Role of GTP hydrolysis in microtubule dynamics: information from a slowly hydrolyzable analogue, GMPCPP

Microtubule capture by CENP-E silences BubR1-dependent mitotic checkpoint signaling

The mitotic checkpoint is the major cell cycle control mechanism for maintaining chromosome content in multicellular organisms. Prevention of premature onset of anaphase requires activation at unattached kinetochores of the BubR1 kinase, which acts with other components to generate a diffusible “stop anaphase” inhibitor. Not only does direct binding of BubR1 to the centromere-associated kinesin family member CENP-E activate its essential kinase, binding of a motorless fragment of CENP-E is shown here to constitutively activate BubR1 bound at kinetochores, producing checkpoint signaling that is not silenced either by spindle microtubule capture or the tension developed at those kinetochores by other components. Using purified BubR1, microtubules, and CENP-E, microtubule capture by the CENP-E motor domain is shown to silence BubR1 kinase activity in a ternary complex of BubR1–CENP-E–microtubule. Together, this reveals that CENP-E is the signal transducing linker responsible for silencing BubR1-dependent mitotic checkpoint signaling through its capture at kinetochores of spindle microtubules.

Biochemical character of the purified OsRAA1, a novel rice protein with GTP-binding activity, and its expression pattern in Oryza sativa

OsRAA1, as previously reported, is a novel conserved protein in plants and plays an important role in rice root development. Overexpression of OsRAA1 results in reduced growth of primary roots and an increased number of adventitious roots. The biochemical functions and expression patterns of OsRAA1, however, remain poorly understood. To obtain purified OsRAA1 for biochemical analysis, the coding region was amplified by RT-PCR and expressed as a fusion protein with glutathione S-transferase in Escherichia coli. The antibodies to OsRAA1 were prepared by a synthetic 15-residue peptide (YYEDPSLFQFHKRGS) cross-linked with bovine serum albumin. Results of isotope labeling experiments suggested that OsRAA1 had binding activities with [alpha-32P]-GTP. The immunoprecipitation data showed that OsRAA1 had tissue-specific expression in roots and spikes rather than young shoots and leaves, which was consistent with its transcriptional expression. Our results indicate that OsRAA1 GTP-binding activity may contribute to the regulation mechanism of root development mediated by OsRAA1.

Interactions between CLIP-170, tubulin, and microtubules: implications for the mechanism of Clip-170 plus-end tracking behavior

CLIP-170 belongs to a group of proteins (+TIPs) with the enigmatic ability to dynamically track growing microtubule plus-ends. CLIP-170 regulates microtubule dynamics in vivo and has been implicated in cargo-microtubule interactions in vivo and in vitro. Though plus-end tracking likely has intimate connections to +TIP function, little is known about the mechanism(s) by which this dynamic localization is achieved. Using a combination of biochemistry and live cell imaging, we provide evidence that CLIP-170 tracks microtubule plus-ends by a preassociation, copolymerization, and regulated release mechanism. As part of this analysis, we find that CLIP-170 has a stronger affinity for tubulin dimer than for polymer, and that CLIP-170 can distinguish between GTP- and GDP-like polymer. This work extends the previous analysis of CLIP-170 behavior in vivo and complements the existing fluorescence microscope characterization of CLIP-170 interactions with microtubules in vitro. In particular, these data explain observations that CLIP-170 localizes to newly polymerized microtubules in vitro but cannot track microtubule plus-ends in vitro. These observations have implications for the functions of CLIP-170 in regulating microtubule dynamics.

Mechanochemical model of microtubule structure and self-assembly kinetics

Microtubule self-assembly is largely governed by the chemical kinetics and thermodynamics of tubulin-tubulin interactions. An important aspect of microtubule assembly is that hydrolysis of the beta-tubulin-associated GTP promotes protofilament curling. Protofilament curling presumably drives the transition from tip structures associated with growth (sheetlike projections and blunt ends) to those associated with shortening (rams' horns and frayed ends), and transitions between these structures have been proposed to be important for growth-shortening transitions. However, previous models for microtubule dynamic instability have not considered such structures or mechanics explicitly. Here we present a three-dimensional model that explicitly incorporates mechanical stress and strain within the microtubule lattice. First, we found that the model recapitulates three-dimensional tip structures and rates of assembly and disassembly for microtubules grown under standard conditions, and we propose that taxol may stabilize microtubule growth by reducing flexural rigidity. Second, in contrast to recent suggestions, it was determined that sheetlike tips are more likely to undergo catastrophe than blunt tips. Third, partial uncapping of the tubulin-GTP cap provides a possible mechanism for microtubule pause events. Finally, simulations of the binding and structural effects of XMAP215 produced the experimentally observed growth and shortening rates, and tip structure.

The mechanism, function and regulation of depolymerizing kinesins during mitosis

Kinesins are motor proteins that use the hydrolysis of ATP to do mechanical work. Most of these motors translocate cargo along the surface of the microtubule (MT). However, a subfamily of these motors (Kin-I kinesins) can destabilize MTs directly from their ends. This distinct ability makes their activity crucial during mitosis, when reordering of the MT cytoskeleton is most evident. Recently, much work has been done to elucidate the structure and mechanism of depolymerizing kinesins, particularly those of the mammalian kinesin mitotic centromere-associated kinesin (MCAK). In addition, new regulatory factors have been discovered that shed light on the regulation and precise role of Kin-I kinesins during mitosis.

A molecular-mechanical model of the microtubule

Dynamic instability of MTs is thought to be regulated by biochemical transformations within tubulin dimers that are coupled to the hydrolysis of bound GTP. Structural studies of nucleotide-bound tubulin dimers have recently provided a concrete basis for understanding how these transformations may contribute to MT dynamic instability. To analyze these ideas, we have developed a molecular-mechanical model in which structural and biochemical properties of tubulin are used to predict the shape and stability of MTs. From simple and explicit features of tubulin, we define bond energy relationships and explore the impact of their variations on integral MT properties. This modeling provides quantitative predictions about the GTP cap. It specifies important mechanical features underlying MT instability and shows that this property does not require GTP-hydrolysis to alter the strength of tubulin-tubulin bonds. The MT plus end is stabilized by at least two layers of GTP-tubulin subunits, whereas the minus end requires at least one; this and other differences between the ends are explained by asymmetric force balances. Overall, this model provides a new link between the biophysical characteristics of tubulin and the physiological behavior of MTs. It will also be useful in building a more complete description of MT dynamics and mechanics.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Interactions of tobacco microtubule-associated protein MAP65-1b with microtubules

Tobacco microtubule associated protein (MAP65) (NtMAP65s) constitute a family of microtubule-associated proteins with apparent molecular weight around 65 kDa that collectively induce microtubule bundling and promote microtubule assembly in vitro. They are associated with most of the tobacco microtubule arrays in situ. Recently, three NtMAP65s belonging to the NtMAP65-1 subfamily have been cloned. Here we investigated in vitro the biochemical properties of one member of this family, the tobacco NtMAP65-1b. We demonstrated that recombinant NtMAP65-1b is a microtubule-binding and a microtubule-bundling protein. NtMAP65-1b has no effect on microtubule polymerization rate and binds microtubules with an estimated equilibrium constant of dissociation (K(d)) of 0.57 micro m. Binding of NtMAP65-1b to microtubules occurs through the carboxy-terminus of tubulin, as NtMAP65-1b was no longer able to bind subtilisin-digested tubulin. In vitro, NtMAP65-1b stabilizes microtubules against depolymerization induced by cold, but not against katanin-induced destabilization. The biological implications of these results are discussed.

Mechanism of Microtubule Stabilization by Doublecortin

Neurons undertake an amazing journey from the center of the developing mammalian brain to the outer layers of the cerebral cortex. Doublecortin, a component of the microtubule cytoskeleton, is essential in postmitotic neurons and was identified because its mutation disrupts human brain development. Doublecortin stabilizes microtubules and stimulates their polymerization but has no homology with other MAPs. We used electron microscopy to characterize microtubule binding by doublecortin and visualize its binding site. Doublecortin binds selectively to 13 protofilament microtubules, its in vivo substrate, and also causes preferential assembly of 13 protofilament microtubules. This specificity was explained when we found that doublecortin binds between the protofilaments from which microtubules are built, a previously uncharacterized binding site that is ideal for microtubule stabilization. These data reveal the structural basis for doublecortin's binding selectivity and provide insight into its role in maintaining microtubule architecture in maturing neurons.

Energetics of the cooperative assembly of cell division protein FtsZ and the nucleotide hydrolysis switch

FtsZ is the first protein recruited to the bacterial division site, where it forms the cytokinetic Z ring. We have determined the functional energetics of FtsZ assembly, employing FtsZ from the thermophilic Archaea Methanococcus jannaschii bound to GTP, GMPCPP, GDP, or GMPCP, under different solution conditions. FtsZ oligomerizes in a magnesium-insensitive manner. FtsZ cooperatively assembles with magnesium and GTP or GMPCPP into large polymers, following a nucleated condensation polymerization mechanism, under nucleotide hydrolyzing and non-hydrolyzing conditions. The effect of temperature on the critical concentration indicates polymer elongation with an apparent heat capacity change of -800 +/- 100 cal mol-1 K-1 and positive enthalpy and entropy changes, compatible with axial hydrophobic contacts of each FtsZ in the polymer, and predicts optimal polymer stability near 75 degrees C. Assembly entails the binding of one medium affinity magnesium ion and the uptake of one proton per FtsZ. Interestingly, GDP- or GMPCP-liganded FtsZ cooperatively form helically curved polymers, with an elongation only 1-2 kcal mol-1 more unfavorable than the straight polymers formed with nucleotide triphosphate, suggesting a physiological requirement for FtsZ polymerization inhibitors. This GTP hydrolysis switch should provide the basic properties for FtsZ polymer disassembly and its functional dynamics.

The majority of the Saccharomyces cerevisiae septin complexes do not exchange guanine nucleotides

We show here that affinity-purified Saccharomyces cerevisiae septin complexes contain stoichiometric amounts of guanine nucleotides, specifically GTP and GDP. Using a (15)N-dilution assay read-out by liquid chromatography-tandem mass spectrometry, we determined that the majority of the bound guanine nucleotides do not turn over in vivo during one cell cycle period. In vitro, the isolated S. cerevisiae septin complexes have similar GTP binding and hydrolytic properties to the Drosophila septin complexes (Field, C. M., al-Awar, O., Rosenblatt, J., Wong, M. L., Alberts, B., and Mitchison, T. J. (1996) J. Cell Biol. 133, 605-616). In particular, the GTP turnover of septins is very slow when compared with the GTP turnover for Ras-like GTPases. We conclude that bound GTP and GDP play a structural, rather then regulatory, role for the majority of septins in proliferating cells as GTP does for alpha-tubulin.

EB1-microtubule interactions in Xenopus egg extracts: role of EB1 in microtubule stabilization and mechanisms of targeting to microtubules

EB1 targets to polymerizing microtubule ends, where it is favorably positioned to regulate microtubule polymerization and confer molecular recognition of the microtubule end. In this study, we focus on two aspects of the EB1-microtubule interaction: regulation of microtubule dynamics by EB1 and the mechanism of EB1 association with microtubules. Immunodepletion of EB1 from cytostatic factor-arrested M-phase Xenopus egg extracts dramatically reduced microtubule length; this was complemented by readdition of EB1. By time-lapse microscopy, EB1 increased the frequency of microtubule rescues and decreased catastrophes, resulting in increased polymerization and decreased depolymerization and pausing. Imaging of EB1 fluorescence revealed a novel structure: filamentous extensions on microtubule plus ends that appeared during microtubule pauses; loss of these extensions correlated with the abrupt onset of polymerization. Fluorescent EB1 localized to comets at the polymerizing plus ends of microtubules in cytostatic factor extracts and uniformly along the lengths of microtubules in interphase extracts. The temporal decay of EB1 fluorescence from polymerizing microtubule plus ends predicted a dissociation half-life of seconds. Fluorescence recovery after photobleaching also revealed dissociation and rebinding of EB1 to the microtubule wall with a similar half-life. EB1 targeting to microtubules is thus described by a combination of higher affinity binding to polymerizing ends and lower affinity binding along the wall, with continuous dissociation. The latter is likely to be attenuated in interphase. The highly conserved effect of EB1 on microtubule dynamics suggests it belongs to a core set of regulatory factors conserved in higher organisms, and the complex pattern of EB1 targeting to microtubules could be exploited by the cell for coordinating microtubule behaviors.

Mobility of taxol in microtubule bundles

Mobility of taxol inside microtubules was investigated using fluorescence recovery after photobleaching on flow-aligned bundles. Bundles were made of microtubules with either GMPCPP or GTP at the exchangeable site on the tubulin dimer. Recovery times were sensitive to bundle thickness and packing, indicating that taxol molecules are able to move laterally through the bundle. The density of open binding sites along a microtubule was varied by controlling the concentration of taxol in solution for GMPCPP samples. With >63% sites occupied, recovery times were independent of taxol concentration and, therefore, inversely proportional to the microscopic dissociation rate, k(off). It was found that 10k(off)(GMPCPP) approximately equal k(off)(GTP), consistent with, but not fully accounting for, the difference in equilibrium constants for taxol on GMPCPP and GTP microtubules. With <63% sites occupied, recovery times decreased as approximately [Tax](-1/5) for both types of microtubules. We conclude that the diffusion of taxol inside the microtubule bundle is hindered by rebinding events when open sites are within approximately 7 nm of each other.

Identification of XMAP215 as a microtubule-destabilizing factor in Xenopus egg extract by biochemical purification

Microtubules (MTs) polymerized with GMPCPP, a slowly hydrolyzable GTP analogue, are stable in buffer but are rapidly depolymerized in Xenopus egg extracts. This depolymerization is independent of three previously identified MT destabilizers (Op18, katanin, and XKCM1/KinI). We purified the factor responsible for this novel depolymerizing activity using biochemical fractionation and a visual activity assay and identified it as XMAP215, previously identified as a prominent MT growth-promoting protein in Xenopus extracts. Consistent with the purification results, we find that XMAP215 is necessary for GMPCPP-MT destabilization in extracts and that recombinant full-length XMAP215 as well as an NH2-terminal fragment have depolymerizing activity in vitro. Stimulation of depolymerization is specific for the MT plus end. These results provide evidence for a robust MT-destabilizing activity intrinsic to this microtubule-associated protein and suggest that destabilization may be part of its essential biochemical functions. We propose that the substrate in our assay, GMPCPP-stabilized MTs, serves as a model for the pause state of MT ends and that the multiple activities of XMAP215 are unified by a mechanism of antagonizing MT pauses.

Dynamics and mechanics of the microtubule plus end

An important function of microtubules is to move cellular structures such as chromosomes, mitotic spindles and other organelles around inside cells. This is achieved by attaching the ends of microtubules to cellular structures; as the microtubules grow and shrink, the structures are pushed or pulled around the cell. How do the ends of microtubules couple to cellular structures, and how does this coupling regulate the stability and distribution of the microtubules? It is now clear that there are at least three properties of a microtubule end: it has alternate structures; it has a biochemical transition defined by GTP hydrolysis; and it forms a distinct target for the binding of specific proteins. These different properties can be unified by thinking of the microtubule as a molecular machine, which switches between growing and shrinking modes. Each mode is associated with a specific end structure on which end-binding proteins can assemble to modulate dynamics and couple the dynamic properties of microtubules to the movement of cellular structures.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

The kinesin-related protein MCAK is a microtubule depolymerase that forms an ATP-hydrolyzing complex at microtubule ends

MCAK belongs to the Kin I subfamily of kinesin-related proteins, a unique group of motor proteins that are not motile but instead destabilize microtubules. We show that MCAK is an ATPase that catalytically depolymerizes microtubules by accelerating, 100-fold, the rate of dissociation of tubulin from microtubule ends. MCAK has one high-affinity binding site per protofilament end, which, when occupied, has both the depolymerase and ATPase activities. MCAK targets protofilament ends very rapidly (on-rate 54 micro M(-1).s(-1)), perhaps by diffusion along the microtubule lattice, and, once there, removes approximately 20 tubulin dimers at a rate of 1 s(-1). We propose that up to 14 MCAK dimers assemble at the end of a microtubule to form an ATP-hydrolyzing complex that processively depolymerizes the microtubule.

Self-organisation and forces in the microtubule cytoskeleton

Modern microscopy techniques allow us to observe specifically tagged proteins in live cells. We can now see directly that many cellular structures, for example mitotic spindles, are in fact dynamic assemblies. Their apparent stability results from out-of-equilibrium stochastic interactions at the molecular level. Recent studies have shown that the spindles can form even after centrosomes are destroyed, and that they can even form around DNA-coated beads devoid of kinetochores. Moreover, conditions have been produced in which microtubule asters interact even in the absence of chromatin. Together, these observations suggest that the spindle can be experimentally deconstructed, and that its defining characteristics can be studied in a simplified context, in the absence of the full division machinery.

Repeat motifs of tau bind to the insides of microtubules in the absence of taxol

The tau family of microtubule-associated proteins has a microtubule-binding domain which includes three or four conserved sequence repeats. Pelleting assays show that when tubulin and tau are co- assembled into microtubules, the presence of taxol reduces the amount of tau incorporated. In the absence of taxol, strong binding sites for tau are filled by one repeat motif per tubulin dimer; additional tau molecules bind more weakly. We have labelled a repeat motif with nanogold and used three-dimensional electron cryomicroscopy to compare images of microtubules assembled with labelled or unlabelled tau. With kinesin motor domains bound to the microtubule outer surface to distinguish between alpha- and beta-tubulin, we show that the gold label lies on the inner surface close to the taxol binding site on beta-tubulin. Loops within the repeat motifs of tau have sequence similarity to an extended loop which occupies a site in alpha-tubulin equivalent to the taxol-binding pocket in beta-tubulin. We propose that loops in bound tau stabilize microtubules in a similar way to taxol, although with lower affinity so that assembly is reversible.

EB1-microtubule interactions in Xenopus egg extracts: role of EB1 in microtubule stabilization and mechanisms of targeting to microtubules

EB1 targets to polymerizing microtubule ends, where it is favorably positioned to regulate microtubule polymerization and confer molecular recognition of the microtubule end. In this study, we focus on two aspects of the EB1-microtubule interaction: regulation of microtubule dynamics by EB1 and the mechanism of EB1 association with microtubules. Immunodepletion of EB1 from cytostatic factor-arrested M-phase Xenopus egg extracts dramatically reduced microtubule length; this was complemented by readdition of EB1. By time-lapse microscopy, EB1 increased the frequency of microtubule rescues and decreased catastrophes, resulting in increased polymerization and decreased depolymerization and pausing. Imaging of EB1 fluorescence revealed a novel structure: filamentous extensions on microtubule plus ends that appeared during microtubule pauses; loss of these extensions correlated with the abrupt onset of polymerization. Fluorescent EB1 localized to comets at the polymerizing plus ends of microtubules in cytostatic factor extracts and uniformly along the lengths of microtubules in interphase extracts. The temporal decay of EB1 fluorescence from polymerizing microtubule plus ends predicted a dissociation half-life of seconds. Fluorescence recovery after photobleaching also revealed dissociation and rebinding of EB1 to the microtubule wall with a similar half-life. EB1 targeting to microtubules is thus described by a combination of higher affinity binding to polymerizing ends and lower affinity binding along the wall, with continuous dissociation. The latter is likely to be attenuated in interphase. The highly conserved effect of EB1 on microtubule dynamics suggests it belongs to a core set of regulatory factors conserved in higher organisms, and the complex pattern of EB1 targeting to microtubules could be exploited by the cell for coordinating microtubule behaviors.

Structural microtubule cap: stability, catastrophe, rescue, and third state

Microtubules polymerize from GTP-liganded tubulin dimers, but are essentially made of GDP-liganded tubulin. We investigate the tug-of-war resulting from the fact that GDP-liganded tubulin favors a curved configuration, but is forced to remain in a straight one when part of a microtubule. We point out that near the end of a microtubule, the proximity of the end shifts the balance in this tug-of-war, with some protofilament bending as result. This somewhat relaxes the microtubule lattice near its end, resulting in a structural cap. This structural cap thus is a simple mechanical consequence of two well-established facts: protofilaments made of GDP-liganded tubulin have intrinsic curvature, and microtubules are elastic, made from material that can yield to forces, in casu its own intrinsic forces. We explore possible properties of this structural cap, and demonstrate 1) how it allows both polymerization from GTP-liganded tubulin and rapid depolymerization in its absence; 2) how rescue can occur; 3) how a third, meta-stable intermediate state is possible and can explain some experimental results; and 4) how the tapered tips observed at polymerizing microtubule ends are stabilized during growth, though unable to accommodate a lateral cap. This scenario thus supports the widely accepted GTP-cap model by suggesting a stabilizing mechanism that explains the many aspects of dynamic instability.

Quantum computation in brain microtubules: decoherence and biological feasibility

The Penrose-Hameroff orchestrated objective reduction (orch. OR) model assigns a cognitive role to quantum computations in microtubules within the neurons of the brain. Despite an apparently "warm, wet, and noisy" intracellular milieu, the proposal suggests that microtubules avoid environmental decoherence long enough to reach threshold for "self-collapse" (objective reduction) by a quantum gravity mechanism put forth by Penrose. The model has been criticized as regards the issue of environmental decoherence, and a recent report by Tegmark finds that microtubules can maintain quantum coherence for only 10(-13) s, far too short to be neurophysiologically relevant. Here, we critically examine the decoherence mechanisms likely to dominate in a biological setting and find that (1) Tegmark's commentary is not aimed at an existing model in the literature but rather at a hybrid that replaces the superposed protein conformations of the orch. OR theory with a soliton in superposition along the microtubule; (2) recalculation after correcting for differences between the model on which Tegmark bases his calculations and the orch. OR model (superposition separation, charge vs dipole, dielectric constant) lengthens the decoherence time to 10(-5)-10(-4) s; (3) decoherence times on this order invalidate the assumptions of the derivation and determine the approximation regime considered by Tegmark to be inappropriate to the orch. OR superposition; (4) Tegmark's formulation yields decoherence times that increase with temperature contrary to well-established physical intuitions and the observed behavior of quantum coherent states; (5) incoherent metabolic energy supplied to the collective dynamics ordering water in the vicinity of microtubules at a rate exceeding that of decoherence can counter decoherence effects (in the same way that lasers avoid decoherence at room temperature); (6) microtubules are surrounded by a Debye layer of counterions, which can screen thermal fluctuations, and by an actin gel that might enhance the ordering of water in bundles of microtubules, further increasing the decoherence-free zone by an order of magnitude and, if the dependence on the distance between environmental ion and superposed state is accurately reflected in Tegmark's calculation, extending decoherence times by three orders of magnitude; (7) topological quantum computation in microtubules may be error correcting, resistant to decoherence; and (8) the decohering effect of radiative scatterers on microtubule quantum states is negligible. These considerations bring microtubule decoherence into a regime in which quantum gravity could interact with neurophysiology.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

The human EMAP-like protein-70 (ELP70) is a microtubule destabilizer that localizes to the mitotic apparatus

In this report, we show that the echinoderm microtubule (MT)-associated protein (EMAP) and related EMAP-like proteins (ELPs) share a similar domain organization with a highly conserved hydrophobic ELP (HELP) domain and a large tryptophan-aspartic acid (WD) repeat domain. To determine the function of mammalian ELPs, we generated antibodies against a 70-kDa human ELP and showed that ELP70 coassembled with MTs in HeLa cell extracts and colocalized with MTs in the mitotic apparatus. To determine whether ELP70 bound to MTs directly, human ELP70 was expressed and purified to homogeneity from baculovirus-infected Sf9 cells. Purified ELP70 bound to purified MTs with a stoichiometry of 0.40 +/- 0.04 mol of ELP70/mol of tubulin dimer and with an intrinsic dissociation constant of 0.44 +/- 0.13 microm. Using a nucleated assembly assay and video-enhanced differential interference contrast microscopy, we demonstrated that ELP70 reduced seeded nucleation, reduced the growth rate, and promoted MT catastrophes in a concentration-dependent manner. As a result, ELP70-containing MTs were significantly shorter than MTs assembled from tubulin alone. These data indicate that ELP70 is a novel MT destabilizer. A lateral destabilization model is presented to describe ELP70's effects on microtubules.

Dominant-lethal alpha-tubulin mutants defective in microtubule depolymerization in yeast

The dynamic instability of microtubules has long been understood to depend on the hydrolysis of GTP bound to beta-tubulin, an event stimulated by polymerization and necessary for depolymerization. Crystallographic studies of tubulin show that GTP is bound by beta-tubulin at the longitudinal dimer-dimer interface and contacts particular alpha-tubulin residues in the next dimer along the protofilament. This structural arrangement suggests that these contacts could account for assembly-stimulated GTP hydrolysis. As a test of this hypothesis, we examined, in yeast cells, the effect of mutating the alpha-tubulin residues predicted, on structural grounds, to be involved in GTPase activation. Mutation of these residues to alanine (i.e., D252A and E255A) created poisonous alpha-tubulins that caused lethality even as minor components of the alpha-tubulin pool. When the mutant alpha-tubulins were expressed from the galactose-inducible promoter of GAL1, cells rapidly acquired aberrant microtubule structures. Cytoplasmic microtubules were largely bundled, spindle assembly was inhibited, preexisting spindles failed to completely elongate, and occasional, stable microtubules were observed unattached to spindle pole bodies. Time-lapse microscopy showed that microtubule dynamics had ceased. Microtubules containing the mutant proteins did not depolymerize, even in the presence of nocodazole. These data support the view that alpha-tubulin is a GTPase-activating protein that acts, during microtubule polymerization, to stimulate GTP hydrolysis in beta-tubulin and thereby account for the dynamic instability of microtubules.

Probing the transducin nucleotide binding site with GDP analogues

An affinity study between the G protein of the visual photoreceptor, transducin, and eight different non-hydrolyzable GDP analogues is described. Imidodiphosphate derivatives have been shown to exhibit good affinities to transducin. This very important heterotrimeric G protein is shown to be highly restrictive with regard to structural modifications of the nucleotide at the pyrophosphate moiety, at the 3' position on ribose, as well as at the N1 position of the purine.

NCD activation of tubulin polymerization

Tubulin dimer (tT) was purified from turkey erythrocytes. The motor domain of Drosophila non-claret disjunctional protein, NCD(335-700), was expressed in E. coli and purified. At 37 degrees C in the presence of GTP, the rate of polymerization of tT to microtubule (tMt) is accelerated over threefold by the presence of NCD(335-700). At 10 degrees C, the rate of tT polymerization is increased from zero, within experimental error, in the absence of NCD(335-700) to rates near those observed at 37 degrees C when NCD(335-700) is present. The NCD(335-700) concentration dependence of the rate indicated the reactive species was NCD(335-700)(n).tT, with n approximately 2. At 10 degrees C in the absence of GTP, polymerization does not occur, but tT activates NCD(335-700) MgATPase activity 10-fold. For the same conditions, using mians-NCD(335-700), which is modified with 2-(4'-maleimidylanilino) naphthalene-6-sulfonic acid, the apparent K(D) for binding to tT is 2.3 x 10(-5) M in the presence of MgADP. Replacing ADP with AMPPNP or ATP has a negligible effect on K(D). Mians-NCD(335-700) binding to tMt is 10-fold stronger than to tT. The above data indicate NCD(335-700) binds at a functional site on tT. The stoichiometry is consistent with the formation of NCD(335-700)(2).tT which in vitro accelerates self-assembly initiation and/or polymerization by binding a second tT in a position favorable for tubulin-tubulin interaction. The data suggest that in vivo functional NCD binding to microtubule involves one motor domain binding to alpha- and beta-subunits at the interface of two different tubulin dimers in a protofilament.

How motor proteins influence microtubule polymerization dynamics

The interplay between microtubules and microtubule-based motors is fundamental to basic aspects of cellular function, such as the intracellular transport of organelles and alterations in cellular morphology during cell locomotion and division. Motor proteins are unique in that they couple nucleotide hydrolysis to force production that can do work. The force transduction by proteins belonging to the kinesin and dynein superfamilies has been thought only to power movement of these motors along the surface of microtubules; however, a growing body of evidence, both genetic and biochemical, suggests that motors can also directly influence the polymerization dynamics of microtubules. For example, at the vertebrate kinetochore, motors interact directly with microtubule ends and modulate polymerization dynamics to orchestrate chromosome movements during mitosis. Although a role for motors in regulating microtubule length has been established, the mechanisms used by motors to promote microtubule growth or shrinkage are unclear, as is an understanding of why cells might choose motors to control dynamics rather than a variety of non-motor proteins known to affect microtubule stability. Elucidation of the exact mechanisms by which motors alter the exchange of tubulin subunits at microtubule ends in vitro may shed light on how microtubule stability is regulated to produce the array of dynamic behavior seen in cells.

Helical tubes of FtsZ from Methanococcus jannaschii

Bacterial cell division depends on the formation of a cytokinetic ring structure, the Z-ring. The bacterial tubulin homologue FtsZ is required for Z-ring formation. FtsZ assembles into various polymeric forms in vitro, indicating a structural role in the septum of bacteria. We have used recombinant FtsZ1 protein from M. jannaschii to produce helical tubes and sheets with high yield using the GTP analogue GMPCPP [guanylyl-(alpha,beta)-methylene-diphosphate]. The sheets appear identical to the previously reported Ca++-induced sheets of FtsZ from M. jannaschii that were shown to consist of 'thick'-filaments in which two protofilaments run in parallel. Tubes assembled either in Ca++ or in GMPCPP contain filaments whose dimensions indicate that they could be equivalent to the 'thick'-filaments in sheets. Some tubes are hollow but others are filled by additional protein density. Helical FtsZ tubes differ from eukaryotic microtubules in that the filaments curve around the filament axis with a pitch of approximately 430 A for Ca++-induced tubes or 590 - 620 A for GMPCPP. However, their assembly in vitro as well-ordered polymers over distances comparable to the inner circumference of a bacterium may indicate a role in vivo. Their size and stability make them suitable for use in motility assays.

The interaction of TOGp with microtubules and tubulin

TOGp is the human homolog of XMAP215, a Xenopus microtubule-associated protein that promotes rapid microtubule assembly at plus ends. These proteins are thought to be critical for microtubule assembly and/or mitotic spindle formation. To understand how TOGp interacts with the microtubule lattice, we cloned full-length TOGp and various truncations for expression in a reticulocyte lysate system. Based on microtubule co-pelleting assays, the microtubule binding domain is contained within a basic 600-amino acid region near the N terminus, with critical domains flanking a region homologous to the microtubule binding domain found in the related proteins Stu2p (S. cerevisiae) and Dis1 (S. pombe). Both full-length TOGp and the N-terminal fragment show enhanced binding to microtubule ends. Full-length TOGp also binds altered polymer lattice structures including parallel protofilament sheets, antiparallel protofilament sheets induced with zinc ions, and protofilament rings, suggesting that TOGp binds along the length of individual protofilaments. The C-terminal region of TOGp has a low affinity for microtubule polymer but binds tubulin dimer. We propose a model to explain the microtubule-stabilizing and/or assembly-promoting functions of the XMAP215/TOGp family of microtubule-associated proteins based on the binding properties we have identified.

Structural transitions at microtubule ends correlate with their dynamic properties in Xenopus egg extracts

Microtubules are dynamically unstable polymers that interconvert stochastically between growing and shrinking states by the addition and loss of subunits from their ends. However, there is little experimental data on the relationship between microtubule end structure and the regulation of dynamic instability. To investigate this relationship, we have modulated dynamic instability in Xenopus egg extracts by adding a catastrophe-promoting factor, Op18/stathmin. Using electron cryomicroscopy, we find that microtubules in cytoplasmic extracts grow by the extension of a two- dimensional sheet of protofilaments, which later closes into a tube. Increasing the catastrophe frequency by the addition of Op18/stathmin decreases both the length and frequency of the occurrence of sheets and increases the number of frayed ends. Interestingly, we also find that more dynamic populations contain more blunt ends, suggesting that these are a metastable intermediate between shrinking and growing microtubules. Our results demonstrate for the first time that microtubule assembly in physiological conditions is a two-dimensional process, and they suggest that the two-dimensional sheets stabilize microtubules against catastrophes. We present a model in which the frequency of catastrophes is directly correlated with the structural state of microtubule ends.

Straight and curved conformations of FtsZ are regulated by GTP hydrolysis

FtsZ assembles in vitro into protofilaments that can adopt two conformations-the straight conformation, which can assemble further into two-dimensional protofilament sheets, and the curved conformation, which forms minirings about 23 nm in diameter. Here, we describe the structure of FtsZ tubes, which are a variation of the curved conformation. In the tube the curved protofilament forms a shallow helix with a diameter of 23 nm and a pitch of 18 or 24 degrees. We suggest that this shallow helix is the relaxed structure of the curved protofilament in solution. We provide evidence that GTP favors the straight conformation while GDP favors the curved conformation. In particular, exclusively straight protofilaments and protofilament sheets are assembled in GMPCPP, a nonhydrolyzable GTP analog, or in GTP following chelation of Mg, which blocks GTP hydrolysis. Assembly in GDP produces exclusively tubes. The transition from straight protofilaments to the curved conformation may provide a mechanism whereby the energy of GTP hydrolysis is used to generate force for the constriction of the FtsZ ring in cell division.

Molecular characteristics of the centrosome

As an organizer of the microtubule cytoskeleton in animals, the centrosome has an important function. From the early light microscopic observation of the centrosome to examination by electron microscopy, the centrosome field is now in an era of molecular identification and precise functional analyses. Tables compiling centrosomal proteins and reviews on the centrosome are presented here and demonstrate how active the field is. However, despite this intense research activity, many classical questions are still unanswered. These include those regarding the precise function of centrioles, the mechanism of centrosome duplication and assembly, the origin of the centrosome, and the regulation and mechanism of the centrosomal microtubule nucleation activity. Fortunately, these questions are becoming elucidated based on experimental data discussed here. Given the fact that the centrosome is primarily a site of microtubule nucleation, special focus is placed on the process of microtubule nucleation and on the regulation of centrosomal microtubule nucleation capacity during the cell cycle and in some tissues.

Microtubule dynamics: Controlling split ends

The rapid switching between growth and shrinkage at microtubule ends is important for many cellular processes. Recent studies on the structure of the microtubule and on the mechanism of action of the microtubule regulators XKCM1 and OP18 have revealed how these switching events are regulated.

Characterization of two related Drosophila gamma-tubulin complexes that differ in their ability to nucleate microtubules

gamma-tubulin exists in two related complexes in Drosophila embryo extracts (Moritz, M., Y. Zheng, B.M. Alberts, and K. Oegema. 1998. J. Cell Biol. 142:1- 12). Here, we report the purification and characterization of both complexes that we name gamma-tubulin small complex (gammaTuSC; approximately 280,000 D) and Drosophila gammaTuRC ( approximately 2,200,000 D). In addition to gamma-tubulin, the gammaTuSC contains Dgrip84 and Dgrip91, two proteins homologous to the Spc97/98p protein family. The gammaTuSC is a structural subunit of the gammaTuRC, a larger complex containing about six additional polypeptides. Like the gammaTuRC isolated from Xenopus egg extracts (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578-583), the Drosophila gammaTuRC can nucleate microtubules in vitro and has an open ring structure with a diameter of 25 nm. Cryo-electron microscopy reveals a modular structure with approximately 13 radially arranged structural repeats. The gammaTuSC also nucleates microtubules, but much less efficiently than the gammaTuRC, suggesting that assembly into a larger complex enhances nucleating activity. Analysis of the nucleotide content of the gammaTuSC reveals that gamma-tubulin binds preferentially to GDP over GTP, rendering gamma-tubulin an unusual member of the tubulin superfamily.

CLIP-170 highlights growing microtubule ends in vivo

A chimera with the green fluorescent protein (GFP) has been constructed to visualize the dynamic properties of the endosome-microtubule linker protein CLIP170 (GFP-CLIP170). GFP-CLIP170 binds in stretches along a subset of microtubule ends. These fluorescent stretches appear to move with the growing tips of microtubules at 0.15-0.4 microm/s, comparable to microtubule elongation in vivo. Analysis of speckles along dynamic GFP-CLIP170 stretches suggests that CLIP170 treadmills on growing microtubule ends, rather than being continuously transported toward these ends. Drugs affecting microtubule dynamics rapidly inhibit movement of GFP-CLIP170 dashes. We propose that GFP-CLIP170 highlights growing microtubule ends by specifically recognizing the structure of a segment of newly polymerized tubulin.

Taxol selectively blocks microtubule dependent NF-kappaB activation by phorbol ester via inhibition of IkappaBalpha phosphorylation and degradation

Activation of the NF-kappa-B transcription factors has been shown to be directly influenced by changes in the microtubule cytoskeleton network. To better understand cytoskeletal regulation of NF-kappaB, experiments were performed to determine whether the microtubule (MT) stabilizing agent taxol could modulate NF-kappaB activation in the presence of different NF-kappa-B inducers. Pretreatment of murine NIH3T3 and human 293 cells with 5 microM taxol resulted in complete inhibition of phorbol, 12-myristate, 13-acetate (PMA) mediated NF-kappaB activation, detected as the loss of DNA binding and reduced NF-kappaB dependent reporter gene activity. Furthermore, in COS-7 and NIH3T3 cells, PMA-induced Ikappa-Balpha turnover was dramatically reduced in taxol treated cells, mediated via the inhibition of IkappaBalpha phosphorylation. However, taxol did not prevent TNF-alpha induced Ikappa-Balpha phosphorylation, degradation, or NF-kappaB activation, indicating that TNF-alpha acts through a microtubule-independent pathway. In vitro kinase assays with PMA stimulated cell extracts demonstrated that taxol reduced protein kinase C activity by 30%, thus implicating the loss of PKC activity as a possible regulatory target of taxol-mediated suppression of NF-kappa-B. Since PMA causes modulation of cytoarchitecture through PKC activation, microtubule integrity and cell morphology was analysed by indirect immunofluorescence. Both PMA and nocodazole, a MT depolymerizing agent, caused microtubule depolymerization, whereas TNF-alpha did not alter MT integrity; concomitant taxol treatment blocked both nocodazole and PMA induced depolymerization of MTs, as well as NF-kappaB induction, thus demonstrating a link between microtubule depolymerization and NF-kappaB activation. These observations illustrate a novel biological activity of taxol as a selective inhibitor of NF-kappa-B activity, suggesting a link between the state of microtubule integrity and gene regulation.

Dynamic localization of CLIP-170 to microtubule plus ends is coupled to microtubule assembly

CLIP-170 is a cytoplasmic linker protein that localizes to plus ends of microtubules in vivo. In this study, we have characterized the microtubule-binding properties of CLIP-170, to understand the mechanism of its plus end targeting. We show that the NH2-terminal microtubule-interacting domain of CLIP-170 alone localizes to microtubule plus ends when transfected into cells. Association of CLIP-170 with newly-formed microtubules was observed in cells microinjected with biotinylated tubulin, used as a tracer for growing microtubules. Using in vitro assays, association of CLIP-170 with recently polymerized tubulin is also seen. Cross-linking and sedimentation velocity experiments suggest association of CLIP-170 with nonpolymerized tubulin. We conclude from these experiments that the microtubule end targeting of CLIP-170 is closely linked to tubulin polymerization.

Kin I kinesins are microtubule-destabilizing enzymes

Using in vitro assays with purified proteins, we show that XKCM1 and XKIF2, two distinct members of the internal catalytic domain (Kin I) kinesin subfamily, catalytically destabilize microtubules using a novel mechanism. Both XKCM1 and XKIF2 influence microtubule stability by targeting directly to microtubule ends where they induce a destabilizing conformational change. ATP hydrolysis recycles XKCM1/XKIF2 for multiple rounds of action by dissociating a XKCM1/ XKIF2-tubulin dimer complex released upon microtubule depolymerization. These results establish Kin I kinesins as microtubule-destabilizing enzymes, distinguish them mechanistically from kinesin superfamily members that use ATP hydrolysis to translocate along microtubules, and have important implications for the regulation of microtubule dynamics and for the intracellular functions and evolution of the kinesin superfamily.

Dissociation of the tubulin-sequestering and microtubule catastrophe-promoting activities of oncoprotein 18/stathmin

Oncoprotein 18/stathmin (Op18) has been identified recently as a protein that destabilizes microtubules, but the mechanism of destabilization is currently controversial. Based on in vitro microtubule assembly assays, evidence has been presented supporting conflicting destabilization models of either tubulin sequestration or promotion of microtubule catastrophes. We found that Op18 can destabilize microtubules by both of these mechanisms and that these activities can be dissociated by changing pH. At pH 6.8, Op18 slowed microtubule elongation and increased catastrophes at both plus and minus ends, consistent with a tubulin-sequestering activity. In contrast, at pH 7.5, Op18 promoted microtubule catastrophes, particularly at plus ends, with little effect on elongation rates at either microtubule end. Dissociation of tubulin-sequestering and catastrophe-promoting activities of Op18 was further demonstrated by analysis of truncated Op18 derivatives. Lack of a C-terminal region of Op18 (aa 100-147) resulted in a truncated protein that lost sequestering activity at pH 6.8 but retained catastrophe-promoting activity. In contrast, lack of an N-terminal region of Op18 (aa 5-25) resulted in a truncated protein that still sequestered tubulin at pH 6.8 but was unable to promote catastrophes at pH 7.5. At pH 6. 8, both the full length and the N-terminal-truncated Op18 bound tubulin, whereas truncation at the C-terminus resulted in a pronounced decrease in tubulin binding. Based on these results, and a previous study documenting a pH-dependent change in binding affinity between Op18 and tubulin, it is likely that tubulin sequestering observed at lower pH resulted from the relatively tight interaction between Op18 and tubulin and that this tight binding requires the C-terminus of Op18; however, under conditions in which Op18 binds weakly to tubulin (pH 7.5), Op18 stimulated catastrophes without altering tubulin subunit association or dissociation rates, and Op18 did not depolymerize microtubules capped with guanylyl (alpha, beta)-methylene diphosphonate-tubulin subunits. We hypothesize that weak binding between Op18 and tubulin results in free Op18, which is available to interact with microtubule ends and thereby promote catastrophes by a mechanism that likely involves GTP hydrolysis.

Structural changes at microtubule ends accompanying GTP hydrolysis: information from a slowly hydrolyzable analogue of GTP, guanylyl (alpha,beta)methylenediphosphonate

Microtubules are dynamic polymers that interconvert between periods of slow growth and fast shrinkage. The energy driving this nonequilibrium behavior comes from the hydrolysis of GTP, which is required to destabilize the microtubule lattice. To understand the mechanism of this destabilization, cryo-electron microscopy was used to compare the structure of the ends of shrinking microtubules assembled in the presence of either GTP or the slowly hydrolyzable analogue guanylyl (alpha,beta)methylenediphosphonate (GMPCPP). Depolymerization was induced by cold or addition of calcium. With either nucleotide, we have observed curled oligomers at the ends of shrinking microtubules. However, GDP oligomers were consistently more curved than GMPCPP oligomers. This difference in curvature between depolymerizing GDP and GMPCPP protofilaments suggests that GTP hydrolysis is accompanied by an increase in curvature of the protofilaments, thereby destabilizing the lateral interactions between tubulin subunits in the microtubule lattice.

Microtubule polymerization dynamics

The polymerization dynamics of microtubules are central to their biological functions. Polymerization dynamics allow microtubules to adopt spatial arrangements that can change rapidly in response to cellular needs and, in some cases, to perform mechanical work. Microtubules utilize the energy of GTP hydrolysis to fuel a unique polymerization mechanism termed dynamic instability. In this review, we first describe progress toward understanding the mechanism of dynamic instability of pure tubulin and then discuss the function and regulation of microtubule dynamic instability in living cells.

A putative beta-tubulin phosphate-binding motif is involved in lateral microtubule protofilament interactions

We have investigated the role of a putative GTP-binding beta-tubulin motif in microtubule polymerization. A peptide containing residues 126-142 of the beta-tubulin subunit (peptide G) was synthesised and an antibody against it raised. Peptide G prevents the binding of GTP to tubulin and also microtubule polymerization but not the formation of vinblastine-induced tubulin spirals, suggesting that it may prevent lateral but not longitudinal tubulin-tubulin interactions. The antibody to peptide G shows little reaction with the interphase microtubule network, mitotic spindles or midbody of cultured cells, whereas it clearly reacts with vinblastine-induced paracrystals. These results suggest that this putative phosphate-binding site present in beta-tubulin could be involved in the lateral tubulin-tubulin interactions along the microtubule structure.

A metastable intermediate state of microtubule dynamic instability that differs significantly between plus and minus ends

The current two-state GTP cap model of microtubule dynamic instability proposes that a terminal crown of GTP-tubulin stabilizes the microtubule lattice and promotes elongation while loss of this GTP-tubulin cap converts the microtubule end to shortening. However, when this model was directly tested by using a UV microbeam to sever axoneme-nucleated microtubules and thereby remove the microtubule's GTP cap, severed plus ends rapidly shortened, but severed minus ends immediately resumed elongation (Walker, R.A., S. Inoué, and E.D. Salmon. 1989. J. Cell Biol. 108: 931-937). To determine if these previous results were dependent on the use of axonemes as seeds or were due to UV damage, or if they instead indicate an intermediate state in cap dynamics, we performed UV cutting of self-assembled microtubules and mechanical cutting of axoneme-nucleated microtubules. These independent methods yielded results consistent with the original work: a significant percentage of severed minus ends are stable after cutting. In additional experiments, we found that the stability of both severed plus and minus ends could be increased by increasing the free tubulin concentration, the solution GTP concentration, or by assembling microtubules with guanylyl-(alpha,beta)-methylene-diphosphonate (GMPCPP). Our results show that stability of severed ends, particularly minus ends, is not an artifact, but instead reveals the existence of a metastable kinetic intermediate state between the elongation and shortening states of dynamic instability. The kinetic properties of this intermediate state differ between plus and minus ends. We propose a three-state conformational cap model of dynamic instability, which has three structural states and four transition rate constants, and which uses the asymmetry of the tubulin heterodimer to explain many of the differences in dynamic instability at plus and minus ends.

Nanomolar concentrations of nocodazole alter microtubule dynamic instability in vivo and in vitro

Previous studies demonstrated that nanomolar concentrations of nocodazole can block cells in mitosis without net microtubule disassembly and resulted in the hypothesis that this block was due to a nocodazole-induced stabilization of microtubules. We tested this hypothesis by examining the effects of nanomolar concentrations of nocodazole on microtubule dynamic instability in interphase cells and in vitro with purified brain tubulin. Newt lung epithelial cell microtubules were visualized by video-enhanced differential interference contrast microscopy and cells were perfused with solutions of nocodazole ranging in concentration from 4 to 400 nM. Microtubules showed a loss of the two-state behavior typical of dynamic instability as evidenced by the addition of a third state where they exhibited little net change in length (a paused state). Nocodazole perfusion also resulted in slower elongation and shortening velocities, increased catastrophe, and an overall decrease in microtubule turnover. Experiments performed on BSC-1 cells that were microinjected with rhodamine-labeled tubulin, incubated in nocodazole for 1 h, and visualized by using low-light-level fluorescence microscopy showed similar results except that nocodazole-treated BSC-1 cells showed a decrease in catastrophe. To gain insight into possible mechanisms responsible for changes in dynamic instability, we examined the effects of 4 nM to 12 microM nocodazole on the assembly of purified tubulin from axoneme seeds. At both microtubule plus and minus ends, perfusion with nocodazole resulted in a dose-dependent decrease in elongation and shortening velocities, increase in pause duration and catastrophe frequency, and decrease in rescue frequency. These effects, which result in an overall decrease in microtubule turnover after nocodazole treatment, suggest that the mitotic block observed is due to a reduction in microtubule dynamic turnover. In addition, the in vitro results are similar to the effects of increasing concentrations of GDP-tubulin (TuD) subunits on microtubule assembly. Given that nocodazole increases tubulin GTPase activity, we propose that nocodazole acts by generating TuD subunits that then alter dynamic instability.

Posttranslational modification of tubulin by palmitoylation: I. In vivo and cell-free studies

It is well established that microtubules interact with intracellular membranes of eukaryotic cells. There is also evidence that tubulin, the major subunit of microtubules, associates directly with membranes. In many cases, this association between tubulin and membranes involves hydrophobic interactions. However, neither primary sequence nor known posttranslational modifications of tubulin can account for such an interaction. The goal of this study was to determine the molecular nature of hydrophobic interactions between tubulin and membranes. Specifically, I sought to identify a posttranslational modification of tubulin that is found in membrane proteins but not in cytoplasmic proteins. One such modification is the covalent attachment of the long chain fatty acid palmitate. The possibility that tubulin is a substrate for palmitoylation was investigated. First, I found that tubulin was palmitoylated in resting platelets and that the level of palmitoylation of tubulin decreased upon activation of platelets with thrombin. Second, to obtain quantities of palmitoylated tubulin required for protein structure analysis, a cell-free system for palmitoylation of tubulin was developed and characterized. The substrates for palmitoylation were nonpolymerized tubulin and tubulin in microtubules assembled with the slowly hydrolyzable GTP analogue guanylyl-(alpha, beta)-methylene-diphosphonate. However, tubulin in Taxol-assembled microtubules was not a substrate for palmitoylation. Likewise, palmitoylation of tubulin in the cell-free system was specifically inhibited by the antimicrotubule drugs Colcemid, podophyllotoxin, nocodazole, and vinblastine. These experiments identify a previously unknown posttranslational modification of tubulin that can account for at least one type of hydrophobic interaction with intracellular membranes.

How tubulin subunits are lost from the shortening ends of microtubules

Microtubules exhibit dynamic instability, switching between persistent states of growth and shortening at their ends. The switch between growth and shortening has been proposed to depend on end conformation where growing ends have "straight" tubulin protofilaments stabilized by a terminal cap of GTP-tubulin, while-shortening ends have lost their GTP-tubulin cap, allowing terminal GDP-tubulin dimers to curve inside-out and peel rapidly away from the microtubule lattice. This "conformational cap" model predicts that tubulin dissociation from shortening ends is a two-step process where the average lengths of curved GDP-tubulin protofilaments at a depolymerizing end will depend on the ratio of the rate of peeling to the rate of breakage of the longitudinal bonds between adjacent curved dimers. We have tested this model for the plus and minus ends of microtubules assembled with pure porcine tubulin off the ends of axoneme fragments in standard assembly buffer. Individual microtubule ends were imaged using video-enhanced differential interference contrast light microscopy. The rate of rapid shortening was systematically increased by isothermal dilution into assembly buffer containing various concentrations of Mg2+ or Ca2+ ions. At 1 mM Mg2+ and no Ca2+, shortening occurred at 20 (plus) and 45 (minus) microns/min. The ends appeared similar in contrast to growing ends and the core of the microtubule and the ends appeared blunt or slightly frayed by negative stain electron microscopy. Above 20 mM Mg2+ or above 5 mM Ca2+, microtubule shortening occurred at 60 (plus) and 115 (minus) microns/min or faster and "knobs" were distinctly visible at depolymerizing ends, particularly at the faster minus ends, and knob contrast remained constant during many micrometers of rapid shortening. Negative stain electron microscopy revealed that these knobs were "blossoms" of inside-out curved protofilaments, some extending for several helical turns (30 to 60 dimers in length) at constant curvature from the ends. At these high shortening velocities, the peeling of curved protofilaments was confined to within several dimers of the end of the microtubule cylinder, suggesting that dimer curling and protofilament peeling is constrained to the tip by interactions between adjacent straight protofilaments. Depolymerization is produced by conformational changes in GDP-tubulin since microtubules assembled with a slowly hydrolizable analog of GTP, GMPCPP, are stable even at 20 mM Mg2+ or 5 mM Ca2+. Monte Carlo simulations show that the ratio of the peeling to breakage rate constants can control the steady-state average length of curved GDP-tubulin protofilaments at the depolymerizing end.