Assembly of chick brain MAP2-tubulin microtubule protein. Characterization of the protein and the MAP2-dependent addition of tubulin dimers

Modeling of sensing potency of cytoskeletal systems decorated with metabolic enzymes

The highly dynamic cytoskeleton interacts with enzymes and other proteins that are involved in metabolic or signaling pathways. These interactions can influence the structural and functional characteristics of the partners at the microscopic level of individual proteins and polymers. In this work the functional consequences of such interactions have been studied at the macroscopic level in order to evaluate the integrative and regulatory roles of the metabolic pathways associated with the microtubule cytoskeleton. Here we present mathematical models of the interactions between a hypothetical metabolic pathway and microtubule assembly, and explore for the first time the functional consequences of these interactions in distinct situations. The models include kinetic constants of the individual steps and testable, relevant parameters which allow the quantification of the coupled processes at the microscopic and macroscopic levels. For example our kinetic model for the self-assembly of microtubules reproduces the alteration of the time-dependent turbidity caused by pyruvate kinase binding. Our data reveal the power of a mechanistic description of a filamentous system to explain how cells sense the state of metabolic and other pathways.

The effects of Tubulin denaturation on the characterization of its polymerization behavior

We report here upon a simulation study examining the effect of a dynamic mode of tubulin denaturation upon the kinetic and thermodynamic characterisation of the polymer formed for two idealized models of a tubulin polymerization reaction: (i) an irreversibly polymerizing system; and (ii) a reversibly polymerizing system. The effects of each denaturation mode upon the two model systems behavior are highlighted by interpretation of the data in terms of the classical Oosawa reversible condensation polymerization model. We reveal here findings which suggest that the measurement strategy in concert with Tubulin's instability over the time course of the experiment may bias the results obtained so as to make an irreversible system's behavior conform to the equilibrium model or alternatively distort the results obtained from a truly reversible system to produce values of the critical concentration quite seriously in error. It was also found that Tubulin denaturation may seriously distort parameter estimates gained from a kinetic characterization of the system (e.g. nucleus size and growth rate constant).

Hydrolysis of GTP associated with the formation of tubulin oligomers is involved in microtubule nucleation

Hydrolysis of GTP is known to accompany microtubule assembly. Here we show that hydrolysis of GTP is also associated with the formation of linear oligomers of tubulin, which are precursors (prenuclei) in microtubule assembly. The hydrolysis of GTP on these linear oligomers inhibits the lateral association of GTP-tubulin that leads to the formation of a bidimensional lattice. Therefore GTP hydrolysis interferes with the nucleation of microtubules. Linear oligomers are also formed in mixtures of GTP-tubulin and GDP-tubulin. The hydrolysis of GTP associated with heterologous interactions between GTP-tubulin and GDP-tubulin in the cooligomer takes place at a threefold faster rate than upon homologous interactions between GTP-tubulins. The implication of these results in a model of vectorial GTP hydrolysis in microtubule assembly is discussed.

Differences in the regulation of microtubule dynamics by microtubule-associated proteins MAP1B and MAP2

The regulation of microtubule dynamics in vitro by microtubule-associated proteins (MAPs) was examined, using purified porcine MAP1B and MAP2. MAP1B has a significantly smaller effect on the observed critical concentration for microtubule assembly than MAP2. Assembly is faster in the presence of either MAP, and the resulting microtubules are shorter, indicating that nucleation is substantially promoted by the MAPs. Both MAPs stabilise the microtubule lattice as observed from podophyllotoxin-induced disassembly, but the effect of MAP1B is weaker than the effect of MAP2. At steady-state of assembly MAP1B still allows microtubule dynamic instability to occur as inferred from microtubule length changes. The comparison of the effects of MAP1B and MAP2 indicates that the reduction of the observed critical concentration is attributable to the reduction of the depolymerisation rate and correlates with the extent of suppression of dynamic instability. Numerical simulations illustrate that microtubule dynamics are strongly influenced by relatively small changes in the strength of a limited subset of subunit interactions in the lattice. The observed characteristic differences between the MAPs may be important for the regulation of distinct populations of microtubules which coexist in the same cell, where differences in stability and dynamics may be essential for their different spatial roles as, for example, in developing neurons.

Polymerization of microtubule-associated protein tau under near-physiological conditions

Neurofibrillary tangles, which form in certain degenerating neurons in the brains of patients with Alzheimer's disease, are amassed from filaments having a straight or paired helical morphology. Solubilization of these filaments reveals that they are composed of the microtubule-associated protein tau. It has not previously been shown, however, that tau will assemble to form filaments of similar morphology under conditions representative of the intracellular environment. We have succeeded in forming such filaments using tau purified from porcine or rat microtubules. The filaments are relatively straight with narrowing at irregular intervals, and are about 10 nm wide, a morphology similar to that of straight filaments seen in Alzheimer's disease neurofibrillary tangles. At tau concentrations of 1-10 microM, in vitro assembly occurs at physiological pH, ionic strength, temperature, and reducing potential, and each one of these factors modulates the reaction. Assembly is judged to be only slowly reversible by the exponential rather than normal distribution of filament lengths, and by the limited disassembly observed under conditions which inhibit polymerization. Tau purified directly from whole brain tissue rather than from microtubules does not polymerize under conditions described in this report.

Microtubule associated protein 1B (MAP1B) promotes efficient tubulin polymerisation in vitro

The effect of MAP1B on tubulin polymerisation has been examined in reconstitution experiments using purified tubulin and MAP1B. Under the assembly conditions used, tubulin alone was incapable of polymerising, but addition of MAP1B resulted in rapid assembly into microtubules. The kinetics of MAP1B-promoted microtubule assembly examined using pseudo-first-order plots show that assembly is described by a single reaction rate. The calculated association rate constant for MAP1B was about 200 x 1096) M-1.s-1 and this constant was one order of magnitude higher when compared with that for MAP2-promoted assembly.

Unique functional characteristics of the polymerization and MAP binding regulatory domains of plant tubulin

An understanding of the regulation of microtubule polymerization and dynamics in plant cells requires biochemical information on the structures, functions, and molecular interactions of plant tubulin and microtubule-associated proteins (MAPs) that regulate microtubule function. We have probed the regulatory domain and polymerization domain of purified maize tubulin using MAP2, an extensively characterized mammalian neuronal MAP. MAP2 bound to the surface of preformed, taxol-stabilized maize microtubules, with binding saturation occurring with one MAP2 molecule per five to six tubulin dimers, as it does with mammalian microtubules. MAP2 binding and dissociation analyses revealed two affinity classes of binding sites on maize microtubules: a high-affinity site 12 dimers apart that may be homologous to the mammalian MAP2 binding site and an additional low-affinity site also 12 dimers apart that may be homologous to the mammalian tau binding site. MAP2 corrected in vitro folding errors in taxol-stabilized maize microtubules and reduced the critical concentration of maize tubulin polymerization eightfold, from 8.3 to 1.0 microM. However, MAP2 dissociated much more readily from maize microtubules than from mammalian microtubules and induced the assembly of maize tubulin into aberrant helical ribbon polymers that remained stable for prolonged periods. Our results indicated that MAP2 binds to maize tubulin via a partially specific, low-fidelity interaction that reflects unique structural and functional properties of the polymerization and regulatory domains of plant tubulin and possibly of the tubulin binding domains of undocumented MAPs that regulate microtubule function in plant cells.

Sulphonate buffers affect the recovery of microtubule-associated proteins MAP1 and MAP2: evidence that MAP1A promotes microtubule assembly

The influence of two commonly used sulphonate buffers, PIPES and MES, on the in vitro assembly of bovine brain microtubule protein was examined. Microtubule assembly was monitored by turbimetry and, after centrifugation, the polymerised protein was analysed by SDS-PAGE and western blotting. Assembly in MES when compared with PIPES resulted in a higher recovery of microtubule proteins at both pH 6.4 and pH 6.9 and in an altered protein composition. The buffer pH affected the total amount of protein polymerised but did not significantly affect the protein composition. At both pH conditions the recovery of HMW-MAPs was markedly increased in MES buffer and this increase was mostly due to an increase in the amount of MAP1.

The unpolymerized form of tubulin is the target for microtubule inhibition by photoactivated tetra(4-sulfonatophenyl)porphine

Several porphyrins, including tetra(4-sulfonatophenyl)porphine, sensitize cells to photoinactivation. The treatment leads to an accumulation of cells in mitosis, directly or indirectly due to a perturbation of the mitotic spindle. The present work relates to the target for this mode of action. Cells incubated with tetra(4-sulfonatophenyl)porphine were exposed to light and the microtubules were quantified 30 min after light exposure. The amount of microtubules decreased with increasing fluences. The reduction in the amount of microtubules after light exposure was enhanced by prior treatment with nocodazole (1 microgram/ml for 20 min) or low temperature (1 degree C for 60 min). When nocodazole was combined with the photochemical treatment the extent of the inhibition of microtubule formation was dose-dependent only for the lowest fluences applied. Additional light exposure did not further reduce the amount of microtubules 30 min after light exposure. The results presented indicate that the unpolymerized fraction of tubulin is the target for photochemical inhibition of microtubule formation.

Identification of an inhibitor of microtubule assembly present in juvenile brain which displays a novel mechanism of action involving suppression of self-nucleation

An inhibitor of microtubule assembly has been identified and partially purified from microtubule-depleted brain extracts from day-old chicks and 4-month-old calf. This inhibitor suppresses the self-nucleation of microtubules in vitro with minimal effect upon the final extent of assembly. It may have a developmental role in vivo as it is not detected in adult brain from either cattle or rabbit.

Kinetics of GTP hydrolysis during the assembly of chick brain MAP2-tubulin microtubule protein

The kinetics of GTP hydrolysis during microtubule assembly have been examined using chick brain MAP2-tubulin microtubule protein in a NaCl-supplemented buffer. The elongating microtubules terminate in a 'GTP cap', since the kinetics of GTP hydrolysis are slower than those of subunit addition. GTP hydrolysis is (a) stoichiometric, (b) occurs as a vectorial wave as the initial rate of hydrolysis is proportional to the molar concentration of microtubule ends and not to the initial rate of subunit addition, and (c) either does not occur, or occurs only at a much lower rate, in the terminal subunits.

Assembly of chick brain MAP2-tubulin microtubule protein. Analysis of tubulin subunit flux rates by immunofluorescence microscopy

A filter-based immunofluorescence-microscopy method for obtaining microtubule lengths has been developed and evaluated. Kinetic constants and mean lengths obtained show close agreement with those obtained by complementary methods applied to chick brain MAP2-tubulin microtubule protein in NaCl-supplemented buffer. The filter-based method has been used to estimate tubulin subunit flux (Jon) resulting from isothermal dilution of microtubule populations to various free tubulin concentrations, (c). This experimental Jon(c) plot is significantly different from that predicted by a variety of theoretical models, but is consistent with a 'lateral cap' model of dynamic instability [Bayley, Schilstra & Martin (1990) J. Cell. Sci. 95, 33-48] adapted to accommodate the observed vectorial GTP hydrolysis.

Phosphoinositide 3-kinase: a new effector in signal transduction?

Interest in phosphoinositide 3-kinase (PI 3-kinase) has been fuelled by its identification as a major phosphotyrosyl protein detected in cells following growth factor stimulation and oncogenic transformation. It is found complexed with activated growth factor receptors and non-receptor tyrosine kinases, thus suggesting that it participates in the signal transduction pathways initiated by the activation of tyrosine kinases. PI 3-kinase phosphorylates the 3-position in the inositol ring of the well known inositol phospholipids in vitro giving phosphatidylinositol 3-phosphate, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate [PtdIns3P, PtdIns(3,4)P2 and PtdIns(3,4,5)P3], respectively. The cellular levels of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 rapidly increase in circumstances where PI 3-kinase becomes complexed with tyrosine kinases. Accumulation of the same lipids also occurs in platelets and neutrophils following stimulation of G-protein linked alpha-thrombin and chemotactic peptide receptors, respectively, leading to speculation that one or both of these lipids is a new second messenger whose function is not yet known. This review brings together recent information on the isolation, characterization and regulation of PI 3-kinase, the cellular occurrence of 3-phosphorylated inositol phospholipids and possible functions of the PI 3-kinase pathway in cell signalling.