Mutations of oncoprotein 18/stathmin identify tubulin-directed regulatory activities distinct from tubulin association

Combining optogenetics with sensitive FRET imaging to monitor local microtubule manipulations

Optogenetic methods for switching molecular states in cells are increasingly prominent tools in life sciences. Förster Resonance Energy Transfer (FRET)-based sensors can provide quantitative and sensitive readouts of altered cellular biochemistry, e.g. from optogenetics. However, most of the light-inducible domains respond to the same wavelength as is required for excitation of popular CFP/YFP-based FRET pairs, rendering the techniques incompatible with each other. In order to overcome this limitation, we red-shifted an existing CFP/YFP-based OP18 FRET sensor (COPY) by employing an sYFP2 donor and mScarlet-I acceptor. Their favorable quantum yield and brightness result in a red-shifted FRET pair with an optimized dynamic range, which could be further enhanced by an R125I point mutation that stimulates intramolecular interactions. The new sensor was named ROPY and it visualizes the interaction between the microtubule regulator stathmin/OP18 and free tubulin heterodimers. We show that through phosphorylation of the ROPY sensor, its tubulin sequestering ability can be locally regulated by photo-activatable Rac1 (PARac1), independent of the FRET readout. Together, ROPY and PARac1 provide spatiotemporal control over free tubulin levels. ROPY/PARac1-based optogenetic regulation of free tubulin levels allowed us to demonstrate that depletion of free tubulin prevents the formation of pioneer microtubules, while local upregulation of tubulin concentration allows localized microtubule extensions to support the lamellipodia.

The human T-cell leukemia virus type-1 tax oncoprotein dissociates NF-κB p65RelA-Stathmin complexes and causes catastrophic mitotic spindle damage and genomic instability

Genomic instability is a hallmark of many cancers; however, the molecular etiology of chromosomal dysregulation is not well understood. The human T-cell leukemia virus type-1 (HTLV-1) oncoprotein Tax activates NF-κB-signaling and induces DNA-damage and aberrant chromosomal segregation through diverse mechanisms which contribute to viral carcinogenesis. Intriguingly, Stathmin/oncoprotein-18 (Op-18) depolymerizes tubulin and interacts with the p65^RelA subunit and functions as a cofactor for NF-κB-dependent transactivation. We thus hypothesized that the dissociation of p65^RelA-Stathmin/Op-18 complexes by Tax could lead to the catastrophic destabilization of microtubule (MT) spindle fibers during mitosis and provide a novel mechanistic link between NF-κB-signaling and genomic instability. Here we report that the inhibition of Stathmin expression by the retroviral latency protein, p30^II, or knockdown with siRNA-stathmin, dampens Tax-mediated NF-κB transactivation and counters Tax-induced genomic instability and cytotoxicity. The Tax-G148V mutant, defective for NF-κB activation, exhibited reduced p65^RelA-Stathmin binding and diminished genomic instability and cytotoxicity. Dominant-negative inhibitors of NF-κB also prevented Tax-induced multinucleation and apoptosis. Moreover, cell clones containing the infectious HTLV-1 ACH. p30^II mutant provirus, impaired for p30^II production, exhibited increased multinucleation and the accumulation of cytoplasmic tubulin aggregates following nocodozole-treatment. These findings allude to a mechanism whereby NF-κB-signaling regulates tubulin dynamics and mitotic instability through the modulation of p65^RelA-Stathmin/Op-18 interactions, and support the notion that p30^II enhances the survival of Tax-expressing HTLV-1-transformed cells.

Manipulation of the Host Cell Cytoskeleton by Chlamydia

Chlamydiae are obligate intracellular pathogens. They undergo a biphasic developmental cycle differentiating between the infectious but metabolically quiescent elementary body and the vegetative, but non-infectious reticulate body. Chlamydia spends a significant portion of its development in the non-infectious stage, demanding an effective strategy of manipulating the host cells to ensure its intracellular survival and replication. A common target of all Chlamydia species studied so far is the host cell cytoskeleton, with past and recent findings revealing crucial roles in invasion, inclusion maintenance, nutrient acquisition, and egress. The molecular details of how Chlamydia co-opts the cytoskeleton is becoming clearer, with bacterial factors and their corresponding host cell targets identified.

Sep(t)arate or not – how some cells take septin-independent routes through cytokinesis

Cytokinesis is the final step of cell division, and is a process that requires a precisely coordinated molecular machinery to fully separate the cytoplasm of the parent cell and to establish the intact outer cell barrier of the daughter cells. Among various cytoskeletal proteins involved, septins are known to be essential mediators of cytokinesis. In this Commentary, we present recent observations that specific cell divisions can proceed in the absence of the core mammalian septin SEPT7 and its Drosophila homolog Peanut (Pnut) and that thus challenge the view that septins have an essential role in cytokinesis. In the pnut mutant neuroepithelium, orthogonal cell divisions are successfully completed. Similarly, in the mouse, Sept7-null mutant early embryonic cells and, more importantly, planktonically growing adult hematopoietic cells undergo productive proliferation. Hence, as discussed here, mechanisms must exist that compensate for the lack of SEPT7 and the other core septins in a cell-type-specific manner. Despite there being crucial non-canonical immune-relevant functions of septins, septin depletion is well tolerated by the hematopoietic system. Thus differential targeting of cytokinesis could form the basis for more specific anti-proliferative therapies to combat malignancies arising from cell types that require septins for cytokinesis, such as carcinomas and sarcomas, without impairing hematopoiesis that is less dependent on septin.

Microtubule-associated proteins and tubulin interaction by isothermal titration calorimetry

Microtubules play an important role in a number of vital cell processes such as cell division, intracellular transport, and cell architecture. The highly dynamic structure of microtubules is tightly regulated by a number of stabilizing and destabilizing microtubule-associated proteins (MAPs), such as tau and stathmin. Because of their importance, tubulin-MAPs interactions have been extensively studied using various methods that provide researchers with complementary but sometimes contradictory thermodynamic data. Isothermal titration calorimetry (ITC) is the only direct thermodynamic method that enables a full thermodynamic characterization (stoichiometry, enthalpy, entropy of binding, and association constant) of the interaction after a single titration experiment. This method has been recently applied to study tubulin-MAPs interactions in order to bring new insights into molecular mechanisms of tubulin regulation. In this chapter, we review the technical specificity of this method and then focus on the use of ITC in the investigation of tubulin-MAPs binding. We describe technical issues which could arise during planning and carrying out the ITC experiments, in particular with fragile proteins such as tubulin. Using examples of stathmin and tau, we demonstrate how ITC can be used to gain major insights into tubulin-MAP interaction.

Proteins implicated in the increase of adhesivity induced by suberoylanilide hydroxamic acid in leukemic cells

We have previously shown that suberoylanilide hydroxamic acid (SAHA) treatment increases the adhesivity of leukemic cells to fibronectin at clinically relevant concentrations. Now, we present the results of the proteomic analysis of SAHA effects on leukemic cell lines using 2-DE and ProteomLab PF2D system. Histone acetylation at all studied acetylation sites reached the maximal level after 5 to 10 h of SAHA treatment. No difference in histone acetylation between subtoxic and toxic SAHA doses was observed. SAHA treatment induced cofilin phosphorylation at Ser3, an increase in vimentin and paxillin expression and a decrease in stathmin expression as confirmed by western-blotting and immunofluorescence microscopy. The interaction of cofilin with 14-3-3 epsilon was documented using both Duolink system and coimmunoprecipitation. However, this interaction was independent of cofilin Ser3 phosphorylation and the amount of 14-3-3-ε-bound cofilin did not rise following SAHA treatment. SAHA-induced increase in the cell adhesivity was associated with an increase in PAK phosphorylation in CML-T1 cells and was abrogated by simultaneous treatment with IPA-3, a PAK inhibitor. The effects of SAHA on JURL-MK1 cells were similar to those of other histone deacetylase inhibitors, tubastatin A and sodium butyrate. The proteome analysis also revealed several potential non-histone targets of histone deacetylases.

Suspected leukemia oncoproteins CREB1 and LYL1 regulate Op18/STMN1 expression

Stathmin (STMN1) is a microtubule destabilizing protein with a key role in cell cycle progression and cell migration that is up-regulated in several cancers and may contribute to the malignant phenotype. However, the factors that regulate its expression are not well understood. Loss as well as gain-of-function p53 mutations up-regulate STMN1 and in acute myelogenous leukemia where p53 is predominantly wild-type, STMN1 is also over-expressed. Here we show regulatory control of STMN1 expression by the leucine zipper transcription factor (TF) CREB1 and the basic helix-loop-helix TF LYL1. By ChIP-chip experiments we demonstrate in vivo the presence of LYL1 and CREB1 in close proximity on the STMN1 promoter and using promoter assays we reveal co-regulation of STMN1 by CREB1 and LYL1. By contrast, TAL1, another suspected oncoprotein in leukemia and close relative of LYL1, exerts no regulatory effect on the STMN1 promoter. NLI, LMO2 and GATA2 are previously described co-activators of Tal1/Lyl1-E47 transcriptional complexes and potentiate Lyl1 activation of the STMN1 promoter while having no effect on TAL1 transactivation. Promoter mutations that abrogate CREB1 proximal binding or mutations of the DNA-binding domain of CREB1 abolish LYL1 transcriptional activation. These results show that CRE and Ebox sites function as coordinated units and support previous evidence of joint CREB1-and LYL1 transcription events activating an aberrant subset of promoters in leukemia. CREB1 or LYL1 shRNA knock-down down-regulate STMN1 expression. Because down-regulation of STMN1 has been shown to have anti-proliferative effects, while CREB1 and LYL1 are suspected oncoproteins, interference with CREB1-LYL1 interactions may complement standard chemotherapy and yield additional beneficial effects.

Paclitaxel and docetaxel resistance: molecular mechanisms and development of new generation taxanes

Taxanes represent one of the most promising classes of anticancer agents. Unfortunately, their clinical success has been limited by the insurgence of cellular resistance, mainly mediated by the expression of the MDR phenotype or by microtubule alterations. However, the remarkable relevance of paclitaxel and docetaxel in clinical oncology stimulated intensive efforts in the last decade to identify new derivatives endowed with improved activities towards resistant tumor cells, resulting in a huge number of novel natural and synthetic taxanes. Among them, several structurally different derivatives were found to exhibit a promising behavior against the MDR phenotype in terms of either MDR inhibiting properties, or enhanced cytotoxicity compared to parental drugs, or both. On the other hand, only in more recent years have the first taxanes retaining activity against resistant cancer cells bearing alterations of the tubulin/microtubule system emerged. This review describes the main molecular mechanisms of resistance to paclitaxel and docetaxel identified so far, focusing on the advances achieved in the development of new taxanes potentially useful for the treatment of resistant tumors.

Cellular mechanisms of resistance to anthracyclines and taxanes in cancer: intrinsic and acquired

Taxanes and anthracyclines are two of the most potent and broadly effective classes of chemotherapeutic agents. However, resistance to these agents is common and significantly limits their potential. As such, there is a great need to understand the mechanisms underlying de novo and acquired resistance to these agents. Beyond the resistance barrier lies even greater potential to significantly alter the natural course of human cancer. This review discusses what we currently understand about the mechanisms of resistance to taxanes and anthracyclines. Preclinical models suggest a role for ATP-binding cassette transporters, tubulin isoforms, microtubule-associated proteins, tubulin gene mutations, and mitotic checkpoint signaling proteins in resistance to taxanes. Preclinical models also suggest that drug transport proteins, antioxidant defenses, apoptotic signaling, and topoisomerase modulation may mediate anthracycline resistance. Many of these hypotheses remain untested in appropriately designed clinical studies, but limited clinical evidence will be reviewed. Epothilones represent a novel class of non-taxane microtubule stabilizing agents with distinct drug-resistance profiles. Potential mechanisms behind these differences and their potential role in the treatment of both taxane- and anthracycline-refractory patients are discussed.

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.

Stathmin and its phosphoprotein family: general properties, biochemical and functional interaction with tubulin

Stathmin, also referred to as Op18, is a ubiquitous cytosolic phosphoprotein, proposed to be a small regulatory protein and a relay integrating diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation and activities. It interacts with several putative downstream target and/or partner proteins. One major action of stathmin is to interfere with microtubule dynamics, by inhibiting the formation of microtubules and/or favoring their depolymerization. Stathmin (S) interacts directly with soluble tubulin (T), which results in the formation of a T2S complex which sequesters free tubulin and therefore impedes microtubule formation. However, it has been also proposed that stathmin's action on microtubules might result from the direct promotion of catastrophes, which is still controversial. Phosphorylation of stathmin regulates its biological actions: it reduces its affinity for tubulin and hence its action on microtubule dynamics, which allows for example progression of cells through mitosis. Stathmin is also the generic element of a protein family including the neural proteins SCG10, SCLIP and RB3/RB3'/RB3". Interestingly, the stathmin-like domains of these proteins also possess a tubulin binding activity in vitro. In vivo, the transient expression of neural phosphoproteins of the stathmin family leads to their localization at Golgi membranes and, as previously described for stathmin and SCG10, to the depolymerization of interphasic microtubules. Altogether, the same mechanism for microtubule destabilization, that implies tubulin sequestration, is a common feature likely involved in the specific biological roles of each member of the stathmin family.

Altered levels and regulation of stathmin in paclitaxel-resistant ovarian cancer cells

Two paclitaxel(Ptx)-resistant ovarian cancer cell lines, 1A9/Ptx-10 and 1A9/Ptx-22, isolated from the 1A9 cell line (a clone of the A2780 line) by continuous exposure to Ptx and verapamil, have point mutations in their major beta-tubulin gene and in one or both alleles of their TP53 gene. These cells were examined for alterations in cell cycle regulators and the tubulin-binding protein stathmin. Unlike parental cells, neither 1A9/Ptx-10 nor 1A9/Ptx-22 expressed detectable levels of p21(WAF1/Cip1), a putative transcriptional regulator of stathmin, but did overexpress stathmin and Bcl2. No differences were noted in the expression levels of proliferative cell nuclear antigen or tyrosine-phosphorylated p34Cdc2. Ptx treatment altered little the expression of stathmin in the parental cell line, although it increased p21(WAF1/Cip1) levels several-fold. Infection of Ptx-resistant lines with a wild-type TP53-bearing adenovirus (AdWTp53) changed cell cycle distribution and increased the levels of p21(WAF1/Cip1), but caused no changes in stathmin levels. Microtubule drug resistance in ovarian carcinoma may be associated with altered p53/21(WAF1/Cip1) regulatory pathways for stathmin expression and function.

Mechanisms of Taxol resistance related to microtubules

Since its approval by the FDA in 1992 for the treatment of ovarian cancer, the use of Taxol has dramatically increased. Although treatment with Taxol has led to improvement in the duration and quality of life for some cancer patients, the majority eventually develop progressive disease after initially responding to Taxol treatment. Drug resistance represents a major obstacle to improving the overall response and survival of cancer patients. This review focuses on mechanisms of Taxol resistance that occur directly at the microtubule, such as mutations, tubulin isotype selection and post-translational modifications, and also at the level of regulatory proteins. A review of tubulin structure, microtubule dynamics, the mechanism of action of Taxol and its binding site on the microtubule are included, so that the reader can evaluate Taxol resistance in context.

Thermodynamics of the Op18/stathmin-tubulin interaction

Op18/stathmin (stathmin) is an intrinsically disordered protein involved in the regulation of the microtubule filament system. One function of stathmin is to sequester tubulin dimers into assembly incompetent complexes, and recent studies revealed two tubulin binding sites per stathmin molecule. Using high sensitivity isothermal titration calorimetry, we document that at 10 degrees C and under the conditions of 80 mM PIPES, pH 6.8, 1 mM EGTA, 1 mM MgCl2, 1 mM GTP these two binding sites are of equal affinity with an equilibrium binding constant of K0 = 6.0 x 10(6) m(-1). The obtained large negative molar heat capacity change of deltaCp0 = -860 cal mol(-1) K(-1) (referring to tubulin) for the tubulin-stathmin binding equilibrium suggests that the hydrophobic effect is the major driving force of the binding reaction. Replacing GTP by GDP on beta-tubulin had no significant effect on the thermodynamic parameters of the tubulin-stathmin binding equilibrium. The proposed pH-sensitive dual function of stathmin was further evaluated by circular dichroism spectroscopy and nuclear magnetic resonance. At low temperatures, stathmin was found to be extensively helical but devoid of any stable tertiary structure. However, in complex with two tubulin subunits stathmin adopts a stable conformation. Both the stability and conformation of the individual proteins and complexes were not significantly affected by small changes in pH. A 4-fold decrease in affinity of stathmin for tubulin was revealed at pH 7.5 compared with pH 6.8. This decrease could be attributed to a weaker binding of the C terminus of stathmin. These findings do not support the view that stathmin works as a pH-sensitive protein.

Fluorescence correlation spectroscopy analysis of the dynamics of tubulin interaction with RB3, a stathmin family protein

We have used fluorescence correlation spectroscopy to analyze the interaction of GTP-tubulin with rhodamine-labeled RB3, a neural protein of the stathmin family, and to determine the kinetic pathway of the association process. RB3 displayed slow association-dissociation kinetics with tubulin depending on the square of the tubulin concentration. The values of the apparent association and dissociation rate constants of the complex of two tubulin dimers and RB3 are determined to be (3.52+/-0.14)x10(-3) micro;M(-2)/s and (1.9+/-0.6)x10(-3) s(-1) respectively. The value of the equilibrium dissociation constant for the first tubulin-RB3 interaction is estimated to be >or=7 microM at 20 degrees C.

Molecular dissection of GTP exchange and hydrolysis within the ternary complex of tubulin heterodimers and Op18/stathmin family members

The ubiquitous Op18 and the neural RB3 and SCG10 proteins are members of the oncoprotein18/stathmin family of microtubule regulators. These proteins bind two tubulin heterodimers via two imperfect helical repeats to form a complex of heterodimers aligned head-to-tail. Here we have analyzed GTP exchange and GTP hydrolysis at the exchangeable GTP-binding site (E-site) of tubulin heterodimers in complex with Op18, RB3, or SCG10. These proteins stimulate a low and indistinguishable rate of GTP hydrolysis, and our results show that GTP exchange is blocked at both E-sites of the ternary complex, whereas GTP hydrolysis only occurs at one of the two E-sites. Results from mutational analysis of clusters of hydrophobic residues within the first helical repeat of Op18 suggest that GTP is hydrolyzed at the E-site that is interfaced between the head-to-tail arranged heterodimers, which is consistent with predicted GTPase productive interactions between the two tubulin heterodimers. Our mutational analysis has also indicated that Op18/stathmin family members actively restrain the otherwise potent GTPase productive interactions that are generated by longitudinal interactions within protofilaments. We conclude that tubulin heterodimers in complex with Op18/stathmin family members are subject to allosteric effects that prevent futile cycles of GTP hydrolysis.

SCG10-related neuronal growth-associated proteins in neural development, plasticity, degeneration, and aging

Neuronal growth-associated proteins (nGAPs) are in general neuron-specific gene products whose expression correlates tightly with neuronal process outgrowth and/or regeneration, and are mostly good downstream targets of neurotrophin stimulation. Expression of genes encoding nGAPs such as GAP-43, SCG10, and stathmin is upregulated following lesioning of cortical and hippocampal regions of the adult rat brain. In the brains of aged animals, however, the magnitude of the response is reduced, whereas the time course of the response is mostly unchanged when compared with that for brains of young ones. Expression of GAP-43 and stathmin is reduced by aging, and is also changed in age-related neurodegenerative conditions such as Alzheimer's disease in humans. Certain nGAPs are induced during long-term potentiation (LTP) and also during critical periods of song-learning and ocular dominance column formation in birds and cats, respectively. Recent evidence further supports the idea that functional synaptic modulation is often associated with remodeling of synaptic structures. These results suggest that neurotrophin-responsive nGAPs serve as molecular markers of neuronal plasticity during development and aging, and that the neuronal plasticity decreases, at least in certain neuronal circuits, in the aged brain and neurodegenerative diseases. Recent findings on the roles of stathmin and SCG10-related proteins in microtubule destabilization and its functional block by phosphorylation further support the importance of the SCG10 family proteins in neuronal cytoskeletal regulation, particularly as to microtubule dynamics. We summarize here a decade of research on SCG10 and its related molecules with special interests to brain aging and disease.

MAP4 counteracts microtubule catastrophe promotion but not tubulin-sequestering activity in intact cells

Microtubules are polar polymers that continually switch between phases of elongation and shortening, a property referred to as dynamic instability. The ubiquitous microtubule associated protein 4 (MAP4) shows rescue-promoting activity during in vitro assembly of microtubules (i.e., promotes transitions from shortening to elongation), but its regulatory role in intact cells is poorly defined. Here, we demonstrate that ectopic MAP4 promotes outgrowth of extended MTs during beta1-integrin-induced cell spreading. An inducible cotransfection protocol was employed to further analyze the regulatory role of MAP4 in human leukemia cells with microtubules partially destabilized by either ectopic tubulin-sequestering proteins or proteins that promote catastrophes (i.e., transitions from elongation to shortening). Coexpression of proteins that sequester free tubulin heterodimers with different efficiencies was found to abolish microtubule stabilization by MAP4. In contrast, however, the microtubule-stabilizing activity of MAP4 was found to suppress the activities of two distinct and specific catastrophe promoters, namely, XKCM1 and a nonsequestering truncation derivative of Op18/stathmin. These observations reveal specificity in the microtubule-stabilizing activity of MAP4 that differentiates between two mechanistically distinct types of MT destabilization.

The effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration

Stathmin is a phosphorylation-regulated tubulin-binding protein. In vitro and in vivo studies using nonphosphorylatable and pseudophosphorylated mutants of stathmin have questioned the view that stathmin might act only as a tubulin-sequestering factor. Stathmin was proposed to effectively regulate microtubule dynamic instability by increasing the frequency of catastrophe (the transition from steady growth to rapid depolymerization), without interacting with tubulin. We have used a noninvasive method to measure the equilibrium dissociation constants of the T(2)S complexes of tubulin with stathmin, pseudophosphorylated (4E)-stathmin, and diphosphostathmin. At both pH 6.8 and pH 7.4, the relative sequestering efficiency of the different stathmin variants depends on the concentration of free tubulin, i.e. on the dynamic state of microtubules. This control is exerted in a narrow range of tubulin concentration due to the highly cooperative binding of tubulin to stathmin. Changes in pH affect the stability of tubulin-stathmin complexes but do not change stathmin function. The 4E-stathmin mutant mimics inactive phosphorylated stathmin at low tubulin concentration and sequesters tubulin almost as efficiently as stathmin at higher tubulin concentration. We propose that stathmin acts solely by sequestering tubulin, without affecting microtubule dynamics, and that the effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration.

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 oncoprotein 18/stathmin family of microtubule destabilizers

The past several years have seen major advances in our understanding of the mechanisms of microtubule destabilization by oncoprotein18/stathmin (Op18/stathmin) and related proteins. New structural information has clearly shown how members of the Op18/stathmin protein family bind tubulin dimers and suggests models for how these proteins stimulate catastrophe, the transition from microtubule growth to shortening. Regulation of Op18/stathmin by phosphorylation continues to capture much attention. Studies suggest that phosphorylation occurs in a localized fashion, resulting in decreased microtubule destabilizing activity near chromatin or microtubule polymer. A spatial gradient of inactive Op18/stathmin associated with chromatin or microtubules could contribute significantly to mitotic spindle assembly.

Differential effect of two stathmin/Op18 phosphorylation mutants on Xenopus embryo development

Stathmin/Op18 destabilizes microtubules in vitro and regulates microtubule polymerization in vivo. Both a microtubule catastrophe-promoting activity and a tubulin sequestering activity were demonstrated for stathmin in vitro, and both could contribute to microtubule depolymerization in vivo. Stathmin activity can be turned down by extensive phosphorylation on its four phosphorylatable serines, and down-regulation of stathmin activity by phosphorylation is necessary for cells to proceed through mitosis. We show here that microinjection of a nonphosphorylatable Ser to Ala (4A) quadruple mutant in Xenopus two-cell stage embryos results in cell cleavage arrest in the injected blastomeres and aborted development, whereas injection of a pseudo-phosphorylated Ser to Glu quadruple mutant (4E) does not prevent normal development. Addition of these mutants to mitotic cytostatic factor-arrested extracts in which spindle assembly was induced led to a dramatic reduction of spindle size with 4A stathmin, and to a moderate increase with 4E stathmin, but both localized to spindle poles. Interestingly, the microtubule assembly-dependent phosphorylation of endogenous stathmin was abolished in the presence of 4A stathmin, but not of 4E stathmin. Altogether, this shows that the phosphorylation-mediated regulation of stathmin activity during the cell cycle is essential for early Xenopus embryonic development.

Phosphorylation disrupts the central helix in Op18/stathmin and suppresses binding to tubulin

Protein phosphorylation represents a ubiquitous control mechanism in living cells. The structural prerequisites and consequences of this important post-translational modification, however, are poorly understood. Oncoprotein 18/stathmin (Op18) is a globally disordered phosphoprotein that is involved in the regulation of the microtubule (MT) filament system. Here we document that phosphorylation of Ser63, which is located within a helix initiation site in Op18, disrupts the transiently formed amphipathic helix. The phosphoryl group reduces tubulin binding 10-fold and suppresses the MT polymerization inhibition activity of Op18's C-terminal domain. Op18 represents an example where phosphorylation occurs within a regular secondary structural element. Together, our findings have implications for the prediction of phosphorylation sites and give insights into the molecular behavior of a globally disordered protein.

The catastrophe-promoting activity of ectopic Op18/stathmin is required for disruption of mitotic spindles but not interphase microtubules

Oncoprotein18/stathmin (Op18) is a microtubule (MT) destabilizing protein that is inactivated during mitosis by phosphorylation at four Ser-residues. Op18 has at least two functions; the N-terminal region is required for catastrophe-promotion (i.e., transition from elongation to shortening), while the C-terminal region is required to inhibit MT-polymerization rate in vitro. We show here that a "pseudophosphorylation" derivative of Op18 (i.e., four Ser- to Glu-substitutions at phosphorylation sites) exhibits a selective loss of catastrophe-promoting activity. This is contrasted to authentic phosphorylation, which efficiently attenuates all activities except tubulin binding. In intact cells, overexpression of pseudophosphorylated Op18, which is not phosphorylated by endogenous kinases, is shown to destabilize interphase MTs but to leave spindle formation untouched. To test if the mitotic spindle is sensitive only to the catastrophe-promoting activity of Op18 and resistant to C-terminally associated activities, N- and C-terminal truncations with defined activity-profiles were employed. The cell-cycle phenotypes of nonphosphorylatable mutants (i.e., four Ser- to Ala-substitutions) of these truncation derivatives demonstrated that catastrophe promotion is required for interference with the mitotic spindle, while the C-terminally associated activities are sufficient to destabilize interphase MTs. These results demonstrate that specific Op18 derivatives with defined activity-profiles can be used as probes to distinguish interphase and mitotic MTs.

Mutational analysis of op18/stathmin-tubulin-interacting surfaces. Binding cooperativity controls tubulin GTP hydrolysis in the ternary complex

Oncoprotein 18 (Op18) is a microtubule regulator that forms a ternary complex with two tubulin heterodimers. Dispersed regions of Op18 are involved in two-site cooperative binding and subsequent modulation of tubulin GTPase activity. Here we have analyzed specific determinants of Op18 that govern both stoichiometry and positive cooperativity in tubulin binding and consequent stimulatory and inhibitory effects on tubulin GTPase activity. The data revealed that the central and C-terminal regions of Op18 contain overlapping binding-motifs contacting both tubulin heterodimers, suggesting that these regions of Op18 are wedged into the previously noted 1-nm gap between the two longitudinally arranged tubulin heterodimers. Both the N- and C-terminal flanks adjacent to the central region are involved in stabilizing the ternary complex, but only the C-terminal flank does so by imposing positive binding cooperativity. Within the C-terminal flank, deletion of a 7-amino acid region attenuated positive binding cooperativity and resulted in a switch from stimulation to inhibition of tubulin GTP hydrolysis. This switch can be explained by attenuated binding cooperativity, because Op18 under these conditions may block longitudinal contact surfaces of single tubulins with consequent interference of tubulin-tubulin interaction-dependent GTP hydrolysis. Together, our results suggest that Op18 links two tubulin heterodimers via longitudinal contact surfaces to form a ternary GTPase productive complex.

The 4 A X-ray structure of a tubulin:stathmin-like domain complex

Phosphoproteins of the stathmin family interact with the alphabeta tubulin heterodimer (tubulin) and hence interfere with microtubule dynamics. The structure of the complex of GDP-tubulin with the stathmin-like domain of the neural protein RB3 reveals a head-to-tail assembly of two tubulins with a 91-residue RB3 alpha helix in which each copy of an internal duplicated sequence interacts with a different tubulin. As a result of the relative orientations adopted by tubulins and by their alpha and beta subunits, the tubulin:RB3 complex forms a curved structure. The RB3 helix thus most likely prevents incorporation of tubulin into microtubules by holding it in an assembly with a curvature very similar to that of the depolymerization products of microtubules.

Spindle assembly and the art of regulating microtubule dynamics by MAPs and Stathmin/Op18

The way that microtubules reorganize from their long, stable interphase configuration to form the mitotic spindle remains a challenging and unsolved question. It is now widely recognized that microtubule polymerization during the cell cycle is regulated by a balance between microtubule-stabilizing and-destabilizing factors. Stabilizing factors include a large group of microtubule-associated proteins (MAPs; e.g. MAP4, XMAP215, XMAP230/XMAP4 and XMAP310) and the destabilizing factors are a growing family of proteins (e.g. Stathmin/Op18 and XKCM1). Recent studies have allowed a mechanistic dissection of how these stabilizing and destabilizing factors regulate microtubule dynamics and spindle assembly.

Op18/stathmin caps a kinked protofilament-like tubulin tetramer

Oncoprotein 18/stathmin (Op18), a regulator of microtubule dynamics, was recombinantly expressed and its structure and function analysed. We report that Op18 by itself can fold into a flexible and extended alpha-helix, which is in equilibrium with a less ordered structure. In complex with tubulin, however, all except the last seven C-terminal residues of Op18 are tightly bound to tubulin. Digital image analysis of Op18:tubulin electron micrographs revealed that the complex consists of two longitudinally aligned alpha/beta-tubulin heterodimers. The appearance of the complex was that of a kinked protofilament-like structure with a flat and a ribbed side. Deletion mapping of Op18 further demonstrated that (i) the function of the N-terminal part of the molecule is to 'cap' tubulin subunits to ensure the specificity of the complex and (ii) the complete C-terminal alpha-helical domain of Op18 is necessary and sufficient for stable Op18:tubulin complex formation. Together, our results suggest that besides sequestering tubulin, the structural features of Op18 enable the protein specifically to recognize microtubule ends to trigger catastrophes.

Microtubule dynamics and tubulin interacting proteins

Microtubule dynamics are crucial in generation of the mitotic spindle. During the transition from interphase to mitosis, there is an increase in the frequency of microtubule catastrophes. Recent work has identified two proteins, Op 18/stathmin and XKCM1, which can promote microtubule catastrophes in vitro and in cells or extracts. Although both of these proteins share the ability to bind tubulin dimers, their mechanisms of action in destabilizing microtubules are distinct.

Model for stathmin/OP18 binding to tubulin

Stathmin/OP18 is a regulatory phosphoprotein that controls microtubule (MT) dynamics. The protein does not have a defined three-dimensional structure, although it contains three distinct regions (an unstructured N-terminus, N: 1-44; a region with high helix propensity, H 1: 44-89; and a region with low helix propensity, H 2: 90-142). The full protein and a combination of H 1 and H 2 inhibits tubulin polymerization, while the combination of H 1 and the N-terminus is less efficient. None of the individual three regions alone are functional in this respect. However, all of them cross-link to alpha-tubulin, but only full-length stathmin produces high-molecular-weight products. Mass spectrometry analysis of alpha-tubulin-stathmin/OP18 and its truncation products shows that full-length stathmin/OP18 binds to the region around helix 10 of alpha-tubulin, a region that is involved in longitudinal interactions in the MT, sequestering the dimer and possibly linking two tubulin heterodimers. In the absence of the N-terminus, stathmin/OP18 binds to only one molecule of alpha-tubulin, at the top of the free tubulin heterodimer, preventing polymerization.

Op18/stathmin mediates multiple region-specific tubulin and microtubule-regulating activities

Oncoprotein18/stathmin (Op18) is a regulator of microtubule (MT) dynamics that binds tubulin heterodimers and destabilizes MTs by promoting catastrophes (i.e., transitions from growing to shrinking MTs). Here, we have performed a deletion analysis to mechanistically dissect Op18 with respect to (a) modulation of tubulin GTP hydrolysis and exchange, (b) tubulin binding in vitro, and (c) tubulin association and MT-regulating activities in intact cells. The data reveal distinct types of region-specific Op18 modulation of tubulin GTP metabolism, namely inhibition of nucleotide exchange and stimulation or inhibition of GTP hydrolysis. These regulatory activities are mediated via two-site cooperative binding to tubulin by multiple nonessential physically separated regions of Op18. In vitro analysis revealed that NH(2)- and COOH-terminal truncations of Op18 have opposite effects on the rates of tubulin GTP hydrolysis. Transfection of human leukemia cells with these two types of mutants result in similar decrease of MT content, which in both cases appeared independent of a simple tubulin sequestering mechanism. However, the NH(2)- and COOH-terminal-truncated Op18 mutants regulate MTs by distinct mechanisms as evidenced by morphological analysis of microinjected newt lung cells. Hence, mutant analysis shows that Op18 has the potential to regulate tubulin/MTs by more than one specific mechanism.