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A Wegner

Publications and source records attributed to A Wegner.

At least 73 records · Page 4Linked to original sources
PubMed1983

Similar affinities of ADP and ATP for G-actin at physiological salt concentrations.

The equilibrium constant for the exchange of ATP and ADP at G-actin was determined by fluorimetric titration of G-actin-bound epsilon-ATP by ATP or ADP. The affinity of ATP for G-actin was found to be only about 3-fold higher than the affinity of ADP for G-actin at 37 degrees C, pH 7.5 and physiologically relevant salt concentrations (100 mmol K+/l, 0.8 mmol Mg2+/l, less than 0.01 mmol Ca2+/l).

Actins↗
PubMed1983

Treadmilling of actin.

Actin filaments can assemble at the barbed end and disassemble simultaneously at the pointed end. A higher monomer concentration is required to balance the association of actin monomers and the dissociation of filament subunits at the pointed end than at the barbed end. This treadmilling reaction or disparity of the apparent affinity of the two ends for monomers is caused by a continuous hydrolysis of adenosine triphosphate occurring during the association of a monomer with a filament end. In this article, in vitro investigations on treadmilling are reviewed and emerging physiological implications are discussed.

Actins↗
PubMed1983

12-fold difference between the critical monomer concentrations of the two ends of actin filaments in physiological salt conditions.

We determined the critical monomer concentrations at which association and dissociation reactions are balanced at the two ends of actin filaments. For measurement of the critical concentration of the pointed end, interference with the high dynamics of the barbed end was excluded by capping the barbed ends with an actin filament capping protein isolated from bovine brain. The critical concentration of the pointed end (1.5 microM) was found to be 12- to 15-fold higher than the critical concentration of the barbed end (0.10-0.12 microM) at a temperature of 37 degrees C and physiological salt concentrations (100 mM KCl/1-2 mM MgCl2/0.3 mM EGTA or 0.2 mM CaCl2, pH 7.5).

Actins↗
PubMed1982

Fragmentation of actin filaments.

The kinetics of actin polymerization were analyzed by taking into account nucleation, elongation, and spontaneous fragmentation of filaments. Polymerization curves measured in the presence of potassium (40 mM) were found to be in good agreement with curves calculated for the assumption that nucleation and elongation but no fragmentation reactions occur. Polymerization curves measured in the presence of calcium (1.8 mM) or magnesium (0.6 mM MgCl2 and 0.5 mM EGTA) could only be stimulated by calculated curves when spontaneous fragmentation was assumed to occur in addition to nucleation and elongation. The experiments reported in this study that even in the absence of ultrasonication or shear forces actin filaments may break spontaneously and that the extent of fragmentation depends strongly on the experimental conditions. Spontaneous fragmentation changes the shape of the polymerization curves significantly. When fragmentation of filaments takes place, a relatively long lag phase of polymerization is observed that is followed by a strongly increasing polymerization rate to reach the final constant value quickly. On the other hand, when filaments are formed exclusively by nucleation, the polymerization curves approach the final constant value slowly after a relatively short initial lag phase.

Actins↗
PubMed1981

Interaction of tropomyosin-troponin with actin filaments.

The assembly of actin filaments with tropomyosin-troponin was investigated by means of light scattering. Binding curves of tropomyosin-troponin [consisting of all three subunits (holotroponin)] and of tropomyosin-troponin-T-I to actin filaments were analyzed by separating the affinity of tropomyosin-troponin for actin filaments and the affinity for the end-to-end contact of tropomyosin molecules. Under the experimental conditions (42.4 degrees C, 300 mM KCl), tropomyosin-holotroponin in the absence of calcium and tropomyosin-troponin-T-I had similar affinities for actin filaments whereas tropomyosin-holotroponin in the presence of calcium was found to bind more weakly. Tropomyosin-holotroponin and tropomyosin-troponin-T-I bound about 200-300-fold more strongly to binding sites with adjacent tropomyosin-troponin units than to isolated sites on actin filaments. The equilibrium constant for isolated association with actin filaments was more than 2-fold higher for tropomyosin-holotroponin in the absence of calcium (15 400 M-1) and tropomyosin-troponin-T-I (17 500 M-1) than for tropomyosin-holotroponin in the presence of calcium (6600 M-1). Binding curves of mixtures of tropomyosin-holotroponin in the presence of calcium and of tropomyosin-troponin-T-I were measured and analyzed on the basis of a model of cooperative binding of two types of large ligands to a one-dimensional homogeneous lattice. The results provided information on the strength of the end-to-end contacts of tropomyosin-troponin units in different positions on an actin filament. It was found that a tropomyosin-troponin unit binds adjacently to another unit in a different position on an actin filament about 2-fold more weakly than adjacent to a unit in the same position. With the aid of these results, it was possible to obtain information of the equilibrium distribution of tropomyosin-troponin in the two positions on actin filaments. Generation of a sequence of tropomyosin-troponin units in a different position on actin filaments was found to be 4-fold less favored than elongation of an existing sequence (cooperativity parameter sigma = 1/4). Shifting of tropomyosin-troponin on actin filaments appears to be accompanied by small free-energy changes in the various interactions of the components of actin-tropomyosin-troponin filaments and not to be an all-or-none reaction

Actins↗
PubMed1980

Effect of the state of oxidation of cysteine 190 of tropomyosin on the assembly of the actin-tropomyosin complex.

Tropomyosin, cross-linked at cysteine 190, was found to bind more weakly to actin filaments than uncross-linked tropomyosin. Cross-linking of tropomyosin can cause actin filaments nearly completely covered with tropomyosin to be uncovered almost completely. The critical monomer concentration of actin is not significantly changed by binding of cross-linked or uncross-linked tropomyosin to actin filaments. The binding curves were analyzed quantitatively, thereby taking into account the polar end-to-end contact of tropomyosin molecules bound by actin and the overlap of the seven subunit binding sites along the actin filament. Under the conditions of the experiment (80 mM KCl, 1 mM MgCl2, pH 7.5, 38-42 degrees C), the equilibrium constant for isolated binding of tropomyosin to actin filaments is in the range 1 x 10(3)-3 x 10(3) M-1. The equilibrium constants for binding of tropomyosin to binding sites along the actin filament with one or two neighbouring tropomyosin molecules are in the range of 10(6) or 10(8) to 10(9) M-1, respectively. The equilibrium constants for binding of tropomyosin to binding sites along the actin filament with one or two neighbouring tropomyosin molecules are in the range of 10(6) or 10(8) to 10(9) M-1, respectively. The equilibrium constants for cross-linked and uncross-linked tropomyosin differ by a factor of only about two. Owing to the highly cooperative binding, these differences are sufficient so that actin filaments nearly completely covered with uncross-linked tropomyosin are uncovered almost completely by cross-linking tropomyosin at cysteine 190.

Actins↗
PubMed1980

[Influence of chronic lead exposure on resistence to bacterial infection (author's transl)].

Suppression by lead of resistance to bacterial or viral infections has been reported by several authors. We have studied, if a decrease of resistance to bacterial infection could be evaluated at blood lead concentrations (PbB), which correspond to the upper levels of environmental or occupational lead exposure regarded as tolerable (PbB = 35 resp. 60 microgram/100 ml). NMRI mice were chronically exposed to lead by feeding with lead acetate containing diets and given a challenge with Salmonella typhimurium. No increase of susceptibility to bacterial infection could be demonstrated at PbB < 90 microgram/100 g. At PbB > 100 microgram/100 g, however, an increase of lethality and a decrease of 50% survival times could be observed after bacterial infection.

Animals↗
PubMed1977

The polymerization reaction of muscle actin.

Recent advances in the studies of the aggregation of G-actin monomers, containing one molecule of ATP, to long filaments of F-actin, with a concomitant hydrolysis of the nucleotide to ADP, are reviewed. With the aid of omega-ATP, the association and dissociation rate constant of the nucleotide could be determined. The binding of the nucleotide is enhanced by the binding of one Ca++ ion, probably at a different site. The delta G value of the Mg++ or Ca++ induced polymerization has been determined to --39 to--59 kJ/mole, the critical protein concentration for the ATP-G-actin to ADP-F-actin conversion is very strongly influenced by the concentration of bivalent cations. The rate constants of the protein monomers, and the rate and equilibrium constants for the propagation step show the process to be extremely cooperative. Actin shows the interesting phenomenon of translocational head-to-tail polymerization, which may be regulated by ATP. The contact sites between the monomers in F-actin have been labeled by chemical modification. Two tryosine residues, 53 and 69, are probably close to one of the two sites. The ATP binding sites has been labeled by an ATP analog, and there is evidence that it is close to the contact site.

Actins↗
PubMed1977

The mechanism of ATP hydrolysis by polymer actin.

The rate of actin polymerization, the rate of nucleotide splitting and the rate of the nucleotide exchange have been measured simultaneously. Correlation of these three measurements demonstrated that nucleotide splitting and exchange were mainly connected with the association and dissociation reactions of actin protomers at the ends of actin filaments and were not caused by release and rebinding of nucleotide molecules at the binding sites along the filament. The observation made by others that the nucleotide exchange was accelerated in the presence of ATP was explained by the translocational head-to-tail polymerization of actin: Due to the simultaneous lengthening of the filament at one end and shortening at the other, nucleotide molecules are incorporated at one end and released at the other. In the absence of ATP, where the head-to-tail polymerization mechanism was not operative nucleotide exchange was brought about by the slow process of length fluctuation of polymers.

Actins↗