An activated Ras protein alters cell adhesion by dephosphorylating Dictyostelium DdCAD-1

Social amoebae establish a protective interface with their bacterial associates by lectin agglutination

Both animals and amoebae use phagocytosis and DNA-based extracellular traps as anti-bacterial defense mechanisms. Whether, like animals, amoebae also use tissue-level barriers to reduce direct contact with bacteria has remained unclear. We have explored this question in the social amoeba Dictyostelium discoideum, which forms plaques on lawns of bacteria that expand as amoebae divide and bacteria are consumed. We show that CadA, a cell adhesion protein that functions in D. discoideum development, is also a bacterial agglutinin that forms a protective interface at the plaque edge that limits exposure of vegetative amoebae to bacteria. This interface is important for amoebal survival when bacteria-to-amoebae ratios are high, optimizing amoebal feeding behavior, and protecting amoebae from oxidative stress. Lectins also control bacterial access to the gut epithelium of mammals to limit inflammatory processes; thus, this strategy of antibacterial defense is shared across a broad spectrum of eukaryotic taxa.

Interface interactions between βγ-crystallin domain and Ig-like domain render Ca2+ -binding site inoperative in abundant perithecial protein of Neurospora crassa

We describe a set of proteins in which a βγ-crystallin domain pairs with an Ig-like domain, and which are confined to microbes, like bacteria, slime molds and fungi. DdCAD-1 (Ca²⁺ -dependent cell adhesion molecule-1) and abundant perithecial protein (APP) represent this class of molecules. Using the crystal structure of APP-NTD (N-terminal domain of APP), we describe its mode of Ca²⁺ binding and provide a generalized theme for correct identification of the Ca²⁺ -binding site within this class of molecules. As a common feature, one of the two Ca²⁺ -binding sites is non-functional in the βγ-crystallin domains of these proteins. While APP-NTD binds Ca²⁺ with a micromolar affinity which is comparable to DdCAD-1, APP surprisingly does not bind Ca²⁺ . Crystal structures of APP and Ca²⁺ -bound APP-NTD reveal that the interface interactions in APP render its Ca²⁺ -binding site inoperative. Thus, heterodomain association provides a novel mode of Ca²⁺ -binding regulation in APP. Breaking the interface interactions (mutating Asp30Ala, Leu132Ala and Ile135Ala) or separation from the Ig-like domain removes the constraints upon the required conformational transition and enables the βγ-crystallin domain to bind Ca²⁺ . In mechanistic detail, our work demonstrates an interdomain interface adapted to distinct functional niches in APP and its homolog DdCAD-1.

The Effect of Overexpressed DdRabS on Development, Cell Death, Vesicular Trafficking, and the Secretion of Lysosomal Glycosidase Enzymes

Rab GTPases are essential regulators of many cellular processes and play an important role in downstream signaling vital to proper cell function. We sought to elucidate the role of novel D. discoideum GTPase RabS. Cell lines over-expressing DdRabS and expressing DdRabS N137I (dominant negative (DN)) proteins were generated, and it was determined that DdRabS localized to endosomes, ER-Golgi membranes, and the contractile vacuole system. It appeared to function in vesicular trafficking, and the secretion of lysosomal enzymes. Interestingly, microscopic analysis of GFP-tagged DdRabS (DN) cells showed differential localization to lysosomes and endosomes compared to GFP-tagged DdRabS overexpressing cells. Both cell lines over-secreted lysosomal glycosidase enzymes, especially β-glucosidase. Furthermore, DdRabS overexpressing cells were defective in aggregation due to decreased cell–cell cohesion and sensitivity to cAMP, leading to abnormal chemotactic migration, the inability to complete development, and increased induced cell death. These data support a role for DdRabS in trafficking along the vesicular and biosynthetic pathways. We hypothesize that overexpression of DdRabS may interfere with GTP activation of related proteins essential for normal development resulting in a cascade of defects throughout these processes.

Vesicular Trafficking Defects, Developmental Abnormalities, and Alterations in the Cellular Death Process Occur in Cell Lines that Over-Express Dictyostelium GTPase, Rab2, and Rab2 Mutants

Small molecular weight GTPase Rab2 has been shown to be a resident of pre-Golgi intermediates and required for protein transport from the ER to the Golgi complex, however, the function of Rab2 in Dictyostelium has yet to be fully characterized. Using cell lines that over-express DdRab2, as well as cell lines over-expressing constitutively active (CA), and dominant negative (DN) forms of the GTPase, we report a functional role in vesicular transport specifically phagocytosis, and endocytosis. Furthermore, Rab2 like other GTPases cycles between an active GTP-bound and an inactive GDP-bound state. We found that this GTP/GDP cycle for DdRab2 is crucial for normal Dictyostelium development and cell-cell adhesion. Similar to Rab5 and Rab7 in C. elegans, we found that DdRab2 plays a role in programmed cell death, possibly in the phagocytic removal of apoptotic corpses.

Looking beyond the Wnt pathway for the deep nature of β-catenin

After two decades of stardom, one would think that β-catenin has revealed all of its most intimate details. Yet the essence of its duality has remained mysterious--how can a single protein both be the core link between cadherins and the cytoskeleton, and the nuclear messenger for Wnt signalling? On the basis of the available evidence and on molecular and evolutionary considerations, I propose that β-catenin was a born nuclear transport receptor, which by interacting with adhesion molecules acquired the property to coordinate nuclear functions with cell-cell adhesion. While Wnt signalling diverted this activity, the original pathway might still function in modern eukaryotes.

Diverse evolutionary paths to cell adhesion

The morphological diversity of animals, fungi, plants, and other multicellular organisms stems from the fact that each lineage acquired multicellularity independently. A prerequisite for each origin of multicellularity was the evolution of mechanisms for stable cell-cell adhesion or attachment. Recent advances in comparative genomics and phylogenetics provide critical insights into the evolutionary foundations of cell adhesion. Reconstructing the evolution of cell junction proteins in animals and their unicellular relatives exemplifies the roles of co-option and innovation. Comparative studies of volvocine algae reveal specific molecular changes that accompanied the evolution of multicellularity in Volvox. Comparisons between animals and Dictyostelium show how commonalities and differences in the biology of unicellular ancestors influenced the evolution of adhesive mechanisms. Understanding the unicellular ancestry of cell adhesion helps illuminate the basic cell biology of multicellular development in modern organisms.

Dictyostelium discoideum paxillin regulates actin-based processes

Paxillin is a key player in integrating the actin cytoskeleton with adhesion, and thus is essential to numerous cellular processes, including proliferation, differentiation, and migration in animal cells. PaxB, the Dictyostelium discoideum orthologue of paxillin, has been shown to be important for adhesion and development, much like its mammalian counterpart. Here, we use the overproduction of PaxB to gain better insight into its role in regulating the actin cytoskeleton and adhesion. We find that PaxB-overexpressing (PaxBOE) cells can aggregate and form mounds normally, but are blocked in subsequent development. This arrest can be rescued by addition of wild-type cells, indicating a non-cell autonomous role for PaxB. PaxBOE cells also have alterations in several actin-based processes, including adhesion, endocytosis, motility, and chemotaxis. PaxBOE cells exhibit an EDTA-sensitive increase in cell-cell cohesion, suggesting that PaxB-mediated adhesion is Ca(2+) or Mg(2+) dependent. Interestingly, cells overexpressing paxB are less adhesive to the substratum. In addition, PaxBOE cells display decreased motility under starved conditions, decreased endocytosis, and are unable to efficiently chemotax up a folate gradient. Taken together, the data suggest that proper expression of PaxB is vital for the regulation of development and actin-dependent processes.