Lactoperoxidase-tubulin interactions

Colchicine binding by the "isolated" beta-monomer of tubulin

At mole ratios of lactoperoxidase to tubulin monomers of 3-4, bovine lactoperoxidase forms 1:1 adducts with both alpha- and beta-tubulin from rat brain, thereby separating the tubulin heterodimer into its monomers. This mixture binds colchicine normally, and we show here by direct photoaffinity labeling that the bulk of the [3H]colchicine becomes attached to beta-tubulin under these conditions. When the alpha-tubulin has been displaced by lactoperoxidase, the ratio of label in beta-tubulin to alpha-tubulin is increased. The amount of label in alpha-tubulin decreases with a corresponding appearance of label in lactoperoxidase. The rate of labeling of beta-tubulin remains slow. We conclude that alpha-tubulin is not necessary for colchicine binding and propose a model wherein the A and C rings of colchicine bind to beta-tubulin, while the B ring faces alpha-tubulin in the dimer.

Interactions of colchicine with tubulin

Colchicine exerts its biological effects through binding to the soluble tubulin heterodimer, the major component of the microtubule. The colchicine-binding abilities of tubulins from a variety of sources are summarized, and the mechanism of colchicine binding to brain tubulin is explored in depth. The relationship between colchicinoid structure and tubulin binding activity provides insight into the structural features of colchicine responsible for high affinity binding to tubulin and is reviewed for analogs in the colchicine series. The thermodynamic and kinetic aspects of the association are described and evaluated in terms of the binding mechanism. Colchicine binding to tubulin results in unusual alterations in the low energy electronic spectra of colchicine. The spectroscopic features of colchicine bound to tubulin are discussed in terms of the nature of the colchicine-tubulin complex. Attempts to locate the high affinity colchicine binding site on tubulin are presented.

Cell-cell interactions in the process of differentiation of thyroid epithelial cells into follicles: a study by microinjection and fluorescence microscopy on in vitro reconstituted thyroid follicles

Thyroid cells, cultured in the presence of thyroid stimulating hormone, reorganized within 36-48 hr into follicular structures, the in vitro reconstituted thyroid follicles or RTF. By microinjection of fluorescent probes either into the neoformed intrafollicular lumen (IL) or into cells forming the follicles, we have studied the development and some functional properties of cell-cell contacts involved in a) the formation of the thyroid follicular lumen and b) the communication between thyrocytes within the follicle. The probes were compounds of either low (Lucifer Yellow: LY) or high molecular weight (Dextran labeled with fluorescein: FITC-Dextran and Cascade Blue conjugated to bovine serum albumin: CB-BSA). LY microinjected into IL of 2-9-day-old RTF was seen to label circular spaces with a diameter ranging from 10 to 100 microns. The cells delimiting the IL remained unlabeled. The fluorescent dye remained concentrated in IL for up to 24 hr. FITC-Dextran or CB-BSA microinjected into IL behaved as LY; the probes were restrained into the lumen. A 2 hr incubation of RTF with iodide induced alterations of the structure of IL; an effect mediated by an organic form of actively trapped iodide. A 15-30 min incubation of RTF in a low CA2+ medium caused the opening of IL visualized by the progressive decrease of the fluorescence of probes preinjected into the lumenal space. The same but more rapid effect was obtained by microinjection of EGTA into the IL. The low Ca2(+)-dependent opening of IL was also demonstrated by the release into the medium of thyroglobulin present in IL. Microinjection of LY in a cell involved in the follicle structure led to the rapid labeling of the other cells forming the follicle but LY did not penetrate the IL. Unlike LY, the distribution of FITC-Dextran or CB-BSA injected into cells delimiting the lumen was restricted to the microinjected cells. Alterations of medium or intralumenal Ca2+ concentration which caused the opening of IL did not affect the cell-to-cell transfer of LY. By using fluorescent probe microinjection, we show that the in vitro thyrocyte histiotypic differentiation leads to the reconstitution of functional intercellular junctions: tight junctions insuring the tightness of the neoformed lumen and gap junctions mediating the cell-to-cell exchange of small molecules. The structure of the thyroid follicles appears to be under the control of both extracellular and intralumenal Ca2+ concentrations.

Reaction of alpha-tubulin with iodotyrosines catalyzed by tubulin:tyrosine ligase: carboxy-terminal labeling of tubulin with [125I]monoiodotyrosine

We have studied the capacity of different iodinated derivatives of phenylalanine and tyrosine to inhibit the incorporation of [3H]tyrosine into tubulin catalyzed by tubulin:tyrosine ligase. In contrast to thyronine and its iodinated derivatives, iodotyrosines were efficient inhibitors. That they also functioned as substrates of the enzyme was shown by the effective incorporation of [125I]mono- and diiodotyrosine into tubulin. The label was shown to be located at the carboxy terminus. Labeling by this method conserves the polymerization capacity of tubulin in contrast with classical radioiodination methods involving oxidation.

Evidence for the presence of a very high concentration of arylsulfatase A in the pig thyroid: identification of arylsulfatase A subunits as the two major glycoproteins in purified thyroid lysosomes

In addition to their general function in cellular homeostasis, thyroid lysosomes play an essential role in the biosynthesis of thyroid hormones by cleaving the macromolecular prohormone, thyroglobulin. In the present work, we have attempted to determine whether the enzyme composition of thyroid lysosomes differs from that of lysosomes from other tissues. Lysosomal enzymes, cathepsin D, beta-D-galactosidase, beta-D-glucosidase, alpha-D-mannosidase, alpha-L-fucosidase, hexosaminidase, and arylsulfatase A and B, were assayed in crude fractions from various pig tissues, heart, brain, liver, kidney, thyroid, adrenals, ovary, and spleen. It appeared that the specific activity of arylsulfatase A was at least 20 times higher in the thyroid than in most other tissues. Thyroid lysosomes purified by isopycnic centrifugation on Percoll gradients contained two major polypeptides with apparent molecular weights of 58,000 and 54,000 representing about 30% of the total protein. These polypeptides were glycosylated and were exclusively found in the intralysosomal soluble fraction obtained by osmotic pressure-dependent lysis. By fractionating intralysosomal soluble proteins by velocity sedimentation on sucrose gradients or gel permeation chromatography we identified a thyroid arylsulfatase A holoenzyme which corresponds to a 120,000 Mr species. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of the gradient or column fractions showed that the 120-kDa protein peak with arylsulfatase A activity essentially contained the 58- and 54-kDa polypeptides in equivalent amounts. In conclusion, arylsulfatase A, a heterodimer of 120 kDa composed of two nonidentical subunits, is the major protein component of thyroid lysosomes. The superabundance of this protein in purified thyroid lysosomes is related to the very high specific activity of the enzyme in the thyroid as compared to other tissues.

Glyceraldehyde-3-phosphate dehydrogenase is a microtubule binding protein in a human colon tumor cell line

A protein which binds to tubulin polymer was isolated from a human colonic tumor cell line. This protein has a molecular mass of 35 kDa, as determined by polyacrylamide slab gel electrophoresis. The protein was purified by affinity chromatography on taxol-stabilized microtubules, and it did not cross-react with anti-MAP2 or anti-tau antibodies. This protein was identified as glyceraldehyde-3-phosphate dehydrogenase by its enzyme activity and immunoblotting experiments. The purified protein caused a pronounced enhancement in the turbidity increase produced by in vitro tubulin polymerization, and electron microscopic observations revealed the presence of bundles of microtubules.

A basic protein from bovine brain that co-precipitates with tubulin in vitro

A 193 kDa protein consisting of 58 kDa subunits, which has pI values of 8.50 and 8.65, was purified from bovine brain cytosol. It formed heavy precipitates with tubulin, and the molar ratio of tubulin dimer to this protein in the precipitate was 3.2. In contrast to microtubules containing ordinary microtubule-associated proteins, these complexes remained stable against cold and 1 mM CaCl2.

Microtubules bind glyceraldehyde 3-phosphate dehydrogenase and modulate its enzyme activity and quaternary structure

Glyceraldehyde 3-phosphate dehydrogenase, a tetramer of 140,000 Da, interacts with in vitro reconstituted microtubules. It results in a partial inhibition of the activity of the microtubule-bound enzyme. After cold depolymerization of the microtubule-glyceraldehyde 3-phosphate dehydrogenase complexes, a fraction of the enzyme is recovered in an active form in the disassembly supernatant; the other fraction devoid of activity, identified by polyacrylamide gel electrophoresis, remains associated with the undepolymerizable microtubule protein pellet. The inactivation of the microtubule-bound enzyme is related to the concentration of microtubule protein. Higher the concentration of microtubule protein, lower the fraction of inactivated enzyme; consequently, glyceraldehyde 3-phosphate dehydrogenase is able to copolymerize quantitatively with microtubule protein through one assembly-disassembly cycle, provided that the concentration of microtubule protein is high. Monomeric glyceraldehyde 3-phosphate dehydrogenase (molecular weight: 35,000) devoid of enzyme activity, prepared by reversible dissociation of the tetrameric enzyme, also binds to microtubules and is quantitatively recovered in the undepolymerizable microtubule protein fraction after cold treatment. These results indicate that interacting with microtubules, glyceraldehyde 3-phosphate dehydrogenase partly dissociates into inactive monomers, this process is regulated by the concentration of assembled microtubule protein, and active and inactive glyceraldehyde 3-phosphate dehydrogenase bound to microtubules have different fate at the step of microtubule disassembly. These data suggest that an association of glyceraldehyde 3-phosphate dehydrogenase to microtubules could play a role in modulating the activity of the glycolytic enzyme in intact cells.

Cyclic AMP-dependent phosphorylation of high molecular mass proteins in pig thyroid cells

Four high molecular mass (H Mr) proteins were found to be phosphorylated in a cyclic AMP-dependent manner in both partially purified pig thyroid membrane fractions and in pig thyroid cells in culture. These phosphoproteins did not correspond to major cellular proteins; they were found in both soluble and particulate subfractions of homogenates from cultured thyroid cells. The molecular mass of the 4 proteins named HMr1 to HMr4 determined by polyacrylamide gel electrophoresis in the in the presence of sodium dodecylsulfate was about 310 000 for HMr1, 250 000 for HMr2, 240 000 for Hmr3 and 220 000 for HMr4. HMr1 comigrated with brain MAP1, whereas HMr3 and HMr4 had the same mobility as alpha-and beta-spectrins, respectively. The 4 high molecular mass phosphoproteins are substrates of cyclic AMP-dependent protein kinase(s) since (a) their phosphorylation was increased by cyclic AMP and not by cyclic GMP or calcium alone or calcium in the presence of calmodulin or phospholipid; (b) the effect of cyclic AMP was prevented by the thermostable inhibitor of cyclic AMP-dependent protein kinases; (c) the purified catalytic subunit of cyclic AMP-dependent protein kinases markedly phosphorylated the 4 HMr proteins. The 32P-labeling of HMr proteins using either endogenous cyclic AMP-dependent protein kinase or the purified catalytic subunit was always lower in cells cultured in the presence of TSH (reassociated in follicle-like structures) than in freshly dispersed cells or cells cultured in basal conditions (cells in monolayer). These results suggest that the 4 high molecular mass thyroid phosphoproteins represent structural components, the phosphorylation of which could vary with the cellular organization.

Association states of tubulin in the presence and absence of microtubule-associated proteins. Analysis by electric birefringence

Electric birefringence has been used to examine the states of association of tubulin in phosphocellulose-purified tubulin or depolymerized microtubule protein solutions at low temperature. In a high electric field (1000-4000 V/cm), tubulin could be orientated (owing to the existence of a permanent and/or induced dipole) and exhibited a positive birefringence (delta n), related to its intrinsic optical anisotropy. The analysis of the relaxation process (depending on hydrodynamic properties of molecules), by measurement of the time decay of delta n, revealed the existence of a multicomponent or polydisperse system, whatever the tubulin solution. Two relaxation times, representative of the smallest and the largest orientated species, were obtained by computer-fitting analysis. The mean values of relaxation time for phosphocellulose-purified tubulin were 0.8 and 8 microseconds. In microtubule protein solutions, large-sized macromolecular species with relaxation time up to 450 microseconds were detected. The largest species (relaxation times ranging from 50 to 450 microseconds) could be eliminated by centrifugation at 3000000 X g for 1 h. Addition of microtubule-associated protein to either pure tubulin or high-speed centrifuged microtubule protein led to a rapid formation of large species analogous to those present in microtubule protein. Molecular dimensions of the relaxing structures were estimated using simple hydrodynamic models and values of rotational diffusion constants calculated from the relaxation times, and compared to those of the structures described in the literature. In conclusion, we have found that (a) phosphocellulose-purified tubulin is not only composed of elementary species (dimers) but also contains tubulin-associated forms of limited size (up to 7-10 dimers), (b) depolymerized microtubule protein solutions contain ring oligomers and structures very much larger, the formation of which is dependent on the presence of microtubule-associated protein.

Bundling of microtubules by glyceraldehyde-3-phosphate dehydrogenase and its modulation by ATP

Glyceraldehyde-3-phosphate dehydrogenase from different origins (brain, muscle, erythrocytes) binds to microtubules polymerized from pure brain tubulin and causes bundle formation in vitro. ATP is shown to dissociate these bundles into individual microtubules, while the dehydrogenase is not displaced from the polymers by this nucleotide. ATP can be replaced by adenosine 5'-(beta, gamma-imido]triphosphate, a nonhydrolyzable analog of ATP. These data are interpreted in terms of dissociation of the glyceraldehyde-3-phosphate dehydrogenase tetramer into dimers by ATP. The enzyme is also efficiently purified by a tubulin-Sepharose affinity chromatography.

Microtubule-associated adenylate cyclase

Twice-cycled bovine brain or rat brain microtubule protein contains an adenylate cyclase activity that passes 0.2 micron filters, is activated 2-7-fold by 30 microM forskolin, shows modest stimulation by fluoride (especially in the presence of added AI3+), but is virtually insensitive to added guanine nucleotides. The activity is insensitive to various hormones or Ca2+/calmodulin. The adenylate cyclase is active with both Mg2+ and Mn2+ but activity is less in the presence of Mg2+ than with Mn2+. The cyclase is inhibited by agonists of the adenosine P site. It is proposed that the catalytic unit of adenylate cyclase and probably small quantities of the guanine nucleotide regulatory protein, Ns, are cycled along with microtubules.

Anti-tubulin antibodies in recent onset type 1 (insulin-dependent) diabetes mellitus: comparison with islet cell antibodies

Antibodies to tubulin, the fundamental protein of microtubules, were studied by radioimmunoassay in patients with Type 1 (insulin-dependent) diabetes of varying duration and in healthy control subjects. Elevated levels of anti-tubulin antibodies were found in 46% of 28 patients with Type 1 diabetes of recent onset (less than or equal to 6 months) and in only 6.2% of 64 patients with long-standing Type 1 diabetes (duration 6-43 years). None of 34 DR3-positive normal subjects and none of 20 Type 2 (non-insulin-dependent) diabetic patients were positive for anti-tubulin antibodies. Anti-tubulin antibody levels were elevated in two out of 26 first-degree relatives of Type 1 diabetic patients. The specificity of the detection of anti-tubulin antibodies was demonstrated by dilution of the sera, competitive binding experiments between labelled and unlabelled tubulin, immunoblotting. Antibodies to tubulin were elevated in 60% of patients with islet cell surface antibodies and there was a significant association between anti-tubulin antibodies and islet-cell surface antibodies. These antibodies, however, recognize different specificities, since adsorption of islet cell surface antibody by rat islets did not alter the anti-tubulin antibody activity. Elevated anti-actin antibody responses were found in two out of 17 and one out of 26 patients with recent onset and long-standing Type 1 diabetes, respectively. In conclusion, anti-tubulin antibodies are detected in a high proportion of patients with diabetes of recent onset, are associated with islet cell surface antibodies and like islet cell surface antibodies decrease or disappear during the course of the disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Visualization of lactoperoxidase binding to microtubule and tubulin

The binding of lactoperoxidase to microtubules and tubulin was shown in both electron micrography and polyacrylamide gel electrophoresis by tracing the enzymatic activity of lactoperoxidase. Lactoperoxidase bound to purified microtubules appeared to distribute evenly on the surface without forming special structures. Both alpha and beta-tubulin separated by sodium dodecylsulfate-polyacrylamide gel electrophoresis bound lactoperoxidase, and could be detected by the use of lactoperoxidase reaction. Electrophoretic study revealed that the interaction between lactoperoxidase and tubulin were not strictly specific and a variety of proteins other than alpha- and beta-tubulin, including actin and neurofilament subunits, bound lactoperoxidase.