Studies on the microtubules in heliozoa. 3. A pressure analysis of the role of these structures in the formation and maintenance of the axopodia of Actinosphaerium nucleofilum (Barrett)

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Early, nonlethal ploidy and genome size quantification using confocal microscopy in zebrafish embryos

Ploidy transitions through whole genome duplication have shaped evolution by allowing the sub- and neo-functionalization of redundant copies of highly conserved genes to express novel traits. The nuclear:cytoplasmic (n:c) ratio is maintained in polyploid vertebrates resulting in larger cells, but body size is maintained by a concomitant reduction in cell number. Ploidy can be manipulated easily in most teleosts, and the zebrafish, already well established as a model system for biomedical research, is therefore an excellent system in which to study the effects of increased cell size and reduced cell numbers in polyploids on development and physiology. Here we describe a novel technique using confocal microscopy to measure genome size and determine ploidy non-lethally at 48 h post-fertilization (hpf) in transgenic zebrafish expressing fluorescent histones. Volumetric analysis of myofiber nuclei using open-source software can reliably distinguish diploids and triploids from a mixed-ploidy pool of embryos for subsequent experimentation. We present an example of this by comparing heart rate between confirmed diploid and triploid embryos at 54 hpf.

The living state: How cellular excitability is controlled by the thermodynamic state of the membrane

The thermodynamic (TD) properties of biological membranes play a central role for living systems. It has been suggested, for instance, that nonlinear pulses such as action potentials (APs) can only exist if the membrane state is in vicinity of a TD transition. Herein, two membrane properties in living systems - excitability and velocity - are analyzed for a broad spectrum of conditions (temperature (T), 3D-pressure (p) and pH-dependence). Based on experimental data from Characean cells and a review of literature we predict parameter ranges in which a transition of the membrane is located (15-35°C below growth temperature; 1-3pH units below pH7; at ∼800atm) and propose the corresponding phase diagrams. The latter explain: (i) changes of AP velocity with T,p and pH.(ii) The existence and origin of two qualitatively different forms of loss of nonlinear excitability ("nerve block", anesthesia). (iii) The type and quantity of parameter changes that trigger APs. Finally, a quantitative comparison between the TD behavior of 2D-lipid model membranes with living systems is attempted. The typical shifts in transition temperature with pH and p of model membranes agree with values obtained from cell physiological measurements. Taken together, these results suggest that it is not specific molecules that control the excitability of living systems but rather the TD properties of the membrane interface. The approach as proposed herein can be extended to other quantities (membrane potential, calcium concentration, etc.) and makes falsifiable predictions, for example, that a transition exists within the specified parameter ranges in excitable cells.

Immunocytochemical Analysis of α-Tubulin Distribution Before and After Rapid Axopodial Contraction in the Centrohelid Raphidocystis contractilis

The centrohelid Raphidocystis contractilis is a heliozoan that has many radiating axopodia, each containing a bundle of microtubules. Although the rapid contraction of the axopodia at nearly a video rate (30 frames/s) is induced by mechanical stimuli, the mechanism underlying this phenomenon in R. contractilis has not yet been elucidated. In the present study, we described for the first time an adequate immunocytochemical fixation procedure for R. contractilis and the cellular distribution of α-tubulin before and after rapid axopodial contraction. We developed a flow-through chamber equipped with a micro-syringe pump that allowed the test solution to be injected at a flow rate below the threshold required to induce rapid axopodial contraction. Next, we used this injection method for evaluating the effects of different combinations of two fixatives (paraformaldehyde or glutaraldehyde) and two buffers (phosphate buffer or PHEM) on the morphological structure of the axopodia. A low concentration of glutaraldehyde in PHEM was identified as an adequate fixative for immunocytochemistry. The distribution of α-tubulin before and after rapid axopodial contraction was examined using immunocytochemistry and confocal laser scanning fluorescence microscopy. Positive signals were initially detected along the extended axopodia from the tips to the bases and were distributed in a non-uniform manner within the axopodia. Conversely, after the induction of a rapid axopodial contraction, these positive signals accumulated in the peripheral region of the cell. These results indicated that axopodial microtubules disassemble into fragments and/ or tubulin subunits during rapid axopodial contraction. Therefore, we hypothesize that the mechanism of extremely rapid axopodial contraction accompanied by cytoskeletal microtubule degradati

Microtubule glycylation promotes attachment of basal bodies to the cell cortex

Motile cilia generate directed hydrodynamic flow that is important for the motility of cells and extracellular fluids. To optimize directed hydrodynamic flow, motile cilia are organized and oriented into a polarized array. Basal bodies (BBs) nucleate and position motile cilia at the cell cortex. Cytoplasmic BB-associated microtubules are conserved structures that extend from BBs. By using the ciliate, Tetrahymena thermophila, combined with EM-tomography and light microscopy, we show that BB-appendage microtubules assemble coincidently with new BB assembly and that they are attached to the cell cortex. These BB-appendage microtubules are specifically marked by post translational modifications of tubulin, including glycylation. Mutations that prevent glycylation shorten BB-appendage microtubules and disrupt BB positioning and cortical attachment. Consistent with the attachment of BB-appendage microtubules to the cell cortex to position BBs, mutations that disrupt the cellular cortical cytoskeleton disrupt the cortical attachment and positioning of BBs. In summary, BB-appendage microtubules promote the organization of ciliary arrays through attachment to the cell cortex.

Transient window of resilience during early development minimizes teratogenic effects of heat in zebrafish embryos

Background: Transient heat shock during early development is an established experimental paradigm for doubling the genome of the zebrafish zygote, which has practical applications in expedited identification of recessive mutations in genetic screens. Despite the simplicity of the strategy and the genetic tractability of zebrafish, heat shock has not been used for genome doubling since the proof‐of‐principle experiments done in the 1980s. This is because of poor survival of embryos that ensue from transient heat shocks and gross developmental abnormalities in the few survivors, which is incompatible with phenotype driven screens. Results: We show that heat shocks during early zebrafish development uncouple the second cycle of DNA and centrosome duplication. Interestingly, the developmental time of the heat shock that triggers the dissociation between DNA and centrosome duplication cycles significantly affect the potential of embryos to survive and attain normal morphology. The potential to develop normally after a heat shock alters in a developmental time span of 2 min in zebrafish embryos, a phenomenon that has not been reported in any species. Conclusions: The existence of heat resilient developmental windows and reduced heat teratogenicity during these windows could be an effective step forward in practical application of transient heat for experimental manipulation of ploidy in zebrafish. More broadly, heat resilience before zygotic genome activation suggests that metazoan embryos may possess innate protective features against heat beyond the canonical heat shock response. Developmental Dynamics 247:992–1004, 2018. © 2018 Wiley Periodicals, Inc.

Zebrafish embryos at the end of pronuclear fusion and before initiation of zygotic mitosis are resistant to teratogenic effects of heat.

The teratogenic heat resilient window exists transiently during the maternally controlled phase of development.

Heat shock during the teratogenic heat resilient window enables generation of morphologically normal zebrafish tetraploids.

Diploidization of haploids by transient heat shocks during the teratogenic heat resilient windows aids in effective generation of gynogenic diploids.

A novel live-cell imaging system reveals a reversible hydrostatic pressure impact on cell-cycle progression

Life is dependent upon the ability of a cell to rapidly respond to changes in the environment. Small perturbations in local environments change the ability of molecules to interact and, hence, communicate. Hydrostatic pressure provides a rapid non-invasive, fully reversible method for modulating affinities between molecules both in vivo and in vitro. We have developed a simple fluorescence imaging chamber that allows intracellular protein dynamics and molecular events to be followed at pressures <200 bar in living cells. By using yeast, we investigated the impact of hydrostatic pressure upon cell growth and cell-cycle progression. While 100 bar has no effect upon viability, it induces a delay in chromosome segregation, resulting in the accumulation of long undivided cells that are also bent, consistent with disruption of the cytoskeletons. This delay is independent of stress signalling and induces synchronisation of cell-cycle progression. Equivalent effects were observed in Candida albicans, with pressure inducing a reversible cell-cycle delay and hyphal growth. We present a simple novel non-invasive fluorescence microscopy-based approach to transiently impact molecular dynamics in order to visualise, dissect and study signalling pathways and cellular processes in living cells.

Summary: Development of a simple fluorescence imaging chamber allowing observation of intracellular protein dynamics and molecular events in living cells at pressure up to 200 bar.

Microtubules grow by the addition of bent guanosine triphosphate tubulin to the tips of curved protofilaments

How microtubules (MTs) grow during the addition of guanosine triphosphate (GTP) tubulin is not clear. McIntosh et al. now show that MTs elongating either in vivo or in vitro end in bent protofilaments that curve out from the microtubule axis, suggesting that GTP-tubulin is bent in solution and must straighten to join the MT wall.

We used electron tomography to examine microtubules (MTs) growing from pure tubulin in vitro as well as two classes of MTs growing in cells from six species. The tips of all these growing MTs display bent protofilaments (PFs) that curve away from the MT axis, in contrast with previously reported MTs growing in vitro whose tips are either blunt or sheetlike. Neither high pressure nor freezing is responsible for the PF curvatures we see. The curvatures of PFs on growing and shortening MTs are similar; all are most curved at their tips, suggesting that guanosine triphosphate–tubulin in solution is bent and must straighten to be incorporated into the MT wall. Variations in curvature suggest that PFs are flexible in their plane of bending but rigid to bending out of that plane. Modeling by Brownian dynamics suggests that PF straightening for MT growth can be achieved by thermal motions, providing a simple mechanism with which to understand tubulin polymerization.

Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophrys sol. Upon addition of Ca2+ ([Ca2+]free = 2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5+/-1.7 microm to 11.0+/-1.3 microm. The threshold level of [Ca2+]free for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus.

Therapeutic effects of colchicine in the management of Peyronie's disease: a randomized double-blind, placebo-controlled study

To determine effectiveness and safety of colchicine in Peyronie's disease. In all, 84 patients with Peyronie's disease who did not have calcified plaque were entered into study. The mean disease duration was 15 months. A medical history was obtained, and physical examination, penile X-ray, and dynamic penile duplex ultrasound were performed. Patients were randomly divided into group 1, those who received 0.5-2.5 mg colchicine daily for 4 months and group 2, who received placebo for the same period. Response to therapy was assessed objectively, during dynamic penile duplex ultrasound, as well as subjectively using International Index of Erectile Function (IIEF) questionnaire and measurements of pain, duration of disease, penile curvature, and plaque size. Differences before and after treatment and among the three Kelami classification groups were assessed. In total, 78 (92.8%) completed the whole treatment schedule. Pain resolved in 60 and 63.6% of the patients treated with colchicine and placebo, respectively (P > 0.05). After therapy, in subjects and controls a reduction in the penile deformity was observed by 17.1 and 18.4% of the patients (P > 0.05), and a decrease in plaque size was noticed by 10.5 and 10%, respectively (P > 0.05). Objective measurements did not demonstrate any difference in plaque size or penile curvature. There were no substantial differences in response to treatment based on duration of disease or within the three Kelami classification groups. Significant drug-related adverse effects occurred in colchicine group and in two cases was treatment discontinued. Colchicine is no better than placebo in improvement of pain, curvature angle, or plaque size in patients with Peyronie's disease.

Reactivation of Ca2+-dependent cytoplasmic contraction in permeabilized cell models of the heliozoon Echinosphaerium akamae

Permeabilized cell models of the large heliozoon Echinosphaerium akamae were prepared by treatment with 100 mM EGTA or 1% Triton X-100. When > 10(-6) M Ca(2+) was added to the EGTA-permeabilized cells, axopodial cytoplasm became contracted and several swellings were formed along the axopodial length. Axonemal microtubules remained intact, while higher concentration of Ca(2+) (> 10(-4) M) induced microtubule disassembly and complete breakdown of the axopodia. In Triton-permeabilized cells, cytoplasmic contraction and relaxation of the cell body were induced repeatedly by successive addition and removal of Ca(2+). The contraction did not require ATP, and was not inhibited by cytochalasin B. Electron microscopy showed, in EGTA-permeabilized axopodia, contractile tubules became granulated by the addition of Ca(2+). From these observations, it is strongly suggested that Ca(2+)-dependent granulation of the contractile tubules is responsible for the axopodial contraction.

High hydrostatic pressure effects in vivo: changes in cell morphology, microtubule assembly, and actin organization

We present the first study of the changes in the assembly and organization of actin filaments and microtubules that occur in epithelial cells subjected to the hydrostatic pressures of the deep sea. Interphase BSC-1 epithelial cells were pressurized at physiological temperature and fixed while under pressure. Changes in cell morphology and cytoskeletal organization were followed over a range of pressures from 1 to 610 atm. At atmospheric pressure, cells were flat and well attached. Exposure of cells to pressures of 290 atm or greater caused cell rounding and retraction from the substrate. This response became more pronounced with increased pressure, but the degree of response varied within the cell population in the pressure range of 290-400 atm. Microtubule assembly was not noticeably affected by pressures up to 290 atm, but by 320 atm, few microtubules remained. Most actin stress fibers completely disappeared by 290 atm. High pressure did not simply induce the overall depolymerization of actin filaments for, concurrent with cell rounding, the number of visible microvilli present on the cell surface increased dramatically. These effects of high pressure were reversible. Cells re-established their typical morphology, microtubule arrays appeared normal, and stress fibers reformed after approximately 1 hour at atmospheric pressure. High pressure may disrupt the normal assembly of microtubules and actin filaments by affecting the cellular regulatory mechanisms that control cytological changes during the transition from interphase into mitosis.

Optic vesicle defects induced by vincristine sulfate: an in vivo and in vitro study in the mouse embryo

The optic vesicle develops from cuboidal neuroepithelial cells which become high-columnar, then become wedge-shaped. Vincristine sulfate, a microtubule inhibitor, was used to study the role of microtubules in optic vesicle formation. Swiss-Webster mice were injected with vincristine sulfate on the eighth day of gestation, placebo females received an equivalent volume of saline, and additional females were not injected. All embryos were harvested on the 10th day of gestation. Additional embryos were cultured by the whole embryo culture technique described by New et al. ('73) beginning on the ninth day of gestation for 24 hours. When embryos were harvested on the 10th day of gestation, crown-rump lengths, developmental stage, and the number of visible anomalies were recorded. Embryos were then examined using light, transmission, and scanning electron microscopy. Embryos exposed to vincristine sulfate in vivo or in vitro were significantly smaller and developmentally delayed when compared to the control groups. The embryos treated in vivo appeared to be more severely affected than those exposed in vitro. Observed malformations were similar in both experimental groups, and consisted mainly of closure defects of the cephalic neural folds and defective formation of the optic vesicles. The optic vesicle defects ranged from complete absence to asymmetrical development. Microtubules appeared to be disorganized, S-shaped, or incorporated into paracrystalline inclusion structures.

Axis determination in eggs of Xenopus laevis: a critical period before first cleavage, identified by the common effects of cold, pressure and ultraviolet irradiation

Exposure of eggs of Xenopus laevis to a temperature of 1.0 degree C for 4 min or a pressure of 8000 psi for 5 min in a critical period before first cleavage results in embryos exhibiting a reduction and loss of structures of the body axis. The deficiencies occur in a craniocaudal progression which is dose dependent. In the extreme, totally axis-deficient embryos with radial symmetry are formed. Maximum sensitivity to cold and pressure occurs at 0.6 of the time from fertilization to first cleavage and extends from approximately 0.4 to 0.8, the period between pronuclear contact and mitosis, and the approximate period of gray crescent formation. The effects of cold and pressure resemble those previously reported for uv irradiation in that (1) the types of axis-deficient embryos produced are morphologically indistinguishable; (2) sensitivity in all cases ends before 0.8; (3) cold and uv effects, although not those of pressure, can be prevented by cotreatment with D2O; and (4) impaired eggs can be rescued by oblique orientation. We interpret these results as follows: during the 0.4-0.8 period the egg reorganizes its contents in a manner critical for subsequent development of the embryonic body axis. The reorganization process involves cytoskeletal elements, some of which are sensitive to cold, pressure, and uv, and protected by D2O. Rescue by oblique orientation can be understood as the result of a gravity-driven reorganization of the egg's contents, supplanting the normal mechanochemical process impaired in treated eggs.

The effects of taxol on microtubular arrays: in vivo effects on heliozoan axonemes

The effects of the antitumor drug taxol on the microtubular axonemes of the heliozoon Actinophrys sol have been investigated. The drug induces polymerization of microtubules as shown by a large increase in the length and number of microtubular arrays. The interaction between microtubules and microtubule-associated proteins is also affected, with the result that the normal geometric patterning within the microtubular arrays is disturbed. This is due to the loss or inactivation of long intermicrotubule links. As a result, arms lose their rigidity. Because the drug stabilizes polymerized microtubules, C-shaped profiles and other signs of poor microtubule preservation are absent in taxol-treated cells.

Production of clones of homozygous diploid zebra fish (Brachydanio rerio)

Homozygous diploid zebra fish have been produced on a large scale by the application of simple physical treatments. Clones of homozygous fish have been produced from individual homozygotes. These clones and associated genetic methods will facilitate genetic analyses of this vertebrate.

Low-temperature induction of calcium-dependent protein phosphorylation in blood platelets

Exposure to low temperature causes platelets to change shape in a manner similar to the shape change that precedes secretagogue-induced serotonin release. Previous studies have shown that two proteins, of approximately 20,000 and approximately 40,000 Mr, become phosphorylated before secretion. We have investigated whether low temperature can induce phosphorylation of these proteins and/or serotonin secretion. The data indicate that low-temperature-induced shape change has no requirement for extracellular calcium, whereas phosphorylation of the two proteins and subsequent serotonin release both have strong calcium requirements. Because cold treatment is thought to influence platelet shape through an effect on microtubules, the events in the shape change-release sequence would seem to be ordered as follows: microtubule disassembly leads to shape change leads to protein phosphorylation leads to secretion.

Intact microtubules are required for rapid turnover of carboxyl-terminal tyrosine of alpha-tubulin in cell cultures

In cultured muscle cells the carboxyl-terminal tyrosine of alpha-tubulin was shown to exchange rapidly with free tyrosine. The rapid turnover of this residue was dependent upon the presence of intact microtubules. Half-life determinations were made by two methods: (i) the cells were pulse-labeled in hypertonic medium, in which the major tyrosine incorporation was post-translational, and then chased with isotonic medium; and (ii) the cells were pulsed and chased in isotonic medium, and the post-translational component of the radioactivity of purified alpha-tubulin was calculated. Both methods yielded a half-life of 37 min or less for the terminal tyrosine residue, whereas the half-life of tubulin itself was shown to be greater than 48 hr.

Vesicle production of heated and stressed erythrocytes

Human erythrocytes develop vesicles by budding when heated to temperatures close to the thermal transition for spectrin. Regularly spaced strings of vesicles also develop if cells heated to 51--54 degrees C are pulled into unstable shapes by flow of liquid between cover slips. These strings of vesicles develop when cells which had attached to the glass are restrained in the flow by a long membrane-bound tether which maintains a connection with the attachment site on the glass. Breakup into regularly spaced vesicles suggests the breakup of a liquid-like cylinder by growth of Rayleigh instabilities. The ratio of length:diameter of the fragments of cylinder on which each disturbance grew ranged from 2.2 to 5.4 to 1 with a peak of 3.2, as measured from scanning electron micrographs. The upper limit of the range is slightly less than the ratio for the disturbance most likely to grow if surface tension and viscosity alone controlled the vesicle formation. Similar vesicle formation when the form-maintaining structures were weakened has been reported in other systems.

Light- and electron-microscopic studies on the formation of "dark" variants of chromaffin cells in the rat adrenal medulla

"Dark" cell formation has been studies in the adrenal medulla with combined light- and electron-microscopic methods. Correlation between light- and electron-microscopic appearance has been demonstrated. It has been pointed out that the formation of "dark" cell artefact can be prevented during immersion fixation and can be produced also during perfusion fixation by appropriate physical conditions. "Dark" cell formation was elicited by prefixation cooling and/or by increased tissular pressure. The nature of the underlying physicochemical change was interpreted as a uniformly distributed sol-gel transition of cytoplasmic macromolecules. The formation of cytoplasmic gel structures can be considered an endothermic process which involves volume increase (+ deltaV) - as may be inferred by their susceptibility to solation by high pressure and low temperature.

Ultrastructural investigation of the mechanism of muscle attachment to the gastropod shell

The ultrastructure of the muscle-shell attachment was investigated in the land pulmonate snails Helix aspersa, Anguispira alternata, in the freshwater pulmonate Laevipex sp., and in the freshwater prosobranch Pomacea paludosa. In all cases, a collagenous intercellular matrix and a specialized epithelium (tendon cells) intervene between the columellar muscle and the shell. These tendon cells are characterized by hemidesmosomes at both apical and basal ends, connected by thick bundles of microfilaments. The tendon cells do not insert into the shell directly by microvilli, as formerly thought, but by an extensive network of extracellular organic fibers.

Pressure-induced depolymerization of spindle microtubules. III. Differential stability in HeLa cells

Evidence from light microscopy (principally polarization microscopy) has demonstrated that hydrostatic pressure can reversibly inhibit mitosis by rapidly depolymerizing the spindle fiber microtubules. We have confirmed this finding in ultrastructural studies of mitotic HeLa cells incubated at 37 degrees C and pressurized at 680 atm (10,000 psi). Althouth there are many spindle microtubules in the cells at atmospheric pressure, electron micographs of cells pressurized for 10 min (and fixed while under pressure in a Landau-Thibodeau chamber) show few microtubules. Pressure has a differential effect on the various types of spindle microtubules. Astral and interpolar MTs appear to be completely depolymerized in pressurized cells, but occasional groups of kinetochore fiber microtubules are seen. Surprisingly, the length and density of microtubules of the stem bodies and midbody of telophase cells appear unchanged by pressurization. In cells fixed 10 min after pressure was released, microtubules were again abundant, the density often appearing to be higher than in control cells. Reorganization seems incomplete, however, since many of the microtubules are randomly oriented. Unexpectedly, kinetochores appeared diffuse and were difficult to identify in sections of pressurized cells. Even after 10 min of recovery at atmospheric pressure, their structure was less distinct than in unpressurized cells.

Pressure-induced depolymerization of spindle microtubules. II. Thermodynamics of in vivo spindle assembly

The present experiments were designed to test whether the simple equilibrium assembly model proposed by Inoué could predict variations in spindle microtubule assembly in response to changes in hydrostatic pressure as it does for changes in temperature. The results were also analyzed according to a model based on nucleated condensation polymerization since this recently appears to be the mechanism by which purified brain microtubules are assembled in vitro. Equilibrium birefringence (BR) of the meiotic metaphase-arrested spindle was measured in vivo as a function of hydrostatic pressure and temperature in Chaetopterus oocytes using a miniature microscope pressure chamber. Increasing pressure in steps to 3,000 psi at temperatures below 22 degrees C did produce decreases in spindle equilibrium BR predictable directly from the simple equilibrium model of spindle assembly. Thermodynamic analysis of the pressure data yielded a value of delta V congruent to 400 ml/mol of polymerizing unit. Theoretical curves based on the nucleated condensation model can also be made to fit the data, but semilog plots of the dependence of the equilibrium constant versus pressure and versus reciprocal temperature are biphasic, suggesting that either the size of the polymerizing unit changes or more than one equilibrium constant governs the assembly reaction. That the same value of delta V, 90 ml/mol, was estimated from both the majority of the spindle BR data and data for the assembly of neural microtubules in vitro supports the possibility that spindle microtubules are assembled by a nucleated condensation mechanism.

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.