NUTRIENTS DO NOT INFLUENCE SWIMMING BEHAVIOR OR SETTLEMENT RATES OF ECTOCARPUS SILICULOSUS (PHAEOPHYCEAE) SPORES

Spores newly released from plurilocular sporangia of Ectocarpus siliculosus (Dillw.) Lyngb. sporophytes were assayed for chemotaxis to nutrients and for settlement stimulation by nutrients. To enable these measurements with relatively small volumes and numbers of released spores, we developed a computer-assisted motion-analysis assay for spore chemotaxis and verified the results with a more standard, capillary tube chemotaxis assay. The presence of a nutrient gradient did not influence the swimming behavior of E. siliculosus spores in the motion-analysis assay, and likewise no chemotactic effect was measured in the capillary tube assay. Microplate settlement assays previously utilized with bacteria and invertebrates were adapted for use with algal spores. E. siliculosus spores settled at higher rates on a hydrophobic plastic surface than on surfaces with either positively or negatively charged hydrophilic coatings. Nutrient mixtures had no effect on the rate of spore settlement on hydrophobic surfaces.     

Diterpene carboxylic acids and a heliangolide from Helianthus angustifolius

Two diterpene carboxylic acids, one a new kaurenoid derivative and one the previously characterized labdane, ()-cis-ozic acid, as well as a     

Intravascular plug formation induced by poly-APS is the principal mechanism of the toxin's lethality in rats/rat tissues

Toxic water soluble polymeric 3-alkylpyridinium salts isolated from the sponge Raniera sarai strongly inhibited AChE in vitro. In vivo, experimental animals died due to plugs formed in microcirculation. The mechanism of this plug formation is unknown. In vitro, the toxin did not affect the coagulation rate, but the rate of platelet aggregation was accelerated in a dose-dependent manner. The hemolytic activity of poly-APS was diminished by the addition of serum proteins in a dose-dependent manner. These results support the conclusion that non-specific binding to proteins is the underlying mechanism of the lethality of poly APS.     

RNA binding is more critical to the suppression of silencing function of Cucumber mosaic virus 2b protein than nuclear localization

Previously, we found that silencing suppression by the 2b protein and six mutants correlated both with their ability to bind to double-stranded (ds) small RNAs (sRNAs) in vitro and with their nuclear/nucleolar localization. To further discern the contribution to suppression activity of sRNA binding and of nuclear localization, we have characterized the kinetics of in vitro binding to a ds sRNA, a single-stranded (ss) sRNA, and a micro RNA (miRNA) of the native 2b protein and eight mutant variants. We have also added a nuclear export signal (NES) to the 2b protein and assessed how it affected subcellular distribution and suppressor activity. We found that in solution native protein bound ds siRNA, miRNA, and ss sRNA with high affinity, at protein:RNA molar ratios ~2:1. Of the four mutants that retained suppressor activity, three showed sRNA binding profiles similar to those of the native protein, whereas the remaining one bound ss sRNA at a 2:1 molar ratio, but both ds sRNAs with 1.5-2 times slightly lower affinity. Three of the four mutants lacking suppressor activity failed to bind to any sRNA, whereas the remaining one bound them at far higher ratios. NES-tagged 2b protein became cytoplasmic, but suppression activity in patch assays remained unaffected. These results support binding to sRNAs at molar ratios at or near 2:1 as critical to the suppressor activity of the 2b protein. They also show that cytoplasmically localized 2b protein retained suppressor activity, and that a sustained nuclear localization was not required for this function.     

Where Killers Meet—Permeabilization of the Outer Mitochondrial Membrane during Apoptosis

Although mitochondria are usually considered as supporters of life, they are also involved in cellular death. Mitochondrial outer membrane permeabilization (MOMP) is a crucial event during apoptosis because it causes the release of proapoptotic factors from the mitochondrial intermembrane space to the cytosol. MOMP is mainly controlled by the Bcl-2 family of proteins, which consists of both proapoptotic and antiapoptotic members. We discuss the current understanding of how activating and inhibitory interactions within this family lead to the activation and oligomerization of MOMP effectors Bax and Bak, which result in membrane permeabilization. The order of events leading to MOMP is then highlighted step by step, emphasizing recent discoveries regarding the formation of Bax/Bak pores on the outer mitochondrial membrane. Besides the Bcl-2 proteins, the mitochondrial organelle contributes to and possibly regulates MOMP, because mitochondrial resident proteins and membrane lipids are prominently involved in the process.Mitochondria are essential for the life of the cell. They produce most of the ATP via oxidative phosphorylation thanks to the respiratory chain that is embedded in the inner mitochondrial membrane. Consequently, mitochondrial dysfunction is implicated in the development of many human diseases, in particular, neurodegenerative disorders (Lin and Beal 2006). Mitochondria are also prominently involved in cell death, because they play a crucial role in many apoptotic responses. Apoptosis is a self-destruction program that is essential during the development of multicellular organisms. Its dysregulation has also been recognized as a main feature of many pathological conditions, especially cancer (Llambi and Green 2011).The executioners of apoptosis are a family of cysteine proteases termed caspases that cleave a variety of cellular targets, resulting in morphological changes, degradation of genomic DNA, and, ultimately, phagocytic removal of the apoptotic cell (Taylor et al. 2008). Caspases are synthesized as inactive zymogens that become activated after regulated limited proteolysis. Two different pathways of apoptotic signaling that result in the activation of executioner caspases 3 and 7 can be distinguished. In the extrinsic pathway, binding of ligands such as FasL or TNFα to a death receptor on the plasma membrane leads to the activation of initiator caspase 8. Active caspase 8 propagates the signal by directly cleaving and thereby activating caspases 3 and 7, which continue a proteolytic cascade ultimately leading to the removal of the cell.The intrinsic pathway, on the other hand, is initiated upon exposure to a number of stress situations, including DNA damage. A subclass of the Bcl-2 protein family termed BH3-only proteins (see below) becomes activated after an internal stress stimulus and translocates to the outer mitochondrial membrane (OMM), where they orchestrate a process called mitochondrial outer membrane permeabilization (MOMP). As an outcome of this process, pores are formed in the OMM, membrane integrity is lost, and contents of the intermembrane space gain access to the cytosol. One of the molecules that is rapidly released to the cytosol is cytochrome c, which is normally a soluble electron carrier between respiratory complexes III and IV. Together with the proapoptotic cytosolic factor APAF1, cytochrome c assembles into a caspase-activating complex termed the “apoptosome.” This complex subsequently activates caspase 9, which is able to cleave caspases 3 and 7, proceeding with the same downstream cascade as in the extrinsic pathway. Other intermembrane space proteins also contribute to cell death after being released into the cytosol (e.g., SMAC/Diablo, which blocks the caspase inhibitor protein XIAP).Remarkably, the two pathways are not completely independent. Cross talk between the extrinsic and intrinsic pathways exists because of caspase 8-dependent cleavage of the BH3-only protein Bid. Upon cleavage, Bid becomes activated, and the truncated version, tBid, translocates to the surface of mitochondria to induce MOMP. In so-called type II cells, this mitochondrial feedback loop is needed to induce apoptosis through the extrinsic pathway, because of the requirement of XIAP antagonism by SMAC.The loss of OMM integrity caused by MOMP is usually considered the point of no return in the whole process, because cells are committed to die once MOMP is initiated. Therefore, this process represents a major checkpoint of apoptosis and must be tightly controlled to ensure that it is initiated at the right time and place. The main molecular players of MOMP belong to the Bcl-2 protein family. Integration of proapoptotic and antiapoptotic signals by the network of Bcl-2 proteins determines whether or not the OMM is permeabilized. In the following sections, we describe in detail the stimulatory and inhibitory protein–protein interactions within this family, discussing various models of how the MOMP effectors, Bax and Bak, become activated. Furthermore, we focus on the actual event of membrane permeabilization, summarizing the current understanding of how pores are formed in the OMM by Bax and Bak oligomers.     

THE CELLULAR IMMUNE RESPONSE OF DAPHNIA MAGNA UNDER HOST–PARASITE GENETIC VARIATION AND VARIATION IN INITIAL DOSE

In invertebrate–parasite systems, the likelihood of infection following parasite exposure is often dependent on the specific combination of host and parasite genotypes (termed genetic specificity). Genetic specificity can maintain diversity in host and parasite populations and is a major component of the Red Queen hypothesis. However, invertebrate immune systems are thought to only distinguish between broad classes of parasite. Using a natural host–parasite system with a well‐established pattern of genetic specificity, the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa, we found that only hosts from susceptible host–parasite genetic combinations mounted a cellular response following exposure to the parasite. These data are compatible with the hypothesis that genetic specificity is attributable to barrier defenses at the site of infection (the gut), and that the systemic immune response is general, reporting the number of parasite spores entering the hemocoel. Further supporting this, we found that larger cellular responses occurred at higher initial parasite doses. By studying the natural infection route, where parasites must pass barrier defenses before interacting with systemic immune responses, these data shed light on which components of invertebrate defense underlie genetic specificity.     

Analysis of the substrate specificity of α-L-arabinofuranosidases by DNA sequencer-aided fluorophore-assisted carbohydrate electrophoresis

Applied Microbiology and Biotechnology - Carbohydrate-active enzyme discovery is often not accompanied by experimental validation, demonstrating the need for techniques to analyze substrate...     

Variation in Apical Hook Length Reflects the Intensity of Sperm Competition in Murine Rodents

Background

Post-copulatory sexual selection has been shown to shape morphology of male gametes. Both directional and stabilizing selection on sperm phenotype have been documented in vertebrates in response to sexual promiscuity.

Methodology

Here we investigated the degree of variance in apical hook length and tail length in six taxa of murine rodents.

Conclusions

Tail sperm length and apical hook length were positively associated with relative testis mass, our proxy for levels of sperm competition, thus indicating directional post-copulatory selection on sperm phenotypes. Moreover, our study shows that increased levels of sperm competition lead to the reduction of variance in the hook length, indicating stabilizing selection. Hence, the higher risk of sperm competition affects increasing hook length together with decreasing variance in the hook length. Species-specific post-copulatory sexual selection likely optimizes sperm morphology.     

Structural and Physiological Analyses of the Alkanesulphonate-Binding Protein (SsuA) of the Citrus Pathogen Xanthomonas citri

Background

The uptake of sulphur-containing compounds plays a pivotal role in the physiology of bacteria that live in aerobic soils where organosulfur compounds such as sulphonates and sulphate esters represent more than 95% of the available sulphur. Until now, no information has been available on the uptake of sulphonates by bacterial plant pathogens, particularly those of the Xanthomonas genus, which encompasses several pathogenic species. In the present study, we characterised the alkanesulphonate uptake system (Ssu) of Xanthomonas axonopodis pv. citri 306 strain (X. citri), the etiological agent of citrus canker.

Methodology/Principal Findings

A single operon-like gene cluster (ssuEDACB) that encodes both the sulphur uptake system and enzymes involved in desulphurisation was detected in the genomes of X. citri and of the closely related species. We characterised X. citri SsuA protein, a periplasmic alkanesulphonate-binding protein that, together with SsuC and SsuB, defines the alkanesulphonate uptake system. The crystal structure of SsuA bound to MOPS, MES and HEPES, which is herein described for the first time, provides evidence for the importance of a conserved dipole in sulphate group coordination, identifies specific amino acids interacting with the sulphate group and shows the presence of a rather large binding pocket that explains the rather wide range of molecules recognised by the protein. Isolation of an isogenic ssuA-knockout derivative of the X. citri 306 strain showed that disruption of alkanesulphonate uptake affects both xanthan gum production and generation of canker lesions in sweet orange leaves.

Conclusions/Significance

The present study unravels unique structural and functional features of the X. citri SsuA protein and provides the first experimental evidence that an ABC uptake system affects the virulence of this phytopathogen.