Coronins are highly conserved among species, but their function is far from being under-stood in detail. Here we will introduce members of the family of coronin like proteins from Drosophila melanogaster, Caenorhabditis elegans and the social amoeba Dictyostelium discoideum. Genetic data from D. discoideum and D. melanogaster revealed that coronins in general are important regulators of many actin-dependent processes.

Coronins, a Versatile Family

The actin cytoskeleton is important for cell morphology and cell motility and its polymerization state can change rapidly in response to a variety of signals. Cytoskeletal dynamics are an essential component of many physiological processes, such as trafficking of intracellular vesicles, cell motility or the migration of cells during immune responses and tissue morphogenesis.¹ They are regulated by various actin-binding proteins. Coronins form one of the many groups of actin cytoskeleton regulators. Coronin was identified in the social amoeba D. discoideum.² It was first observed as a major copurifying protein in a preparation of contracted actomyosin from D. discoideum and was proposed to be an important regulator of the actin cytoskeleton. In order to understand the cellular and biochemical function of coronins, it is important to first consider their general molecular structure. A structural characteristic of coronins is the presence of several WD40-repeats, which in other proteins, such as the β-subunit of G-proteins, form β-propeller structures and mediate protein-protein interactions.³ Recent structural analysis revealed a seven bladed propeller for each WD40 domain in coronin and an overall fold for the WD40-repeat domain as in G-proteins.^4,5

Most eukaryotes express two kinds of coronins referred to as short and long coronins. Short coronins have the above described structure, the long form of coronins contains two complete copies of the basic coronin motif but lacks a coiled coil C-terminal domain that is only present in short coronins and is essential for their oligomerization.⁶ The functional significance of the second copy of the WD40-repeat domain is unknown. Several coronin like proteins have been identified in different invertebrate model organisms, D. discoideum, D.melanogaster and C. elegans. In this review we will summarize the data obtained from these organisms emphasising the results obtained from mutant analysis.

The Coronins of D. Discoideum

D. discoideum is a soil living amoeba frequently used as a model organism in cell and devel-opmental biology. The amoeba shares many physiological functions with mammalian cells and is amenable to genetic manipulation.⁷ Proteome-based phylogeny shows that the amoebozoa deviated from the animal-fungal lineage after the plant-animal split, but D. discoideum seems to have retained more of the diversity of the ancestral genome than do plants, animals or fungi (Fig 1).⁸ The complete protein repertoire of D. discoideum provides a new perspective for studying its cellular and developmental biology. At a systems level, D. discoideum provides a level of complexity that is greater than that of yeast, but much simpler than that of plants or animals.⁹

Figure 1

Proteome based eukaryotic phylogeny. The tree was constructed from a database of 5.279 orthologous protein clusters drawn from the proteomes of 17 eukaryotes shown and was rooted on 159 protein clusters that had representatives from six archaebacterial (more...)

In D. discoideum three coronin like proteins have been identified, two, Coronin 7 (corB) and villidin, belonging to the class of long coronins and one, coronin or corA, the classical coronin, belonging to the short coronins (Fig. 2; Shina MC, Noegel AA unpublished). The crn7 and the coronin gene are located on chromosome 1, the villidin gene vilA is on chromosome 5. Crn7 and coronin conform to the typical coronin structures with two and one highly conserved WD40-repeat domains (Fig. 2; Shina MC, Noegel AA unpublished), respectively, whereas villidin has a fairly unique domain structure consisting of one WD40-repeat domain followed by a gelsolin domain and a head piece (see below).

Figure 2

Comparison of domain structures of representative invertebrate coronins. From the left side, abbreviations indicating organisms, protein names, domain structures and amino acid numbers for each protein are given. All coronin proteins visualized in this (more...)

The Coronin Prototype Controls Actin Dynamics

Coronin localizes to actin rich crown-like structures on the dorsal surface of the cells, to leading edges of locomoting cells, pseudopodia, eupodia and endocytic cortical structures.¹⁰ Detailed analysis using GFP-tagged coronin indicated its dynamic behaviour in motile cells where it continuously shuttled between the cytoplasm and front regions and accumulated in fronts that formed in response to the chemoattractant cyclic AMP (cAMP) paralleling the accumulation of F-actin.¹² Further dissection of the cAMP response revealed however that at the time of coronin recruitment at the cell cortex F-actin starts to disassemble.¹¹ It was therefore concluded that coronin converts the cellular response from actin polymerization to depolymerization. Rosentreter et al reported a similar differential F-actin and coronin-1C accumulation during the formation of cellular protrusions in mammalian cells.^12,13

The involvement of coronin in various actin-driven processes has been also supported by genetic studies. In coronin-null mutants cell motility is reduced to less than half the rate in respect to wild type, cytokinesis is impaired and the rate of pino and phagocytosis is reduced.^14,15 A more detailed analysis of particle and fluid uptake revealed an intimate involvement of coronin in this process. All particle and fluid containing vacuoles are transiently surrounded by a cytoskeletal coat and by coronin. GFP-tagged coronin labeled postlysosomal vacuoles together with filamentous actin.^16-18 This association of coronin and actin at postlysosomal vacuoles emerged to be important not only for endocytosis but also for exocytosis. To show the importance of coronin and actin colocalization at postlysosomal vacuoles, cells were treated with cytochalasin. The depolymerization of actin inhibited the exocytosis rate significantly suggesting that the actin coat facilitates association of the late vacuole with the cell cortex.^17,18

Filamentous actin is also essential for phagocytosis since cytochalasin blocks phagocytosis almost completely. Actin normally distributes along the phagocytic cup after particle attachment, while coronin accumulates at the phagocytic cup within 45 sec after attachment of a particle and separates from the phagosome within 1 min after ingestion is completed. The temporal pattern of coronin association was also seen by proteom analysis.^19,20 The role of coronin in phagocytosis is supported by mutant analysis as in coronin null mutants the phagocytosis rate is reduced by about 70%. Furthermore microscopic examination of coronin null cells showed an unusual appearance. They were mostly irregularly shaped with enlarged cell size. However, the coronin mutants also show that coronin is neither essential for actin assembly nor for the normal localization of the actin filaments. Moreover, the formation of cell-surface extensions called crowns and multicellular development were also observed in the absence of coronin. Although aggregation-competent cells lacking coronin moved more slowly than control cells they were still capable of chemotactic orientation in a cAMP gradient. The chemotactic responsiveness in the absence of coronin is notable because of the sequence relationship of coronin to β-subunits of G-proteins. Because of this relationship a function for coronin in regulating the transmission of signals from chemo-attractant receptors via G-proteins to the actin cytoskeleton has been suggested.²

When D. discoideum wild type cells grow and divide on bacteria, they give rise almost exclusively to mononucleated cells. During axenic growth a portion of cells containing several nuclei is found, indicating that karyokinesis is not always connected with cytokinesis under these conditions. The lack of coronin markedly enhances this tendency. This effect was interpreted in terms of the accumulation of coronin in the distal portions of dividing cells as the protein has been mainly found in polar regions of dividing cells and only some coronin was present in the cortical layer of the ingressing cleavage furrow.²¹ By contrast, Fukui et al reported a coronin accumulation in the progressing cleavage furrow.²²

The inability of coronin cells to divide properly under axenic conditions points to a role of coronin in cell division and illustrates that various, probably independent, actin-based mechanisms are involved in guaranteeing proper cytokinesis. In conclusion, the defects in cell growth, locomotion and cytokinesis show that coronin is involved in more than a single function of the actin system and suggest that the protein plays a general role in the reorganization of the actin cytoskeleton.¹⁴

In recent years D. discoideum has also been used as a host for infection with pathogens like Pseudomonas aeruginosa, Mycobacterium avium, Mycobacterium marinum, Cryptococcus neoformans and Legionella pneumophila and various Dictyostelium mutants have been studied with regard to uptake of bacteria and intracellular growth.²³ The coronin mutant proved to be susceptible to infection with L. pneumophila and supported intracellular growth of the bacteria better than wild type.^24,25 These results are in contrast to those obtained from macrophages where a coat formed by coronin 1A prevents the maturation of phagosomes containing living mycobacteria into lysosomes.²⁶

Coronin 7, a Long Coronin in D. Discoideum

Coronin 7 (DCrn7) is the second coronin like protein in D. discoideum and the orthologue of the human Coronin 7 (Fig. 2). D. discoideum DCrn7 and human Coronin 7 share high homology and both possess two WD40-core repeat structures that are separated by a PST-domain (proline, serine, threonine rich region) of unknown function.^27,28

Predictions for the structure of DCrn7 display a double propeller structure as it was shown for the C. elegans Actin-Interacting-Protein-1 (AIP-1).²⁹ Analysis of DCrn7-GFP fusions and immunofluorescence studies with specific monoclonal antibodies show a localization to actin rich structures in the cells and dot like staining in the cytosol. This contrasts with the location described for the mammalian protein which was primarily found on the Golgi apparatus.^28,30

The 105 kDa D. discoideum DCrn7 is expressed throughout development and especially prominent during the aggregation state. First data from Dcrn7 null cells show a premature development, furthermore, growth in suspension and on agar plates is decreased, cells exhibit a strong phagocytosis increase but do not show any defect in exocytosis. Moreover Dcrn7 null cells exhibit a reduced motility in a cAMP gradient (Shina MC, Noegel AA unpublished data). Like the conventional coronin, DCrn7 accumulates at phagocytic and pinocytic cups leading to the suggestion that DCrn7 participates in the remodelling of the cortical actin cytoskeleton. A comparison of the results from the analysis of the (short) coronin and DCrn7 mutants suggests that in some processes they may act in an antagonistic fashion.

Villidin, an Unusual Coronin in D. Discoideum

Villidin is a 190 kDa protein from D. discoideum containing a coronin like domain at its N-terminus with high homology to the C-terminal WD-domain of the long coronins, which clearly places it in the coronin family, three PH domains in the middle and five gelsolin-like segments at its C-terminus followed by a villin-like headpiece. Villidin protein and mRNA are present in low amounts during growth and early aggregation, but increase during development and reach their highest levels at the tipped mound stage. The protein is present in the cytosol as well as in cytoskeletal and membrane fractions.³¹ GFP-tagged villidin exhibits a similar distribution as native villidin, including a distinct colocalization with Golgi structures and ER membranes. GFP fusions of the C-terminus are uniformly dispersed in the cytoplasm whereas GFP fusions of the N-terminal WD40-repeats codistribute with F-actin and are associated with the Triton-insoluble cytoskeleton.³¹ This suggests that villidin harbours the F-actin binding site in the N-terminal domain whereas the C-terminal headpiece turned out to be inactive in in vitro F-actin binding assays. The headpiece domain is present in many F-actin binding proteins and has been associated with F-actin interaction. Molecular modeling showed that the surface of the villidin headpiece did not harbor the residues that were identified as being essential for an F-actin interaction.³²

Strains lacking villidin grow normally and can develop into fruiting bodies. Although development is not impaired, cell motility is reduced during aggregation and phototaxis during the slug stage is affected.³¹ Interestingly, 55 genes are supposed to be involved in slug behavior and several of the encoded proteins regulate signal transduction pathways involving the intracellular messengers cAMP, cGMP, IP3 and Ca²⁺.^33,34 Phototactic migration depends on the motility of individual cells that require the efficient functioning of their cytoskeleton. It was concluded that villidin plays a role in motility related processes leading to phototactic movement.

D. Melanogaster Coronins Add New Roles

The knowledge of the genome of D. melanogaster offers the ultimate opportunity to clarify processes ranging from the development of an organism to its behavior. During the last century, more than 1300 genes, mostly based on mutant phenotypes, were genetically identified. Interestingly, many of them were found to have counterparts in other metazoans including humans. Among the genetically identified genes, coronin or coro codes for the D. melanogaster orthologue of D. discoideum short coronin. At the protein sequence level coro is showing a considerable identity to a number of coronin like proteins from different organisms across the whole eukaryotic spectrum from yeast to human. Closest orthologues are coronins from mosquito (74% identity across 376 residues) and zebrafish (55% identity across 265 residues), followed by human, Xenopus, mouse and rat (around 54% identity across 265 residues). The protein possesses a WD40 domain, which can form a β-propeller-like structure and a C-terminal end containing a coiled-coil domain, which is implicated in dimerization at the cell periphery.^6,35 Coro transcripts are seen at very high levels in 0-5-hour embryos, but expression is drastically reduced after 15-20-hours and maintains a steady state level until pupal stage. Interestingly, Enabled (another actin binding protein) and Costa (a microtubule binding protein) show the highest correlation of expression to coro. Developmental stages at which these genes show higher levels of expression may be stages in which actin and microtubule structures play significant roles.³⁶

Coro mutants were identified in a GAL4-enhancer trap screen for segmentally modulated expression patterns. Further deletion mutations that were generated by imprecise excision of the P-element affected adult morphogenesis and led to prominent wing and eye phenotypes and all homozygous females were sterile with severe defects in ovary development.³⁷ Closer inspection revealed a disruption of the actin cytoskeleton in imaginal discs and an altered wing disc morphology as it has been seen as result of suppressed Dpp signaling. Dpp (Decapentaplegic), a D. melanogaster TGF-beta orthologue, is normally transported from the source to the recipient cells through endocytosis and exocytosis of Dpp-containing vesicles. Overexpression of Dpp and its receptor Thickvein (Tkv), a type I receptor, in coro deficient cells leads to accumulation of Dpp in endocytic vesicles along the anterior-posterior boundary inhibiting the establishment of the morphogen gradient supporting the proposed vesicle-cytoskeleton interaction in which coro participates.^37-39

The observed phenotypes of the coro mutants were also remarkably similar to phenotypes reported for syntaxin1A (syx1A) alleles.³⁸ Syx1A, a member of the SNARE complex, is required for membrane trafficking associated with synaptic vesicles or other small vesicles such as endosomes. Based on their observations the authors concluded that D. melanogaster coro protein functions with syx1A to mediate trafficking and fusion of F-actin coated vesicles with the membrane. It appears that coro displays the same biological functions like the D. discoideum coronin. In addition, the results on cloning and characterization of coro provide new insights into the role of the actin cytoskeleton in various developmental processes.

D. Melanogaster Pod-1 Links Actin and Microtubules

D. melanogaster in addition possesses a single long coronin like protein pod-1, that is expressed in the nervous system. Sequence analysis of the full-length cDNA for pod-1 showed that the predicted protein is 1074 aa long and 31% identical and 46% similar to C. elegans POD-1 (see below). D. melanogaster pod-1 contains two tandem WD40-repeats that likely mediate F-actin binding⁴⁰ but unlike other long coronins the two WD40 domains are not separated by a PST domain (Fig. 2). The D. melanogaster orthologue of other long coronins has been shown to crosslink actin and microtubules (MTs) in vitro. D. melanogaster pod-1 colocalizes extensively with newly assembled F-actin and often colocalizes with MTs. In developing neurons, pod-1 is concentrated in growing neurites, where it is particularly enriched at the tips of extending axons. The primary defect in embryos completely lacking pod-1 is aberrant axon targeting. Furthermore, the level of pod-1 is critical, as postmitotic neuronal overexpression of pod-1 causes severe defects in axon path finding and induces dramatic changes in cell shape. Costaining for pod-1 and tubulin showed that in latrunculin treated cells, pod-1 relocalized from actin filaments to MTs whereas depolymerization of MTs had no effect on pod-1 localization suggesting that pod-1 has a high affinity for F-actin and a lower affinity for MTs or that its association with MTs may be regulated.

Embryos lacking zygotic expression had serious central nervous system (CNS) axon misguidance, which was however not accompanied by other general defects, suggesting that pod-1 is specifically important for axon guidance. Growth cone filopodia are required for steering but not for the extension of axons.⁴¹ Analysis of the growth cone structure in the mutants showed no obvious disruption, as filopodia were still seen. From this it was concluded that pod-1 is not required for filopodia formation in axonal growth cones.^42,43 In addition, expression was variable as seen in nerves, which reached their targets even without any pod-1. This led to the proposal, that pod-1 is an actin-microtubule crosslinker that has functions in remodelling of the cytoskeleton during axon navigation and pod-1 may facilitate the flow of guidance information to cytoskeletal networks. This finding, together with the endogenous pod-1 localization, is consistent with a role for pod-1 in the interaction of actin filaments and MTs in dynamic cellular structures.

C. Elegans POD-1 Is Essential for Cell Polarity

The genome of C. elegans was the first completely known genome of a multicellular organism. It codes for approximately 20,000 genes. Because the complete cell lineage of the species has been determined, C. elegans has proven especially useful for studying cellular differentiation. Polarization of cells and the asymmetric dissociation of components within cells are important in the development and function of many individual cell types, including epithelial and immune cells. The actin cytoskeleton has been shown to play an important role in establishing or maintaining polarity and the C. elegans embryo has become important for studying the developmental asymmetry establishment.⁴⁴

Among actin regulatory proteins, POD-1 (stands for polarity osmotic defective 1) was identified as a protein required for anterior-posterior (a-p) axis formation during a biochemical screen for actin-interacting proteins.⁴⁵ POD-1 is a protein with homology to the long coronin family and contains two complete copies of the WD-40 domains and one proline-rich region between the coronin domains. Important regulators are the so-called par genes (partitioning defective) that control polarization of the one-cell embryo. POD-1 is polarized along the a-p axis of the one-cell embryo, suggesting it might provide a link between F-actin and the generation of polarity. Elimination of POD-1 protein from embryos results in a par phenotype,loss of a-p polarity but additionally also in physical defects like osmotic sensitivity. This sensitivity to external salt concentration is not found with other published polarity mutants. However, unlike the loss of other polarity or PAR proteins characterized to date, loss of POD-1 leads to dramatic and specific alterations in the internal and external structure of embryonic cells, including the formation of abnormal endocytic vesicles containing large, circular, granule-free structures, membrane protrusions, abnormal eggshells and the deposition of extracellular plaque material.⁴⁶ Analogous defects to those of POD-1 deficient worms have been observed in the D. discoideum coronin mutant, which led to the assumption that POD-1 has a role in intracellular trafficking and cytoskeletal organization.

The study of POD-1 spans two important biological fields of embryonic development and the cytoskeleton and appears to link developmental polarity with cell structure in a way not previously characterized in C. elegans, assuming POD-1 as an upstream effector of PAR-1 distribution. However, PAR-1 is not required for POD-1 asymmetric localization. Based on the fact that D. discoideum coronin and DCrn7 play important roles in endocytosis, colocalization of C. elegans POD-1 with cortical actin and cytoplasmic structures suggests a similar intracellular function and this is playing a major role in polarizing the C. elegans embryo.⁴⁷

Another polarity gene, pod-2, was identified during large-scale screens for conditional embryonic lethal mutants.^46,48 In contrast to POD-1, pod-2 comprises no WD40-repeats but is an Acetyl CoA Carboxylase, an enzyme that is involved in the fatty acid metabolism. A mutation of this gene pod-2 causes also defects in anterior-posterior polarity in one-cell C. elegans embryos. Like loss of POD-1, loss of this gene function also results in embryos sensitive to their osmotic environment. Pod-2 mutant embryos share a number of phenotypes with POD-1 mutant embryos suggesting that these two genes might function in a common pathway to polarize the C. elegans embryo. Other polarity phenotypes and osmotic defects associated with loss of POD-1 are also found in pod-2. Both proteins are found in the cytoplasm and if POD-1 functions in the same pathway as pod-2, then elimination of POD-1 in a pod-2 mutant background should not enhance the polarity defect. Indeed, embryos lacking POD-1 and pod-2 show the same loss of polarity as lacking either gene alone. Although POD-1 and pod-2 share similar polarity and eggshell phenotypes, pod-2 embryos do not display some of the nonpolarity defects associated with POD-1 embryos. They do not form abnormal endocytically derived compartments and do not develop hyaline zones.⁴⁶

Interestingly, there is an orthologue of pod-2 found in D. discoideum, accA (DDB0187917), which is also involved in fatty acid biosynthesis. If there is a link between POD-1 and pod-2 there might also be a direct or indirect cooperation of D. discoideum Crn7 and accA. Moreover, C. elegans POD-1 as well as D. discoideum coronin appear to be both involved in cytokinesis.¹⁴ It is interesting that the BTB protein MEL-26, a substrate-specific adaptor of the CUL-3-based ligase, forms a complex with the coronin like protein POD-1 in C. elegans. It interacts with the entire second WD-domain and the linker region of POD-1 through its MATH-domain and POD-1 is required for proper localization of MEL26 to the cleavage furrow. On the other hand cortical localization of POD-1 is independent of MEL-26. This indicates that the MEL26/POD-1 complex at the cell cortex promotes efficient initiation and ingression of the cytokinesis furrow in vivo.⁴⁹

In addition, the C. elegans genome comprises another orthologue of coronin cor-1, that reveals higher identity and more gene structure similarities to the short coronin of D. discoideum by also containing a single WD-40 domain and a coiled coil domain at the C-terminus.⁴⁰ For this gene five coronin mRNAs were identified in C. elegans. Cor-1 has an alternatively spliced exon and two exons containing cleavage sites different from the conventional splice sites.⁵⁰ Further analysis on this protein has not been carried out.

Summary

Taking all the different aspects together, coronins, apart of their common function in regulating the actin cytoskeleton, differ in their structure and length. Both short and long coronins in invertebrates seem to have almost the same distribution in the cell indicating that they might play roles in the same pathways. In D. discoideum coronins are located in the leading fronts and may be essential for polarization of the cell. This gives rise to the assumption that in loss of either one of the coronin like proteins polarity in these cells is disturbed. In the case of C. elegans there is good evidence for polarity defects, as C. elegans POD-1 deficient embryos show wrong distribution of PAR genes that are differentially distributed in the one-cell embryo. Also, the misguidance of neurons and the establishment of morphogen gradients in D. melanogaster may be linked to a distribution of polarity establishing proteins and even may play itself a role in linking these proteins to the actin cytoskeleton.

Remarkable progress has been made to understand the molecular and cellular function of these highly conserved proteins such as structural analysis and the recent discovery of a N-terminal area in human coronin 1B, providing the platform for the F-actin binding.⁵¹Questions that need to be addressed in the future are how these proteins are regulated and who are their binding partners. Structural analysis of some of these proteins may further reveal significant cues of presumable binding sites. However, much work is still remaining and has to be done to fully understand this interesting class of proteins.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie (FCI).

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