Phosphorus availability and alkaline phosphatase activities in two Israeli fishponds

1. Alkaline phosphatase activities and the release of orthophosphate from endogenous substrates by these enzymes were measured in waters from two commercial fishponds in the watershed area of Lake Kinneret (Northern Israel). These data were compared with results from the lake at seasons of adequate or limited phosphorus supply. In the fishponds, high Relative Phosphatase Activity ratios (>2) and relatively large amounts of orthophosphate extracted from plankton by autoclaving (average 27% of readily available phosphorus) indicated adequate, or even excess, levels of phosphorus availability despite elevated pond productivity (2 to 3 tons carp/ha/yr). Therefore, we suggest that decreasing routine phosphorus fertilization of these ponds would not affect overall productivity but would eventually lower the amounts of phosphorus reaching Lake Kinneret.
2. In general, the R. P. A. ratio may be a useful index to evaluate phosphorus availability for a wide range of natural waters. Values for this ratio of <1 and >2 appear indicative of limited or adequate phosphorus availability respectively.
3. Three sources of orthophosphate, (Pi), readily available to phytoplankton, are indicated: (1) enzymatically released Pi, (2) Pi in intracellular pools and (3) Pi initially present in the water. Although the first source is always important, relatively greater amounts of Pi are contributed by the other fractions in situations of plentiful phosphorus availability.
4. Activity of free dissolved phosphatases was found in filtered samples of fishpond water. However, neither these enzymes or added phosphatases released significant amounts of Pi from the dissolved organic phosphorus compounds in the filtered water.

     

A sensitive radioisotope assay for pectin methyl esterase activity

The preparation of polygalacturonic acid [¹⁴C]-labeled methyl ester (pectic acid ester) is described. This labeled polysaccharide is employed as the substrate in a simple, sensitive, and rapid assay procedure for measuring pectin methyl esterase activity.     

Two distinct sites of interaction form the calponin: gelsolin complex and two calcium switches control its activity

Gelsolin and calponin are well characterized actin-binding proteins that form a tight gelsolin:calponin complex (GCC). We show here that the GCC is formed through two distinct interfaces. One of these is formed between 144-182 of calponin and 25-150 of gelsolin (G1). The second is a calcium-sensitive site centred on calponin's CH domain, and the C-terminal half of gelsolin (G4-6). The behaviour of this second interface is dependent on the presence of calcium and so it is possible that potential GCC-binding partners may be selected by calcium availability. Actin is one such GCC-binding partner and we show that a larger complex is formed with monomeric actin in calcium. The stoichiometry of this complex is determined to be 1 gelsolin/1 calponin/2 G-actins (GCA(2)). Both actin monomers bind the GCC through gelsolin. Both calponin and gelsolin are reported to play signaling roles in addition to their better-characterized actin-binding properties and it is possible that the GCC regulates both of these functions.     

Axis development: the mouse becomes a dachshund.

Targeted deletion of the gene for GDF11, a novel member of the TGFbeta family, has been found to cause an increase in the number of thoracic and lumbar vertebrae in the mouse. This is the first hint that a secreted factor may influence the specification of segment identity.     

Capping and dynamic relation between domains 1 and 2 of gelsolin

Gelsolin is a protein that severs and caps actin filaments. The two activities are located in the N-terminal half of the gelsolin molecules. Severing and subsequent capping requires the binding of domains 2 and 3 (S2–3) to the side of the filaments to position the N-terminal domain 1 (S1) at the barbed end of actin (actin subdomains 1 and 3). The results provide a structural basis for the gelsolin capping mechanism. The effects of a synthetic peptide derived from the sequence of a binding site located in gelsolin S2 on actin properties have been studied. CD and IR spectra indicate that this peptide presented a secondary structure in solution which would be similar to that expected for the native full length gelsolin molecule. The binding of the synthetic peptide induces conformational changes in actin subdomain 1 and actin oligomerization. An increase in the polymerization rate was observed, which could be attributed to a nucleation kinetics effect. The combined effects of two gelsolin fragments, the synthetic peptide derived from an S2 sequence and the purified segment 1 (S1), were also investigated as a molecule model. The two fragments induced nucleation enhancement and inhibited actin depolymerization, two characteristic properties of capping. In conclusion, for the first time it is reported that the binding of a small synthetic fragment is sufficient to promote efficient capping by S1 at the barbed end of actin filaments. ©1998 European Peptide Society and John Wiley & Sons, Ltd.     

Identification of amino acid residues within the N-terminal domain of EspA that play a role in EspA filament biogenesis and function

Enteropathogenic Escherichia coli employs a filamentous type III secretion system, made by homopolymerization of the translocator protein EspA. In this study, we have shown that the N-terminal region of EspA has a role in EspA's protein stability, interaction with the CesAB chaperone, and filament biogenesis and function.