells. In these spherical bodies, actin filaments were present and the spherical bodies were connected to each other by thin strands of actin filaments. Upon CA removal, transvacuolar strands, in which actin filament bundles were aligned, and cytoplasmic streaming reappeared. A nonselective inhibitor for protein kinases, K-252a, delayed the inhibitory effect of CA on cytoplasmic streaming and suppressed the CA-induced formation of the spherical bodies. From these results, it is suggested that phosphatases sensitive to CA regulate cytoplasmic streaming and are involved in the organization of the cytoplasm in root hair cells.Abbreviations APW artificial pond water - CA calyculin A     

A cytochalasin-sensitive actin filament meshwork is a prerequisite for local wound wall deposition inNitella internodal cells

Summary Reorganization of the actin cytoskeleton following cell wall puncturing of characean internodal cells was studied by immunofluorescence and confocal laser scanning microscopy. Injury locally destroyed the parallel subcortical actin filament bundles and cortical actin strands that are characteristic of unwounded regions. At wounds, a delicate three-dimensional interlaced structure of actin strands, with meshes up to 5 m wide, formed by de novo assembly of isolated filaments and by the elongation of residual subcortical actin bundles and cortical actin strands. The actin meshwork persisted for up to 2 h, corresponding to the duration of intense wound wall secretion. Actin filament bundles continuous with the subcortical bundles outside the wound then regenerated, their parallel alignment probably assisted by endoplasmic flow. Cytochalasin D concentrations that arrested cytoplasmic streaming completely inhibited the formation of the actin meshwork, wound wall deposition and recovery of actin bundles. Concentrations that only reduced streaming velocity delayed meshwork formation and wound walls were thinner than in controls. The actual amount of F-actin within the meshwork, however, was clearly greater in the presence of low cytochalasin concentrations. In late stages of recovery, the actin bundles became very thick and intervening spaces became wider thereby forming a conspicuous, three-dimensional lattice that was continuous with interwebbing subcortical bundles and cortical actin around the periphery of the wound. Our experiments suggest that actin meshwork formation is a prerequisite for plasma membrane-directed transport of vesicles involved in wounding-induced exocytosis in characean internodes. Stabilization of the meshwork by subinhibitory concentrations of cytochalasin D is probably caused by actinbinding properties of the drug that either induce bundling or impede function of associated proteins.Abbreviations AFW artificial fresh water - BSA bovine serum albumin - CLSM confocal laser scanning microscope (microscopy) - DIC differential interference contrast - DMSO dimethyl sulfoxide - FITC fluorescein isothiocyanate - MBS m-maleimidobenzoyl N-hydroxy-succinimide ester - PBS phosphate-buffered saline - SCAB subcortical actin bundle     

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.     

Microtubules regulate the organization of actin filaments at the cortical region in root hair cells ofHydrocharis

Summary We studied the mechanism controlling the organization of actin filaments (AFs) inHydrocharis root hair cells, in which reverse fountain streaming occurs. The distribution of AFs and microtubules (MTs) in root hair cells were analyzed by fluorescence microscopy and electron microscopy. AFs and MTs were found running in the longitudinal direction of the cell at the cortical region. AFs were observed in the transvacuolar strand, but not MTs. Ultrastructural studies revealed that AFs and MTs were colocalized and that MTs were closer to the plasma membrane than AFs. To examine if MTs regulate the organization of AFs, we carried out a double inhibitor experiment using cytochalasin B (CB) and propyzamide, which are inhibitors of AFs and MTs, respectively. CB reversibly inhibited cytoplasmic streaming while propyzamide alone had no effect on it. However, after treatment with both CB and propyzamide, removal of CB alone did not lead to recovery of cytoplasmic streaming. In these cells, AFs showed a meshwork structure. When propyzamide was also removed, cytoplasmic streaming and the original organization of AFs were recovered. These results strongly suggest that MTs are responsible for the organization of AFs inHydrocharis root hair cells.     

Regulation of myosin sliding alongChara actin bundles by native skeletal muscle tropomyosin

Summary Native tropomyosin from rabbit skeletal muscle introduced by intracellular perfusion intoChara cells inhibited the cytoplasmic streaming irrespective of the Ca²⁺ concentration. To find the action site of native tropomyosin inChara, the cytoplasmic streaming was reconstituted by introducing isolated endoplasm into actin donorChara cells from which native endoplasm had been removed. The reconstituted streaming was inhibited by pretreatment of the actin donor cells with native tropomyosin but not by that of the endoplasm, suggesting that the native tropomyosin inhibited the cytoplasmic streaming by binding toChara actin bundles. Staining of the actin bundles with FITC-labeled native tropomyosin also showed that the native tropomyosin could bind to the actin bundles. Streaming reconstituted fromChara actin bundles and skeletal muscle myosin was insensitive to Ca²⁺, but became sensitive on application of the native tropomyosin.Abbrevations APW artificial pond water - ATP adenosine 5-triphosphoric acid - BSA bovine serum albumin - EDTA ethylene diamine tetraacetic acid - EGTA ethyleneglycol-bis-(-aminoethylether) N,N,N,N-tetraacetic acid - FITC fluorescein isothiocyanate - FITC-NTM fluorescein isothiocyanate-labeled native tropomyosin - NTM native tropomyosin     

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.     

Microtubules at wound sites of Nitella internodal cells passively co-align with actin bundles when exposed to hydrodynamic forces generated by cytoplasmic streaming

The organization of cortical microtubules at wound sites in Nitella pseudoflabellata(A. Br. & Nordst.) em. R.D.W. and N. flexilis(L.) Ag. internodal cells was examined in relation to the regeneration of actin filament bundles in order to identify the mechanisms by which microtubules are oriented. Actin bundle regrowth occurs prior to that of microtubules, so it was considered possible that microtubule alignment is actin-dependent, perhaps mediated by cross-linking proteins. In all types of wounds investigated, subcortical actin bundles regenerated parallel to the direction of cytoplasmic streaming. Microtubule orientation patterns, however, varied according to the nature of wound formation and the type of wound wall eventually produced. In chloroplast-free windows induced by blue light irradiation, microtubule orientation varied according to the size of the window. Microtubules were randomized in 10- to 30-μm-wide windows where exposure to cytoplasmic flow is minimal, but were aligned more or less parallel to regenerated actin bundles in 80- to 100-μm-wide windows. Where co-alignment between microtubules and actin bundles was obvious after fluorescence labelling, electron micrographs revealed that microtubules and actin bundles were too widely spaced to account for any cross-linkages. Furthermore, treatments that inhibited or reduced cytoplasmic streaming without altering the direction of actin bundles caused randomization of microtubules previously oriented in the streaming direction, even in the presence of taxol. When evenly flat wound walls were induced by 10⁻⁴ M chlortetracycline, microtubules were co-aligned with nearby actin bundles at the surface of the wound wall. At wounds induced by treatment with 5 × 10⁻² M CaCl₂, however, microtubules were randomly oriented and preferentially located in the narrow clefts between the wound-wall protuberances, up to several micrometers away from the actin bundles near the wound-wall tips. These results indicate that microtubules regenerated in wounds are merely co-aligned with actin filament bundles because they are passively aligned by the hydrodynamic forces created by cytoplasmic flow. Received: 4 August 1998 / Accepted: 30 January 1999     

The mechanism of cytoplasmic streaming in characean algal cells: sliding of endoplasmic reticulum along actin filaments

Electron microscopy of directly frozen giant cells of characean algae shows a continuous, tridimensional network of anastomosing tubes and cisternae of rough endoplasmic reticulum which pervade the streaming region of their cytoplasm. Portions of this endoplasmic reticulum contact the parallel bundles of actin filaments at the interface with the stationary cortical cytoplasm. Mitochondria, glycosomes, and other small cytoplasmic organelles enmeshed in the endoplasmic reticulum network display Brownian motion while streaming. The binding and sliding of endoplasmic reticulum membranes along actin cables can also be directly visualized after the cytoplasm of these cells is dissociated in a buffer containing ATP. The shear forces produced at the interface with the dissociated actin cables move large aggregates of endoplasmic reticulum and other organelles. The combination of fast-freezing electron microscopy and video microscopy of living cells and dissociated cytoplasm demonstrates that the cytoplasmic streaming depends on endoplasmic reticulum membranes sliding along the stationary actin cables. Thus, the continuous network of endoplasmic reticulum provides a means of exerting motive forces on cytoplasm deep inside the cell distant from the cortical actin cables where the motive force is generated.     

Filamentous actin in paramecium cells: functional and structural changes correlated with phalloidin affinity labeling in vivo

Rhodaminylated (R)-phalloidin microinjected into Paramecium tetraurelia cells at a final concentration of greater than or equal to 20 micrograms/ml produces considerable functional and structural changes. F-actin bundles (with 20 micrograms/ml phalloidin within 15 min) are formed, which subsequently (greater than 30 min) are sequestered into autophagic vacuoles; simultaneously, the originally intense fluorescence of a narrow cortical layer becomes more and more diminished. When such microinjected cells are processed for electron microscopy, they display concomitant ultrastructural alterations, namely, the formation of transcellular bundles of 5-7 nm-thick filaments, which subsequently appear in autophagosomes, as well as a considerable reduction of filamentous materials in the cortex. This, in turn, entails a considerable restructuring of the cortex, enabling free access of various structural components to the cortex. Higher doses of R-phalloidin abolish cytoplasmic streaming (e.g., 50 micrograms/ml after 20-30 min); although the cells may survive, new secretory organelles (trichocysts) are no longer docked to the cell membrane. In contrast, exocytosis of docked trichocysts (as well as subsequent membrane resealing and retrieval) is not impaired under any conditions. Cortical F-actin may account for the cytoplasmic streaming that may normally guarantee the delivery of new trichocysts to free docking sites at the cell membrane. When docking is inhibited by high R-phalloidin doses, excess free trichocysts are sequestered into autophagosomes (crinophagy). One of the most sensitive cell functions is food vacuole formation (assayed by prelabeling with India ink), which correlates with the presence of R-phalloidin labeling in the cytostomal region and around food vacuoles. The main conclusions from this work are that filamentous actin may be involved in structuring of the cortex and in cytoplasmic streaming, and may therefore influence the formation, and possibly the transcellular transport (cyclosis), of food vacuoles, as well as the docking of trichocysts, whereas it does not play a role in exocytosis per se or in the steps immediately following.