A prehistory of cell adhesion

Comparative study of structures and functional motifs in lectins from the commercially important photosynthetic microorganisms

Photosynthetic microorganisms, specifically cyanobacteria and microalgae, can synthesize a vast array of biologically active molecules, such as lectins, that have great potential for various biotechnological and biomedical applications. However, since the structures of these proteins are not well established, likely due to the presence of intrinsically disordered regions, our ability to better understand their functionality is hampered. We embarked on a study of the carbohydrate recognition domain (CRD), intrinsically disordered regions (IDRs), amino acidic composition, as well as and functional motifs in lectins from cyanobacteria of the genus Arthrospira and microalgae Chlorella and Dunaliella genus using a combination of bioinformatics techniques. This search revealed the presence of five distinctive CRD types differently distributed between the genera. Most CRDs displayed a group-specific distribution, except to C. sorokiniana possessing distinctive CRD probably due to its specific lifestyle. We also found that all CRDs contain short IDRs. Bacterial lectin of Arthrospira prokarionte showed lower intrinsic disorder and proline content when compared to the lectins from the eukaryotic microalgae (Chlorella and Dunaliella). Among the important functions predicted in all lectins were several specific motifs, which directly interacts with proteins involved in the cell-cycle control and which may be used for pharmaceutical purposes. Since the aforementioned properties of each type of lectin were investigated in silico, they need experimental confirmation. The results of our study provide an overview of the distribution of CRD, IDRs, and functional motifs within lectin from the commercially important microalgae.

Adhesion of Dictyostelium Amoebae to Surfaces: A Brief History of Attachments

Dictyostelium amoebae adhere to extracellular material using similar mechanisms to metazoan cells. Notably, the cellular anchorage loci in Amoebozoa and Metazoa are both arranged in the form of discrete spots and incorporate a similar repertoire of intracellular proteins assembled into multicomponent complexes located on the inner side of the plasma membrane. Surprisingly, however, Dictyostelium lacks integrins, the canonical transmembrane heterodimeric receptors that dominantly mediate adhesion of cells to the extracellular matrix in multicellular animals. In this review article, we summarize the current knowledge about the cell-substratum adhesion in Dictyostelium, present an inventory of the involved proteins, and draw parallels with the situation in animal cells. The emerging picture indicates that, while retaining the basic molecular architecture common to their animal relatives, the adhesion complexes in free-living amoeboid cells have evolved to enable less specific interactions with diverse materials encountered in their natural habitat in the deciduous forest soil. Dissection of molecular mechanisms that underlay short lifetime of the cell-substratum attachments and high turnover rate of the adhesion complexes in Dictyostelium should provide insight into a similarly modified adhesion phenotype that accompanies the mesenchymal-amoeboid transition in tumor metastasis.

Evolution of mechanisms controlling epithelial morphogenesis across animals: new insights from dissociation-reaggregation experiments in the sponge Oscarella lobularis

BACKGROUND

The ancestral presence of epithelia in Metazoa is no longer debated. Porifera seem to be one of the best candidates to be the sister group to all other Metazoa. This makes them a key taxon to explore cell-adhesion evolution on animals. For this reason, several transcriptomic, genomic, histological, physiological and biochemical studies focused on sponge epithelia. Nevertheless, the complete and precise protein composition of cell-cell junctions and mechanisms that regulate epithelial morphogenetic processes still remain at the center of attention.

RESULTS

To get insights into the early evolution of epithelial morphogenesis, we focused on morphogenic characteristics of the homoscleromorph sponge Oscarella lobularis. Homoscleromorpha are a sponge class with a typical basement membrane and adhaerens-like junctions unknown in other sponge classes. We took advantage of the dynamic context provided by cell dissociation-reaggregation experiments to explore morphogenetic processes in epithelial cells in a non-bilaterian lineage by combining fluorescent and electron microscopy observations and RNA sequencing approaches at key time-points of the dissociation and reaggregation processes.

CONCLUSIONS

Our results show that part of the molecular toolkit involved in the loss and restoration of epithelial features such as cell-cell and cell-matrix adhesion is conserved between Homoscleromorpha and Bilateria, suggesting their common role in the last common ancestor of animals. In addition, sponge-specific genes are differently expressed during the dissociation and reaggregation processes, calling for future functional characterization of these genes.

Cryptic surface-associated multicellularity emerges through cell adhesion and its regulation

The repeated evolution of multicellularity led to a wide diversity of organisms, many of which are sessile, including land plants, many fungi, and colonial animals. Sessile organisms adhere to a surface for most of their lives, where they grow and compete for space. Despite the prevalence of surface-associated multicellularity, little is known about its evolutionary origin. Here, we introduce a novel theoretical approach, based on spatial lineage tracking of cells, to study this origin. We show that multicellularity can rapidly evolve from two widespread cellular properties: cell adhesion and the regulatory control of adhesion. By evolving adhesion, cells attach to a surface, where they spontaneously give rise to primitive cell collectives that differ in size, life span, and mode of propagation. Selection in favor of large collectives increases the fraction of adhesive cells until a surface becomes fully occupied. Through kin recognition, collectives then evolve a central-peripheral polarity in cell adhesion that supports a division of labor between cells and profoundly impacts growth. Despite this spatial organization, nascent collectives remain cryptic, lack well-defined boundaries, and would require experimental lineage tracking technologies for their identification. Our results suggest that cryptic multicellularity could readily evolve and originate well before multicellular individuals become morphologically evident.

Diverse cell junctions with unique molecular composition in tissues of a sponge (Porifera)

The integrity and organization of animal tissues depend upon specialized protein complexes that mediate adhesion between cells with each other (cadherin-based adherens junctions), and with the extracellular matrix (integrin-based focal adhesions). Reconstructing how and when these cell junctions evolved is central to understanding early tissue evolution in animals. We examined focal adhesion protein homologs in tissues of the freshwater sponge, Ephydatia muelleri (phylum Porifera; class Demospongiae). Our principal findings are that (1) sponge focal adhesion homologs (integrin, talin, focal adhesion kinase, etc.) co-precipitate as a complex, separate from adherens junction proteins; (2) that actin-based structures resembling focal adhesions form at the cell–substrate interface, and their abundance is dynamically regulated in response to fluid shear; (3) focal adhesion proteins localize to both cell–cell and cell–extracellular matrix adhesions, and; (4) the adherens junction protein β-catenin is co-distributed with focal adhesion proteins at cell–cell junctions everywhere except the choanoderm, and at novel junctions between cells with spicules, and between cells with environmental bacteria. These results clarify the diversity, distribution and molecular composition of cell junctions in tissues of E. muelleri, but raise new questions about their functional properties and ancestry.

Adhesion strategies of Dictyostelium discoideum- a force spectroscopy study

Biological adhesion is essential for all motile cells and generally limits locomotion to suitably functionalized substrates displaying a compatible surface chemistry. However, organisms that face vastly varying environmental challenges require a different strategy. The model organism Dictyostelium discoideum (D.d.), a slime mould dwelling in the soil, faces the challenge of overcoming variable chemistry by employing the fundamental forces of colloid science. To understand the origin of D.d. adhesion, we realized and modified a variety of conditions for the amoeba comprising the absence and presence of the specific adhesion protein Substrate Adhesion A (sadA), glycolytic degradation, ionic strength, surface hydrophobicity and strength of van der Waals interactions by generating tailored model substrates. By employing AFM-based single cell force spectroscopy we could show that experimental force curves upon retraction exhibit two regimes. The first part up to the critical adhesion force can be described in terms of a continuum model, while the second regime of the curve beyond the critical adhesion force is governed by stochastic unbinding of individual binding partners and bond clusters. We found that D.d. relies on adhesive interactions based on EDL-DLVO (Electrical Double Layer-Derjaguin-Landau-Verwey-Overbeek) forces and contributions from the glycocalix and specialized adhesion molecules like sadA. This versatile mechanism allows the cells to adhere to a large variety of natural surfaces under various conditions.

Analysis of a vinculin homolog in a sponge (phylum Porifera) reveals that vertebrate-like cell adhesions emerged early in animal evolution

The evolution of cell-adhesion mechanisms in animals facilitated the assembly of organized multicellular tissues. Studies in traditional animal models have revealed two predominant adhesion structures, the adherens junction (AJ) and focal adhesions (FAs), which are involved in the attachment of neighboring cells to each other and to the secreted extracellular matrix (ECM), respectively. The AJ (containing cadherins and catenins) and FAs (comprising integrins, talin, and paxillin) differ in protein composition, but both junctions contain the actin-binding protein vinculin. The near ubiquity of these structures in animals suggests that AJ and FAs evolved early, possibly coincident with multicellularity. However, a challenge to this perspective is that previous studies of sponges-a divergent animal lineage-indicate that their tissues are organized primarily by an alternative, sponge-specific cell-adhesion mechanism called "aggregation factor." In this study, we examined the structure, biochemical properties, and tissue localization of a vinculin ortholog in the sponge Oscarella pearsei (Op). Our results indicate that Op vinculin localizes to both cell-cell and cell-ECM contacts and has biochemical and structural properties similar to those of vertebrate vinculin. We propose that Op vinculin played a role in cell adhesion and tissue organization in the last common ancestor of sponges and other animals. These findings provide compelling evidence that sponge tissues are indeed organized like epithelia in other animals and support the notion that AJ- and FA-like structures extend to the earliest periods of animal evolution.

Surface topology affects wetting behavior of Bacillus subtilis biofilms

The colonization of surfaces by bacterial biofilms constitutes a huge problem in healthcare and industry. When attempting biofilm inactivation or removal, it is crucial to sufficiently wet the biofilm surface with antibacterial agents; however, certain biofilms efficiently resist wetting, and the origin of this behavior remains to date unclear. Here, we demonstrate that, depending on the growth medium used, the model bacterium Bacillus subtilis can form biofilm colonies with distinct surface properties: we find either hydrophilic or two variants of hydrophobic behavior. We show that those differences in biofilm wetting correlate with distinct surface topologies which, in turn, give rise to different physical wetting regimes known from lotus leaves or rose petals. Forming biofilms with different wetting properties may help bacteria to survive in both arid and humid conditions. Furthermore, converting the surface polarity of a biofilm could facilitate their removal from surfaces by increasing their wettability.

A biofilm’s surface structure affects its susceptibility to wetting, influencing persistence and suggesting strategies for biofilm removal. Effective wetting of biofilm surfaces is essential for combatting them with antibacterial agents. Resistance to the procedures currently available for removing biofilms is a huge problem in healthcare. Researchers in Germany led by Oliver Lieleg at the Technical University of Munich found that Bacillus subtilis bacteria can form colonies with different surface structures that either resist or assist wetting. This structural variability in their biofilms may allow the bacteria to adapt to survive in either arid or humid conditions. The insight that environmental conditions can influence biofilm wettability should spur research to control this natural variation. Learning how to convert biofilms into more readily wetted forms may greatly assist their removal in healthcare situations.

Purification and partial characterization of a lectin protein complex, the clathrilectin, from the calcareous sponge Clathrina clathrus

Carbohydrate-binding proteins were purified from the marine calcareous sponge Clathrina clathrus via affinity chromatography on lactose and N-acetyl glucosamine-agarose resins. Proteomic analysis of acrylamide gel separated protein subunits obtained in reducing conditions pointed out several candidates for lectins. Based on amino-acid sequence similarity, two peptides displayed homology with the jack bean lectin Concanavalin A, including a conserved domain shared by proteins in the L-type lectin superfamily. An N-acetyl glucosamine - binding protein complex, named clathrilectin, was further purified via gel filtration chromatography, bioguided with a diagnostic rabbit erythrocyte haemagglutination assay, and its activity was found to be calcium dependent. Clathrilectin, a protein complex of 3200kDa estimated by gel filtration, is composed of monomers with apparent molecular masses of 208 and 180kDa estimated on 10% SDS-PAGE. Nine internal peptides were identified using proteomic analyses, and compared to protein libraries from the demosponge Amphimedon queenslandica and a calcareous sponge Sycon sp. from the Adriatic Sea. The clathrilectin is the first lectin isolated from a calcareous sponge and displays homologies with predicted sponge proteins potentially involved in cell aggregation and interaction with bacteria.

Notch-Mediated Cell Adhesion

Notch family members are generally recognized as signaling molecules that control various cellular responses in metazoan organisms. Early fly studies and our mammalian studies demonstrated that Notch family members are also cell adhesion molecules; however, information on the physiological roles of this function and its origin is limited. In this review, we discuss the potential present and ancestral roles of Notch-mediated cell adhesion in order to explore its origin and the initial roles of Notch family members dating back to metazoan evolution. We hypothesize that Notch family members may have initially emerged as cell adhesion molecules in order to mediate multicellularity in the last common ancestor of metazoan organisms.

IGSF9 family proteins

The Drosophila protein Turtle and the vertebrate proteins immunoglobulin superfamily (IgSF), member 9 (IGSF9/Dasm1) and IGSF9B are members of an evolutionarily ancient protein family. A bioinformatics analysis of the protein family revealed that invertebrates contain only a single IGSF9 family gene, whereas vertebrates contain two to four genes. In cnidarians, the gene appears to encode a secreted protein, but transmembrane isoforms of the protein have also evolved, and in many species, alternative splicing facilitates the expression of both transmembrane and secreted isoforms. In most species, the longest isoforms of the proteins have the same general organization as the neural cell adhesion molecule family of cell adhesion molecule proteins, and like this family of proteins, IGSF9 family members are expressed in the nervous system. A review of the literature revealed that Drosophila Turtle facilitates homophilic cell adhesion. Moreover, IGSF9 family proteins have been implicated in the outgrowth and branching of neurites, axon guidance, synapse maturation, self-avoidance, and tiling. However, despite the few published studies on IGSF9 family proteins, reports on the functions of both Turtle and mammalian IGSF9 proteins are contradictory.

Genome-wide identification, classification and expression analysis of genes encoding putative fasciclin-like arabinogalactan proteins in Chinese cabbage (Brassica rapa L.)

Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), have both predicted AGP-like glycosylated regions and putative fasciclin (FAS) domains, which may function in cell adhesion and communication. Previous studies have identified 21, 27, and 34 FLAs in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and wheat (Triticum aestivum), respectively. In this study, we identified 33 FLAs in the annotated genome of Chinese cabbage (Brassica rapa ssp. pekinensis line Chiifu-401-42). Sequence analysis indicated that FAS domains each contain two highly conserved regions, named H1 and H2, and that 17 FLAs from B. rapa (BrFLAs) possess both of these regions. Prediction of glycosylphosphatidylinositol (GPI) modification sites suggested that 15 BrFLAs were GPI-anchored to the plasma membrane. Additionally, 25 BrFLAs may have been duplicated during the processes that shaped the triplicated genome of the mesopolyploid B. rapa. Expression analyses indicated that BrFLA1, BrFLA11, BrFLA13, BrFLA28 and BrFLA32 were specifically expressed in inflorescence. Meanwhile, BrFLA9 (homologous to AtFLA12) is specifically expressed in stem, and BrFLA6/22 (homologous to AtFLA11) is also highly expressed in stem, suggesting BrFLA6/9/22 may have the same functions as AtFLA11/12 in A. thaliana. Taken together, the identification and bioinformatic analysis of FLAs in B. rapa will open the way for studying their biological functions in plant growth and development as well as evolutionary history of this gene family from A. thaliana to B. rapa.

Impact of the carbazole derivative wiskostatin on mechanical stability and dynamics of motile cells

Many essential functions in eukaryotic cells like phagocytosis, division, and motility rely on the dynamical properties of the actin cytoskeleton. A central player in the actin system is the Arp2/3 complex. Its activity is controlled by members of the WASP (Wiskott-Aldrich syndrome protein) family. In this work, we investigated the effect of the carbazole derivative wiskostatin, a recently identified N-WASP inhibitor, on actin-driven processes in motile cells of the social ameba Dictyostelium discoideum. Drug-treated cells exhibited an altered morphology and strongly reduced pseudopod formation. However, TIRF microscopy images revealed that the overall cortical network structure remained intact. We probed the mechanical stability of wiskostatin-treated cells using a microfluidic device. While the total amount of F-actin in the cells remained constant, their stiffness was strongly reduced. Furthermore, wiskostatin treatment enhanced the resistance to fluid shear stress, while spontaneous motility as well as chemotactic motion in gradients of cAMP were reduced. Our results suggest that wiskostatin affects the mechanical integrity of the actin cortex so that its rigidity is reduced and actin-driven force generation is impaired.

Biological activities of ACL-I and physicochemical properties of ACL-II, lectins isolated from the marine sponge Axinella corrugata

Lectin II from the marine sponge Axinella corrugata (ACL-II) was purified by affinity chromatography on rabbit erythrocytic stroma incorporated into a polyacrylamide gel, followed by gel filtration on Ultrogel AcA 44 column. Purified ACL-II is a lectin with an Mr of 80 kDa and 78 kDa, estimated by SDS-PAGE and by FPLC on Superose 12 HR column, respectively. ACL-II mainly agglutinates native rabbit erythrocytes and this hemagglutinating activity is independent of Ca(2+), Mg(2+) and Mn(2+), but is inhibited by d-galactose, chitin and N-acetyl derivatives, with the exception of GalNAc. ACL-II is stable for up to 65 °C for 30 min, with a better stability at a pH range of 2 to 6. In contrast, ACL-I displays a strong mitogenic and cytotoxic effect.

A fasciclin-like arabinogalactan-protein (FLA) mutant of Arabidopsis thaliana, fla1, shows defects in shoot regeneration

BACKGROUND

The fasciclin-like arabinogalactan-proteins (FLAs) are an enigmatic class of 21 members within the larger family of arabinogalactan-proteins (AGPs) in Arabidopsis thaliana. Located at the cell surface, in the cell wall/plasma membrane, they are implicated in many developmental roles yet their function remains largely undefined. Fasciclin (FAS) domains are putative cell-adhesion domains found in extracellular matrix proteins of organisms from all kingdoms, but the juxtaposition of FAS domains with highly glycosylated AGP domains is unique to plants. Recent studies have started to elucidate the role of FLAs in Arabidopsis development. FLAs containing a single FAS domain are important for the integrity and elasticity of the plant cell wall matrix (FLA11 and FLA12) and FLA3 is involved in microspore development. FLA4/SOS5 with two FAS domains and two AGP domains has a role in maintaining proper cell expansion under salt stressed conditions. The role of other FLAs remains to be uncovered.

METHOD/PRINCIPAL FINDINGS

Here we describe the characterisation of a T-DNA insertion mutant in the FLA1 gene (At5g55730). Under standard growth conditions fla1-1 mutants have no obvious phenotype. Based on gene expression studies, a putative role for FLA1 in callus induction was investigated and revealed that fla1-1 has a reduced ability to regenerate shoots in an in vitro shoot-induction assay. Analysis of FLA1p:GUS reporter lines show that FLA1 is expressed in several tissues including stomata, trichomes, the vasculature of leaves, the primary root tip and in lateral roots near the junction of the primary root.

CONCLUSION

The results of the developmental expression of FLA1 and characterisation of the fla1 mutant support a role for FLA1 in the early events of lateral root development and shoot development in tissue culture, prior to cell-type specification.

The cell adhesion molecule DdCAD-1 regulates morphogenesis through differential spatiotemporal expression in Dictyostelium discoideum

During development of Dictyostelium, multiple cell types are formed and undergo a coordinated series of morphogenetic movements guided by their adhesive properties and other cellular factors. DdCAD-1 is a unique homophilic cell adhesion molecule encoded by the cadA gene. It is synthesized in the cytoplasm and transported to the plasma membrane by contractile vacuoles. In chimeras developed on soil plates, DdCAD-1-expressing cells showed greater propensity to develop into spores than did cadA-null cells. When development was performed on non-nutrient agar, wild-type cells sorted from the cadA-null cells and moved to the anterior zone. They differentiated mostly into stalk cells and eventually died, whereas the cadA-null cells survived as spores. To assess the role of DdCAD-1 in this novel behavior of wild-type and mutant cells, cadA-null cells were rescued by the ectopic expression of DdCAD-1-GFP. Morphological studies have revealed major spatiotemporal changes in the subcellular distribution of DdCAD-1 during development. Whereas DdCAD-1 became internalized in most cells in the post-aggregation stages, it was prominent in the contact regions of anterior cells. Cell sorting was also restored in cadA(-) slugs by exogenous recombinant DdCAD-1. Remarkably, DdCAD-1 remained on the surface of anterior cells, whereas it was internalized in the posterior cells. Additionally, DdCAD-1-expressing cells migrated slower than cadA(-) cells and sorted to the anterior region of chimeric slugs. These results show that DdCAD-1 influences the sorting behavior of cells in slugs by its differential distribution on the prestalk and prespore cells.

Uncovering a role for the tail of the Dictyostelium discoideum SadA protein in cell-substrate adhesion

Previous work from our laboratory showed that the Dictyostelium discoideum SadA protein plays a central role in cell-substrate adhesion. SadA null cells exhibit a loss of adhesion, a disrupted actin cytoskeleton, and a cytokinesis defect. How SadA mediates these phenotypes is unknown. This work addresses the mechanism of SadA function, demonstrating an important role for the C-terminal cytoplasmic tail in SadA function. We found that a SadA tailless mutant was unable to rescue the sadA adhesion deficiency, and overexpression of the SadA tail domain reduced adhesion in wild-type cells. We also show that SadA is closely associated with the actin cytoskeleton. Mutagenesis studies suggested that four serine residues in the tail, S924/S925 and S940/S941, may regulate association of SadA with the actin cytoskeleton. Glutathione S-transferase pull-down assays identified at least one likely interaction partner of the SadA tail, cortexillin I, a known actin bundling protein. Thus, our data demonstrate an important role for the carboxy-terminal cytoplasmic tail in SadA function and strongly suggest that a phosphorylation event in this tail regulates an interaction with cortexillin I. Based on our data, we propose a model for the function of SadA.

Armadillo-repeat protein functions: questions for little creatures

Armadillo (ARM)-repeat proteins form a large family with diverse and fundamental functions in many eukaryotes. ARM-repeat proteins have largely been characterised in multicellular organisms and much is known about how a subset of these proteins function. The structure of ARM-repeats allows proteins containing them to be functionally very versatile. Are the ARM-repeat proteins in 'little creatures' as multifunctional as their better-studied relatives? The time is now right to start analysing ARM-repeat proteins in these new systems to better understand their cell biology. Here, we review recent advances in understanding the many cellular roles of both well-known and novel ARM-repeat proteins.

Evolution of complexity in the integrin adhesome

Integrin-mediated adhesion is as ancient as multicellularity, but it was not always as complex as it is in humans. Here, I examine the extent of conservation of 192 adhesome proteins across the genomes of nine model organisms spanning one and a half billion years of evolution. The work reveals that Rho GTPases, lipid- and serine/threonine-kinases, and phosphatases existed before integrins, but tyrosine phosphorylation developed concomitant with integrins. The expansion of specific functional groups such as GAPs, GEFs, adaptors, and receptors is demonstrated, along with the expansion of specific protein domains, such as SH3, PH, SH2, CH, and LIM. Expansion is due to gene duplication and creation of families of paralogues. Apparently, these paralogues share few partners and create new sets of interactions, thus increasing specificity and the repertoire of integrin-mediated signaling. Interestingly, the average number of interactions positively correlates with the evolutionary age of proteins. While shedding light on the evolution of adhesome complexity, this analysis also highlights the relevance and creates a framework for studying integrin-mediated adhesion in simpler model organisms.

Genome analysis of the unicellular green alga Chlamydomonas reinhardtii Indicates an ancient evolutionary origin for key pattern recognition and cell-signaling protein families

The evolution of specific cell signaling and adhesion domains may have played an important role in the transition to a multicellular existence in the metazoans. Genomic analysis indicates that several signaling domains predominately found in animals are also present in the unicellular green alga, Chlamydomonas reinhardtii. A large group of proteins is present, containing scavenger receptor cysteine-rich (SRCR) and C-type lectin domains, which function in ligand binding and play key roles in the innate immune system of animals. Chlamydomonas also contains a large family of putative tyrosine kinases, suggesting an important role for phosphotyrosine signaling in the green algae. These important signaling domains may therefore be widespread among eukaryotes and most probably evolved in ancestral eukaryotes before the divergence of the Opisthokonts (the animal and fungal lineage).

Early evolution of animal cell signaling and adhesion genes

In stark contrast to the rapid morphological radiation of eumetazoans during the Cambrian explosion, the simple body plan of sponges (Phylum Porifera) emerged from the Cambrian relatively unchanged. Although the genetic and developmental underpinnings of these disparate evolutionary outcomes are unknown, comparisons between modern sponges and eumetazoans promise to reveal the extent to which critical genetic factors were present in their common ancestors. Two particularly interesting classes of genes in this respect are those involved in cell signaling and adhesion. These genes help guide development and morphogenesis in modern eumetazoans, but the timing and sequence of their origins is unknown. Here, we demonstrate that the sponge Oscarella carmela, one of the earliest branching animals, expresses core components of the Wnt, transforming growth factor beta, receptor tyrosine kinase, Notch, Hedgehog, and Jak/Stat signaling pathways. Furthermore, we identify sponge homologs of nearly every major eumetazoan cell-adhesion gene family, including those that encode cell-surface receptors, cytoplasmic linkers, and extracellular-matrix proteins. From these data, we infer that key signaling and adhesion genes were in place early in animal evolution, before the divergence of sponge and eumetazoan lineages.

CHDL: A cadherin-like domain in Proteobacteria and Cyanobacteria

We identified a cadherin-like domain (CHDL) using computational analysis. The CHDL domain is mostly distributed in Proteobacteria and Cyanobacteria, although it is also found in some eukaryotic proteins. Prediction of three-dimensional protein folding indicated that the CHDL domain has an immunoglobulin beta-sandwich fold and belongs to the cadherin superfamily. The CHDL domain does not have LDRE and DxNDN motifs, which are conserved in the cadherin domain, but has three other motifs: PxAxxD, DxDxD and YT-V/I-S/T-D, which might contribute to forming a calcium-binding site. The identification of this cadherin-like domain indicates that the cadherin superfamily may exhibit wider sequence and structural diversity than previously appreciated. Domain architecture analysis revealed that the CHDL domain is also associated with other adhesion domains as well as enzyme domains. Based on computational analysis and previous experimental data, we predict that the CHDL domain has calcium-binding and also carbohydrate-binding activity.