Actin-based organelle movement

Actin isovariant ACT7 controls root meristem development in Arabidopsis through modulating auxin and ethylene responses

The meristem is the most functionally dynamic part in a plant. The shaping of the meristem requires constant cell division and elongation, which are influenced by hormones and the cytoskeletal component, actin. Although the roles of hormones in modulating meristem development have been extensively studied, the role of actin in this process is still elusive. Using the single and double mutants of the vegetative class actin, we demonstrate that actin isovariant ACT7 plays an important role in root meristem development. In the absence of ACT7, but not ACT8 and ACT2, depolymerization of actin was observed. Consistently, the act7 mutant showed reduced cell division, cell elongation, and meristem length. Intracellular distribution and trafficking of auxin transport proteins in the actin mutants revealed that ACT7 specifically functions in the root meristem to facilitate the trafficking of auxin efflux carriers PIN1 and PIN2, and consequently the transport of auxin. Compared with act7, the act7act8 double mutant exhibited slightly enhanced phenotypic response and altered intracellular trafficking. The altered distribution of auxin in act7 and act7act8 affects the response of the roots to ethylene, but not to cytokinin. Collectively, our results suggest that ACT7-dependent auxin-ethylene response plays a key role in controlling Arabidopsis root meristem development.

Homogentisic acid induces cytoskeleton and extracellular matrix alteration in alkaptonuric cartilage

Alkaptonuria (AKU) is an ultra-rare disease caused by the deficient activity of homogentisate 1,2-dioxygenase enzyme, leading the accumulation of homogentisic acid (HGA) in connective tissues implicating the formation of a black pigmentation called "ochronosis." Although AKU is a multisystemic disease, the most affected tissue is the articular cartilage, which during the pathology appears to be highly damaged. In this study, a model of alkaptonuric chondrocytes and cartilage was realized to investigate the role of HGA in the alteration of the extracellular matrix (ECM). The AKU tissues lost its architecture composed of collagen, proteoglycans, and all the proteins that characterize the ECM. The cause of this alteration in AKU cartilage is attributed to a degeneration of the cytoskeletal network in chondrocytes caused by the accumulation of HGA. The three cytoskeletal proteins, actin, vimentin, and tubulin, were analyzed and a modification in their amount and disposition in AKU chondrocytes model was identified. Cytoskeleton is involved in many fundamental cellular processes; therefore, the aberration in this complex network is involved in the manifestation of AKU disease.

Tissue Engineering of Cartilage Using a Random Positioning Machine

Articular cartilage is a skeletal tissue of avascular nature and limited self-repair capacity. Cartilage-degenerative diseases, such as osteoarthritis (OA), are difficult to treat and often necessitate joint replacement surgery. Cartilage is a tough but flexible material and relatively easy to damage. It is, therefore, of high interest to develop methods allowing chondrocytes to recolonize, to rebuild the cartilage and to restore joint functionality. Here we studied the in vitro production of cartilage-like tissue using human articular chondrocytes exposed to the Random Positioning Machine (RPM), a device to simulate certain aspects of microgravity on Earth. To screen early adoption reactions of chondrocytes exposed to the RPM, we performed quantitative real-time PCR analyses after 24 h on chondrocytes cultured in DMEM/F-12. A significant up-regulation in the gene expression of IL6, RUNX2, RUNX3, SPP1, SOX6, SOX9, and MMP13 was detected, while the levels of IL8, ACAN, PRG4, ITGB1, TGFB1, COL1A1, COL2A1, COL10A1, SOD3, SOX5, MMP1, and MMP2 mRNAs remained unchanged. The STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) analysis demonstrated among others the importance of these differentially regulated genes for cartilage formation. Chondrocytes grown in DMEM/F-12 medium produced three-dimensional (3D) spheroids after five days without the addition of scaffolds. On day 28, the produced tissue constructs reached up to 2 mm in diameter. Using specific chondrocyte growth medium, similar results were achieved within 14 days. Spheroids from both types of culture media showed the typical cartilage morphology with aggrecan positivity. Intermediate filaments form clusters under RPM conditions as detected by vimentin staining after 7 d and 14 d. Larger meshes appear in the network in 28-day samples. Furthermore, they were able to form a confluent chondrocyte monolayer after being transferred back into cell culture flasks in 1 g conditions showing their suitability for transplantation into joints. Our results demonstrate that the cultivation medium has a direct influence on the velocity of tissue formation and tissue composition. The spheroids show properties that make them interesting candidates for cellular cartilage regeneration approaches in trauma and OA therapy.

Photodynamic exposure of Rose-Bengal inhibits Tau aggregation and modulates cytoskeletal network in neuronal cells

The intracellular Tau aggregates are known to be associated with Alzheimer’s disease. The inhibition of Tau aggregation is an important strategy for screening of therapeutic molecules in Alzheimer's disease. Several classes of dyes possess a unique property of photo-excitation, which is applied as a therapeutic measure against numerous neurological dysfunctions. Rose Bengal is a Xanthene dye, which has been widely used as a photosensitizer in photodynamic therapy. The aim of this work was to study the protective role of Rose Bengal against Tau aggregation and cytoskeleton modulations. The aggregation inhibition and disaggregation potency of Rose Bengal and photo-excited Rose Bengal were observed by in-vitro fluorescence, circular dichroism, and electron microscopy. Rose Bengal and photo-excited Rose Bengal induce minimal cytotoxicity in neuronal cells. In our studies, we observed that Rose Bengal and photo-excited Rose Bengal modulate the cytoskeleton network of actin and tubulin. The immunofluorescence studies showed the increased filopodia structures after photo-excited Rose Bengal treatment. Furthermore, Rose Bengal treatment increases the connections between the cells. Rose Bengal and photo-excited Rose Bengal treatment-induced actin-rich podosome-like structures associated with cell membranes. The in-vivo studies on UAS E-14 Tau mutant Drosophila suggested that exposure to Rose Bengal and photo-excited Rose Bengal efficiency rescues the behavioural and memory deficit in flies. Thus, the overall results suggest that Rose Bengal could have a therapeutic potency against Tau aggregation.

A Disturbance in the Force: Cellular Stress Sensing by the Mitochondrial Network

As a highly dynamic organellar network, mitochondria are maintained as an organellar network by delicately balancing fission and fusion pathways. This homeostatic balance of organellar dynamics is increasingly revealed to play an integral role in sensing cellular stress stimuli. Mitochondrial fission/fusion balance is highly sensitive to perturbations such as loss of bioenergetic function, oxidative stress, and other stimuli, with mechanistic contribution to subsequent cell-wide cascades including inflammation, autophagy, and apoptosis. The overlapping activity with m-AAA protease 1 (OMA1) metallopeptidase, a stress-sensitive modulator of mitochondrial fusion, and dynamin-related protein 1 (DRP1), a regulator of mitochondrial fission, are key factors that shape mitochondrial dynamics in response to various stimuli. As such, OMA1 and DRP1 are critical factors that mediate mitochondrial roles in cellular stress-response signaling. Here, we explore the current understanding and emerging questions in the role of mitochondrial dynamics in sensing cellular stress as a dynamic, responsive organellar network.

F-actin distribution and function during sexual development in Eimeria maxima

To determine the involvement of the actin cytoskeleton in macrogametocyte growth and oocyst wall formation, freshly purified macrogametocytes and oocysts were stained with Oregon Green 514 conjugated phalloidin to visualize F-actin microfilaments, while Evans blue staining was used to detect type 1 wall forming bodies (WFB1s) and the outer oocyst wall. The double-labelled parasites were then analysed at various stages of sexual development using three-dimensional confocal microscopy. The results showed F-actin filaments were distributed throughout the entire cytoplasm of mature Eimeria maxima macrogametocytes forming a web-like meshwork of actin filaments linking the type 1 WFBs together into structures resembling ‘beads on a string’. At the early stages of oocyst wall formation, F-actin localization changed in alignment with the egg-shaped morphology of the forming oocysts with F-actin microfilaments making direct contact with the WFB1s. In tissue oocysts, the labelled actin cytoskeleton was situated underneath the forming outer layer of the oocyst wall. Treatment of macrogametocytes in vitro with the actin depolymerizing agents, Cytochalasin D and Latrunculin, led to a reduction in the numbers of mature WFB1s in the cytoplasm of the developing macrogametocytes, indicating that the actin plays an important role in WFB1 transport and oocyst wall formation in E. maxima.

Effect of Cytochalasin B, Lantrunculin B, Colchicine, Cycloheximid, Dimethyl Sulfoxide and Ion Channel Inhibitors on Biospeckle Activity in Apple Tissue

The biospeckle phenomenon is used for non-destructive monitoring the quality of fresh fruits and vegetables, but in the case of plant tissues there is a lack of experimentally confirmed information about the biological origin of the biospeckle activity (BA). As a main sources of BA, processes associated with the movement inside the cell, such as cytoplasmic streaming, organelle movement and intra- and extracellular transport mechanisms, are considered. The aim of this study is to investigate the effect of metabolism inhibitors, connected with intracellular movement such as cytochalasin B, lantrunculin B, colchicine, cycloheximid, dimethyl sulfoxide (DMSO) and mixture of ion channel inhibitors, injected into apples, on BA. Two methods of BA analysis based on cross-correlation coefficient and Laser Speckle Contrast Analysis (LASCA) were used. DMSO, lantrunculin B and mixture of ion channel inhibitors have a significant effect on BA, and approximately 74 % of BA of apple tissue is potentially caused by biological processes. Results indicate that the functioning of actin microfilaments and ion channels significantly affect BA.

A protein phosphatase 2A catalytic subunit modulates blue light-induced chloroplast avoidance movements through regulating actin cytoskeleton in Arabidopsis

Chloroplast avoidance movements mediated by phototropin 2 (phot2) are one of most important physiological events in the response to high-fluence blue light (BL), which reduces damage to the photosynthetic machinery under excess light. Protein phosphatase 2A-2 (PP2A-2) is an isoform of the catalytic subunit of PP2A, which regulates a number of developmental processes. To investigate whether PP2A-2 was involved in high-fluence BL-induced chloroplast avoidance movements, we first analyzed chloroplast migration in the leaves of the pp2a-2 mutant in response to BL. The data showed that PP2A-2 might act as a positive regulator in phot2-mediated chloroplast avoidance movements, but not in phot1-mediated chloroplast accumulation movements. Then, the effect of okadaic acid (OA) and cantharidin (selective PP2A inhibitors) on high-fluence BL response was further investigated in Arabidopsis thaliana mesophyll cells. Within a certain concentration range, exogenously applied OA or cantharidin inhibited the high-fluence BL-induced chloroplast movements in a concentration-dependent manner. Actin depolymerizing factor (ADF)/cofilin phosphorylation assays demonstrated that PP2A-2 can activate/dephosphorylate ADF/cofilin, an actin-binding protein, in Arabidopsis mesophyll cells. Consistent with this observation, the experiments showed that OA could inhibit ADF1 binding to the actin and suppress the reorganization of the actin cytoskeleton after high-fluence BL irradiation. The adf1 and adf3 mutants also exhibited reduced high-fluence BL-induced chloroplast avoidance movements. In conclusion, we identified that PP2A-2 regulated the activation of ADF/cofilin, which, in turn, regulated actin cytoskeleton remodeling and was involved in phot2-mediated chloroplast avoidance movements.

Rho kinase-dependent CCL20 induced by dynamic compression of human chondrocytes

OBJECTIVE

Mechanical stimulation of cartilage affects tissue homeostasis and chondrocyte function. The chondrocyte phenotype is dependent on cell shape, which is largely determined by the actin cytoskeleton. Reorganization of the actin cytoskeleton results from Rho GTPase activation. The purpose of this study was to examine the roles of both actin and Rho in mechanotransduction in chondrocytes.

METHODS

We embedded human articular chondrocytes in 2 x 6-mm agarose discs at 5 x 10(6) cells/ml and subjected the discs to unconfined dynamic compression at 0.5 Hz. By comparing samples with and without dynamic compression, we identified Rho activation according to the GTP-bound active RhoA measured in cell lysates. We identified rearrangements in filamentous actin structures using fluorescence-labeled phalloidin and confocal microscopy of fixed samples. We identified altered gene expression using TaqMan quantitative reverse transcription-polymerase chain reaction analysis. We tested for a requirement for Rho signaling by performing the dynamic compression in the presence of Rho kinase inhibitors.

RESULTS

RhoA activation occurred within 5-10 minutes of dynamic compression. Rho kinase-dependent actin reorganization occurred within 20 minutes after application of dynamic compression and was apparent as "punctate" F-actin structures that were visible under confocal microscopy. We identified early-phase mechanoresponsive genes (CCL20 and inducible nitric oxide synthase) that were highly up-regulated within 1 hour of dynamic compression in a Rho kinase-dependent and actin-dependent manner.

CONCLUSION

Together, these results are the first demonstration that the Rho-Rho kinase pathway and actin cytoskeletal reorganization are required for changes in the expression of genes involved in human chondrocyte mechanotransduction.

Involvement of the cytoskeletal elements in articular cartilage homeostasis and pathology

The cytoskeleton of all cells is a three-dimensional network comprising actin microfilaments, tubulin microtubules and intermediate filaments. Studies in many cell types have indicated roles for these cytoskeletal proteins in many diverse cellular processes including alteration of cell shape, movement of organelles, migration, endocytosis, secretion, cell division and extracellular matrix assembly. The cytoskeletal networks are highly organized in structure enabling them to fulfil their biological functions. This review will primarily focus on the organization and function of the three major cytoskeletal networks in articular cartilage chondrocytes. Articular cartilage is a major load-bearing tissue of the synovial joint; it is well known that the cytoskeleton acts as a physical interface between the chondrocytes and the extracellular matrix in 'sensing' mechanical stimuli. The effect of mechanical load on cytoskeletal element expression and organization will also be reviewed. Abnormal mechanical load is widely believed to be a risk factor for the development of osteoarthritis. Several studies have intimated that the major cytoskeletal networks are disorganized or often absent in osteoarthritic cartilage chondrocytes. The implications and possible reasoning for this are more widely discussed and placed into context with their potential relevance to disease and therapeutic strategies.

Plant Prevacuolar/Endosomal Compartments

Multivesicular endosomes or prevacuolar compartments (PVCs) are membrane-bound organelles that play an important role in mediating protein traffic in the secretory and endocytic pathways of eukaryotic cells. PVCs function as an intermediate compartment for sorting proteins from the Golgi apparatus to vacuoles, sending missorted proteins back to the Golgi from the PVC, and receiving proteins from plasma membrane in the endocytic pathway. PVCs have been identified as multivesicular bodies in mammalian cells and yeast and more recently in plant cells. Whereas much is known about PVC-mediated protein trafficking and PVC biogenesis in mammalian cells and yeast, relatively little is known about the molecular mechanism of plant PVCs. In this review, we summarize and discuss our understanding of the plant PVC and compare it with its counterparts in yeast and mammalian cells.

Presence of multiple sites containing polar material in spherical Escherichia coli cells that lack MreB

In rod-shaped bacteria, certain proteins are specifically localized to the cell poles. The nature of the positional information that leads to the proper localization of these proteins is unclear. In a screen for factors required for the localization of the Shigella sp. actin assembly protein IcsA to the bacterial pole, a mutant carrying a transposon insertion in mreB displayed altered targeting of IcsA. The phenotype of cells containing a transposon insertion in mreB was indistinguishable from that of cells containing a nonpolar mutation in mreB or that of wild-type cells treated with the MreB inhibitor A22. In cells lacking MreB, a green fluorescent protein (GFP) fusion to a cytoplasmic derivative of IcsA localized to multiple sites. Secreted full-length native IcsA was present in multiple faint patches on the surfaces of these cells in a pattern similar to that seen for the cytoplasmic IcsA-GFP fusion. EpsM, the polar Vibrio cholerae inner membrane protein, also localized to multiple sites in mreB cells and colocalized with IcsA, indicating that localization to multiple sites is not unique to IcsA. Our results are consistent with the requirement, either direct or indirect, for MreB in the restriction of certain polar material to defined sites within the cell and, in the absence of MreB, with the formation of ectopic sites containing polar material.

Bacterial cell polarity: a "swarmer-stalked" tale of actin

The actin cytoskeleton is important for cell polarity and morphogenesis in eukaryotic organisms. A recent article describes an unexpected requirement for the actin-like protein MreB in the polarization of the bacterium Caulobacter crescentus. More surprisingly, the formation of a filamentous MreB structure that traverses the length of the cell is sufficient for randomized polar localization of cell-fate proteins. In this article, we discuss the significance of these findings and the possible mechanisms by which an actin-like cytoskeleton could mediate cell polarity in bacteria.

Effect of the arbuscular mycorrhizal fungus Glomus intraradices on expression of cytoskeletal proteins in tomato roots

The expression patterns of the cytoskeletal proteins α-, β-, and γ-tubulin, actin, and myosin were investigated in young tomato roots and older roots at different intensities of mycorrhizal colonization. The relative level of cytoskeletal proteins was estimated by protein blotting and immunostaining. The contribution of plant α-, β-, and γ-tubulin to the total protein pool was higher in uncolonized 2-week-old roots than in 10-week-old roots, whereas the contribution of actin remained constant. The level of plant tubulin expression was clearly higher in mycorrhizal root systems than in uncolonized older root systems. These results indicate that tubulins are more involved in plant cell differentiation than actin. Myosin of approximately 230 kDa was expressed in the roots of 10-week-old wild-type tomato but not in young or mycorrhizal tomatoes. In contrast, a smaller ca. 170 kDa myosin was consistently present in all root samples. Indirect immunofluorescence microscopy showed that plant myosin was located particularly along the periarbuscular membranes surrounding the arbuscule branches. In uncolonized roots, myosin was associated mainly with membranes adjacent to plant cell walls. These data provide novel evidence that myosin expression and localization in root cells responds to mycorrhizal colonization.Key words: actin, myosin, arbuscular mycorrhizal fungi, protein expression, tubulin.

Importance of mitochondrial dynamics during meiosis and sporulation

Opposing fission and fusion events maintain the yeast mitochondrial network. Six proteins regulate these membrane dynamics during mitotic growth-Dnm1p, Mdv1p, and Fis1p mediate fission; Fzo1p, Mgm1p, and Ugo1p mediate fusion. Previous studies established that mitochondria fragment and rejoin at distinct stages during meiosis and sporulation, suggesting that mitochondrial fission and fusion are required during this process. Here we report that strains defective for mitochondrial fission alone, or both fission and fusion, complete meiosis and sporulation. However, visualization of mitochondria in sporulating cultures reveals morphological defects associated with the loss of fusion and/or fission proteins. Specifically, mitochondria collapse to one side of the cell and fail to fragment during presporulation. In addition, mitochondria are not inherited equally by newly formed spores, and mitochondrial DNA nucleoid segregation defects give rise to spores lacking nucleoids. This nucleoid inheritance defect is correlated with an increase in petite spore colonies. Unexpectedly, mitochondria fragment in mature tetrads lacking fission proteins. The latter finding suggests either that novel fission machinery operates during sporulation or that mechanical forces generate the mitochondrial fragments observed in mature spores. These results provide evidence of fitness defects caused by fission mutations and reveal new phenotypes associated with fission and fusion mutations.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.

Mitochondrial dynamics in filamentous fungi

Mitochondria are essential organelles of eukaryotic cells. They grow continuously throughout the cell cycle and are inherited by daughter cells upon cell division. Inheritance of mitochondria and maintenance of mitochondrial distribution and morphology require active transport of the organelles along the cytoskeleton and depend on membrane fission and fusion events. Many of the molecular components and cellular mechanisms mediating these complex processes have been conserved during evolution across the borders of the fungal and animal kingdoms. During the past few decades, several constituents of the cellular machinery mediating mitochondrial behavior have been identified and functionally characterized. Here, we review the contributions of fungi, with special emphasis on the filamentous fungus Neurospora crassa, to our current understanding of mitochondrial morphogenesis and inheritance.

ATP-dependent membrane assembly of F-actin facilitates membrane fusion

We recently established an in vitro assay that monitors the fusion between latex-bead phagosomes and endocytic organelles in the presence of J774 macrophage cytosol (). Here, we show that different reagents affecting the actin cytoskeleton can either inhibit or stimulate this fusion process. Because the membranes of purified phagosomes can assemble F-actin de novo from pure actin with ATP (), we focused here on the ability of membranes to nucleate actin in the presence of J774 cytosolic extracts. For this, we used F-actin sedimentation, pyrene actin assays, and torsional rheometry, a biophysical approach that could provide kinetic information on actin polymerization and gel formation. We make two major conclusions. First, under our standard in vitro conditions (4 mg/ml cytosol and 1 mM ATP), the presence of membranes actively catalyzed the assembly of cytosolic F-actin, which assembled into highly viscoelastic gels. A model is discussed that links these results to how the actin may facilitate fusion. Second, cytosolic actin paradoxically polymerized more under ATP depletion than under high-ATP conditions, even in the absence of membranes; we discuss these data in the context of the well described, large increases in F-actin seen in many cells during ischemia.

Oxygen, homeostasis, and metabolic regulation

Even a cursory review of the literature today indicates that two views dominate experimental approaches to metabolic regulation. Model I assumes that cell behavior is quite similar to that expected for a bag of enzymes. Model II assumes that 3-D order and structure constrain metabolite behavior and that metabolic regulation theory has to incorporate structure to ever come close to describing reality. The phosphagen system may be used to illustrate that both approaches lead to very productive experimentation and significant advances are being made within both theoretical frameworks. However, communication between the two approaches or the two 'groups' is essentially nonexistent and in many cases (our own for example) some experiments are done in one framework and some in the other (implying some potential schizophrenia in the field). In our view, the primary paradox and problem which no one has solved so far is that essentially all metabolite concentrations are remarkably stable (are homeostatic) over large changes in pathway fluxes. For muscle cells O2 is one of the most perfectly homeostatic of all even though O2 delivery and metabolic rate usually correlate in a 1:1 fashion. Four explanations for this behavior are given by traditional metabolic regulation models. Additionally, there is some evidence for universal O2 sensors which could help to get us out of the paradox. In contrast, proponents of an ultrastructurally dominated view of the cell assume intracellular perfusion or convection as the main means for accelerating enzyme-substrate encounter and as a way to account for the data which have been most perplexing so far: the striking lack of correlation between changes in pathway reaction rates and changes in concentrations of pathway substrates and intermediates, including oxygen. The polarization illustrated by these two views of living cells extends throughout the metabolic regulation field (and has caused the field to progress along two surprisingly independent paths with minimal communication between them). The time may have come when cross talk between the two fields may be useful.

The chondrocyte cytoskeleton in mature articular cartilage: structure and distribution of actin, tubulin, and vimentin filaments

We investigated the structure of the chondrocyte cytoskeleton in intact tissue sections of mature bovine articular cartilage using confocal fluorescence microscopy complemented by protein extraction and immunoblotting analysis. Actin microfilaments were present inside the cell membrane as a predominantly cortical structure. Vimentin and tubulin spanned the cytoplasm from cell to nuclear membrane, the vimentin network appearing finer compared to tubulin. These cytoskeletal structures were present in chondrocytes from all depth zones of the articular cartilage. However, staining intensity varied from zone to zone, usually showing more intense staining for the filament systems at the articular surface compared to the deeper zones. These results obtained on fluorescently labeled sections were also corroborated by protein contents extracted and observed by immunoblotting. The observed cytoskeletal structures are compatible with some of the proposed cellular functions of these systems and support possible microenvironmental regulation of the cytoskeleton, including that due to physical forces from load-bearing, which are known to vary through the depth layers of articular cartilage.

Role of MMM1 in maintaining mitochondrial morphology in Neurospora crassa

Mmm1p is a protein required for maintenance of mitochondrial morphology in budding yeast. It was proposed that it is required to mediate the interaction of the mitochondrial outer membrane with the actin cytoskeleton. We report the cloning and characterization of MMM1 of the filamentous fungus Neurospora crassa, an organism that uses microtubules for mitochondrial transport. Mutation of the mmm-1 gene leads to a temperature-sensitive slow growth phenotype and female sterility. Mutant cells harbor abnormal giant mitochondria at all stages of the asexual life cycle, whereas actin filament-depolymerizing drugs have no effect on mitochondrial morphology. The MMM1 protein has a single transmembrane domain near the N terminus and exposes a large C-terminal domain to the cytosol. The protein can be imported into the outer membrane in a receptor-dependent manner. Our findings suggest that MMM1 is a factor of general importance for mitochondrial morphology independent of the cytoskeletal system used for mitochondrial transport.

Rab GTPases coordinate endocytosis

Endocytosis is characterized by vesicular transport along numerous pathways. Common steps in each pathway include membrane budding to form vesicles, transport to a particular destination, and ultimately docking and fusion with the target membrane. Specificity of vesicle targeting is rendered in part by associated Rab GTPases. This review summarizes current knowledge about Rab GTPase functions in the endocytic pathways and provides insight into the regulation of Rab GTPase activity and mechanisms of Rab protein function. Functional assays have identified some Rab proteins that operate on individual pathways, but Rab proteins in several pathways remain controversial or have not been identified. Control of Rab GTPase activity is exerted through multiple levels of regulation. Significant new information pertaining to Rab protein function in regulating transport has emerged. Remarkably, Rab5 GTPase links budding, cytoskeletal transport and docking/fusion activities. This paradigm will most likely be generally applicable to other Rab GTPase pathways. Together with the cross-talk between different Rab proteins and their effectors, this may provide an integrated system for the general coordination of endocytic pathways to maintain organelle homeostasis.

The metabolic implications of intracellular circulation

Two views currently dominate research into cell function and regulation. Model I assumes that cell behavior is quite similar to that expected for a watery bag of enzymes and ligands. Model II assumes that three-dimensional order and structure constrain and determine metabolite behavior. A major problem in cell metabolism is determining why essentially all metabolite concentrations are remarkably stable (are homeostatic) over large changes in pathway fluxes-for convenience, this is termed the [s] stability paradox. For muscle cells, ATP and O(2) are the most perfectly homeostatic, even though O(2) delivery and metabolic rate correlate in a 1:1 fashion. In total, more than 60 metabolites are known to be remarkably homeostatic in differing metabolic states. Several explanations of [s] stability are usually given by traditional model I studies-none of which apply to all enzymes in a pathway, and all of which require diffusion as the means for changing enzyme-substrate encounter rates. In contrast, recent developments in our understanding of intracellular myosin, kinesin, and dyenin motors running on actin and tubulin tracks or cables supply a mechanistic basis for regulated intracellular circulation systems with cytoplasmic streaming rates varying over an approximately 80-fold range (from 1 to >80 micrometer x sec(-1)). These new studies raise a model II hypothesis of intracellular perfusion or convection as a primary means for bringing enzymes and substrates together under variable metabolic conditions. In this view, change in intracellular perfusion rates cause change in enzyme-substrate encounter rates and thus change in pathway fluxes with no requirement for large simultaneous changes in substrate concentrations. The ease with which this hypothesis explains the [s] stability paradox is one of its most compelling features.

Vesicle transport: the role of actin filaments and myosin motors

The transport of vesicles and the retention of organelles at specific locations are fundamental processes in cells. Actin filaments and myosin motors have been shown to be required for both of these tasks. Most of the organelles in cells associate with actin filaments and some of the myosin motors required for movement on actin filaments have been identified. Myosin V has been shown to transport endoplasmic reticulum (ER) vesicles in neurons, pigment granules in melanocytes, and the vacuole in yeast. Myosin I has been shown to be involved in the transport of Golgi-derived vesicles in epithelial cells. Myosin VI has been shown to be associated with Golgi-derived vesicles, and cytoplasmic vesicles in living Drosophila embryos. Myosin II may be a vesicle motor but its role in vesicle transport has not been resolved. Secretory vesicles, endosomes and mitochondria appear to be transported on actin filaments but the myosin motors on these organelles have not been identified. Mitochondria in yeast may be transported by the dynamic assembly of an actin "tail." The model that has unified all of these findings is the concept that long-range movement of vesicles occurs on microtubules and short-range movement on actin filaments. The details of how the microtubule-dependent and the actin-dependent motors are coordinated are important questions in the field. There is now strong evidence that two molecular motors, kinesin and myosin V, interact with each other and perhaps function as a complex on vesicles. An understanding of the interrelationship of microtubules and actin filaments and the motors that move cargo on them will ultimately establish how vesicles and organelles are transported to their specific locations in cells.

Actin-organelle interaction: association with chloroplast in arabidopsis leaf mesophyll cells

The role of the cytoskeleton in the regulation of chloroplast motility and positioning has been investigated by studying: (1) structural relationship of actin microfilaments, microtubules, and chloroplasts in cryofixed and freeze-substituted leaf cells of Arabidopsis; and (2) the effects of anti-actin (Latrunculin B; LAT-B) and anti-microtubule (Oryzalin) drugs on intracellular distribution of chloroplasts. Immunolabeling of leaf cells with two plant-actin specific antibodies, which react equivalently with all the expressed Arabidopsis actins, revealed two arrangements of actin microfilaments: longitudinal arrays of thick actin bundles and randomly oriented thin actin filaments that extended from the bundles. Chloroplasts were either aligned along the actin bundles or closely associated with the fine filaments. Baskets of actin microfilaments were also observed around the chloroplasts. The leaf cells labeled with an anti-tubulin antibody showed dense transverse arrays of cortical microtubules that exhibited no apparent association with chloroplasts. The application of LAT-B severely disrupted actin filaments and their association with chloroplasts. In addition, LAT-B induced aberrant aggregation of chloroplasts in the mesophyll and bundle sheath cells. Double labeling of LAT-B treated cells with anti-actin and anti-tubulin antibodies revealed that the microtubules in these cells were unaffected. Moreover, depolymerization of microtubules with Oryzalin did not affect the distribution of chloroplasts. These results provide evidence for the involvement of actin, but not tubulin, in the movement and positioning of chloroplasts in leaf cells. We propose that using motor molecules, some chloroplasts migrate along the actin cables directly, while others are pulled along the cables by the fine actin filaments. The baskets of microfilaments may anchor the chloroplasts during streaming and allow control over proper three-dimensional orientation to light.

Mitochondrial dynamics in yeast

Proteins that control mitochondrial dynamics in yeast are being identified at a rapid pace. These proteins include cytoskeletal elements that regulate organelle distribution and inheritance and several outer membrane proteins that are required to maintain the branched, mitochondrial reticulum. Interestingly, three of the high molecular weight GTPases encoded by the yeast genome are required for mitochondrial integrity and are potential regulators of mitochondrial branching, distribution, and membrane fusion. The recent finding that mtDNA mixing is restricted in the mitochondrial matrix has stimulated the hunt for the molecular machinery that anchors mitochondrial nucleoids in the organelle. Considering that many aspects of mitochondrial structure and behavior are strikingly similar in different cell types, the functional analyses of these yeast proteins should provide general insights into the mechanisms governing mitochondrial dynamics in all eukaryotes.