The Dictyostelium cytoskeleton

Structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum

The crystal structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum was solved by Ca(2+)/S-SAD phasing and refined at 1.89 Å resolution. ABP34 is a calcium-regulated actin-binding protein that cross-links actin filaments into bundles. Its in vitro F-actin-binding and F-actin-bundling activities were confirmed by a co-sedimentation assay and transmission electron microscopy. The co-localization of ABP34 with actin in cells was also verified. ABP34 adopts a two-domain structure with an EF-hand-containing N-domain and an actin-binding C-domain, but has no reported overall structural homologues. The EF-hand is occupied by a calcium ion with a pentagonal bipyramidal coordination as in the canonical EF-hand. The C-domain structure resembles a three-helical bundle and superposes well onto the rod-shaped helical structures of some cytoskeletal proteins. Residues 216-244 in the C-domain form part of the strongest actin-binding sites (193-254) and exhibit a conserved sequence with the actin-binding region of α-actinin and ABP120. Furthermore, the second helical region of the C-domain is kinked by a proline break, offering a convex surface towards the solvent area which is implicated in actin binding. The F-actin-binding model suggests that ABP34 binds to the side of the actin filament and residues 216-244 fit into a pocket between actin subdomains -1 and -2 through hydrophobic interactions. These studies provide insights into the calcium coordination in the EF-hand and F-actin-binding site in the C-domain of ABP34, which are associated through interdomain interactions.

Actin binding domains direct actin-binding proteins to different cytoskeletal locations

BACKGROUND

Filamin (FLN) and non-muscle alpha-actinin are members of a family of F-actin cross-linking proteins that utilize Calponin Homology domains (CH-domain) for actin binding. Although these two proteins have been extensively characterized, little is known about what regulates their binding to F-actin filaments in the cell.

RESULTS

We have constructed fusion proteins consisting of green fluorescent protein (GFP) with either the entire cross-linking protein or its actin-binding domain (ABD) and examined the localization of these fluorescent proteins in living cells under a variety of conditions. The full-length fusion proteins, but not the ABD's complemented the defects of cells lacking both endogenous proteins indicating that they are functional. The localization patterns of filamin (GFP-FLN) and alpha-actinin (GFP-alphaA) were overlapping but distinct. GFP-FLN localized to the peripheral cell cortex as well as to new pseudopods of unpolarized cells, but was observed to localize to the rear of polarized cells during cAMP and folate chemotaxis. GFP-alphaA was enriched in new pseudopods and at the front of polarized cells, but in all cases was absent from the peripheral cortex. Although both proteins appear to be involved in macropinocytosis, the association time of the GFP-probes with the internalized macropinosome differed. Surprisingly, the localization of the GFP-actin-binding domain fusion proteins precisely reflected that of their respective full length constructs, indicating that the localization of the protein was determined by the actin-binding domain alone. When expressed in a cell line lacking both filamin and alpha-actinin, the probes maintain their distinct localization patterns suggesting that they are not functionally redundant.

CONCLUSION

These observations strongly suggest that the regulation of the binding of these proteins to actin filaments is built into the actin-binding domains. We suggest that different actin binding domains have different affinities for F-actin filaments in functionally distinct regions of the cytoskeleton.

Dictyostelium CBP3 associates with actin cytoskeleton and is related to slug migration

Calcium-binding protein 3 (CBP3) expression was up-regulated under the control of the actin 15 promoter and down-regulated by RNA interference in Dictyostelium discoideum. The overexpression of CBP3 accelerated cell aggregation and formed small aggregates and fruiting body. CBP3-inhibited cells showed uneven aggregation and increased slug trail lengths toward the directed light, whereas CBP3-overexpressing cells showed the opposite phenomena. Under dark condition, the enhanced slug trail length was also observed in the CBP3-inhibited cells. Yeast two-hybrid screening identified actin 8 as interacting protein with CBP3. The interaction between CBP3 and actin was confirmed by beta-galactosidase assay and surface plasmon resonance. CBP3 was associated with Triton X-100-insoluble cytoskeleton in the presence of Ca(2+) and the interaction of CBP3 with cytoskeleton was increased by the addition of Ca(2+). Using fluorescence microscopy, CBP3 was also shown to associate with the actin cytoskeleton during development. Subcellular fractionation indicated that CBP3 was enriched in cytosolic fraction. Taken together, these results suggest that CBP3 interacts with actin cytoskeleton and has a role during cell aggregation and slug migration of Dictyostelium.

A cell number counting factor regulates Akt/protein kinase B to regulate Dictyostelium discoideum group size

Little is known about how individual cells can organize themselves to form structures of a given size. During development, Dictyostelium discoideum aggregates in dendritic streams and forms groups of approximately 20,000 cells. D. discoideum regulates group size by secreting and simultaneously sensing a multiprotein complex called counting factor (CF). If there are too many cells in a stream, the associated high concentration of CF will decrease cell-cell adhesion and increase cell motility, causing aggregation streams to break up. The pulses of cyclic AMP (cAMP) that mediate aggregation cause a transient translocation of Akt/protein kinase B (Akt/PKB) to the leading edge of the plasma membrane and a concomitant activation of the kinase activity, which in turn stimulates motility. We found that countin- cells (which lack bioactive CF) and wild-type cells starved in the presence of anticountin antibodies (which block CF activity) showed a decreased level of cAMP-stimulated Akt/PKB membrane translocation and kinase activity compared to parental wild-type cells. Recombinant countin has the bioactivity of CF, and a 1-min treatment of cells with recombinant countin potentiated Akt/PKB translocation to membranes and Akt/PKB activity. Western blotting of total cell lysates indicated that countin does not affect the total level of Akt/PKB. Fluorescence microscopy of cells expressing an Akt/PKB pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion protein indicated that recombinant countin and anti-countin antibodies do not obviously alter the distribution of Akt/PKB PH-GFP when it translocates to the membrane. Our data indicate that CF increases motility by potentiating the cAMP-stimulated activation and translocation of Akt/PKB.

A novel partner for Dictyostelium filamin is an α-helical developmentally regulated protein

The filamins are a family of highly homologous actincrosslinking proteins that stabilize three-dimensional actin networks, link them to membrane proteins and direct intracellular signaling reactions to the actin scaffold through interaction with various binding partners. Here, we describe the first Dictyostelium filamin-interacting protein to be isolated - FIP, a 229.8 kDa protein with two α-helical coiled coil domains. FIP was identified in a yeast two-hybrid screen using the rod domain of filamin as bait. FIP can also be coimmunoprecipitated with filamin from cellular extracts. Deletion analysis located the interaction domain of FIP to a C-terminal region; by contrast, in filamin rods, repeats 2-4 interacted with the recombinant FIP protein. The 7 kb transcript of FIP is upregulated during early development. Monoclonal antibodies raised against a bacterially expressed FIP polypeptide recognize a 230 kDa developmentally regulated protein in western blots. Immunofluorescence analysis shows a punctate staining pattern in the cytosol and, in cell fractionation experiments, FIP is mainly found in the cytosolic fraction. A fusion protein composed of GFP and the C-terminal part localizes to the plasma membrane and is associated with the cytoskeleton. Expression of the fusion protein affects development and influences the size of the multicellular aggregates and the phototactic behavior of slugs. Thus, FIP might provide a candidate link between the dynamic actin cytoskeleton and signal transduction events during the multicellular stages of Dictyostelium amoebae.

Identification of a second member of the ponticulin gene family and its differential expression pattern

We have identified a homologue (ponB) of the ponticulin gene (ponA), an F-actin binding protein, in the expressed sequence tag library generated to mRNA isolated from fusion-competent cells of Dictyostelium discoideum. PonB is predicted to have many of the same characteristics as ponticulin. Both proteins are predicted to possess a cleaved signal peptide, a glycosyl anchor, an amphipathic beta-strand structure and six conserved cysteines. Because of the sequence similarity and predicted conserved structures, this gene constitutes the second member of a ponticulin gene family. Unlike ponticulin, ponB is not expressed in axenically grown cells or during the asexual reproductive phase of D. discoideum. PonB is expressed by cells grown on bacterial lawns and by cells induced to be fusion-competent, i.e., gametes. The expression of ponB correlates with the appearance of a new F-actin binding activity in cell lysates of bacterially grown ponA(-) cells. By immunofluorescence microscopy, ponB appears to be localized to vesicles and to the plasma membrane of bacterially grown cells. Because ponticulin is the major high-affinity link between the plasma membrane and the cytoskeleton, the ponticulin gene family is likely to be part of the redundant system of proteins involved in connecting the cytoskeleton to the plasma membrane.

Cells respond to and bind countin, a component of a multisubunit cell number counting factor

In Dictyostelium discoideum counting factor (CF), a secreted approximately 450-kDa complex of polypeptides, inhibits group and fruiting body size. When the gene encoding countin (a component of CF) was disrupted, cells formed large groups. We find that recombinant countin causes developing cells to form small groups, with an EC(50) of approximately 3 ng/ml, and affects cAMP signal transduction in the same manner as semipurified CF. Recombinant countin increases cell motility, decreases cell-cell adhesion, and regulates gene expression in a manner similar to the effect of CF. However, countin does not decrease adhesion or group size to the extent that semipurified CF does. A 1-min exposure of developing cells to countin causes an increase in F-actin polymerization and myosin phosphorylation and a decrease in myosin polymerization, suggesting that countin activates a rapid signal transduction pathway. (125)I-Labeled countin has countin bioactivity, and binding experiments suggest that vegetative and developing cells have approximately 53 cell-surface sites that bind countin with a K(D) of approximately 1.5 ng/ml or 60 pm. We hypothesize that countin regulates cell development through the same pathway as CF and that other proteins within the complex may modify the activity of countin and/or have independent size-regulating activities.

Eukaryotic cell locomotion depends on the propagation of self-organized reaction-diffusion waves and oscillations of actin filament assembly

Actin filament (F-actin) assembly kinetics determines the locomotion and shape of crawling eukaryotic cells, but the nature of these kinetics and their determining reactions are unclear. Live BHK21 fibroblasts, mouse melanoma cells, and Dictyostelium amoebae, locomoting on glass and expressing Green Fluorescent Protein-actin fusion proteins, were examined by confocal microscopy. The cells demonstrated three-dimensional bands of F-actin, which propagated throughout the cytoplasm at rates usually ranging between 2 and 5 microm/min in each cell type and produced lamellipodia or pseudopodia at the cell boundary. F-actin's dynamic behavior and supramolecular spatial patterns resembled in detail self-organized chemical waves in dissipative, physico-chemical systems. On this basis, the present observations provide the first evidence of self-organized, and probably autocatalytic, chemical reaction-diffusion waves of reversible actin filament assembly in vertebrate cells and a comprehensive record of wave and locomotory dynamics in vegetative-stage Dictyostelium cells. The intensity and frequency of F-actin wavefronts determine locomotory cell projections and the rotating oscillatory waves, which structure the cell surface. F-actin assembly waves thus provide a fundamental, deterministic, and nonlinear mechanism of cell locomotion and shape, which complements mechanisms based exclusively on stochastic molecular reaction kinetics.

A cell number-counting factor regulates the cytoskeleton and cell motility in Dictyostelium

Little is known about how a morphogenetic rearrangement of a tissue is affected by individual cells. Starving Dictyostelium discoideum cells aggregate to form dendritic streams, which then break up into groups of approximately 2 x 10(4) cells. Cell number is sensed at this developmental stage by using counting factor (CF), a secreted complex of polypeptides. A high extracellular concentration of CF indicates that there is a large number of cells, which then causes the aggregation stream to break up. Computer simulations indicated that stream breakup could be caused by CF decreasing cell-cell adhesion and/or increasing cell motility, and we observed that CF does indeed decrease cell-cell adhesion. We find here that CF increases cell motility. In Dictyostelium, motility is mediated by actin and myosin. CF increases the amounts of polymerized actin and the ABP-120 actin-crosslinking protein. Partially inhibiting motility by using drugs that interfere with actin polymerization reduces stream dissipation, resulting in fewer stream breaks and thus larger groups. CF also potentiates the phosphorylation and redistribution of myosin while repressing its basal level of assembly. The computer simulations indicated that a narrower distribution of group sizes results when a secreted factor modulates both adhesion and motility. CF thus seems to induce the morphogenesis of streams into evenly sized groups by increasing actin polymerization, ABP-120 levels, and myosin phosphorylation and decreasing adhesion and myosin polymerization.

F-actin assembly in Dictyostelium cell locomotion and shape oscillations propagates as a self-organized reaction-diffusion wave

The crawling locomotion and shape of eukaryotic cells have been associated with the stochastic molecular dynamics of actin and its protein regulators, chiefly Arp2/3 and Rho family GTPases, in making a cytoskeleton meshwork within cell extensions. However, the cell's actin-dependent oscillatory shape and extension dynamics may also yield insights into locomotory mechanisms. Confocal observations of live Dictyostelium cells, expressing a green fluorescent protein-actin fusion protein, demonstrate oscillating supramolecular patterns of filamentous actin throughout the cell, which generate pseudopodia at the cell edge. The distinctively dissipative spatio-temporal behavior of these structures provides strong evidence that reversible actin filament assembly propagates as a self-organized, chemical reaction-diffusion wave.

Characterization of CD36/LIMPII homologues in Dictyostelium discoideum

The CD36/LIMPII family is ubiquitously expressed in higher eukaryotes and consists of integral membrane proteins that have in part been characterized as cell adhesion receptors, scavenger receptors, or fatty acid transporters. However, no physiological role has been defined so far for the members of this family that localize specifically to vesicular compartments rather than to the cell surface, namely lysosomal integral membrane protein type II (LIMPII) from mammals and LmpA from the amoeba Dictyostelium discoideum. LmpA, the first described CD36/LIMPII homologue from lower eukaryotes, has initially been identified as a suppressor of the profilin-minus phenotype. We report the discovery and initial characterization of two new CD36/LIMPII-related proteins, both of which share several features with LmpA: (i) their size is considerably larger than that of the CD36/LIMPII proteins from higher eukaryotes; (ii) they show the characteristic "hairpin" topology of this protein family; (iii) they are heavily N-glycosylated; and (iv) they localize to vesicular structures of putative endolysosomal origin. However, they show intriguing differences in their developmental regulation and exhibit different sorting signals of the di-leucine or tyrosine-type in their carboxyl-terminal tail domains. These features make them promising candidates as a paradigm for the study of the function and evolution of the as yet poorly understood CD36/LIMPII proteins.

Toward the functional analysis of the Dictyostelium discoideum genome

Dictyostelium discoideum is a useful model for molecular studies of cell biology and development. The 34-megabase Dictyostelium genome is currently being sequenced through the efforts of an international consortium. The genome is expected to encode 8-10,000 genes, including all those required for a free-living eukaryote capable of multicellular development. A complete description of the Dictyostelium genome will open the way toward the application of genome-based experimental approaches to studies of cell biology and development in this organism, and allow detailed physiological and evolutionary comparisons to other species.

Optimal development of Chlamydophila psittaci in L929 fibroblast and BGM epithelial cells requires the participation of microfilaments and microtubule-motor proteins

The cytoskeleton is involved in several cellular activities, including internalization and transport of foreign particles. Although particular functions to each cytoskeleton component have been described, interactions between those components seem to occur. The involvement of the different host cell cytoskeletal components in uptake and development of Chlamydophila psittaci is incompletely understood. In this study, the participation of the microfilament network along with the kinesin and dynein microtubule motor proteins in the internalization and further development of Chlamydophila psittaci were investigated in L929 fibroblast and BGM epithelial cells. Cytochalasin D disruption of actin filaments, and blockage of the motor proteins through the introduction of monoclonal antibodies into the host cells were carried out, either single or combined, at different moments around bacterial inoculation, and Chlamydophila infectivity determined 24 h post- inoculation by direct immunofluorescence. Our results show that, although Chlamydophila Ipsittaci can make use of both microfilament-dependent and independent entry pathways in both cell types, Chlamydophila internalization and development in the fibroblast cells mainly concerned processes mediated by microfilaments while in the epithelial cells mechanisms that require microtubule motor proteins were the ones predominantly involved. Evidence that mutual participation of the actin and tubulin networks in both host cells are required for optimal growth of Chlamydophila psittaci is also presented.

Cloning and characterization of beta-COP from Dictyostelium discoideum

We have isolated a cDNA coding for beta-COP from Dictyostelium discoideum by polymerase chain reaction using degenerate primers derived from rat beta-COP. The complete cDNA clone has a size of 2.8 kb and codes for a protein with a calculated molecular mass of 102 kDa. Dictyostelium beta-COP exhibits highest homology to mammalian beta-COP, but it is considerably smaller due to a shortened variable region that is thought to form a linker between the highly conserved N- and C-terminal domains. Dictyostelium beta-COP is encoded by a single gene, which is transcribed at moderate levels into two RNAs that are present throughout development. To localize the protein, full-length beta-COP was fused to GFP and expressed in Dictyostelium cells. The fusion protein was detected on vesicles distributed all over the cells and was strongly enriched in the perinuclear region. Based on coimmunofluorescence studies with antibodies directed against the Golgi marker comitin, this compartment was identified as the Golgi apparatus. Beta-COP distribution in Dictyostelium was not brefeldin A sensitive being most likely due to the presence of a brefeldin A resistance gene. However, upon DMSO treatment we observed a reversible disassembly of the Golgi apparatus. In mammalian cells DMSO treatment had a similar effect on beta-COP distribution.

Reaction-diffusion waves of actin filament polymerization/depolymerization in Dictyostelium pseudopodium extension and cell locomotion

Cell surface movements and the intracellular spatial patterns and dynamics of actin filament (F-actin) were investigated in living and formalin-fixed cells of Dictyostelium discoideum by confocal microscopy. Excitation waves of F-actin assembly developed and propagated several micrometers at up to 26 microm/min in cells which had been intracellularly loaded with fluorescently labeled actin monomer. Wave propagation and extinction corresponded with the initiation and attenuation of pseudopodium extension and cell advance, respectively. The identification of chemical waves was supported by the ring, sphere, spiral and scroll wave patterns, which were observed in the extensions of fixed cells stained with phalloidin-rhodamine, and by the similar, asymmetrical [F-actin] distribution in wavefronts in living and fixed cells. These F-actin patterns and dynamics in Dictyostelium provide evidence for a new supramolecular state of actin, which propagates as a self-organized, reaction-diffusion wave of reversible F-actin assembly and affects pseudopodium extension. Actin's properties of oscillation and self-organization might also fundamentally determine the nature of the eukaryotic cell's reactions of adaptation, timing and signal response.

Dictyostelium discoideum: a new host model system for intracellular pathogens of the genus Legionella

The soil amoeba Dictyostelium discoideum is a haploid eukaryote that, upon starvation, aggregates and enters a developmental cycle to produce fruiting bodies. In this study, we infected single-cell stages of D. discoideum with different Legionella species. Intracellular growth of Legionella in this new host system was compared with their growth in the natural host Acanthamoeba castellanii. Transmission electron microscopy of infected D. discoideum cells revealed that legionellae reside within the phagosome. Using confocal microscopy, it was observed that replicating, intracellular, green fluorescent protein (GFP)-tagged legionellae rarely co-localized with fluorescent antibodies directed against the lysosomal protein DdLIMP of D. discoideum. This indicates that the bacteria inhibit the fusion of phagosomes and lysosomes in this particular host system. In addition, Legionella infection of D. discoideum inhibited the differentiation of the host into the multicellular fruiting stage. Co-culture studies with profilin-minus D. discoideum mutants and Legionella resulted in higher rates of infection when compared with infections of wild-type amoebae. Because the amoebae are amenable to genetic manipulation as a result of their haploid genome and because a number of cellular markers are available, we show for the first time that D. discoideum is a valuable model system for studying intracellular pathogenesis of microbial pathogens.

The actin cytoskeleton of Dictyostelium: a story told by mutants

Actin-binding proteins are effectors of cell signalling and coordinators of cellular behaviour. Research on the Dictyostelium actin cytoskeleton has focused both on the elucidation of the function of bona fide actin-binding proteins as well as on proteins involved in signalling to the cytoskeleton. A major part of this work is concerned with the analysis of Dictyostelium mutants. The results derived from these investigations have added to our understanding of the role of the actin cytoskeleton in growth and development. Furthermore, the studies have identified several cellular and developmental stages that are particularly sensitive to an unbalanced cytoskeleton. In addition, use of GFP fusion proteins is revealing the spatial and temporal dynamics of interactions between actin-associated proteins and the cytoskeleton.

Severe developmental defects in Dictyostelium null mutants for actin-binding proteins

The actin cytoskeleton is implicated in many cellular processes, such as cell adhesion, locomotion, contraction and cytokinesis, which are central to any development. The extent of polymerization, cross-linking, and bundling of actin is regulated by several actin-binding proteins. Knock-out mutations in these proteins have revealed in many cases only subtle, if any, defects in development, suggesting that the actin system is redundant, with multiple proteins sharing overlapping functions. The apparent redundancy may, however, reflect limitations of available laboratory assays in assessing the developmental role of a given protein. By using a novel assay, which reproduces conditions closer to the natural ones, we have re-examined the effects of disruption of many actin-binding proteins, and show here that deletion of alpha-actinin, interaptin, synexin, 34-kDa actin-bundling protein, and gelation factor affect to varying degrees the efficiency of Dictyostelium cells to complete development and form viable spores. No phenotypic defects were found in hisactophilin or comitin null mutants.

Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin distinguished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface loop suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins, and indeed a GST-loop construct binds actin. Probably as a consequence, binding of MyoK to actin was not only ATP- but also salt-dependent. Moreover, as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dynamic regions of the cortex. Absence of MyoK resulted in a drop of cortical tension whereas overexpression led to significantly increased tension. Absence and overexpression of MyoK dramatically affected the cortical actin cytoskeleton and resulted in reduced initial rates of phagocytosis. Cells lacking MyoK showed excessive ruffling, mostly in the form of large lamellipodia, accompanied by a thicker basal actin cortex. At early stages of development, aggregation of myoK null cells was slowed due to reduced motility. Altogether, the data indicate a distinctive role for MyoK in the maintenance and dynamics of the cell cortex.

Morphogenetic cell movement in Dictyostelium

Dictyostelium morphogenesis starts with the chemotactic aggregation of starving individual cells. The cells move in response to propagating waves of the chemoattractant cyclic AMP initiated by cells in the aggregation centre. During aggregation the cells begin to differentiate into several types with different signalling and chemotactic properties. These cell types sort out from each other to form an axial pattern in the slug. There is now good evidence that periodic chemotactic signals not only control aggregation, but also later stages of morphogenesis. These signals take the form of target patterns, spirals, multi-armed spirals and scroll waves. I will discuss their role in the control of cell movement during mound and slug formation and in the formation of the fruiting body.

Dictyostelium as model system for studies of the actin cytoskeleton by molecular genetics

The actin cytoskeleton is an essential structure for most movements at the cellular and intracellular level. Whereas for contraction a muscle cell requires a rather static organisation of cytoskeletal proteins, cell motility of amoeboid cells relies on a tremendously dynamic turnover of filamentous networks in a matter of seconds and at distinct regions inside the cell. The best model system for studying cell motility is Dictyostelium discoideum. The cells live as single amoebae but can also start a developmental program that leads to multicellular stages and differentiation into simple types of tissues. Thus, cell motility can be studied on single cells and on cells in a tissue-like aggregate. The ability to combine protein purification and biochemistry with fairly easy molecular genetics is a unique feature for investigation of the cytoskeleton and cell motility. The actin cytoskeleton in Dictyostelium harbours essentially all classes of actin-binding proteins that have been found throughout eukaryotes. By conventional mutagenesis, gene disruption, antisense approaches, or gene replacements many genes that code for cytoskeletal proteins have been disrupted, and altered phenotypes in transformants that lacked one or more of those cytoskeletal proteins allowed solid conclusions about their in vivo function. In addition, tagging the proteins or selected domains with green fluorescent protein allows the monitoring of protein redistribution during cell movement. Gene tagging by restriction enzyme mediated integration of vectors and the ongoing international genome and cDNA sequencing projects offer the chance to understand the dynamics of the cytoskeleton by identification and functional characterisation of all proteins involved.

Three actin cross-linking proteins, the 34 kDa actin-bundling protein, alpha-actinin and gelation factor (ABP-120), have both unique and redundant roles in the growth and development of Dictyostelium

The contribution of three actin cross-linking proteins, alpha-actinin (alphaA), gelation factor (ABP-120), and the 34 kDa actin-bundling protein to cellular functions has been studied in three single mutant (alphaA-, 120-, and 34-) and three double mutant (alphaA-/120-, 34-/alphaA-, 34-/120-) strains of Dictyostelium generated by homologous recombination. Strains alphaA-/120- and 34-/alphaA- exhibited a reduced rate of pinocytosis, grew to lower saturation densities, and produced small cells in shaking cultures. All strains grew normally in bacterial suspensions and on agar plates with a bacterial lawn. Slow growth under conditions of reduced temperature and increased osmolarity was observed in single mutants 34- and alphaA-, respectively, as well as in some of the double mutant strains. Motility, chemotaxis, and development were largely unaltered in 34-/alphaA- and 34-/120- cells. However, 34-/alphaA- cells showed enhanced aggregation when starved in suspension. Moreover, morphogenesis was impaired in both double mutant strains and fruiting bodies of aberrant morphology were observed. These defects were reverted by re-expression of one of the lacking cross-linking proteins. The additive and synthetic phenotypes of these mutations indicate that actin cross-linking proteins serve both unique and overlapping functions in the actin cytoskeleton.

Chemoattractant-mediated transient activation and membrane localization of Akt/PKB is required for efficient chemotaxis to cAMP in Dictyostelium

Chemotaxis-competent cells respond to a variety of ligands by activating second messenger pathways leading to changes in the actin/myosin cytoskeleton and directed cell movement. We demonstrate that Dictyostelium Akt/PKB, a homologue of mammalian Akt/PKB, is very rapidly and transiently activated by the chemoattractant cAMP. This activation takes place through G protein-coupled chemoattractant receptors via a pathway that requires homologues of mammalian p110 phosphoinositide-3 kinase. pkbA null cells exhibit aggregation-stage defects that include aberrant chemotaxis, a failure to polarize properly in a chemoattractant gradient and aggregation at low densities. Mechanistically, we demonstrate that the PH domain of Akt/PKB fused to GFP transiently translocates to the plasma membrane in response to cAMP with kinetics similar to those of Akt/PKB kinase activation and is localized to the leading edge of chemotaxing cells in vivo. Our results indicate Akt/PKB is part of the regulatory network required for sensing and responding to the chemoattractant gradient that mediates chemotaxis and aggregation.

Identification of a suppressor of the Dictyostelium profilin-minus phenotype as a CD36/LIMP-II homologue

Profilin is an ubiquitous G-actin binding protein in eukaryotic cells. Lack of both profilin isoforms in Dictyostelium discoideum resulted in impaired cytokinesis and an arrest in development. A restriction enzyme-mediated integration approach was applied to profilin-minus cells to identify suppressor mutants for the developmental phenotype. A mutant with wild-type-like development and restored cytokinesis was isolated. The gene affected was found to code for an integral membrane glycoprotein of a predicted size of 88 kD containing two transmembrane domains, one at the NH2 terminus and the other at the COOH terminus. It is homologous to mammalian CD36/LIMP-II and represents the first member of this family in D. discoideum, therefore the name DdLIMP is proposed. Targeted disruption of the lmpA gene in the profilin-minus background also rescued the mutant phenotype. Immunofluorescence revealed a localization in vesicles and ringlike structures on the cell surface. Partially purified DdLIMP bound specifically to PIP2 in sedimentation and gel filtration assays. A direct interaction between DdLIMP and profilin could not be detected, and it is unclear how far upstream in a regulatory cascade DdLIMP might be positioned. However, the PIP2 binding of DdLIMP points towards a function via the phosphatidylinositol pathway, a major regulator of profilin.

RacF1, a novel member of the Rho protein family in Dictyostelium discoideum, associates transiently with cell contact areas, macropinosomes, and phagosomes

Using a PCR approach we have isolated racF1, a novel member of the Rho family in Dictyostelium. The racF1 gene encodes a protein of 193 amino acids and is constitutively expressed throughout the Dictyostelium life cycle. Highest identity (94%) was found to a RacF2 isoform, to Dictyostelium Rac1A, Rac1B, and Rac1C (70%), and to Rac proteins of animal species (64-69%). To investigate the role of RacF1 in cytoskeleton-dependent processes, we have fused it at its amino-terminus with green fluorescent protein (GFP) and studied the dynamics of subcellular redistribution using a confocal laser scanning microscope and a double-view microscope system. GFP-RacF1 was homogeneously distributed in the cytosol and accumulated at the plasma membrane, especially at regions of transient intercellular contacts. GFP-RacF1 also localized transiently to macropinosomes and phagocytic cups and was gradually released within <1 min after formation of the endocytic vesicle or the phagosome, respectively. On stimulation with cAMP, no enrichment of GFP-RacF1 was observed in leading fronts, from which it was found to be initially excluded. Cell lines were obtained using homologous recombination that expressed a truncated racF1 gene lacking sequences encoding the carboxyl-terminal region responsible for membrane targeting. These cells displayed normal phagocytosis, endocytosis, and exocytosis rates. Our results suggest that RacF1 associates with dynamic structures that are formed during pinocytosis and phagocytosis. Although RacF1 appears not to be essential, it might act in concert and/or share functions with other members of the Rho family in the regulation of a subset of cytoskeletal rearrangements that are required for these processes.

Chemoattractant-induced lamellipod extension

The mammary adenocarcinoma cell line MTLn3 is chemotactic towards epidermal growth factor (EGF), and this induced motility is thought to promote breast cancer invasion and metastasis. Stimulation of MTLn3 cells with EGF results in the extension of a flat, thin structure filled with filamentous actin and termed a lamellipod. Lamellipod extension is dependent on actin polymerization and is localized to the border of adherent cells. The structure of EGF-stimulated lamellipods in MTLn3 cells is well suited to analysis of chemoattractant-stimulated protrusion. Actin polymerization occurs within 200 nm of the extending edge of the lamellipod. Although extension of the lamellipod is not dependent upon interaction with the substratum, stabilization of the extended lamellipod is dependent on an adhesive substratum. Dorsal ruffling is suppressed during lamellipod extension. Tyrosine phosphorylation is reduced in preexisting focal contacts compared to new contacts induced by EGF stimulation. The coordination of turnover of focal contacts with lamellipod extension is proposed to result in polarized cell motility in response to gradients of chemoattractants.

Ethane-freezing/methanol-fixation of cell monolayers: a procedure for improved preservation of structure and antigenicity for light and electron microscopies

In order to dissect at the ultrastructural level the morphology of highly dynamic processes such as cell motility, membrane trafficking events, and organelle movements, it is necessary to fix/stop time-dependent events in the millisecond range. Ideally, immunoelectron microscopical labeling experiments require the availability of high-affinity antibodies and accessibility to all compartments of the cell. The biggest challenge is to define an optimum between significant preservation of the antigenicity in the fixed material without compromising the intactness of fine structures. Here, we present a procedure which offers an opportunity to unify preparation of cell monolayers for immunocytochemistry in fluorescence and electron microscopy. This novel strategy combines a rapid ethane-freezing technique with a low temperature methanol-fixation treatment (EFMF) and completely avoids chemical fixatives. It preserves the position and delicate shape of cells and organelles and leads to improved accessibility of the intracellular antigens and to high antigenicity preservation. We illustrate the establishment of this procedure using Dictyostelium discoideum, a powerful model organism to study molecular mechanisms of membrane trafficking and cytoskeleton.

Interaptin, an actin-binding protein of the alpha-actinin superfamily in Dictyostelium discoideum, is developmentally and cAMP-regulated and associates with intracellular membrane compartments

In a search for novel members of the alpha-actinin superfamily, a Dictyostelium discoideum genomic library in yeast artificial chromosomes (YAC) was screened under low stringency conditions using the acting-binding domain of the gelation factor as probe. A new locus was identified and 8.6 kb of genomic DNA were sequenced that encompassed the whole abpD gene. The DNA sequence predicts a protein, interaptin, with a calculated molecular mass of 204,300 D that is constituted by an actin-binding domain, a central coiled-coil rod domain and a membrane-associated domain. In Northern blot analyses a cAMP-stimulated transcript of 5.8 kb is expressed at the stage when cell differentiation occurs. Monoclonal antibodies raised against bacterially expressed interaptin polypeptides recognized a 200-kD developmentally and cAMP-regulated protein and a 160-kD constitutively expressed protein in Western blots. In multicellular structures, interaptin appears to be enriched in anterior-like cells which sort to the upper and lower cups during culmination. The protein is located at the nuclear envelope and ER. In mutants deficient in interaptin development is delayed, but the morphology of the mature fruiting bodies appears normal. When starved in suspension abpD- cells form EDTA-stable aggregates, which, in contrast to wild type, dissociate. Based on its domains and location, interaptin constitutes a potential link between intracellular membrane compartments and the actin cytoskeleton.

A Dictyostelium discoideum homologue to Tcp-1 is essential for growth and development

Tcp-1 (t-complex polypeptide 1 gene) was first identified in the mouse as relevant for tail-less and embryonic lethal phenotypes. Since then, its homologous sequences have been isolated in several other species, and the yeast Tcp-1 has been shown to encode a molecular chaperon for actin and tubulin. In a random sample of genes expressed in the gamete of Dictyostelium discoideum (Dd), we encountered a sequence containg the TCP1 motifs. The complete ORF of the gene (DdTcp-1) showed more than 60% similarity to TCP-1 of several organisms, including human. DdTcp-1 was found to be expressed in both sexually mature and immature cells at the growth phase. Although the sexual process itself was not affected, antisense interference of this gene resulted in severe retardation of cell growth, leading to the complete cessation of division. In addition, the antisense transformants stopped asexual development at the finger stage. These results suggest an important function of DdTcp-1 in growth and development of this organism.

The actomyosin cytoskeleton of amoebae of the cellular slime molds acrasis rosea and protostelium mycophaga: structure, biochemical properties, and function

In amoebae of the cellular slime molds (mycetozoans) Acrasis rosea and Protostelium mycophaga, bundles of F-actin radiate from the endoplasm-ectoplasm interface into the pseudopodia, where G-actin is also located. We conclude that these actin bundles form a core scaffold driving pseudopod extension which is subsequently completed by filling with a more loosely organized meshwork of F-actin. Some bipolar, elongate amoebae of A. rosea also contained long bundles of F-actin that traverse the cells lengthwise and remotely resemble stress fibers. Rodlets of F-actin were scattered in the body of amoebae of A. rosea or formed star-shaped or polygonal complexes near or around contractile vacuoles, where they may play a role in contraction. In total protein extracts analyzed by SDS-PAGE and immunoblots the actins migrated like the rabbit skeletal muscle control. The relative proportion of actin in total protein extracts was 7.9% for A. rosea and 34.5% for P. mycophaga. We detected four or five isoactins in extracts of both species and we determined that the genome of each species contains approximately six actin genes. Whether they are all expressed or if posttranslational modifications occur remains to be determined. Myosin II was enriched in actomyosin extracts; its Mr was 187.8 kDa for A. rosea and 220.7 kDa for P. mycophaga. Cell models ("ghosts") contracted upon the addition of ATP. We conclude that amoebae of A. rosea and P. mycophaga, although behaving differently from those of Dictyostelium discoideum, contain the basic repertoire of molecules that enable pseudopod extension by actin polymerization and ATP-induced contraction of the cell cortex. Copyright 1998 Academic Press.

MAP kinase function in amoeboid chemotaxis

Mutants lacking the MAP kinase DdERK2 show reduced chemotactic responses to folate and cAMP. Analysis of cAMP chemotaxis shows that Dderk2- cells are defective in chemotaxis to high concentrations of cAMP. This defect is due to an inability to repolarize in the continued presence of high concentrations of cAMP. Under these conditions, the speed of movement of mutant cells remains low. Instead of generating a leading pseudopod, mutant cells generate transient crown-like structures over multiple regions of the cell surface. These structures differ from pseudopods in that they contain myosin II as well as F actin and coronin. These studies identify a role for MAP kinases in coordinating the formation of cell projections generated in response to chemoattractants.

Motoring along the hyphae: molecular motors and the fungal cytoskeleton

This review discusses molecular motors that use the microfilament and microtubule cytoskeletal systems in filamentous fungi. There has been an explosion in our knowledge of kinesins over the past year, because of the integration of genetic and biochemical data. The recognition of possible interactions between septation genes and cytokinesis has also advanced our understanding of microfilament-based cytoskeletal systems. We review recent findings on microfilament motors, including conventional and unconventional myosins, and the microtubule motors of the kinesin family and cytoplasmic dynein. The roles that these molecules play in hyphal morphogenesis and organelle transport provide an insight into cytoskeletal-based transport systems.

Photosensory and thermosensory responses in Dictyostelium slugs are specifically impaired by absence of the F-actin cross-linking gelation factor (ABP-120)

Chemotactic aggregation of starving amoebae of Dictyostelium discoideum leads to formation of a motile, multicellular organism - the slug - whose anterior tip controls its phototactic and thermotactic behaviour. To determine whether proteins that regulate the in vitro assembly of actin are involved in these responses, we tested phototaxis and thermotaxis in mutant slugs in which the gene encoding one of five actin-binding proteins had been disrupted. Of the proteins tested - severin, alpha-actinin, fimbrin, the 34 kD actin-bundling protein and the F-actin cross-linking gelation factor (ABP-120) - only ABP-120 proved essential for normal phototaxis and thermotaxis in the multicellular slugs. The related human protein ABP-280 is required for protein phosphorylation cascades initiated by lysophosphatidic acid and tumor necrosis factor alpha. The repeating segments constituting the rod domains of ABP-120 and ABP-280 may be crucial for the function of both proteins in specific signal transduction pathways by mediating interactions with regulatory proteins.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

Molecular cloning of actin genes in Trichomonas vaginalis and phylogeny inferred from actin sequences

The parasitic protozoan Trichomonas vaginalis is known to contain the ubiquitous and highly conserved protein actin. A genomic library and a cDNA library have been screened to identify and clone the actin gene(s) of T. vaginalis. The nucleotide sequence of one gene and its flanking regions have been determined. The open reading frame encodes a protein of 376 amino acids. The sequence is not interrupted by any introns and the promoter could be represented by a 10 bp motif close to a consensus motif also found upstream of most sequenced T. vaginalis genes. The five different clones isolated from the cDNA library have similar sequences and encode three actin proteins differing only by one or two amino acids. A phylogenetic analysis of 31 actin sequences by distance matrix and parsimony methods, using centractin as outgroup, gives congruent trees with Parabasala branching above Diplomonadida.

Evidence for physical and functional interactions among two Saccharomyces cerevisiae SH3 domain proteins, an adenylyl cyclase-associated protein and the actin cytoskeleton

In a variety of organisms, a number of proteins associated with the cortical actin cytoskeleton contain SH3 domains, suggesting that these domains may provide the physical basis for functional interactions among structural and regulatory proteins in the actin cytoskeleton. We present evidence that SH3 domains mediate at least two independent functions of the Saccharomyces cerevisiae actin-binding protein Abp1p in vivo. Abp1p contains a single SH3 domain that has recently been shown to bind in vitro to the adenylyl cyclase-associated protein Srv2p. Immunofluorescence analysis of Srv2p subcellular localization in strains carrying mutations in either ABP1 or SRV2 reveals that the Abp1p SH3 domain mediates the normal association of Srv2p with the cortical actin cytoskeleton. We also show that a site in Abp1p itself is specifically bound by the SH3 domain of the actin-associated protein Rvs167p. Genetic analysis provides evidence that Abp1p and Rvs167p have functions that are closely interrelated. Abp1 null mutations, like rvs167 mutations, result in defects in sporulation and reduced viability under certain suboptimal growth conditions. In addition, mutations in ABP1 and RVS167 yield similar profiles of genetic "synthetic lethal" interactions when combined with mutations in genes encoding other cytoskeletal components. Mutations which specifically disrupt the SH3 domain-mediated interaction between Abp1p and Srv2p, however, show none of the shared phenotypes of abp1 and rvs167 mutations. We conclude that the Abp1p SH3 domain mediates the association of Srv2p with the cortical actin cytoskeleton, and that Abp1p performs a distinct function that is likely to involve binding by the Rvs167p SH3 domain. Overall, work presented here illustrates how SH3 domains can integrate the activities of multiple actin cytoskeleton proteins in response to varying environmental conditions.

The structure, function, and assembly of actin filament bundles

The cellular organization, function, and molecular composition of selected biological systems with prominent actin filament bundles are reviewed. An overall picture of the great variety of functions served by actin bundles emerges from this overview. A unifying theme is that the actin cross-linking proteins are conserved throughout the eukaryotic kingdom and yet assembled in a variety of combinations to produce actin bundles of differing functions. Mechanisms of actin bundle formation in vitro are considered illustrating the variety of physical and chemical driving forces in this exceedingly complex process. Our limited knowledge regarding the formation of actin filament bundles in vivo is contrasted with the elegant biophysical studies performed in vitro but nonetheless reveals that interactions with membranes, nucleation sites, and other organizational components must contribute to formation of actin bundles in vivo.

Myosin light chain kinase (MLCK) gene disruption in Dictyostelium: a role for MLCK-A in cytokinesis and evidence for multiple MLCKs

We have created a strain of Dictyostelium that is deficient for the Ca2+/calmodulin-independent MLCK-A. This strain undergoes cytokinesis less efficiently than wild type, which results in an increased frequency of multinucleate cells when grown in suspension. The MLCK-A-cells are able, however, to undergo development and to cap crosslinked surface receptors, processes that require myosin heavy chain. Phosphorylated regulatory light chain (RLC) is still present in MLCK-A-cells, indicating that Dictyostelium has one or more additional protein kinases capable of phosphorylating RLC. Concanavalin A treatment was found to induce phosphorylation of essentially all of the RLC in wild-type cells, but RLC phosphorylation levels in MLCK-A-cells are unaffected by concanavalin A. Thus MLCK-A is regulated separately from the other MLCK(s) in the cell.