Discoidin-binding polysaccharide from Dictyostelium discoideum

Extracts of Dictyostelium discoideum grown axenically in a chemically defined medium were evaluated for binding to discoidin I and discoidin II, endogenous lectins of this slime mold. Binding activity was measured by competitive inhibition of 125I-lactosyl-bovine serum albumin binding to the immobilized lectins. With the solubilization procedure used extracts of vegetative cells and of early aggregates had no significant inhibitory activity, but an abundant discoidin-binding substance was detected in late aggregates and fruiting bodies. This material was purified by ethanol and acid precipitation followed by precipitation with discoidin. It is a polysaccharide composed of 77% galactose, 15% N-acetylgalactosamine, 5% glucose, and 3% N-acetylglucosamine and may be a biologically functional ligand for the slime mold lectins, in particular discoidin II. Use of axenic cells was critical in these experiments, since extracts of Escherichia coli and Klebsiella aerogenes, commonly used as food for D. discoideum, were found to contain substances that react with discoidin. This would complicate isolation of endogenous discoidin ligands from cells raised on bacteria.     

Regulation of the synthesis of two carbohydrate-binding proteins in Dictyostelium discoideum

The relative rate of de novo synthesis of two membrane-associated carbohydrate-binding proteins (CBP) has been examined during Dictyostelium development. The results show that the relative rate of CBP synthesis is minimal during the vegetative stage and increases to represent approximately 3.5 to 5% of newly synthesized protein during the aggregation stage after which the relative rate decreases. Analysis of the relative rates of synthesis of CBP-26 and CBP-24 indicate that at the peak period of synthesis (approximately 5 to 9 h of development) CBP-26 is synthesized at a rate which is approximately eight times greater than CBP-24. In addition, we have examined the relative amount of CBP-26 and CBP-24 mRNA during development as assayed by its ability to direct CBP synthesis in in vitro protein-synthesizing systems. We show that there is no detectable CBP mRNA in vegetative cells and that during the pre-aggregating stages, assayable CBP mRNA appears and accumulates with a maximal level at the period of peak in vivo CBP synthesis. These results suggest that the rate at CBP synthesis in vivo is controlled by the relative amount of functional mRNA.     

cAMP-dependent protein kinase from Dictyostelium discoideum

The cAMP-dependent protein kinase (cAK) from Dictyostelium discoideum is an enzyme composed of one catalytic and one regulatory subunit. Upon binding of cAMP, the holoenzyme dissociates to liberate free active catalytic subunits. The cAK is developmentally regulated, ranging from very little activity in vegetative cells to maximal expression in postaggregative cells. Although there is no immunological cross-reaction between the subunits of cAKs from Dictyostelium and from other organisms, they share several biochemical properties. A complete cDNA for the regulatory subunit has been cloned and sequenced. Only one copy of the gene for the regulatory subunit is present per haploid genome. On the basis of the comparison of the structure of the cAK from Dictyostelium with its counterparts in yeast and higher eukaryotes, we propose a model for the evolution of cyclic-nucleotide-binding proteins.     

Uracil-DNA glycosylase activity from Dictyostelium discoideum

We have isolated and partially characterized a uracil-DNA glycosylase activity from the cellular slime mold, Dictyostelium discoideum. This glycosylase has a broad pH optimum (6.5-8.5) and is fully active in 10 mM EDTA or in 5 mM Mg2+. Its molecular weight by gel filtration is about 55 000. This enzyme activity may work in concert with previously described apurinic/apyrimidinic (AP) endonuclease activities in the excision repair of uracil from the DNA of this lower eukaryote.     

Repression of rRNA synthesis during slug reconstruction in Dictyostelium discoideum

When cells dissociated from "slugs" are allowed to reaggregate, they reconstruct slugs. During this process, the cells showed a marked decrease in the ratio of labeled RNA lacking poly(A) to labeled total RNA as compared with that of cells in normal slugs. Irrespective of the change in total labeled RNA, the ratio remained low even after 6 h of reconstruction. Sucrose density gradient analysis of RNA showed that the synthesis of high molecular weight rRNA (26S and 17S) was considerably repressed in reconstructed slugs as compared with normal slugs. Polyacrylamide gel electrophoresis of low molecular weight rRNA revealed that the synthesis of 5S rRNA, but not 4S tRNA, was repressed.     

盘基网柄菌肌动蛋白的生物信息学分析

盘基网柄菌Dictyostelium discoideum是目前黏菌中研究最清楚的模式生物,其捕食过程与肌动蛋白的多聚化密切相关。为探讨盘基网柄菌肌动蛋白的序列特征,本研究利用生物信息学方法分析了盘基网柄菌32条肌动蛋白的蛋白-蛋白相互作用(protein-protein interaction)和可能含有的保守基序。结果表明:盘基网柄菌32条肌动蛋白与其他蛋白存在一组复杂相互作用关系和5组比较简单的相互作用关系;利用MEME SUITE分别分析盘基网柄菌32条肌动蛋白序列的保守基序和与actin17呈最佳匹配的21种生物肌动蛋白的保守基序,结果共获得6个保守基序,即motif1,motif2,motif3,Motif1,Motif2,Motif3。其中motif1,Motif1,Motif3为本研究新发现的保守基序,这3个保守基序可定位于Profilin-actin-VASP202–244(PDB ID:3CHW)三维结构的重要位置。以上结果表明actin3,actin10,actin14,actin15,actin17,actin31可能为盘基网柄菌比较重要的肌动蛋白;motif1,Motif1,Motif3可能是盘基网柄菌肌动蛋白在进化中重要的保守基序。     

Structural requirements of dictyopyrones isolated from Dictyostelium spp. in the regulation of Dictyostelium development and in anti-leukemic activity

Cellular slime molds are fascinating to the field of developmental biology, and have long been used as excellent model organisms for the study of various aspects of multicellular development. We have recently isolated alpha-pyronoids, named dictyopyrones A-D (1-4), from various species of Dictyostelium cellular slime molds, and it was shown that compound 3 may regulate Dictyostelium development. In this study, we synthesized dictyopyrones A-D (1-4) and their analogues, investigated the physiological role of the molecules in cell growth and morphogenesis in D. discoideum, and further verified their effects on human leukemia K562 cells. Nitrogen-containing compounds 22 and 37 strongly inhibited cell growth in K562 leukemia cells, indicating that these compounds may be utilized as novel lead compounds for anti-leukemic agents.     

Acid DNAase activity from Dictyostelium discoideum

We have isolated and partially characterized an acid endonuclease activity from the cellular slime mold, Dictyostelium discoideum. This activity comprises more than 90% of the nonspecific DNA-endonuclease activity of the vegetative cells. Its molecular weight is about 44 000, and its activity is enhanced 7-fold by Mg2+. The pH optimum for the nicking activity depends upon NaCl concentrations, being at pH 5.0 in 207 mM NaCl, and at pH 5.8 in 7 mM NaCl. Large quantities of this enzyme activity are released into the growth medium or buffer, with detectable amounts appearing within 15 min of incubation.     

New actin-binding proteins from Dictyostelium discoideum

Dictyostelium discoideum contains a soluble actin-binding protein that caps actin filaments at their fast growing ends. The purified protein consists of two subunits with 34 kd and 32 kd apparent mol. wts. Like similar proteins from Acanthamoeba and bovine brain the capping protein from D. discoideum acts in a Ca²⁺ -independent manner. It lacks severing activity as indicated by its inability to disrupt the stress fibers and the microfilament network in detergent-extracted cells. Two actin-binding proteins from a plasma membrane-enriched fraction were labeled with [¹²⁵I]actin using a gel overlay technique. One of these proteins, with an apparent mol. wt. of 17 kd in SDS-polyacrylamide gels, has been purified from high-salt extracts, the other protein with an apparent mol. wt. of 31 kd has been purified from Triton X-100 extracted membranes. Monoclonal antibodies were raised against D. discoideum severin, α-actinin, the larger subunit of the capping protein, and the 17-kd membrane-associated protein. Immunoblotting of proteins from whole cell lysates showed that all these actin-binding proteins were present in both growth phase and aggregation-competent cells.     

Chemiluminescence in Dictyostelium discoideum

Abstract Oxygen radicals generated during oxidative metabolism participate in chemical reactions resulting in light emission. Chemiluminescence is used therefore to measure their production. We have shown that starvation and heat shock induce chemiluminescence in Dictyostelium discoideum . The peak light emission was found to occur about 4 h after the onset of starvation. The optimum temperature for chemiluminescence by starving amoebae was about 33°C. The heat shock inducibility of chemiluminescence was maximal at the beginning of development. Our results are consistent with suggestions that the product(s) of perturbed mitochondrial metabolism might be intracellular signal(s) controlling gene expression in stressed cells. They also suggest a role for intracellular stress signal(s) in the initiation of development in Dictyostelium by starvation.     

Three steps in prespore differentiationin Dictyostelium discoideum with different requirements of cellular interaction

Abstract. Extracellular cAMP and a secreted factors have been known to be involved in prespore differentiation of Dictyostelium discoideum . Here we show that cAMP, a secreted factor(s) and some other interactions are required for prespore differentiation and that they work in completely different periods; a secreted factor(s) and other interactions are required only in the stages earlier than the cAMP-dependent stage. According to the results the process of prespore differentiation can be dissected into three sequential stages, stage I, II and III. The processes in stage I and II depend on high cell density. The requirement for high cell density in stage II could be replaced with a secreted factor(s) in conditioned medium, whereas it could not in stage I. The factor(s) in conditioned medium does not appear to be cAMP, ammonia, or methionine. In contrast to these two stages, the process in stage III, the last stage, proceeds even at low cell density if cAMP is supplied, where other interactions would be negligible. Therefore cells that have proceeded to the end of stage II are considered to have acquired a competence to differentiate to prespore cells without further cellular interactions other than cAMP.
cAMP pulses are not essential for the processes of any stage of prespore differentiation, since they proceed in the presence of caffeine, an inhibitor of cAMP pulse production, or in a mutant strain (Frigid A) which is deficient in cAMP relay systems.     

Specific mRNA destabilization in Dictyostelium discoideum requires RNA synthesis.

We tested the effects of inhibitors of protein and RNA synthesis on the disaggregation-mediated destabilization of prespore mRNAs in Dictyostelium discoideum. Incubating disaggregated cells with daunomycin to inhibit RNA synthesis prevented the loss of prespore mRNAs, whereas the inhibitor decreased or did not affect levels of the common mRNAs CZ22 and actin. Protein synthesis inhibitors varied in their effects. Cycloheximide, which inhibited protein synthesis almost completely, prevented the loss of the prespore mRNAs, but puromycin, which inhibited protein synthesis less well, did not. These results indicate that the process of specific mRNA destabilization requires the synthesis of RNA and possibly of protein.     

cDNA sequence of cyclophilin from Dictyostelium discoideum

A cDNA encoding a protein homologous to cyclophilins from other species has been isolated from a Dictyostelium discoideum cDNA library. From the deduced amino acid sequence a protein with a molecular mass of 19 kD and 64% identity with human cyclophilin is predicted. Southern blot analysis indicates that there is one cyclophilin gene in the D. discoideum genome. The mRNA is present in all developmental stages.     

Aggregation in Dictyostelium discoideum

Cultured peritoneal macrophages have previously been shown to release a potent mitogen for mesenchymal cells. Peritoneal macrophages are derived from peripheral blood monocytes, one of the principal inflammatory cells associated with numerous tissue responses to injury. Cultured human monocytes can be activated by endotoxin or concanavalin A to secrete a potent growth factor(s) that is active on human smooth muscle cells, human fibroblasts and 3T3 cells. The optimal conditions for activation of monocyte release of this monocyte-derived growth factor(s) (MDGF) were to expose 5-day-old monocyte cultures (initially plated at 6.8 × 10⁵ cells/ml medium) to 10 μg/ml endotoxin or 6 μg/ml concanavalin A for approximately 20 hr. Monocytes can secrete MDGF into serum-free medium supplemented with 0.15% bovine serum albumin. MDGF stimulates both DNA synthesis and increase in cell number and is trypsin-sensitive, heat labile and nondialyzable. The relationship of MDGF to other monocyte products and its potential importance in wound repair and atherogenesis are discussed.