Effects of fire on sward structure and grazing in western Serengeti, Tanzania

In Serengeti fire is used as a management tool to improve the forage quality for large herbivores. However, little is known of the effects of fire on grazing resources particularly sward structure, its influence on herbivore forage patch selection and utilization to the relative amount of phytomass consumed in burnt and nonburnt patches. From September 2003 to July 2004, consumption of phytomass by large herbivores was assessed with eight samplings in six grassland sites in the Western Corridor in Serengeti National Park. Each site had burnt and nonburnt plots. Movable cages were used to exclude grazing between samplings and plant material harvests were used to assess phytomass and sward structure changes in time. Nonburnt grasslands had consistently larger phytomass at all sampling events whereas the ratio for live leaf/total phytomass was higher in burnt grassland at early postfire stages, but declined later in the season. Moreover, periodic consumption of both total phytomass and different phytomass components shifted between burnt and nonburnt grasslands, but there were also large site-specific responses. The shift appears to be related to the balance between the amount of phytomass available and the quality of the forage in terms of the ratio between live and total phytomass. The study highlights the significance of maintaining mosaics of burnt and nonburnt areas with an adequate provision of forage amount and quality all year round.     

Is Ffh required for export of secretory proteins?

In Escherichia coli, protein export from the cytoplasm may occur via the signal recognition particle (SRP)-dependent pathway or the Sec-dependent pathway. Membrane proteins utilize the SRP-dependent route, whereas many secretory proteins use the cytoplasmic Sec machinery. To examine the possibility that signal peptide hydrophobicity governs which targeting route is utilized, we used a series of PhoA signal sequence mutants which vary only by incremental hydrophobicity changes. We show that depletion of SRP, but not trigger factor, affects all the mutants examined. These results suggest secretory proteins with a variety of signal sequences, as well as membrane proteins, require SRP for export.     

Stereochemistry of the peroxisomal branched-chain fatty acid alpha- and beta-oxidation systems in patients suffering from different peroxisomal disorders

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid derived from dietary sources and broken down in the peroxisome to pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) via alpha-oxidation. Pristanic acid then undergoes beta-oxidation in peroxisomes. Phytanic acid naturally occurs as a mixture of (3S,7R,11R)- and (3R,7R,11R)-diastereomers. In contrast to the alpha-oxidation system, peroxisomal beta-oxidation is stereospecific and only accepts (2S)-isomers. Therefore, a racemase called alpha-methylacyl-CoA racemase is required to convert (2R)-pristanic acid into its (2S)-isomer. To further investigate the stereochemistry of the peroxisomal oxidation systems and their substrates, we have developed a method using gas-liquid chromatography-mass spectrometry to analyze the isomers of phytanic, pristanic, and trimethylundecanoic acid in plasma from patients with various peroxisomal fatty acid oxidation defects. In this study, we show that in plasma of patients with a peroxisomal beta-oxidation deficiency, the relative amounts of the two diastereomers of pristanic acid are almost equal, whereas in patients with a defect of alpha-methylacyl-CoA racemase, (2R)-pristanic acid is the predominant isomer. Furthermore, we show that in alpha-methylacyl-CoA racemase deficiency, not only pristanic acid accumulates, but also one of the metabolites of pristanic acid, 2610-trimethylundecanoic acid, providing direct in vivo evidence for the requirement of this racemase for the complete degradation of pristanic acid.     

A Drosophila homolog of cyclase-associated proteins collaborates with the Abl tyrosine kinase to control midline axon pathfinding

We demonstrate that Drosophila capulet (capt), a homolog of the adenylyl cyclase-associated protein that binds and regulates actin in yeast, associates with Abl in Drosophila cells, suggesting a functional relationship in vivo. We find a robust and specific genetic interaction between capt and Abl at the midline choice point where the growth cone repellent Slit functions to restrict axon crossing. Genetic interactions between capt and slit support a model where Capt and Abl collaborate as part of the repellent response. Further support for this model is provided by genetic interactions that both capt and Abl display with multiple members of the Roundabout receptor family. These studies identify Capulet as part of an emerging pathway linking guidance signals to regulation of cytoskeletal dynamics and suggest that the Abl pathway mediates signals downstream of multiple Roundabout receptors.     

Axon sorting in the optic tract requires HSPG synthesis by ext2 (dackel) and extl3 (boxer)

Retinal ganglion cell (RGC) axons are topographically ordered in the optic tract according to their retinal origin. In zebrafish dackel (dak) and boxer (box) mutants, some dorsal RGC axons missort in the optic tract but innervate the tectum topographically. Molecular cloning reveals that dak and box encode ext2 and extl3, glycosyltransferases implicated in heparan sulfate (HS) biosynthesis. Both genes are required for HS synthesis, as shown by biochemical and immunohistochemical analysis, and are expressed maternally and then ubiquitously, likely playing permissive roles. Missorting in box can be rescued by overexpression of extl3. dak;box double mutants show synthetic pathfinding phenotypes that phenocopy robo2 mutants, suggesting that Robo2 function requires HS in vivo; however, tract sorting does not require Robo function, since it is normal in robo2 null mutants. This genetic evidence that heparan sulfate proteoglycan function is required for optic tract sorting provides clues to begin understanding the underlying molecular mechanisms.     

Selective photoaffinity labeling identifies the signal peptide binding domain on SecA

SecA, an ATPase crucial to the Sec-dependent translocation machinery in Escherichia coli, recognizes and directly binds the N-terminal signal peptide of an exported preprotein. This interaction plays a central role in the targeting and transport of preproteins via the SecYEG channel. Here we identify the signal peptide binding groove (SPBG) on SecA addressing a key issue regarding the SecA-preprotein interaction. We employ a synthetic signal peptide containing the photoreactive benzoylphenylalanine to efficiently and specifically label SecA containing a unique Factor Xa site. Comparison of the photolabeled fragment from the subsequent proteolysis of several SecAs, which vary only in the location of the Factor Xa site, reveals one 53 residue segment in common with the entire series. The covalently modified SecA segment produced is the same in aqueous solution and in lipid vesicles. This spans amino acid residues 269 to 322 of the E. coli protein, which is distinct from a previously proposed signal peptide binding site, and contributes to a hydrophobic peptide binding groove evident in molecular models of SecA.     

Overexpression of the Large-Conductance,Ca2+-Activated K+ (BK) Channel Shortens Action Potential Duration in HL-1 Cardiomyocytes

Long QT syndrome is characterized by a prolongation of the interval between the Q wave and the T wave on the electrocardiogram. This abnormality reflects a prolongation of the ventricular action potential caused by a number of genetic mutations or a variety of drugs. Since effective treatments are unavailable, we explored the possibility of using cardiac expression of the large-conductance, Ca²⁺-activated K⁺ (BK) channel to shorten action potential duration (APD). We hypothesized that expression of the pore-forming α subunit of human BK channels (hBKα) in HL-1 cells would shorten action potential duration in this mouse atrial cell line. Expression of hBKα had minimal effects on expression levels of other ion channels with the exception of a small but significant reduction in Kv11.1. Patch-clamped hBKα expressing HL-1 cells exhibited an outward voltage- and Ca²⁺-sensitive K⁺ current, which was inhibited by the BK channel blocker iberiotoxin (100 nM). This BK current phenotype was not detected in untransfected HL-1 cells or in HL-1 null cells sham-transfected with an empty vector. Importantly, APD in hBKα-expressing HL-1 cells averaged 14.3 ± 2.8 ms (n = 10), which represented a 53% reduction in APD compared to HL-1 null cells lacking BKα expression. APD in the latter cells averaged 31.0 ± 5.1 ms (n = 13). The shortened APD in hBKα-expressing cells was restored to normal duration by 100 nM iberiotoxin, suggesting that a repolarizing K⁺ current attributed to BK channels accounted for action potential shortening. These findings provide initial proof-of-concept that the introduction of hBKα channels into a cardiac cell line can shorten APD, and raise the possibility that gene-based interventions to increase hBKα channels in cardiac cells may hold promise as a therapeutic strategy for long QT syndrome.