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At least 343 records · Page 19Linked to original sources
PubMed1990

Microtubule solutions display nematic liquid crystalline structure.

We report a study of the spontaneous formation of ordered arrays of microtubules in solution. Form birefringence and anisotropic light-scattering appear rapidly and spontaneously when tubulin, initially present in homogeneous solution, self-assembles into microtubules. This phenomenon is reversible and occurs at protein concentrations of a few milligrams per ml, in the presence or absence of microtubule-associated proteins. Light and electron microscopic examination reveals that extensive regions of these birefringent solutions consist of nearly parallel microtubules. Measurement of the order parameter, S, yields a value of 0.81 +/- 0.05, indicating a high degree of alignment. Comparison of these observations to qualitative predictions developed from the theory of Onsager ((1949) Ann. N.Y. Acad. Sci. 51, 627-659) leads to the conclusion that microtubules form a nematic liquid crystalline phase in vitro under ordinary conditions. Simultaneous spectrophotometric observation of turbidity (a measure of microtubule assembly) and birefringence shows that the parallel ordering lags only slightly behind assembly, thus demonstrating that much microtubule growth must occur by addition of tubulin to the ends of microtubules that are already aligned. These observations of anisotropy are important to the understanding of microtubule dynamics in vitro.

Animals↗
PubMedDate not supplied

[Polymorphism of fibrin equilibrium oligomers in the presence of fragment D].

The methods of viscosimetry, the Rayleigh light-scattering and analytical ultracentrifugation were applied to study the physicochemical mechanism of the effect of fragment D on the structure of fibrin equilibrium oligomers. Using the values of intrinsic viscosity, weight average molecular masses and mass/length ratio it was shown that when producing an antipolymerization effect the fragment D retains the three-dimensional organization of fibrin polymers, i.e. rigid rod-like single- and double-stranded protofibrillas. The paper has proved that along with the traditional mechanism of inhibiting self-assembly of of the double-stranded structure due to the competition of fragment D with fibrin monomer for central domain E there is an alternative attributed to its attachment to a peripheral region of the fibrin monomer. The second mechanism is the only one which occurs in the region of single-stranded pseudoprotofibrillas existence. The role of alpha C-domains in protein-protein interactions is also discussed.

Animals↗
PubMed1987

[Effect of phosphorylation of myosin light chains on the interaction of myosin minifilaments with F-actin].

The effect of phosphorylation of light chains-2 (LC2) of rabbit skeletal muscle myosin on the interaction of myosin minifilaments with F-actin as well as on the actin-stimulated Mg2+-ATPase of minifilaments was studied. It was shown that in the absence of KCl the degree of F-actin-induced stimulation of myosin minifilament Mg2+-ATPase with phosphorylated LC2 exceeds 2-4-fold that with unphosphorylated LC2. Phosphorylation of LC2 considerably increases the rate of actin-stimulated Mg2+-ATPase reaction of myosin minifilaments but exerts only a very weak influence on the affinity of minifilaments for F-actin. After addition of KCl the differences in the actin-stimulated Mg2+-ATPase activity disappear in a great degree; in the presence of 50 mM KCl they do not exceed 50%. It was assumed that the observed specific influence of LC2 phosphorylation on the kinetic parameters of actin-stimulated Mg2+-ATPase reaction of myosin minifilaments is due to unique properties of the minifilaments (e.g., their ability to ordered self-assembly as a result of interaction between the heads of myosin molecules) which reflect their structural peculiarities.

Actins↗
PubMed1987

Dependent sequences of gene expression controlling spore formation in Bacillus subtilis.

Sporulation in Bacillus subtilis is a simple form of differentiation that involves the formation of a two-cell organism; mother cell and prospective spore. It is regulated by at least 50 operons, some mono- and some polycistronic. These are expressed in a dependent sequence which branches at an early stage so that there are independent lines of expression in both cells; further ramifications then occur in both cell compartments. Three DNA-binding proteins and two sigma factors have so far been identified as probable regulators of the dependent sequence. Most of the operons of the sequence are expressed in the first four hours and the last two stages occur mainly by self-assembly of proteins.

Bacillus subtilis↗
PubMed1988

[Effect of fibronectin on fibrinogen conversion into fibrin].

The effects of fibronectin on fibrinogen clotting induced by thrombin or reptilase and on fibrin monomer polymerization in a pure system in the absence of factor XIIIa were studied. It was shown that within a broad range of concentrations and molar ratios of the mixed proteins, fibronectin does not alter significantly the fibrinogen clotting time either under thrombin or under reptilase action. The effect of fibronectin on the fibrin self-assembly consists in a slight acceleration of this process, whose degree is directly dependent on the fibronectin/fibrin monomer molar ratio as well as on the absolute fibrin monomer content at a constant molar ratio. The stimulating effect of fibronectin is amplified by Ca2+. The experimental results suggest that fibronectin can noncovalently bind the fibrin monomer and/or intermediate polymers in the non-enzymatic phase of fibrinogen conversion to fibrin.

Batroxobin↗
PubMed1988

The effect of nonenzymatic glucosylation on the binding of the main noncollagenous NC1 domain to type IV collagen.

Type IV collagen has the ability to self-assemble by amino end, carboxyl end, and lateral associations to complex network-like structures which can be visualized by rotary shadowing. The main noncollagenous NC1 domain which is located at the carboxyl end of type IV collagen molecules binds to itself to form dimers and also binds along the length of type IV collagen. The latter binding initiates lateral assembly. Following in vitro nonenzymatic glucosylation of the isolated NC1 domain, binding to the helix-rich domain of type IV collagen was impaired. In turbidity experiments, the nonenzymatically glucosylated NC1 domain minimally suppressed the development of turbidity of collagen solutions when compared to control NC1 domain. In rotary shadowing experiments nonenzymatically glucosylated NC1 domain did not significantly inhibit lateral associations or networks formed by type IV collagen, whereas control NC1 domain caused a drastic decrease in laterally assembled structures. These data suggest that nonenzymatic glucosylation of the NC1 domain may interfere with normal assembly of type IV collagen in diabetes mellitus and may be related to abnormal functions of basement membranes in this pathological condition.

Animals↗
PubMed1988

Spatial organization of collagen in annelid cuticle: order and defects.

The epidermis of Paralvinella grasslei (Polychaete, Annelida) is covered by an extracellular matrix, the cuticle, mainly composed as in other annelids of superimposed layers of non-striated collagen fibrils. The collagen fibrils of annelid cuticle are shown to be composed of parallel and sinuous microfibrils (thin sections and freeze-fracture replicas). The 3-dimensional organization of collagen is characterized by 2 different types of geometrical order: (a) Fibrils form a quasiorthogonal network, whose structure is comparable to that of a "plywood"; (b) Fibrils are helical, and goniometric studies show that microfibrils present a definite order within each fibril, which is termed "cylindrical twist". These 2 characteristics are those which have recently been evidenced in "blue phases", i.e., liquid crystals which are closely related to cholesteric liquid crystalline phases. Non-fluid analogues of cholesteric liquids are widespread among invertebrate cuticles and the presence of blue phase analogues suggests that a self-assembly mechanisms is involved in cuticle morpho-genesis, which is derived from that governing blue phase growth. The cuticular network presents local rearrangements of fibrils called "defects", despite the fact that they are elaborate structures which trigonal and pentagonal singularities. Branched fibrils are regularly observed. We discuss the involvement of these pattern disruptions in the cuticle growth process.

Animals↗
PubMed1988

Assembly of type I collagen fibrils de novo. Between 37 and 41 degrees C the process is limited by micro-unfolding of monomers.

The effects of temperature on the assembly of collagen fibrils were examined in a system in which collagen monomers are generated de novo and in a physiological buffer by specific enzymic cleavage of type I pC-collagen, an intermediate in the normal processing of type I procollagen to type I collagen. Increasing the temperature of the reaction in the range of 29-35 degrees C decreased the turbidity lag and increased the rate of propagation as assayed by turbidity. The effect of temperature on the turbidity propagation rate gave a linear Arrhenius plot with a negative slope. The predicted value of the activation energy of propagation was 113 kJ/mol. However, the effects of temperature on the rate of assembly above 37 degrees C were opposite to the effects seen at temperatures below 37 degrees C. In the range of 37-41 degrees C, the turbidity propagation rate decreased markedly with temperature. Also, the turbidity lag increased. Therefore, much longer times were required for monomers to reach equilibrium with fibrils. A large fraction of the collagen monomers remaining in solution at temperatures above 37 degrees C was sensitive to rapid digestion by trypsin and alpha-chymotrypsin. Cooling the solutions to 25 degrees C made the monomers resistant to protease digestion. The results are consistent with the conclusion that, although formation of collagen fibrils is a classical example of an entropy-driven process of self-assembly, the rate of assembly between 37 and 41 degrees C is limited by reversible micro-unfolding of the monomer.

Cells, Cultured↗
PubMed1978

Protein-lipid interactions and the role of water.

The rigidity of the three-dimensional structure of a native protein is dependent on the network of hydrogen-bonded groups which provide the scaffolding for the other interactions. The structure is stabilized by the hydrophobic interactions of the nonpolar side chains. The latter are formed by the very unfavorable entropy change that occurs in water but not in less-polar solvents. It is unlikely that any solvent other than water can produce the same folding of a polypeptide chain to form the active native structure. Water plays a unique role, since it alone is responsible for the heat capacity changes observed when nonpolar groups are transferred from an aqueous to a nonaqueous environment, as exists in the interior of a protein. The need to juxtapose like groups and to avoid making contact among unlike groups imposes severe restrictions on the binding of small or large molecules to proteins. Consequently there must be proper pairing of polarities as well as close fitting of ligands for strong binding to occur. This is clearly evident from the x-ray studies of proteins containing subunits or prosthetic groups. The thermodynamic parameters observed in the most complex protein reactions--i.e., self-assembly systems--resemble rather well those observed in micelle association reactions or even in the solution of nonpolar gases in water. This interaction--hydrophobic--can be looked upon as the controlling reaction which stabilized the organized structures of most cellular entities aside from nucleic acids--i.e., membranes and organelles.

Butanes↗
PubMedDate not supplied

[Cooperative interaction of serum albumin with quaternized poly-4-vinyl pyridine and structure of the complexes].

Interaction of bovine serum albumin (BSA) with quaternized poly-4-vinyl pyridine (PE) in aqueous solutions at pH 7 was studied. It was shown that in a wide range of the ratios of the components (nBSA/nPE) soluble stable cooperative complexes were formed. At the same time a certain critical content of the protein exists at which the system loses its homogeneity. Complex formation is not accompanied by protein denaturation. At smaller nBSA/nPE ratios non-homogeneous distribution of protein globulas among polyelectrolite macromolecules was found; this corresponded to the "all or none" principle. Using ultracentrifugation technique viscosimetric measurements and electron microscopy it was shown that the soluble complexes exist in the form of rode-like particles consisting of protein globules stabilized by polycation chains. Such particle can be considered as a model of nucleoprotein complex. At certain crytical nBSA/nPE rations the rod-like particles aggregate with additional number of BSA-molecules and form more complicate soluble and insoluble cooperative complexes. Possible structural models of the complexes described were suggested and the thermodinamic and kinetic cryteria of their self-assembly were discussed.

Drug Stability↗
PubMed1977

[Quaternary structure of oligomeric immunoglobulin A forms from human blood serum].

A character of forces stabilyzing quaternary structure of dimer and more high molecular human immunoglobulin A oligomers is found to be different. Quaternary structure of IgA dimer is formed when joining subunits with disulfide bonds and is stabilized by non-covalent interactions between them. Disulfide bonds play a main part in the formation of trimers and tetramers. Dimer IgA reconstructs by 40% from subunits with intact interchain S--S bonds. The addition of exogenous J-chain does not significantly affect the process of dimer self-assembling from subunits with recovered and intact interchain disulfide bonds.

Chemical Phenomena↗
PubMed1979

The binding of ribosomal protein S4 does not change the gross conformation of the 16 S RNA.

The binding of ribosomal protein S4 to the 16 S RNA does not result in a large shape or conformational change in the 16 S RNA under the conditions of reconstitution. The sedimentation coefficient, frictional coefficient ratio, and effective hydrodynamic radius of the 16 S RNA.protein S4 complex are very similar to those obtained for the 16 S RNA free in solution. Only subtle conformational differences were obtained in the comparison of the complex and free 16 S RNA by circular dichroism. Thus, extensive organization of the 16 S RNA by ribosomal protein S4 is not a step in the process of self-assembly of the 30 S subunit.

Circular Dichroism↗
PubMedDate not supplied

[Spectrum differentiation as a method of studying protein preparations with a high turbidity level].

The studies on a model of a mixture of fibrin-monomer with glycogen and of fibrin gels of different turbidity show that in the first and second derivatives of the absorption spectra the contribution of turbidity is several orders lower. The derivative spectra are shown to be valuable technique to study the phenomena that are accompanied by substantial changes in turbidity: protein association and self-assembly of supermolecular structure.

Fibrin↗
PubMed1978

Monoclonal IgG anticoagulants delaying fibrin aggregation in two patients with systemic lupus erythematosus (SLE).

There is paucity of information regarding the prolonged plasma thrombin time known to occur in some patients with systemic lupus erythematosus. Detailed investigations of plasma from two such patients disclosed that IgG accounted for this defect in each case. IgG isolated from plasma of either patient possessed the property of delaying fibrin aggregation and prolonging the clotting times of fibrinogen. Preincubation of IgG from either patient with anti-IgG or anti-Fab (rabbit) serum abolished this anticoagulant property. Moreover, the anticoagulant IgG from the first patient was neutralized with anit-k chain and anti-IgG3, that from the second patient with anti-lambda chain and anti-IgG1 serum. These anticoagulants were also dissimilar with respect to their interactions with fibrin(ogen). IgG from the first patient had no anticoagulant activity against fibrin(ogen) species lacking intact Aalpha chains. IgG from the second patient displayed undiminished anticoagulant effect on such fibrin(ogen) species. We conclude that each anticoagulant interacted with a distinct region(s) on the fibrinogen molecule and that these interactions affect or involve sites that participate in the fibrin self-assembly process.

Adolescent↗
PubMed1976

Biogenesis of matrix vesicles in cartilage growth plates.

Although the structure and function of cartilage matrix vesicles have been comprehensively documented in the literature, the method of vesicle production and export to the extra-cellular matrix is less well understood. The existing data are grouped into four general hypotheses of matrix vesicle biogenesis, with a consideration of the relative merits and weaknesses of each postulate. The hypotheses are: a) budding from cellular processes; b) extrusion of preformed structures; c) cellular degeneration and disintegration; and c)subunit secretion and extracellular assembly. Each of these possibilities offers conceptual advantages and disadvantages, so that it is difficult to adopt a monistic stance. Previous notions of vesicle biogenesis have derived from statis morphologic and analytical data, which are necessarily limiting when attempting to draw kinetic conclusions. Radiotracer studies of lipid synthesis and transport in cartilage lead to the conclusion that matrix vesicles in the growth plate are derived from chondrocytes; the present data favor the hypotheses of budding from cellular processes and/or subunit secretion with extracellular self-assembly.

Cartilage↗
PubMed1977

Myogenesis and contraction in the early embryonic heart of the rainbow trout. An electron microscopic study.

Myogenesis in the embryonic heart of the rainbow trout, Salmo gairdneri (Rich.), was investigated electron microscopically from the 29th to the 41st somite stage. Thick and thin myofilaments are formed simultaneously as well as precursors of Z-lines, to which the thin filaments are attached. The genesis of filaments takes place in the region around the intracellular yolk droplets. The first myofibrils appear by the 33rd somite stage, probably formed by a mechanism of self-assembly in which the binding sites of actin and myosin participate. A- and I-bands do not develop before the 38th osmite stage. The contraction already begins during the 33rd somite stage in the middle of the tubular heart. Gradually, the peristaltic waves spread increasingly to other parts of the heart. In the 41st somite stage the entire heart is contractile and all myocytes contain myofibrils.

Animals↗
PubMedDate not supplied

[Determination of fibrin and fibrinogen split products by their anticoagulant activity].

A new diagnostic method to determine fibrinogen and fibrin splitting products is based on the fact that high-molecular fragments acting as specific inhibitors of fibrin self-assembly compose these products. Fibrinogen and fibrin splitting products are detected and determined quantitatively by prolongation of coagulation time in the test with the standard monomeric fibrin. The number of units of anticoagulating activity in the studied urine are the results of the determinations. Sensitivity of the method if necessary may be increased by adding of a certain amount of the fibrinogen and fibrin splitting products to the test that makes a favourable background for the action of the products. The method is rather sensitive, reliable and simple. It may be used in ordinary clinical laboratories. The method proved to be quite satisfactory when testing renal diseases at the clinic. It is of great significance for early detection of the transplanted kidney rejection. The new method is compared with the known immunological method which consist in determination of the inhibitory effect of the fibrinogen and fibrin splitting products on agglutination of erythrocytes of specific antibodies; the erythrocytes being subjected to "tanning" and "loading" with fibrinogen. A satisfactory correlation is found between the results of two methods.

Adult↗