The cellulose-binding activity of the PsB multiprotein complex is required for proper assembly of the spore coat and spore viability in Dictyostelium discoideum

Investigating the Role of Translationally Control Tumor Protein in Growth, Development and Differentiation of Dictyostelium discoideum

Translationally controlled tumor protein (TCTP) is a multifunctional protein implicated in various types of cellular processes involving growth and development of an organism. Here, we identified tctp gene in Dictyostelium discoideum and unraveled its function. The sequence analysis of D. discoideum TCTP (DdTCTP) showed its conservation among eukaryotes. Transcript of DdTCTP was highly expressed at the initial time points of development and protein is localized both in the cytoplasm and nucleus. Disruption of tctp was achieved by BSR cassette using double homologous recombination method. Abrogation of tctp resulted in reduced cell proliferation but increased cell size. Additionally, development was delayed by 4 h wherein small-sized aggregates and fruiting bodies were produced by tctp^– cells while larger aggregates and fruiting bodies were produced by tctp^OE cells concordant with the fact that TCTP regulates prestalk/prespore ratio and cell-type differentiation. tctp^– cells produced round spores with reduced viability and stalk cells are arranged in septate pattern as compared to polyhedral manner of wild type. Abrogation of tctp resulted in aberrant localization of cell type specific markers and show low proclivity toward prespore/spore region, in presence of wild type cells.

Dependence of stress resistance on a spore coat heteropolysaccharide in Dictyostelium

In Dictyostelium, sporulation occurs synchronously as prespore cells approach the apex of the aerial stalk during culmination. Each prespore cell becomes surrounded by its own coat comprised of a core of crystalline cellulose and a branched heteropolysaccharide sandwiched between heterogeneous cysteine-rich glycoproteins. The function of the heteropolysaccharide, which consists of galactose and N-acetylgalactosamine, is unknown. Two glycosyltransferase-like genes encoding multifunctional proteins, each with predicted features of a heteropolysaccharide synthase, were identified in the Dictyostelium discoideum genome. pgtB and pgtC transcripts were modestly upregulated during early development, and pgtB was further intensely upregulated at the time of heteropolysaccharide accumulation. Disruption of either gene reduced synthase-like activity and blocked heteropolysaccharide formation, based on loss of cytological labeling with a lectin and absence of component sugars after acid hydrolysis. Cell mixing experiments showed that heteropolysaccharide expression is spore cell autonomous, suggesting a physical association with other coat molecules during assembly. Mutant coats expressed reduced levels of crystalline cellulose based on chemical analysis after acid degradation, and cellulose was heterogeneously affected based on flow cytometry and electron microscopy. Mutant coats also contained elevated levels of selected coat proteins but not others and were sensitive to shear. Mutant spores were unusually susceptible to hypertonic collapse and damage by detergent or hypertonic stress. Thus, the heteropolysaccharide is essential for spore integrity, which can be explained by a role in the formation of crystalline cellulose and regulation of the protein content of the coat.

Dictyostelium Hip1r contributes to spore shape and requires epsin for phosphorylation and localization

Clathrin-coated pits assemble on the plasma membrane to select and sequester proteins within coated vesicles for delivery to intracellular compartments. Although a host of clathrin-associated proteins have been identified, much less is known regarding the interactions between clathrin-associated proteins or how individual proteins influence the function of other proteins. In this study, we present evidence of a functional relationship between two clathrin-associated proteins in Dictyostelium, Hip1r and epsin. Hip1r-null cells form fruiting bodies that yield defective spores that lack the organized fibrils typical of wild-type spores. This spore coat defect leads to formation of round, rather than ovoid, spores in Hip1r-null cells that exhibit decreased viability. Like Hip1r-null cells, epsin-null cells also construct fruiting bodies with round spores, but these spores are more environmentally robust. Double-null cells that harbor deletions in both epsin and Hip1r form fruiting bodies, with spores identical in shape and viability to Hip1r single-null cells. In the growing amoeba, Hip1r is phosphorylated and localizes to puncta on the plasma membrane that also contain epsin. Both the phosphorylation state and localization of Hip1r into membrane puncta require epsin. Moreover, expression of the N-terminal ENTH domain of epsin is sufficient to restore both the phosphorylation and the restricted localization of Hip1r within plasma membrane puncta. The results from this study reveal a novel interaction between two clathrin-associated proteins during cellular events in both growing and developing Dictyostelium cells.

Colony sectorization of Metarhizium anisopliae is a sign of ageing

Spontaneous phenotypic degeneration resulting in sterile sectors is frequently observed when culturing filamentous fungi on artificial medium. Sterile sectors from two different strains of the insect pathogenic fungus Metarhizium anisopliae were investigated and found to contain reduced levels of cAMP and destruxins (insecticidal peptides). Microarray analysis using slides printed with 1730 clones showed that compared to wild-type, sterile sectors down-regulated 759 genes and upregulated 27 genes during growth in Sabouraud glucose broth or on insect cuticle. The differentially expressed genes are largely involved in cell metabolism (18.8 %), cell structure and function (13.6 %) and protein metabolism (8.8 %). Strong oxidative stress was demonstrated in sectorial cultures using the nitro blue tetrazolium assay and these cultures show other syndromes associated with ageing, including mitochondrial DNA alterations. However, genes involved in deoxidation and self-protection (e.g. heat-shock proteins, HSPs) were also upregulated. Further evidence of physiological adaptation by the degenerative sectorial cultures included cell-structure reorganization and the employment of additional signalling pathways. In spite of their very similar appearance, microarray analysis identified 181 genes differentially expressed between the two sectors, and the addition of exogenous cAMP only restored conidiation in one of them. Most of the differentially expressed genes were involved in catabolic or anabolic pathways, but the latter included genes for sporulation. Compared to the mammalian ageing process, sectorization in M. anisopliae showed many similarities, including similar patterns of cAMP production, oxidative stress responses and the involvement of HSPs. Thus, a common molecular machinery for ageing may exist throughout the eukaryotes.

Proteomics opens doors to the mechanisms of developmentally regulated secretion

The program of multicellular development in Dictyostelium discoideum culminates with the assembly of a rugged, environmentally resistant spore coat around each spore cell. After synthesis, the proteins that will constitute the coat are stored in prespore vesicles (PSVs) until an unknown developmental signal triggers the PSVs to move to the cell surface where they fuse with the plasma membrane and secrete their cargo by exocytosis. These events occur synchronously in 80% of the cells in each developing multicellular aggregate, and thus the system offers a unique opportunity to study the developmental regulation of protein secretion in situ. Proteomic analysis of purified PSVs identified many of the constituent proteins, which in turn has lead to novel hypotheses and new experimental avenues regarding the molecular mechanisms regulating secretion from the PSVs.

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

Formation of the outer layer of the Dictyostelium spore coat depends on the inner-layer protein SP85/PsB

The Dictyostelium spore is surrounded by a 220 microm thick trilaminar coat that consists of inner and outer electron-dense layers surrounding a central region of cellulose microfibrils. In previous studies, a mutant strain (TL56) lacking three proteins associated with the outer layer exhibited increased permeability to macromolecular tracers, suggesting that this layer contributes to the coat permeability barrier. Electron microscopy now shows that the outer layer is incomplete in the coats of this mutant and consists of a residual regular array of punctate electron densities. The outer layer is also incomplete in a mutant lacking a cellulose-binding protein associated with the inner layer, and these coats are deficient in an outer-layer protein and another coat protein. To examine the mechanism by which this inner-layer protein, SP85, contributes to outer-layer formation, various domain fragments were overexpressed in forming spores. Most of these exert dominant negative effects similar to the deletion of outer-layer proteins, but one construct, consisting of a fusion of the N-terminal and Cys-rich C1 domain, induces a dense mat of novel filaments at the surface of the outer layer. Biochemical studies show that the C1 domain binds cellulose, and a combination of site-directed mutations that inhibits its cellulose-binding activity suppresses outer-layer filament induction. The results suggest that, in addition to a previously described early role in regulating cellulose synthesis, SP85 subsequently contributes a cross-bridging function between cellulose and other coat proteins to organize previously unrecognized structural elements in the outer layer of the coat.

Sphingosine-1-phosphate lyase has a central role in the development of Dictyostelium discoideum

Sphingosine-1-phosphate, a product of sphingomyelin degradation, is an important element of signal transduction pathways that regulate cell proliferation and cell death. We have demonstrated additional roles for sphingosine-1-phosphate in growth and multicellular development. The specific disruption in Dictyostelium discoideum of the sphingosine-1-phosphate lyase gene, which encodes the enzyme that catalyzes sphingosine-1-phosphate degradation, results in a mutant strain with aberrant morphogenesis, as well as an increase in viability during stationary phase. The absence of sphingosine-1-phosphate lyase affects multiple stages throughout development, including the cytoskeletal architecture of aggregating cells, the ability to form migrating slugs, and the control of cell type-specific gene expression and terminal spore differentiation. This pleiotropic effect, which is due to the loss of sphingosine-1-phosphate lyase, establishes sphingolipids as pivotal regulatory molecules in a wide range of processes in multicellular development.

Cellulose-binding modules from extracellular matrix proteins of Dictyostelium discoideum stalk and sheath

Cellulose-binding modules (CBMs) of two extracellular matrix proteins, St15 and ShD, from the slime mold Dictyostelium discoideum were expressed in Escherichia coli. The expressed proteins were purified to > 98% purity by extracting inclusion bodies at pH 11.5 and refolding proteins at pH 7.5. The two refolded CBMs bound tightly to amorphous phosphoric acid swollen cellulose (PASC), but had a low affinity toward xylan. Neither protein exhibited cellulase activity. St15, the stalk-specific protein, had fourfold higher binding affinity toward microcrystalline cellulose (Avicel) than the sheath-specific ShD CBM. St15 is unusual in that it consists of a solitary CBM homologous to family IIa CBMs. Sequence analysis of ShD reveals three putative domains containing: (a) a C-terminal CBM homologous to family IIb CBMs; (b) a Pro/Thr-rich linker domain; and (c) a N-terminal Cys-rich domain. The biological functions and potential role of St15 and ShD in building extracellular matrices during D. discoideum development are discussed.