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

Nuclear Envelope Dynamics in Dictyostelium Amoebae

In the last decades, the study of many nuclear envelope components in Dictyostelium amoebae has revealed conserved mechanisms of nuclear envelope dynamics that root back unexpectedly deep into the eukaryotic tree of life. In this review, we describe the state of the art in nuclear envelope research in this organism starting from early work on nuclear pore complexes to characterization of the first true lamin in a non-metazoan organism and its associated nuclear envelope transmembrane proteins, such as the HeH-family protein Src1 and the LINC complex protein Sun1. We also describe the dynamic processes during semi-closed mitosis, including centrosome insertion into the nuclear envelope, and processes involved in the restoration of nuclear envelope permeability around mitotic exit and compare them to the situation in cells with open or fully closed mitosis.

Inner membrane complex 1l protein of Plasmodium falciparum links membrane lipids with cytoskeletal element 'actin' and its associated motor 'myosin'

The inner membrane complex (IMC) is a defining feature of apicomplexans comprising of lipid and protein components involved in gliding motility and host cell invasion. Motility of Plasmodium parasites is accomplished by an actin and myosin based glideosome machinery situated between the parasite plasma membrane (PPM) and IMC. Here, we have studied in vivo expression and localization of a Plasmodium falciparum (Pf) IMC protein 'PfIMC1l' and characterized it functionally by using biochemical assays. We have identified cytoskeletal protein 'actin' and motor protein 'myosin' as novel binding partners of PfIMC1l, alongside its interaction with the lipids 'cholesterol' and 'phosphatidyl-inositol 4, 5 bisphosphate' (PIP2). While actin and myosin compete for interaction with PfIMC1l, actin and either of the lipids (cholesterol or PIP2) simultaneously bind PfIMC1l. Interestingly, PfIMC1l showed enhanced binding with actin in the presence of calcium ions, and displayed direct binding with calcium. Based on our in silico analysis and experimental data showing PfIMC1l-actin/myosin and PfIMC1l-lipid interactions, we propose that this protein may anchor the IMC membrane with the parasite gliding apparatus. Considering its binding with key proteins involved in motility viz. myosin and actin (with calcium dependence), we suggest that PfIMC1l may have a role in the locomotion of Plasmodium.

A polychromatic 'greenbeard' locus determines patterns of cooperation in a social amoeba

Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Cheater impact can be diminished if cooperators display a tag (‘greenbeard') and recognise and preferentially direct cooperation towards other tag carriers. Despite its popular appeal, the feasibility of such greenbeards has been questioned because the complex patterns of partner-specific cooperative behaviours seen in nature require greenbeards to come in different colours. Here we show that a locus (‘Tgr') of a social amoeba represents a polychromatic greenbeard. Patterns of natural Tgr locus sequence polymorphisms predict partner-specific patterns of cooperation by underlying variation in partner-specific protein–protein binding strength and recognition specificity. Finally, Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners. These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation.

Cooperation can be stabilized against exploitation if cooperators can reliably recognize each other. Here, Gruenheit and colleagues show that different alleles of the Tgr locus of the social amoeba Dictyostelium discoideum underlie the ability of different strains to recognize and cooperate with socially compatible individuals.

Balanced cortical stiffness is important for efficient migration of Dictyostelium cells in confined environments

Dictyostelium discoideum cells resemble in many aspects human leukocytes and serve as a model to study actin cytoskeleton dynamics and cell migration of highly motile cells. Dictyostelium cells deficient in the actin-binding protein filamin (ddFLN) showed a surprisingly subtle change in phenotype with no or only minor effects in single cell motility. These findings were in contrast to the strong actin-crosslinking activities measured for filamin in vitro. In the present study, we set out to revisit the role of ddFLN in cell migration. For this purpose, we examined migration of wild-type, ddFLN-null and ddFLN-overexpressing cells under different conditions. In addition to cyclic-AMP chemotaxis assays using micropipettes, we explored cell migration under more confined conditions: an under-agarose 2D assay and a 3D assay employing a collagen matrix that was adapted from assays for leukocytes. Using 3D migration conditions, cells deficient in ddFLN displayed only a minor impairment of motility, similar to the results obtained for migration in 2D. However, cells overexpressing ddFLN showed a remarkable decrease in the speed of migration in particular in 3D environments. We suggest that these results are in line with an increased stiffening of the cortex due to the crosslinking activity of overexpressed ddFLN. Our conclusion is that the absolute level of ddFLN is critical for efficient migration. Furthermore, our results show that under conditions of increased mechanical stress, Dictyostelium cells, like leukocytes, switch to a bleb-based mode of movement.

Scenario-targeted toxicity assessment through multiple endpoint bioassays in a soil posing unacceptable environmental risk according to regulatory screening values

Lanestosa is a chronically polluted site (derelict mine) where the soil (Lanestosa (LA) soil) exceeds screening values (SVs) of regulatory policies in force (Basque Country; Europe) for Zn, Pb and Cd. A scenario-targeted toxicity assessment was carried out on the basis of a multi-endpoint bioassay approach. Acute and chronic toxicity bioassays were conducted with selected test species (Vibrio fischeri, Dictyostelium discoideum, Lactuca sativa, Raphanus sativus and Eisenia fetida) in combination with chemical analysis of soils and elutriates and with bioaccumulation studies in earthworms. Besides, the toxicity profile was compared with that of the mine runoff (RO) soil and of a fresh artificially polluted soil (LAAPS) resembling LA soil pollutant profile. Extractability studies in LA soil revealed that Pb, Zn and Cd were highly available for exchange and/or release into the environment. Indeed, Pb and Zn were accumulated in earthworms and LA soil resulted to be toxic. Soil respiration, V. fischeri, vegetative and developmental cycles of D. discoideum and survival and juvenile production of E. fetida were severely affected. These results confirmed that LA soil had unacceptable environmental risk and demanded intervention. In contrast, although Pb and Zn concentrations in RO soil revealed also unacceptable risk, both metal extractability and toxicity were much lower than in LA soil. Thus, within the polluted site, the need for intervention varied between areas that posed dissimilar risk. Besides, since LAAPS, with a high exchangeable metal fraction, was the most toxic, ageing under in situ natural conditions seemingly contributed to attenuate LA soil risk. As a whole, combining multi-endpoint bioassays with scenario-targeted analysis (including leaching and ageing) provides reliable risk assessment in soils posing unacceptable environmental risk according to SVs, which is useful to optimise the required intervention measures.

The actinin family of actin cross-linking proteins - a genetic perspective

Actinins are one of the major actin cross-linking proteins found in virtually all cell types and are the ancestral proteins of a larger family that includes spectrin, dystrophin and utrophin. Invertebrates have a single actinin-encoding ACTN gene, while mammals have four. Mutations in all four human genes have now been linked to heritable diseases or traits. ACTN1 mutations cause macrothrombocytopenia, a platelet disorder characterized by excessive bleeding. ACTN2 mutations have been linked to a range of cardiomyopathies, and ACTN4 mutations cause a kidney condition called focal segmental glomerulosclerosis. Intriguingly, approximately 16 % of people worldwide are homozygous for a nonsense mutation in ACTN3 that abolishes actinin-3 protein expression. This ACTN3 null allele has undergone recent positive selection in specific human populations, which may be linked to improved endurance and adaptation to colder climates. In this review we discuss the human genetics of the ACTN gene family, as well as ACTN gene knockout studies in several model organisms. Observations from both of these areas provide insights into the evolution and cellular functions of actinins.

Leaps and lulls in the developmental transcriptome of Dictyostelium discoideum

BACKGROUND

Development of the soil amoeba Dictyostelium discoideum is triggered by starvation. When placed on a solid substrate, the starving solitary amoebae cease growth, communicate via extracellular cAMP, aggregate by tens of thousands and develop into multicellular organisms. Early phases of the developmental program are often studied in cells starved in suspension while cAMP is provided exogenously. Previous studies revealed massive shifts in the transcriptome under both developmental conditions and a close relationship between gene expression and morphogenesis, but were limited by the sampling frequency and the resolution of the methods.

RESULTS

Here, we combine the superior depth and specificity of RNA-seq-based analysis of mRNA abundance with high frequency sampling during filter development and cAMP pulsing in suspension. We found that the developmental transcriptome exhibits mostly gradual changes interspersed by a few instances of large shifts. For each time point we treated the entire transcriptome as single phenotype, and were able to characterize development as groups of similar time points separated by gaps. The grouped time points represented gradual changes in mRNA abundance, or molecular phenotype, and the gaps represented times during which many genes are differentially expressed rapidly, and thus the phenotype changes dramatically. Comparing developmental experiments revealed that gene expression in filter developed cells lagged behind those treated with exogenous cAMP in suspension. The high sampling frequency revealed many genes whose regulation is reproducibly more complex than indicated by previous studies. Gene Ontology enrichment analysis suggested that the transition to multicellularity coincided with rapid accumulation of transcripts associated with DNA processes and mitosis. Later development included the up-regulation of organic signaling molecules and co-factor biosynthesis. Our analysis also demonstrated a high level of synchrony among the developing structures throughout development.

CONCLUSIONS

Our data describe D. discoideum development as a series of coordinated cellular and multicellular activities. Coordination occurred within fields of aggregating cells and among multicellular bodies, such as mounds or migratory slugs that experience both cell-cell contact and various soluble signaling regimes. These time courses, sampled at the highest temporal resolution to date in this system, provide a comprehensive resource for studies of developmental gene expression.

Iron metabolism and resistance to infection by invasive bacteria in the social amoeba Dictyostelium discoideum

Dictyostelium cells are forest soil amoebae, which feed on bacteria and proliferate as solitary cells until bacteria are consumed. Starvation triggers a change in life style, forcing cells to gather into aggregates to form multicellular organisms capable of cell differentiation and morphogenesis. As a soil amoeba and a phagocyte that grazes on bacteria as the obligate source of food, Dictyostelium could be a natural host of pathogenic bacteria. Indeed, many pathogens that occasionally infect humans are hosted for most of their time in protozoa or free-living amoebae, where evolution of their virulence traits occurs. Due to these features and its amenability to genetic manipulation, Dictyostelium has become a valuable model organism for studying strategies of both the host to resist infection and the pathogen to escape the defense mechanisms. Similarly to higher eukaryotes, iron homeostasis is crucial for Dictyostelium resistance to invasive bacteria. Iron is essential for Dictyostelium, as both iron deficiency or overload inhibit cell growth. The Dictyostelium genome shares with mammals many genes regulating iron homeostasis. Iron transporters of the Nramp (Slc11A) family are represented with two genes, encoding Nramp1 and Nramp2. Like the mammalian ortholog, Nramp1 is recruited to phagosomes and macropinosomes, whereas Nramp2 is a membrane protein of the contractile vacuole network, which regulates osmolarity. Nramp1 and Nramp2 localization in distinct compartments suggests that both proteins synergistically regulate iron homeostasis. Rather than by absorption via membrane transporters, iron is likely gained by degradation of ingested bacteria and efflux via Nramp1 from phagosomes to the cytosol. Nramp gene disruption increases Dictyostelium sensitivity to infection, enhancing intracellular growth of Legionella or Mycobacteria. Generation of mutants in other "iron genes" will help identify genes essential for iron homeostasis and resistance to pathogens.

ACAP-A/B are ArfGAP homologs in dictyostelium involved in sporulation but not in chemotaxis

Arfs and Arf GTPase-activating proteins (ArfGAPs) are regulators of membrane trafficking and actin dynamics in mammalian cells. In this study, we identified a primordial Arf, ArfA, and two ArfGAPs (ACAP-A/B) containing BAR, PH, ArfGAP and Ankyrin repeat domains in the eukaryote Dictyostelium discoideum. In vitro, ArfA has similar nucleotide binding properties as mammalian Arfs and, with GTP bound, is a substrate for ACAP-A and B. We also investigated the physiological functions of ACAP-A/B by characterizing cells lacking both ACAP-A and B. Although ACAP-A/B knockout cells showed no defects in cell growth, migration or chemotaxis, they exhibited abnormal actin protrusions and ∼50% reduction in spore yield. We conclude that while ACAP-A/B have a conserved biochemical mechanism and effect on actin organization, their role in migration is not conserved. The absence of an effect on Dictyostelium migration may be due to a specific requirement for ACAPs in mesenchymal migration, which is observed in epithelial cancer cells where most studies of mammalian ArfGAPs were performed.

The annexins of Dictyostelium

Annexins are a highly conserved ubiquitous family of Ca2+- and phospholipid-binding proteins present in nearly all eukaryotic cells. Analysis of the Dictyostelium genome revealed the presence of two annexin genes, the annexin C1 gene (nxnA) giving rise to two isoforms of 47 and 51 kDa (previously synexin), and the annexin C2 gene (nxnB) coding for a 56-kDa protein with 33% sequence identity to annexin C1. Annexin C2 is expressed at very low and constant levels throughout development. Quantification by real-time PCR indicated that it is present in about 35-fold lower amounts compared to annexin C1. We have used a GFP-tagged annexin C2 to study its cellular distribution and dynamics. In cell fractionation studies, annexin C2 cofractionates with annexin C1 and is enriched in the 100,000 g pellet. Like annexin C1, GFP-AnxC2 stains the plasma membrane. In addition it is present in the perinuclear region and overlaps to some degree with the Golgi apparatus, whereas annexin C1 is present on intracellular membranes resembling endosomal membranes and in the nucleus. Annexin C2 is not observed in the nucleus. An annexin C1 mutant (SYN-) which shows a defect during multicellular development can be rescued by full-length annexin C1, whereas overexpression of GFP-AnxC2 did not rescue the developmental defect The data support the concept that annexins, although having a highly conserved structure, participate in different functions in a cell.

Drosophila alpha-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

Role of calcium-dependent actin-bundling proteins: characterization of Dictyostelium mutants lacking fimbrin and the 34-kilodalton protein

Actin-bundling proteins organize actin filaments into densely packed bundles. In Dictyostelium discoideum two abundant proteins display calcium-regulated bundling activity, fimbrin and the 34-kDa protein (ABP34). Using a GFP fusion we observed transient localization of fimbrin at the phagocytic cup and macropinosomes. The distribution of truncated constructs encompassing the EF hands and the first actin-binding domain (EA1) or both actin-binding domains devoid of EF hands (A1A2) was indistinguishable from that of the full length protein. The role of fimbrin and a possible functional overlap with ABP34 was investigated in fim- and double 34-/fim- mutants. Except for a moderate cell size defect, fim- mutants did not show defects in growth, endocytosis, exocytosis, and chemotaxis. Double mutants were characterized by a small cell size and a defect in morphogenesis resulting in small fruiting bodies and a low spore yield. The cell size defect could not be overcome by expression of fimbrin fragments EA1 or A1A2, suggesting that both bundling activity and regulation by calcium are important. Induction of filopod formation in 34-/fim- cells was not impaired, indicating that both proteins are dispensable for this process. We searched in the Dictyostelium genome database for fimbrin-like proteins that could compensate for the fimbrin defect and identified three unconventional fimbrins and two more proteins with actin-binding domains of the type present in fimbrins.

Manifestations of multicellularity: Dictyostelium reports in

The recent release of the Dictyostelium genome sequence is important because Dictyostelium has become a much-favoured model system for cell and developmental biologists. The sequence has revealed a remarkably high total number of approximately 12 500 genes, only a thousand fewer than are encoded by Drosophila. Previous protein-sequence comparisons suggested that Dictyostelium is evolutionarily closer to animals and fungi than to plants, and the global protein sequence comparison, now made possible by the genome sequence, confirms this. This review focuses on several classes of proteins that are shared by Dictyostelium and animals: a highly sophisticated array of microfilament components, a large family of G-protein-coupled receptors and a diverse set of SH2 domain-containing proteins. The presence of these proteins strengthens the case for a relatively close relationship with animals and extends the range of problems that can be addressed using Dictyostelium as a model organism.

Drosophila non-muscle alpha-actinin is localized in nurse cell actin bundles and ring canals, but is not required for fertility

The single copy Drosophila alpha-actinin gene is alternatively spliced to generate three different isoforms that are expressed in larval muscle, adult muscle and non-muscle cells, respectively. We have generated novel alpha-actinin alleles, which specifically remove the non-muscle isoform. Homozygous mutant flies are viable and fertile with no obvious defects. Using a monoclonal antibody that recognizes all three splice variants, we compared alpha-actinin distribution in wild type and mutant embryos and ovaries. We found that non-muscle alpha-actinin was present in young embryos and in the embryonic central nervous system. In ovaries, non-muscle alpha-actinin was localized in the nurse cell subcortical cytoskeleton, cytoplasmic actin cables and ring canals. In the mutant, alpha-actinin expression remained in muscle tissues, but also in a subpopulation of epithelial cells in both embryos and ovaries. This suggests that various populations of non-muscle cells regulate alpha-actinin expression in different ways. We also show that ectopically expressed adult muscle-specific alpha-actinin localizes to all F-actin containing structures in the nurse cells in the absence of endogenous non-muscle alpha-actinin.

Dd-STATb, a Dictyostelium STAT protein with a highly aberrant SH2 domain, functions as a regulator of gene expression during growth and early development

Dictyostelium, the only known non-metazoan organism to employ SH2 domain:phosphotyrosine signaling, possesses STATs (signal transducers and activators of transcription) and protein kinases with orthodox SH2 domains. Here, however, we describe a novel Dictyostelium STAT containing a remarkably divergent SH2 domain. Dd-STATb displays a 15 amino acid insertion in its SH2 domain and the conserved and essential arginine residue, which interacts with phosphotyrosine in all other known SH2 domains, is substituted by leucine. Despite these abnormalities, Dd-STATb is biologically functional. It has a subtle role in growth, so that Dd-STATb-null cells are gradually lost from the population when they are co-cultured with parental cells, and microarray analysis identified several genes that are either underexpressed or overexpressed in the Dd-STATb null strain. The best characterised of these,discoidin 1, is a marker of the growth-development transition and it is overexpressed during growth and early development of Dd-STATb null cells. Dimerisation of STAT proteins occurs by mutual SH2 domain:phosphotyrosine interactions and dimerisation triggers STAT nuclear accumulation. Despite its aberrant SH2 domain, the Dd-STATb protein sediments at the size expected for a homodimer and it is constitutively enriched in the nucleus. Moreover, these properties are retained when the predicted site of tyrosine phosphorylation is substituted by phenylalanine. These observations suggest a non-canonical mode of activation of Dd-STATb that does not rely on orthodox SH2 domain:phosphotyrosine interactions.

Cross-linking of actin filaments by myosin II is a major contributor to cortical integrity and cell motility in restrictive environments

Cells are frequently required to move in a local environment that physically restricts locomotion, such as during extravasation or metastatic invasion. In order to model these events, we have developed an assay in which vegetative Dictyostelium amoebae undergo chemotaxis under a layer of agarose toward a source of folic acid [Laevsky, G. and Knecht, D. A. (2001). Biotechniques 31, 1140-1149]. As the concentration of agarose is increased from 0.5% to 3% the cells are increasingly inhibited in their ability to move under the agarose. The contribution of myosin II and actin cross-linking proteins to the movement of cells in this restrictive environment has now been examined. Cells lacking myosin II heavy chain (mhcA-) are unable to migrate under agarose overlays of greater than 0.5%, and even at this concentration they move only a short distance from the trough. While attempting to move, the cells become stretched and fragmented due to their inability to retract their uropods. At higher agarose concentrations, the mhcA- cells protrude pseudopods under the agarose, but are unable to pull the cell body underneath. Consistent with a role for myosin II in general cortical stability, GFP-myosin dynamically localizes to the lateral and posterior cortex of cells moving under agarose. Cells lacking the essential light chain of myosin II (mlcE-), have no measurable myosin II motor activity, yet were able to move normally under all agarose concentrations. Mutants lacking either ABP-120 or alpha-actinin were also able to move under agarose at rates similar to wild-type cells. We hypothesize that myosin stabilizes the actin cortex through its cross-linking activity rather than its motor function and this activity is necessary and sufficient for the maintenance of cortical integrity of cells undergoing movement in a restrictive environment. The actin cross-linkers alpha-actinin and ABP-120 do not appear to play as major a role as myosin II in providing this cortical integrity.

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.

Calcium regulation of actin crosslinking is important for function of the actin cytoskeleton in Dictyostelium

The actin cytoskeleton is sensitive to changes in calcium, which affect contractility, actin-severing proteins, actin-crosslinking proteins and calmodulin-regulated enzymes. To dissect the role of calcium control on the activity of individual proteins from effects of calcium on other processes, calcium-insensitive forms of these proteins were prepared and introduced into living cells to replace a calcium-sensitive form of the same protein. Crosslinking and bundling of actin filaments by the Dictyostelium 34 kDa protein is inhibited in the presence of micromolar free calcium. A modified form of the 34 kDa protein with mutations in the calcium binding EF hand (34 kDa deltaEF2) was prepared using site-directed mutagenesis and expressed in E. coli. Equilibrium dialysis using [(45)Ca]CaCl(2) revealed that the wild-type protein is able to bind one calcium ion with a Kd of 2.4 microM. This calcium binding is absent in the 34 kDa deltaEF2 protein. The actin-binding activity of the 34 kDa deltaEF2 protein was equivalent to wildtype but calcium insensitive in vitro. The wild-type and 34 kDa deltaEF2 proteins were expressed in 34-kDa-null and 34 kDa/alpha-actinin double null mutant Dictyostelium strains to test the hypothesis that calcium regulation of actin crosslinking is important in vivo. The 34 kDa deltaEF2 failed to supply function of the 34 kDa protein important for control of cell size and for normal growth to either of these 34-kDa-null strains. Furthermore, the distribution of the 34 kDa protein and actin were abnormal in cells expressing 34 kDa deltaEF2. Thus, calcium regulation of the formation and/or dissolution of crosslinked actin structures is required for dynamic behavior of the actin cytoskeleton important for cell structure and growth.

Loss of the F-actin binding and vesicle-associated protein comitin leads to a phagocytosis defect

Comitin is an F-actin binding and membrane-associated protein from Dictyostelium discoideum, which is present on Golgi and vesicle membranes and changes its localization in response to agents affecting the cytoskeleton. To investigate its in vivo functions we have generated knockout mutants by gene replacement. Based on comitin's in vitro functions we examined properties related to vesicular transport and microfilament function. Whereas cell growth, pinocytosis, secretion, chemotaxis, motility, and development were unaltered, comitin-lacking cells were impaired in the early steps of phagocytosis of Saccharomyces cerevisiae particles and of Escherichia coli, whereas uptake of latex beads was unaffected. Furthermore, the lack of comitin positively affected survival of pathogenic bacteria. Mutant cells also showed an altered response to hyperosmotic shock in comparison to the wild type. The redistribution of comitin during hyperosmotic shock in wild-type cells and its presence on early phagosomes suggest a direct involvement of comitin in these processes.

Genetic analysis of the requirements for alpha-actinin function

Null alpha-actinin mutations in Drosophila are lethal and produce conspicuous defects in muscle structure and function. Here, we used transgene rescue to examine the requirements for alpha-actinin function in vivo. First, we tested the ability of a cDNA-based transgene encoding the adult muscle isoform of alpha-actinin under control of the heterologous ubiquitin promoter to rescue the lethality of null alpha-actinin mutations. Successful rescue indicated that alternative splicing, which also generates larval muscle and non-muscle isoforms, was not essential for viability and that there were no strict spatial or temporal requirements for alpha-actinin expression. Secondly, chimeric transgenes, with functional domains of alpha-actinin replaced by similar domains from spectrin, were tested for their ability to rescue alpha-actinin mutants. Replacement of either the actin binding domain or the EF hand calcium binding domain yielded inactive proteins, indicating that these conserved domains were not functionally equivalent. Thirdly, the length of alpha-actinin was modified by adding a 114 amino acid structural repeat from alpha-spectrin to the center of the rod domain of alpha-actinin. Addition of this sequence module was expected to increase the length of the native alpha-actinin molecule by at least 15%. yet was fully compatible with alpha-actinin function as measured by rescued lethality and flight. Thus, unexpectedly, the exact length of alpha-actinin was not critical to its function in the muscle Z disk.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.

Filamins as integrators of cell mechanics and signalling

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.