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Diphtheria toxin (DT) and its N-terminal fragment A (FA) catalyse the transfer of the ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD) into a covalent linkage with eukaryotic elongation factor 2 (eEF2). DT-induced cytotoxicity is versatile, and it includes DNA cleavage and the depolymerisation of actin filaments. The inhibition of the ADP-ribosyltransferase (ADPrT) activity of FA did not affect the deoxyribonuclease activity of FA or its interaction with actin. The toxin entry rate into cells (HUVEC) was determined by measuring the ADP-ribosyltransferase activity. DT uptake was nearly 80% after 30 min. The efficiency was determined as Km = 2.2 nM; Vmax = 0.25 pmol.min−1. The nuclease activity was tested with hyperchromicity experiments, and it was concluded that G-actin has an inhibitory effect on DT nuclease activity. In thepresence of DT and mutant of diphtheria toxin (CRM197), F-actin depolymerisation was determined with gel filtration, WB and fluorescence techniques. In the presence of DT and CRM197, 60–65% F-actin depolymerisation was observed. An in vitro FA-actin interaction and F-actin depolymerisation were reported in our previous paper. The present study thus confirms the depolymerisation of actin cytoskeleton in vivo.
Kinetics of the smooth muscle calponin-F-actin interaction was studied by stopped- flow measurements of light scattering and fluorescence intensity of pyrene-labelled F-actin. The intensity and character of the changes in light scattering, and thus the mode of calponin binding to actin filaments leading to changes in their shape and bun­dling, depend on the molar ratio of the two proteins. Parallel measurements of pyrene-fluorescence quenching upon calponin binding revealed that intrinsic conformational changes in actin filaments are delayed relative to the binding process and are not markedly influenced by the mode of calponin binding. Bundling of actin filaments by calponin was not correlated with fluorescence changes and thus with al­terations in the structure of actin filaments.
The effect of triethyllead (TriEL) on the morphology and motile activity of Walker 256 carcinosarcoma cells was investigated. It was found that both 2 and 5 μM TriEL affected the cellular motility in a dose- and time- dependent manner. Initially, 2 μM TriEL caused the formation of blebs instead of lamellipodia at the front of some cells and stimulated the migration of Walker cells, but after 2 hours of 2 μM TriEL treatment, a reduction of cellular motility was observed. In the presence of 5 μM TriEL, Walker 256 carcinosarcoma cells rounded up, and their rate of movement was reduced. Moreover, the treatment of Walker carcinosarcoma cells with TriEL caused the disruption of microtubules and affected the F-actin distribution at both concentrations. At a concentration of 2 μM TriEL, the actin staining intensity was greatest in the tail of front-tail polarised blebbing cells and the actin layer was very thin at the leading edge. The control cells showed linear cortical F-actin distribution and somewhat less intense cytoplasmic staining at the same TriEL concentration. Cells treated with 5 μM TriEL showed an under-membrane pattern of actin distribution.
Myosin can be precipitated from soluble fraction under different assay conditions. This paper describes a new method for precipitating myosin V from rat brain soluble fraction. Brains were homogenized in 50 mM imidazole/HCl buffer, pH 8.0, containing 10 mM EDTA/EGTA, 250 mM sucrose, 1 mM DTT and 1 mM benzamidine, centrifuged at 45000 × g for 40 min and the supernatant was frozen at -20 °C. Forty-eight hours later, the supernatant was thawed, centrifuged at 45000 × g for 40 min and the precipitate was washed in 20 mM imidazole buffer pH 8.0. SDS/PAGE analysis showed four polypeptides in the precipitate: 205, 150, 57 and 43 kDa. The precipitate presented high Mg2+-ATPase activity, which co-purifies with p205. This polypeptide was recognized by a specific myosin V antibody and was proteolised by calpain, generating two stable polypeptides: p130 and p90. The Mg2+-ATPase activity was not stimulated by calcium in both the absence and presence of exogenous calmodulin and the K+/EDTA-ATPase activity represented 25% of the Mg2+-ATPase activity. In this work, myosin V from rat brain was precipitated by freezing the soluble fraction and was co-purificated with a 45 kDa polypeptide.
The actin cytoskeleton of endosperm and of the mature endosperm chalazal haustorium cell of Rhinanthus serotinus was examined by immunohistochemistry and epifluorescence microscopy. A prominent actin cytoskeleton composed of numerous cross-linked filaments is present at the distal pole of the chalazal haustorium cell. Thick, longitudinally oriented bundles of microfilaments localize in transvacuolar cytoplasmic strands. A meshwork of delicate actin filaments surrounds the large polytene nuclei; some of the filaments radiate from the nuclear envelopes. Abundant and clearly visible actin filaments also occur at the proximal pole of the haustorium cell. A network of microfilaments in cortical cytoplasm and F-actin arrays associated with nuclei are found in endosperm proper cells.
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