Mechanism of ATP hydrolysis by polymeric actin

The lag between polymerization of actin and ATP hydrolysis in actin filaments was analyzed in terms of the mechanism of the hydrolysis reaction. Under the experimental conditions (100 mM KCl and 1 mM MgCl2, or without KCl, 1 mM MgCl2 and 0.4 mM EGTA, 25 degrees C) ATP hydrolysis lagged behind polymerization by about 100 s independently of the concentration of polymerizing filament ends and of the actin monomer concentration. Three models of ATP hydrolysis were compared to experimental data: (i) Random ATP hydrolysis, ATP is assumed to be hydrolyzed at a rate that is independent of the type of the nucleotide bound to adjacent filament subunits. (ii) Cooperative hydrolysis, the rate of ATP hydrolysis is thought to depend on the type of nucleotide bound to adjacent subunits. (iii) Sequential hydrolysis, ATP is assumed to be hydrolyzed only at the interface between ATP-subunits and ADP-subunits. The model of sequential ATP hydrolysis could be excluded. The results were in agreement with random or cooperative ATP hydrolysis. The differences of the rates of ATP hydrolysis by a random or cooperative mechanism are so small that based on the experimental results no distinction between these two mechanisms could be made. All available evidence points towards a mechanism of ATP hydrolysis in which several or perhaps many interfaces between ATP- and ADP-subunits are formed within a filament.

Nucleation of actin polymerization by gelsolin

The time-course of assembly of actin with gelsolin was measured by the fluorescence increase of a fluorescent label covalently linked to actin. The actin concentrations ranged from values far below the critical concentration to values above the critical concentration of the pointed ends of actin filaments. If the concentration of actin was in the range of the critical monomer concentration (0.64 microM), the time-course of the concentration of actin assembled with gelsolin revealed a sigmoidal shape. At higher actin concentrations the time-course of association of actin with gelsolin approximated an exponential curve. The measured time-courses of assembly were quantitatively interpreted by kinetic rate equations. A poor fit was obtained if two actin molecules were assumed to bind to gelsolin to form a 1:2 gelsolin-actin complex and subsequently further actin molecules were assumed to polymerize onto the 1:2 gelsolin-actin complex toward the pointed end. A considerably better agreement between calculated and measured time-courses was achieved if additional creation of actin filaments by fast fragmentation of newly formed actin filaments by not yet consumed gelsolin was assumed to occur. This suggests that both polymerization of actin onto gelsolin and fragmentation of actin filaments contribute to formation of new actin filaments by gelsolin. Furthermore it could be demonstrated that below the critical monomer concentration appreciable amounts of actin are incorporated into gelsolin-actin oligomers.

Intestinal absorption and malnutrition in patients with the acquired immunodeficiency syndrome (AIDS)

The relation of small intestinal dysfunction and malnutrition (body composition and serum index parameters of nutrition) was investigated in 36 male patients with AIDS. Mucosal absorptive capacity was assessed by the 25 g D-xylose test. D-xylose absorption (2 h - serum profile and 5 h - urine) classified 17 patients as having impaired and 19 patients as having normal absorption. In both groups body weight, body mass index as well as body composition analysis indicated malnutrition when compared to healthy male controls (n = 340) or asymptomatic HIV-infected patients (n = 26). Patients with abnormal D-xylose test had more severe malnutrition indicated by a lower body cell mass (17.7 +/- 5.4 vs. 22.5 +/- 4.5 kg; p < 0.01) and an increased ratio of extracellular mass to body cell mass (1.99 +/- 0.82 vs. 1.45 +/- 0.46 p < 0.01). Total serum protein, albumin, cholinesterase activity, cholesterol and LDL were significantly diminished in AIDS-patients with abnormal D-xylose test compared to those with normal D-xylose absorption. Intestinal dysfunction indicated by decreased D-xylose Intestinal dysfunction indicated by decreased D-xylose absorption thus represents an important feature of malnutrition and wasting, and patients with abnormal D-xylose absorption have more profound impairment of body composition, visceral proteins and lipids reflecting malnutrition than patients with unaffected intestinal absorption.

[Primary manifestation of Burkitt's lymphoma of the non-African type in the orbits]

BACKGROUND

Primary orbital involvement in non-African Burkitt's lymphoma does not occur frequently but when it occurs children are mostly affected.

PATIENT AND METHOD

A 83-year-old woman with a primary manifestation of non-African Burkitt's lymphoma in the orbit was presented in our out-patient department.

RESULTS

After radiation therapy no recurrence after 6 months.

CONCLUSION

To our knowledge, this is the first reported adult patient with definite primary involvement of the orbit in non-African Burkitt's lymphoma.

Strategies for nonradioactive methods in the localization of phosphorylated amino acids in proteins

Identification of O-phosphorylated amino acids within the primary structure of regulatory proteins is important in understanding the mechanisms by which their functions are regulated. In many cases radioactive labeling with [32P]phosphate is tedious or sometimes impossible. Therefore, we have established a series of new non-radioactive methods that permit the localization of phosphoserine, phosphothreonine, and phosphotyrosine. After partial hydrolysis of a phosphopeptide or phosphoprotein, phosphoserine, phosphothreonine, or phosphotyrosine are determined by capillary electrophoresis as their dabsyl-derivatives. Chemical modification transforms phosphoserine or phosphothreonine to S-ethyl-cysteine or beta-methyl-S-ethyl-cysteine, respectively, allowing their localization during sequence analysis. We apply solid-phase sequencing to overcome the limitations of the gas-phase sequenator in the case of phosphotyrosine-containing peptides. Liquid chromatography on-line connected to an electrospray mass spectrometer is a powerful new method of increasing importance in the protein chemistry field. It is especially well suited for identification of phosphoserine- or phosphothreonine-containing peptides in a proteolytic digest of a phosphoprotein. In this article we will describe how to work with these new methods practically.

Mechanisms of the cytopathic action of actin-ADP-ribosylating toxins

Clostridium botulinum C2 toxin, Clostridium perfringens iota toxin, and Clostridium spiroforme toxin ADP-ribosylate actin monomers. Toxin-induced ADP-ribosylation disturbs the cellular equilibrium between monomeric and polymeric actin and traps monomeric actin in its unpolymerized form, thereby depolymerizing actin filaments and destroying the microfilament network. Furthermore, the toxins ADP-ribosylate gelsolin actin complexes. These modifications may contribute to the cytopathic action of the toxins.

Occurrence of an actin-inserting domain in tensin

We have previously described a protein called "insertin" that binds strongly to barbed ends of actin filaments and permits polymerization of actin filaments by insertion of actin monomers between the barbed ends and barbed end-bound insertin. We determined the amino acid sequence of insertin and found that the primary structure of insertin is almost identical to amino acid residues 862 to 1212 of the actin-binding protein tensin.

Gelsolin binds to polymeric actin at a low rate

Association of gelsolin with actin filament subunits was investigated by the decrease of the fluorescence intensity of a 7-nitro-2-oxa-1,3-diazole (NBD) label covalently linked to gelsolin. The rate constant of this reaction was found to be 4 x 10(3) M-1 s-1. Binding of NBD-labeled gelsolin to monomeric actin proceeds at a similar low rate. The rate of association of gelsolin that was unmodified to actin filament subunits was estimated too. Unmodified gelsolin was added to a mixture of actin filaments and actin-DNase I complex. The fractions of gelsolin that bound to actin filament subunits or to actin-DNase I complex depended on the relative rates of these two competing reactions. In this way it was possible to estimate the rate constant of association of unmodified gelsolin with actin filament subunits (2 x 10(4) M-1 s-1). Thus, gelsolin associates with actin filament subunits at a rate that is considerably slower than diffusion-controlled and similar to the rate of binding of gelsolin to monomeric actin.

ADP-ribosylation of gelsolin-actin complexes by clostridial toxins

ADP-ribosylation of the 1:1 (G-A) and 1:2 (G-A-A) gelsolin-actin complexes by Clostridium perfringens iota toxin and Clostridium botulinum C2 toxin was studied. Iota toxin ADP-ribosylated actin in the G-A complex from human platelets as effectively as skeletal muscle actin. The Km for NAD (4 microM) was identical for both substrates. C2 toxin ADP-ribosylated actin in the G-A complex with lower efficacy than nonmuscle actin from platelet cytosol. In the G-A-A complex both actin molecules were ADP-ribosylated by iota toxin. The G-A complex bound ADP-ribosylated actin (Ar) to form the G-A-Ar complex in which the weakly bound actin is ADP-ribosylated. Vice versa, ADP-ribosylated 1:1 gelsolin-actin complex (G-Ar) was able to bind unmodified actin to yield the G-Ar-A complex. ADP-ribosylation did not change the nucleation activity of either the G-Ar complex or the G-Ar-A complex. When monomeric actin was added to the G-A-Ar complex, polymerization of actin was delayed by about 10 min. According to a quantitative kinetic analysis, the delay of polymerization corresponded to the rate of dissociation of ADP-ribosylated actin from the G-A-Ar complex. This suggests that the nucleation activity of the G-A-A complex is inhibited by ADP-ribosylation of the weakly bound actin and that the inhibition can be removed by dissociation of ADP-ribosylated actin from the G-A-Ar complex.

Rate constants and equilibrium constants for binding of actin to the 1:1 gelsolin-actin complex

The rate constant and equilibrium constant of association of an actin monomer with 1:1 gelsolin-actin complex isolated from chicken were measured by using fluorescently labeled actin. According to fluorescence stopped-flow experiments, the rate constant of formation of the 1:2 gelsolin-actin complex from 1:1 gelsolin-actin complex and actin was found to be about 2 x 10(7) M-1 s-1 under conditions where gelsolin binds Ca2+. The rate of dissociation of one actin molecule from the 1:2 gelsolin-actin complex was determined by exchange of actin for fluorescently labeled actin. The rate constant of dissociation was about 0.02 s-1. Thus, the equilibrium constant for association of actin with 1:1 gelsolin-actin complex can be calculated to be in the range of 10(9) M-1. The rate of dissociation of actin from 1:2 gelsolin-actin complex was independent of the Ca2+ concentration. Ca2+ affects only the rate of association of actin with 1:1 gelsolin-actin complex.

Random copolymerization of ATP-actin and ADP-actin

The equilibrium of the copolymerization of ATP-actin and ADP-actin was investigated by an analysis of the critical concentrations of mixtures of ATP-actin and ADP-actin. The molar ratio of bound ATP to bound ADP was controlled by the ratio of free ATP and ADP. The experiments were performed under conditions (100 mM KCl, l mM MgCl2, pH 7.5, 25 degrees C) where the ATP hydrolysis following binding of actin monomers to barbed filament ends was so slow that the distribution of ATP or ADP bound to the subunits near the ends of filaments was not affected by ATP hydrolysis. According to the analysis of the critical concentrations, the equilibrium constants for incorporation of ATP-actin or ADP-actin into filaments were independent of the type of nucleotide bound to contiguous subunits.

Rate and mechanism of the assembly of tropomyosin with actin filaments

The rate of assembly of tropomyosin with actin filaments was measured by stopped-flow experiments. Binding of tropomyosin to actin filaments was followed by the change of the fluorescence intensity of a (dimethylamino)naphthalene label covalently linked to tropomyosin and by synchrotron radiation X-ray solution scattering. Under the experimental conditions (2 mM MgCl2, 100 mM KCl, pH 7.5, 25 degrees C) and at the protein concentrations used (2.5-24 microM actin, 0.2-3.4 microM tropomyosin) the half-life time of assembly of tropomyosin with actin filaments was found to be less than 1 s. The results were analyzed quantitatively by a model in which tropomyosin initially binds to isolated sites. Further tropomyosin molecules bind contiguously to bound tropomyosin along the actin filaments. Good agreement between the experimental and theoretical time course of assembly was obtained by assuming a fast preequilibrium between free and isolatedly bound tropomyosin.

Binding of sugar phosphates, inositol phosphates and phosphorylated amino acids to actin

Binding of biological phosphate compounds to actin was investigated by the effect of these compounds on the critical concentration of the pointed ends of gelsolin-capped actin filaments. According to this assay millimolar concentrations of glucose 6-phosphate and the bisphosphorylated sugars fructose 1,6-bisphosphate, fructose 2,6-bisphosphate, glucose 1,6-bisphosphate, sedoheptulose 1,7-bisphosphate and 2,3-bisphosphoglycerate were found to associate with actin. Glycerophosphoinositol phosphates bound to actin if they were present in millimolar concentrations, and if carbon atom 4 of the inositol ring was phosphorylated and carbon atom 5 was free of phosphate. Also phosphoserine and phosphotyrosine were found to interact with actin. Most of the actin-binding compounds stabilized actin filaments by decreasing the critical concentration suggesting that these compounds had a higher affinity for the subunits along actin filaments than for actin monomers. However, 2,3-bisphosphoglycerate and fructose 2,6-bisphosphate increased the critical concentration probably because these sugar phosphates bound to actin monomers thereby inhibiting actin polymerization.

Mechanism of the insertion of actin monomers between the barbed ends of actin filaments and barbed end-bound insertin

Insertin, a protein purified from chicken gizzard smooth muscle, has been shown to retard but not to inhibit actin polymerization at the barbed ends of actin filaments. This effect has been explained by a model in which insertin remains bound to the barbed ends of actin filaments and new actin molecules are inserted into filaments between the barbed ends and barbed end-bound insertin molecules. In this paper we discuss the mechanism of the insertion reaction on a molecular level. A number of simple models were devised and were judged by their agreement with available experimental data. In one class of models insertin was assumed to dissociate from filament ends and to re-associate with the ends. Actin monomers would then bind to a filament end between a dissociation and an association reaction of insertin. In one of the two proposed models in this class insertin binds to an ATP-containing terminal subunit with higher affinity than to an ADP-containing terminal subunit. Dissociation of insertin is brought about by ATP hydrolysis at the terminal filament subunit. Insertion was then thought to re-associate with a filament end following binding of an ATP-containing actin monomer to the filament end. In the other of the two models' insertin was assumed to occur in two conformations which bind to filament ends with different affinities. Association and dissociation of insertin is caused by interconversion between the two forms of insertin. Both models turned out to be incompatible with experimental data. All types of models in which retardation of actin polymerization is brought about by dissociation and re-association of insertin with filament ends can be excluded by a common argument. As 10 nM insertin retards polymerization of 2 microns monomeric actin with maximal efficiency, the rate constant of binding of insertin to a filament end must be considerably higher (greater than 2 microM/10 nM = 200-fold). As the rate of association of actin with a barbed end is almost diffusion-controlled, assembly of insertin with a filament end would have to exceed the rate of a diffusion-controlled reaction. In the other class of models it was assumed that insertin remains permanently bound to filament ends during association or dissociation of an actin molecule and to move towards the terminal subunit of filaments. These models are compatible with experimental data. Thus, models are favoured where insertin remains bound to filament ends during polymerization and depolymerization of actin.

De-ADP-ribosylation actin by Clostridium perfringens iota-toxin and Clostridium botulinum C2 toxin

The reverse reaction of the ADP-ribosylation of actin by Clostridium botulinum C2 toxin and Clostridium perfringens iota-toxin was studied. In the presence of nicotinamide (30-50 mM) C2 toxin and iota-toxin decreased the radioactive labeling of [32P]ADP-ribosylated actin and catalyzed the formation of [32P]NAD. The pH optima for both reactions were 5.5-6.0. Concomitant with the removal of ADP-ribose, the ability of actin to polymerize was restored and actin ATPase activity increased. Neither ADP-ribosylation nor removal of ADP-ribose was observed after treatment of actin with EDTA, indicating that the native structure of actin is required for both reactions. ADP-ribosylation of platelet actin by C2 toxin was reversed by iota-toxin, confirming recent reports that both toxins modify the same amino acid in actin. However, C. botulinum C2 toxin was not able to cleave ADP-ribose from skeletal muscle actin which had been incorporated by iota-toxin, corroborating the different substrate specificities of both toxins.

Antibody mapping of functional domains in vinculin

We have analyzed the functional domain structure of vinculin, a protein involved in linking microfilaments to the cytoplasmic face of cell membranes in animal cells. For this purpose, we used several monoclonal antibodies raised against chicken gizzard vinculin whose epitopes could be assigned to discrete regions in the vinculin sequence by immunoblotting of proteolytic fragments combined with N-terminal amino acid sequencing. Two of these antibodies induced the disruption of stress fibers and changed the number of morphology of focal contacts after microinjection in chicken embryo fibroblasts. Based on the location of its epitope in comparison with vinculin domains previously identified by other groups, we propose that one of these antibodies (15B7) interferes with the binding of vinculin to talin, the most peripheral of the microfilament proteins. The second antibody (14C10) binds within a region comprising three internal repeats and might therefore distort the inner architecture of vinculin. A third antibody (As3) inhibited the binding of F-actin to vinculin in an in vitro assay but had no effect on the microfilament system in cells. These data emphasize the role of vinculin as a key protein in microfilament-membrane linkage and support previous work on a direct interaction between vinculin and actin.

Tropomyosin-troponin complex stabilizes the pointed ends of actin filaments against polymerization and depolymerization

In striated muscle the pointed ends of polar actin filaments are directed toward the center of the sarcomere. Formed filaments keep a constant length of about 1 micron. As polymerization and depolymerization at free pointed ends are not sufficiently slow to account for the constant length of the filaments, we searched for proteins which occur in sarcomeres and can stabilize the pointed ends of actin filaments. We observed that tropomyosin-troponin complex reduces the rate of association and dissociation of actin molecules at the pointed ends more than 30-fold. On the average, every 600 s one association or dissociation reaction has been found to occur at the pointed ends near the critical actin monomer concentration.

Kinetic evidence for insertion of actin monomers between the barbed ends of actin filaments and barbed end-bound insertin, a protein purified from smooth muscle

An actin polymerization-retarding protein was isolated from chicken gizzard smooth muscle. This protein copurified with vinculin on DEAE-cellulose and gel filtration columns. The polymerization-retarding protein could be separated from vinculin by hydroxylapatite chromatography. The isolated polymerization-retarding protein lost its activity within a few days, but was stable for weeks when it was not separated from vinculin. We termed the polymerization-retarding protein "insertin". Because of the instability of the isolated insertin, we investigated the effect of insertin-vinculin on actin polymerization. Insertin-vinculin retarded nucleated actin polymerization maximally fivefold. Polymerization at the pointed ends of gelsolin-capped actin filaments was not affected by insertin-vinculin, suggesting that insertin-vinculin binds to the barbed ends, but not to the pointed ends, of actin filaments. Retarded polymerization was observed even if the actin monomer concentration was between the critical concentrations of the ends of treadmilling actin filaments. As at this low monomer concentration the pointed ends depolymerize, monomers appeared to be inserted at the barbed ends between the terminal subunit and barbed end-bound insertin molecules. Insertin-vinculin was found not to increase the actin monomer concentration to the value of the pointed ends. These observations support the conclusion that insertin is not a barbed end-capping protein but an actin monomer-inserting protein. According to a quantitative analysis of the kinetic data, all observations could be explained by a model in which two insertin molecules were assumed to bind co-operatively to the barbed ends of actin filaments. Actin monomers were found to be inserted between the barbed ends and barbed end-bound insertin molecules at a rate of about 1 x 10(6) M-1 s-1. Insertin may be an essential part of the machinery of molecules that permit treadmilling of actin filaments in living cells by insertion of actin molecules between membranes and actin filaments.

DNA excision-repair ability of embryonic fibroblasts measured by UDS and thymine dimer excorporation in four inbred mice strains

Differences in the excision-repair capability of embryonic fibroblasts of four inbred strains of mice following various degrees of UV irradiation were assessed. Two methods of determination were used: (1) the incorporation of 3H-thymidine during unscheduled DNA synthesis (UDS) as measured by an autoradiographic technique; (2) the rate of excision of thymine dimers (TT) in the acid-insoluble fraction of the cellular DNA as determined by a dimerspecific radioimmuno-assay. Based on UDS, the repair rates of the four strains could be ranked in decreasing order as follows: DBA/2 (DB); C57BL/6J (B6); AKR/N (AK); CBA/J (CB). The calculated rate for DBA/2 (DB) is approximately twice that of CBA/J (CB). The determination of the TT excision rate indicates that 72 h after irradiation a maximum of 50% of the original UV-induced dimers in the DB strain could be repaired. In the three remaining strains the relatively reduced repair rates of 15% - 40% did not differ significantly.

Nonmuscle actin ADP-ribosylated by botulinum C2 toxin caps actin filaments

The effect of nonmuscle actin ADP-ribosylated by botulinum C2 toxin on the polymerization of nonmuscle actin was investigated in order to clarify whether nonmuscle actin is converted into a capping protein by ADP-ribosylation. ADP-ribosylated actin was found to decrease the rate of polymerization of actin filaments which are free at both ends. ADP-ribosylated actin turned out to have no effect on the rate or extent of polymerization at the pointed ends of actin filaments the barbed ends of which were capped by gelsolin. The monomer concentration reached at the final stage of polymerization was similar to the critical concentration of the pointed ends of actin filaments. The results suggest that nonmuscle actin ADP-ribosylated by botulinum C2 toxin acts as a capping protein which binds to the barbed ends to inhibit polymerization.