The stathmin/tubulin interaction in vitro

Specific Ser-Pro phosphorylation by the RNA-recognition motif containing kinase KIS

We present here a first appraisal of the phosphorylation site specificity of KIS (for 'kinase interacting with stathmin'), a novel mammalian kinase that has the unique feature among kinases to possess an RNP type RNA-recognition motif (RRM). In vitro kinase assays using various standard substrates revealed that KIS has a narrow specificity, with myelin basic protein (MBP) and synapsin I being the best in vitro substrates among those tested. Mass spectrometry and peptide sequencing allowed us to identify serine 164 of MBP as the unique site phosphorylated by KIS. Phosphorylation of synthetic peptides indicated the importance of the proline residue at position +1. We also identified a tryptic peptide of synapsin I phosphorylated by KIS and containing a phosphorylatable Ser-Pro motif. Altogether, our results suggest that KIS preferentially phosphorylates proline directed residues but has a specificity different from that of MAP kinases and cdks.

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.

Quantitative measurement of the catastrophe rate of dynamic microtubules

Previous work has shown that catastrophe frequency is the predominant dynamic parameter of microtubules that changes dramatically during the cell cycle. As an alternative to videomicroscopy assays, we have developed a biochemical assay to measure directly the average catastrophe rate of a population of microtubules. In this assay, the growing plus end of the microtubules, polymerized off seeds, are labeled with a brief pulse of alpha-32P-GTP, followed by a cold GTP chase. The rate of loss of 32P label in microtubules measured by this method is equal to the catastrophe frequency at microtubule plus ends measured by videomicroscopy of individual microtubules. Addition of mitotic extract from Xenopus eggs increases the catastrophe rate of purified tubulin by almost 100-fold, while interphase extract alters the catastrophe rate by about 20-fold as compared to pure tubulin. Most of the catastrophe-promoting activities in both mitotic and interphase extracts is found in particulate fractions. High-speed centrifugation of extracts appears to eliminate the components required for increasing microtubule catastrophe, but does not eliminate the cell cycle difference in microtubule dynamics. This assay provides a new approach to quantitate microtubule catastrophe rates. It will be of particular interest to search for catastrophe factors associated with intracellular membranes or other insoluble components.

Xenopus interphase and mitotic microtubule-associated proteins differentially suppress microtubule dynamics in vitro

Based on observations of microtubule dynamics in Xenopus extracts and in vivo, it has been assumed that the pool of interphase microtubule-associated proteins (MAPs) are more potent microtubule stabilizers than their mitotic counterparts. The aim of this study was to test that assumption, and two questions were addressed here. First, are there differences in the composition of interphase and mitotic MAPs? Second, do interphase MAPs more potently promote microtubule assembly than mitotic MAPs? Biochemical purification from Xenopus egg extracts shows that the composition of interphase and mitotic MAPs is similar. XMAP215, XMAP230, and XMAP310, which are the three characterized Xenopus MAPs, show decreased microtubule binding in mitotic extracts, and mitotic MAPs are slightly more phosphorylated than interphase MAPs. Bulk polymerization and time-lapse video microscopy show that microtubules polymerized two times faster in the presence of total interphase MAPs compared with total mitotic MAPs. Interphase but not mitotic MAPs strongly promoted microtubule nucleation in solution. Video microscopy showed that microtubules never underwent catastrophes in the presence of either MAP fraction. It is proposed that the increase in microtubule dynamics at the onset of mitosis results from phosphorylation dependent decreased microtubule stabilization by MAPs, allowing destabilizing factors to increase the catastrophe frequency and dismantle the interphase microtubule network.

Identification of genes regulated during mechanical load-induced cardiac hypertrophy

Cardiac hypertrophy is associated with both adaptive and adverse changes in gene expression. To identify genes regulated by pressure overload, we performed suppressive subtractive hybridization between cDNA from the hearts of aortic-banded (7-day) and sham-operated mice. In parallel, we performed a subtraction between an adult and a neonatal heart, for the purpose of comparing different forms of cardiac hypertrophy. Sequencing more than 100 clones led to the identification of an array of functionally known (70%) and unknown genes (30%) that are upregulated during cardiac growth. At least nine of those genes were preferentially expressed in both the neonatal and pressure over-load hearts alike. Using Northern blot analysis to investigate whether some of the identified genes were upregulated in the load-independent calcineurin-induced cardiac hypertrophy mouse model, revealed its incomplete similarity with the former models of cardiac growth.

Probing the native structure of stathmin and its interaction domains with tubulin. Combined use of limited proteolysis, size exclusion chromatography, and mass spectrometry

Stathmin is a cytosoluble phosphoprotein proposed to be a regulatory relay integrating diverse intracellular signaling pathway. Its interaction with tubulin modulates microtubule dynamics by destabilization of assembled microtubules or inhibition of their polymerization from free tubulin. The aim of this study was to probe the native structure of stathmin and to delineate its minimal region able to interact with tubulin. Limited proteolysis of stathmin revealed four structured domains within the native protein, corresponding to amino acid sequences 22-81 (I), 95-113 (II), 113-128 (III), and 128-149 (IV), which allows us to propose stathmin folding hypotheses. Furthermore, stathmin proteolytic fragments were mixed to interact with tubulin, and those that retained affinity for tubulin were isolated by size exclusion chromatography and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The results indicate that, to interact with tubulin, a stathmin fragment must span a minimal core region from residues 42 to 126, which interestingly corresponds to the predicted alpha-helical "interaction region" of stathmin. In addition, an interacting stathmin fragment must include a short N- or C-terminal extension. The functional significance of these interaction constrains is further validated by tubulin polymerization inhibition assays with fragments designed on the basis of the tubulin binding results. The present results will help to optimize further stathmin structural studies and to develop molecular tools to target its interaction with tubulin.

Disruption of microtubules in living cells by tyrphostin AG-1714

Tyrphostin AG-1714 and several related molecules with the general structure of nitro-benzene malononitrile (BMN) disrupt microtubules in a large variety of cultured cells. This process can be inhibited by the stabilization of microtubules with taxol or by pretreatment of the cells with pervanadate, which inhibits tyrosine phosphatases and increases the overall levels of phosphotyrosine in cells. Unlike other microtubule-disrupting drugs such as nocodazole or colchicine, tyrphostin AG-1714 does not interfere with microtubule polymerization or stability in vitro, suggesting that the effect of this tyrphostin on microtubules is indirect. These results imply an involvement of protein tyrosine phosphorylation in the regulation of overall microtubule dynamics. Tyrphostins of AG-1714 type could thus be powerful tools for the identification of such microtubule regulatory pathways.

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.

Overexpression of stathmin in breast carcinomas points out to highly proliferative tumours

We recently discovered that stathmin was overexpressed in a subgroup of human breast carcinomas. Stathmin is a cytosolic phosphoprotein proposed to act as a relay integrating diverse cell signalling pathways, notably during the control of cell growth and differentiation. It may also be considered as one of the key regulators of cell division for its ability to destabilize microtubules in a phosphorylation-dependent manner. To assess the significance of stathmin overexpression in breast cancer, we evaluated the correlation of stathmin expression, quantified by reverse transcription polymerase chain reaction, with several disease parameters in a large series of human primary breast cancer (n = 133), obtained in strictly followed up women, whose clinico-pathological data were fully available. In agreement with our preliminary survey, stathmin was found overexpressed in a subgroup of tumours (22%). In addition, overexpression was correlated to the loss of steroid receptors (oestrogen, P = 0.0006; progesterone, P = 0.008), and to the Scarff-Bloom-Richardson histopathological grade III (P= 0.002), this latter being ascribable to the mitotic index component (P= 0.02). Furthermore studies at the DNA level indicated that stathmin is overexpressed irrespective of its genomic status. Our findings raise important questions concerning the causes and consequences of stathmin overexpression, and the reasons of its inability to counteract cell proliferation in the overexpression group.

Control of microtubule dynamics by the antagonistic activities of XMAP215 and XKCM1 in Xenopus egg extracts

Microtubules are dynamic polymers that move stochastically between periods of growth and shrinkage, a property known as dynamic instability. Here, to investigate the mechanisms regulating microtubule dynamics in Xenopus egg extracts, we have cloned the complementary DNA encoding the microtubule-associated protein XMAP215 and investigated the function of the XMAP215 protein. Immunodepletion of XMAP215 indicated that it is a major microtubule-stabilizing factor in Xenopus egg extracts. During interphase, XMAP215 stabilizes microtubules primarily by opposing the activity of the destabilizing factor XKCM1, a member of the kinesin superfamily. These results indicate that microtubule dynamics in Xenopus egg extracts are regulated by a balance between a stabilizing factor, XMAP215, and a destabilizing factor, XKCM1.

Decreasing oncoprotein 18/stathmin levels reduces microtubule catastrophes and increases microtubule polymer in vivo

Oncoprotein 18/stathmin (Op18) has been identified recently as a protein which destabilizes microtubules. To characterize the function of Op18 in living cells, we used microinjection of anti-Op18 antibodies or antisense oligonucleotides to block either Op18 activity or expression in interphase newt lung cells. Anti-tubulin staining of cells microinjected with anti-Op18 and fixed 1–2 hours after injection showed an increase in total microtubule polymer. In contrast, microinjection of either non-immune IgG or anti-Op18 preincubated with bacterially-expressed Op18 had little effect on microtubule polymer level. Cells treated with Op18 antisense oligonucleotides for 4 days had (greater than or equal to)50% reduced levels of Op18 with no change in the soluble tubulin level. Measurement of MT polymer level in untreated, antisense or nonsense oligonucleotide treated cells demonstrated that reduced Op18 levels resulted in a 2.5-fold increase in microtubule polymer. Next, the assembly dynamics of individual microtubules at the peripheral regions of living cells were examined using video-enhanced contrast DIC microscopy. Microinjection of antibodies against oncoprotein 18 resulted in a 2.2-fold reduction in catastrophe frequency and a slight reduction in plus end elongation velocity compared to uninjected cells or cells microinjected with non-immune IgG. Preincubation of anti-Op18 antibody with recombinant Op18 greatly diminished the effects of the antibody. Similarly, treatment of cells with antisense oligonucleotides reduced catastrophes 2.5- to 3-fold compared to nonsense oligonucleotide treated or untreated cells. The other parameters of dynamic instability were unchanged after reducing Op18 with antisense oligonucleotides. These studies are consistent with Op18 functioning to regulate microtubule catastrophes during interphase in vivo.

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.

Critical targets of protein kinase C in differentiation of tumour cells

The ultimate target of pharmacological research is to find new drugs for treating human diseases such as cancer. Agents causing differentiation and thus growth arrest should be particularly useful in this regard. A potential target for such anticancer therapy is the enzyme family protein kinase C (PKC), which is involved in the transduction of signals for cell proliferation, differentiation, and apoptosis. Our recent work showing the induction of differentiation in melanoma cells by an activator of one PKC isoform, PKCdelta, touches on several important areas of investigation, which will form the basis of this review: the role of individual isoforms of PKC, their downstream targets and their specific substrates, the mechanism of activation of specific genes involved in the differentiation process, and the molecular basis for the morphological changes associated with differentiation. The central role that PKC plays in these processes points to the need for a greater understanding of the signalling pathways utilized by individual isoforms of this family of enzymes.

Cellular and subcellular localization of stathmin during oocyte and preimplantation embryo development

Stathmin is a 19 kDa cytosolic phosphoprotein, proposed to act as a relay integrating diverse intracellular signaling pathways involved in regulation of cell proliferation, differentiation, and function. To gain further information about its significance during early development, we analyzed stathmin expression and subcellular localization in mouse oocytes and preimplantation embryos. RT-PCR analysis revealed a low expression of stathmin mRNA in unfertilized oocytes and a higher expression at the blastocyst stage. A fine cytoplasmic punctuate fluorescent immunoreactive stathmin pattern was detected in the oocyte, while it evolved toward an increasingly speckled pattern in the two-cell and later four- to eight-cell embryo, with even larger speckles at the morula stage. In blastocysts, stathmin immunoreactivity was fine and intense in inner cell mass cells, whereas it was low and variable in trophectodermal cells. Electron microscopic analysis allowed visualization with more detail of two types of stathmin immunolocalization: small clusters in the cytoplasm of oocytes and blastocyst cells, together with loosely arranged clusters around the outer membrane of cytoplasmic vesicles, corresponding to the immunofluorescent speckles in embryos until the morula stage. In conclusion, it appears from our results that maternal stathmin is accumulated in the oocyte and is relocalized within the oocyte and early preimplantation embryonic cell cytoplasm to interact with specific cytoplasmic membrane formations. Probably newly synthesized, embryonic stathmin is expressed in the blastocyst, where it is localized more uniformly in the cytoplasm mostly of inner cell mass (ICM) cells. These expression and localization patterns are probably related to the particular roles of stathmin at the successive steps of oocyte maturation and early embryonic development. They further support the proposed physiologic importance of stathmin in essential biologic regulation.

Differential, regional, and cellular expression of the stathmin family transcripts in the adult rat brain

Stathmin is a ubiquitous cytosolic phosphoprotein, preferentially expressed in the nervous system, and previously described as a relay integrating diverse intracellular signaling pathways. Stathmin is the generic element of a mammalian protein family including SCG10, SCLIP, and RB3 with its splice variants RB3' and RB3". In contrast with stathmin, SCG10, SCLIP, and RB3/RB3'/RB3" are exclusively expressed in the nervous system, stathmin and SCG10 being mostly expressed during cell proliferation and differentiation, and SCLIP and RB3 rather in mature neural cells. To further understand their specific roles in the CNS, we compared the localization of the stathmin, SCG10, SCLIP, and RB3 transcripts in adult rat brain. Northern blot analysis as well as in situ hybridization experiments showed that all stathmin-related mRNAs are expressed in a wide range of adult rat brain areas. At a regional level, SCG10 and SCLIP appear generally distributed similarly except in a few areas. The pattern of expression of the RB3 transcript is very different from that of the three other members of the stathmin family. Furthermore, unlike SCG10 and SCLIP, which were detected only in neurons, but like stathmin, RB3 was detected in neurons and also in glial cells of the white matter. Altogether, our results suggest distinct roles for each member of the stathmin-related phosphoprotein family, in regard to their specific regional and cellular localization in the rat brain.

The growth-related, translationally controlled protein P23 has properties of a tubulin binding protein and associates transiently with microtubules during the cell cycle

The translationally controlled protein P23 was discovered by the early induction of its rate of synthesis after mitogenic stimulation of mouse fibroblasts. P23 is expressed in almost all mammalian tissues and it is highly conserved between animals, plants and yeast. Based on its amino acid sequence, P23 cannot be attributed to any known protein family, and its cellular function remains to be elucidated. Here, we present evidence that P23 has properties of a tubulin binding protein that associates with microtubules in a cell cycle-dependent manner. (1) P23 is a cytoplasmic protein that occurs in complexes of 100–150 kDa, and part of P23 can be immunoprecipitated from HeLa cell extracts with anti-tubulin antibodies. (2) In immunolocalisation experiments we find P23 associated with microtubules during G1, S, G2 and early M phase of the cell cycle. At metaphase, P23 is also bound to the mitotic spindle, and it is detached from the spindle during metaphase-anaphase transition. (3) A GST-P23 fusion protein interacts with alpha- and beta-tubulin, and recombinant P23 binds to taxol-stabilised microtubules in vitro. The tubulin binding domain of P23 was identified by mutational analysis; it shows similarity to part of the tubulin binding domain of the microtubule-associated protein MAP-1B. (4) Overexpression of P23 results in cell growth retardation and in alterations of cell morphology. Moreover, elevation of P23 levels leads to microtubule rearrangements and to an increase in microtubule mass and stability.

Differential Expression and Activation of p38 Mitogen-Activated Protein Kinase α, β, γ, and δ in Inflammatory Cell Lineages

Four p38 mitogen-activated protein kinases (p38α, β, γ, δ) have been described. To understand the role of p38 family members in inflammation, we determined their relative expression in cells that participate in the inflammatory process. Expression was measured at the level of mRNA by reverse-transcriptase PCR and protein by Western blot analysis. p38α was the dominant form of p38 in monocytes; expression of p38δ was low and p38β was undetected. In macrophages, p38α and p38δ were abundant, but p38β was undetected. p38α and p38δ were also expressed by neutrophils, CD4+ T cells, and endothelial cells. Again, p38β was not detected in neutrophils, although low amounts were present in CD4+ T cells. In contrast, p38β was abundant in endothelial cells. p38γ protein was not detected in any cell type, although p38γ mRNA was present in endothelial cells. Immunokinase assays showed a strong activation of p38α and a lesser activation of p38δ in LPS-stimulated macrophages. Abs specific for mono- and dual-phophorylated forms of p38 suggested that LPS induces dual phosphorylation of p38α, but primarily mono-phosphorylation of p38δ. IL-1β activated p38α and p38β in endothelial cells. However, p38α was the more activated form based on kinase assays and phosphorylation analysis. Expression and activation patterns of p38α in macrophages and endothelial cells suggest that p38α plays a major role in the inflammatory response. Additional studies will be needed to define the contribution of p38δ to macrophage, neutrophil, and T cell functions, and of p38β to signaling in endothelial cells and T cells.

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

Oncoprotein 18/stathmin (Op18) is a recently identified phosphorylation-responsive regulator of the microtubule (MT) system. It was originally proposed that Op18 specifically regulates dynamic properties of MTs by associating with tubulin, but it has subsequently been proposed that Op18 acts simply by sequestering of tubulin heterodimers. We have dissected the mechanistic action of Op18 by generation of two distinct classes of mutants. One class has interruptions of the heptad repeats of a potential coiled-coil region of Op18, and the other involves substitution at all four phosphorylation sites with negatively charged Glu residues. Both types of mutation result in Op18 proteins with a limited decrease in tubulin complex formation. However, the MT-destabilizing activities of the coiled-coil mutants are more severely reduced in transfected leukemia cells than those of the Glu-substituted Op18 derivative, providing evidence for tubulin-directed regulatory activities distinct from tubulin complex formation. Analysis of Op18-mediated regulation of tubulin GTPase activity and taxol-promoted tubulin polymerization showed that while wild-type and Glu-substituted Op18 derivatives are active, the coiled-coil mutants are essentially inactive. This suggests that Op18-tubulin contact involves structural motifs that deliver a signal of regulatory importance to the MT system.

Phosphofructokinase from Dictyostelium discoideum is a potent inhibitor of tubulin polymerization

We identified the nonallosteric phosphofructokinase from the slime mold Dictyostelium discoideum as a potent protein factor that inhibits the rate of polymerization of tubulin at a molar ratio of 1 molecule to about 300 tubulin dimers for half-maximal action (IC50 = 32 nM). This effect was (i) assessed by turbidity measurements, pelleting of microtubules, and electron microscopy, (ii) observed when tubulin assembly was induced by taxol as well as by GTP in the presence of microtubule-associated proteins or glutamate, and (iii) specific as it was not produced by the phosphofructokinase from rabbit muscle. Also in contrast to the latter, neither tubulin nor microtubules modified the catalytic activity of the slime mold isozyme. Immunoelectron microscopy provided further evidence that D. discoideumphosphofructokinase physically interacts with tubulin, leading to the formation of aggregates. The process seems to be reversible since microtubules eventually formed in the presence of the inhibitor with concomitant reduction of tubulin aggregates. Limited proteolysis by subtilisin showed that the hypervariable C-termini of tubulin is not involved in the interaction with the enzyme. The possible physiological relevance of this novel function of D. discoideum phosphofructokinase different from its glycolytic action is discussed.

Induction of integral membrane PAM expression in AtT-20 cells alters the storage and trafficking of POMC and PC1

Peptidylglycine alpha-amidating monooxygenase (PAM) is an essential enzyme that catalyzes the COOH-terminal amidation of many neuroendocrine peptides. The bifunctional PAM protein contains an NH2-terminal monooxygenase (PHM) domain followed by a lyase (PAL) domain and a transmembrane domain. The cytosolic tail of PAM interacts with proteins that can affect cytoskeletal organization. A reverse tetracycline-regulated inducible expression system was used to construct an AtT-20 corticotrope cell line capable of inducible PAM-1 expression. Upon induction, cells displayed a time- and dose-dependent increase in enzyme activity, PAM mRNA, and protein. Induction of increased PAM-1 expression produced graded changes in PAM-1 metabolism. Increased expression of PAM-1 also caused decreased immunofluorescent staining for ACTH, a product of proopiomelanocortin (POMC), and prohormone convertase 1 (PC1) in granules at the tips of processes. Expression of PAM-1 resulted in decreased ACTH and PHM secretion in response to secretagogue stimulation, and decreased cleavage of PC1, POMC, and PAM. Increased expression of a soluble form of PAM did not alter POMC and PC1 localization and metabolism. Using the inducible cell line model, we show that expression of integral membrane PAM alters the organization of the actin cytoskeleton. Altered cytoskeletal organization may then influence the trafficking and cleavage of lumenal proteins and eliminate the ability of AtT-20 cells to secrete ACTH in response to a secretagogue.

Accessory protein regulation of microtubule dynamics throughout the cell cycle

A number of accessory proteins capable of stabilizing or destabilizing microtubule polymers in dividing cells have been identified recently. Many of these accessory proteins are modified and regulated by cell-cycle-dependent phosphorylation. Through this regulation, microtubule dynamics are modified to generate rapid microtubule turnover during mitosis. In general, although some microtubule-stabilizing proteins are inactivated at entry into mitosis, a critical balance between microtubule stabilizers and destabilizers is necessary for assembly of the mitotic spindle.

Identification of MINUS, a small polypeptide that functions as a microtubule nucleation suppressor

In eukaryotic cells, tubulin polymerization must be regulated precisely during cell division and differentiation. To identify new mechanisms involved in cellular microtubule formation, we isolated an activity that suppresses microtubule nucleation in vitro. The activity was due to a small acidic polypeptide of 4.7 kDa which we named MINUS (microtubule nucleation suppressor). MINUS inhibited tau- and taxol-mediated microtubule assembly in vitro and was inactivated by dephosphorylation. The protein was purified to homogeneity from cultured neural (PC12) cells and bovine brain. Microinjection of MINUS caused a transient loss of dynamic microtubules in Vero cells. The results suggest that MINUS acts with a novel mechanism on tubulin polymerization, thus regulating microtubule formation in living cells.

Stathmin interaction with HSC70 family proteins

Stathmin is a ubiquitous cytosolic phosphoprotein participating in the relay and integration of diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation, and activities. It is phosphorylated in response to diverse extracellular signals including hormones and growth factors, and it is highly expressed during development and in diverse tumoral cells and tissues. Stathmin interacts with tubulin and other potential protein partners such as BiP, KIS, CC1 and CC2/tsg101. In our present search for further functional partners of stathmin, we identified proteins in the Hsp70 family, and in particular Hsc70, as interacting with stathmin in vitro. Hsc70 is among the proteins coimmunoprecipitated with stathmin, and it is the main protein retained specifically on stathmin-Sepharose beads identified by one- and two-dimensional electrophoresis and immunoblots. Bovine serum albumin (BSA)-Sepharose did not bind Hsc70, and anti-stathmin antisera specifically inhibited the interaction of Hsc70 with stathmin-Sepharose. The binding of Hsc70 to stathmin is dependent on the phosphorylation status of stathmin, as it did not occur with a "pseudophosphorylated" mutant form of stathmin. This interaction is further dependent on the ATP status of Hsc70. It was inhibited in the presence of ATP-Mg++ but not in the presence of ATP-Mg++ and ethylenediaminetetraacetic acid (EDTA) or of ADP. Our results suggest that the interaction of stathmin with Hsc70 is specific in both proteins and most likely biologically relevant in the context of their functional implication in the control of numerous intracellular signaling and regulatory pathways, and hence of normal cell growth and differentiation.

Pyruvate kinase as a microtubule destabilizing factor in vitro

Endogenous control of microtubule dynamism is essential in many cell types. Numerous microtubule-adhering proteins stabilize the polymer status, while very few protein factors are described with opposite effects. The brain- and muscle-specific M1 isoform of the enzyme pyruvate kinase is investigated here in this respect. Three pieces of evidence indicate antimicrotubular effects of this protein. (1) Pyruvate kinase inhibits taxol-induced tubulin polymerization into microtubules as revealed by turbidimetry. (2) Pelleting experiments show that pyruvate kinase partially disassembles taxol-stabilized microtubules into less sedimentable oligomers leading to the appearance of tubulin in the supernatant fractions. (3) Electron microscopy reveals the kinase-induced formation of great amounts of thread-like tubulin oligomers which tend to accumulate in a light/less sedimentable fraction. Immunoelectron micrographs using labeled antibody against pyruvate kinase provide evidence for the binding of pyruvate kinase to the thread-like oligomeric forms. The present data allow the assumption that pyruvate kinase may display multiple regulatory functions as a glycolytic control enzyme and as a modulator of microtubule dynamism.

Kalirin inhibition of inducible nitric-oxide synthase

Nitric oxide (NO) acts as a neurotransmitter. However, excess NO produced from neuronal NO synthase (nNOS) or inducible NOS (iNOS) during inflammation of the central nervous system can be neurotoxic, disrupting neurotransmitter and hormone production and killing neurons. A screen of a hippocampal cDNA library showed that a unique region of the iNOS protein interacts with Kalirin, previously identified as an interactor with a secretory granule peptide biosynthetic enzyme. Kalirin associates with iNOS in vitro and in vivo and inhibits iNOS activity by preventing the formation of iNOS homodimers. Expression of exogenous Kalirin in pituitary cells dramatically reduces iNOS inhibition of ACTH secretion. Thus Kalirin may play a neuroprotective role during inflammation of the central nervous system by inhibiting iNOS activity.

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.

Balanced regulation of microtubule dynamics during the cell cycle: a contemporary view

Assembly of mitotic and meiotic spindles into an elliptical bipolar shape is an example of morphogenetic processes that involve local chromosomal regulation of microtubule dynamics for proper spatial microtubule assembly. Global microtubule dynamics during the cell cycle and local microtubule dynamics during spindle assembly are regulated by a balance between microtubule stabilizing and destabilizing factors. How a chromosome-induced phosphorylation gradient may be generated and modulate spindle microtubule assembly through balanced regulation of the activity of microtubule-associated proteins and Stathmin/Op 18 is analyzed.

Characterization of a splice variant of metastasis-associated h-mts1 (S100A4) gene expressed in human infiltrating carcinomas of the breast

The h-mts1 (S100A4) is a member of the S100 gene family, coding for a calcium-binding protein. It is a metastasis-associated gene whose expression shows strong correlation with the proliferative potential and invasive and metastatic ability of cancers. In a proportion of human intraductal carcinomas of the breast, a shorter variant h-mts1 transcript (h-mts1v) of approximately 450 nucleotides is expressed. We have characterized the transcript using reverse transcriptase-polymerase chain reaction employing exon-specific oligonucleotides. We show here that the noncoding exon 1a/1b is lost in the variant cDNA. Exons 2 and 3, which code for the protein, seem to be present in the variant isoform. The RT-PCR products obtained using exons 2- and 3-specific oligonucleotides showed a high degree of sequence homology with exons 2 and 3 of the h-mts1 gene. The expression of the variant transcript could be influencing disease progression, albeit not as effectively as the normal unspliced h-mts1 transcript.

The stathmin phosphoprotein family: intracellular localization and effects on the microtubule network

Stathmin is a small regulatory phosphoprotein integrating diverse intracellular signaling pathways. It is also the generic element of a protein family including the neural proteins SCG10, SCLIP, RB3 and its two splice variants RB3′ and RB3″. Stathmin itself was shown to interact in vitro with tubulin in a phosphorylation-dependent manner, sequestering free tubulin and hence promoting microtubule depolymerization. We investigated the intracellular distribution and tubulin depolymerizing activity in vivo of all known members of the stathmin family. Whereas stathmin is not associated with interphase microtubules in HeLa cells, a fraction of it is concentrated at the mitotic spindle. We generated antisera specific for stathmin phosphoforms, which allowed us to visualize the regulation of phosphorylation-dephosphorylation during the successive stages of mitosis, and the partial localization of stathmin phosphorylated on serine 16 at the mitotic spindle. Results from overexpression experiments of wild-type and novel phosphorylation site mutants of stathmin further suggest that it induces depolymerization of interphase and mitotic microtubules in its unphosphorylated state but is inactivated by phosphorylation in mitosis. Phosphorylation of mutants 16A25A and 38A63A on sites 38 and 63 or 16 and 25, respectively, was sufficient for the formation of a functional spindle, whereas mutant 16A25A38A63E retained a microtubule depolymerizing activity. Transient expression of each of the neural phosphoproteins of the stathmin family showed that they are at least partially associated to the Golgi apparatus and not to other major membrane compartments, probably through their different NH2-terminal domains, as described for SCG10. Most importantly, like stathmin and SCG10, overexpressed SCLIP, RB3 and RB3″ were able to depolymerize interphase microtubules. Altogether, our results demonstrate in vivo the functional conservation of the stathmin domain within each protein of the stathmin family, with a microtubule destabilizing activity most likely essential for their specific biological function(s).

Overexpression of the stathmin gene in a subset of human breast cancer

Stathmin is a highly conserved cytosolic phosphoprotein that destabilizes microtubules. Stathmin, which has been proposed as a relay protein integrating diverse cell signalling pathways, acts in vitro as a tubulin-sequestering protein, and its activity is dramatically reduced by phosphorylation. Interestingly, stathmin expression and phosphorylation are regulated during the control of cell growth and differentiation, and there is much evidence suggesting that in vivo stathmin plays a role in the control of microtubule dynamics during mitosis. Stathmin may thus be considered as one of the key regulators of cell division. We examined 50 human primary breast tumours for stathmin mRNA and protein expression and screened for abnormalities in the chromosome region harbouring the stathmin gene. Overexpression of stathmin was found in 15 tumours (30%). At the present stage, no clear correlation emerged between stathmin expression and several prognosis markers. Interestingly, perfect matching was observed between stathmin mRNA overexpression, protein overexpression and strong staining for stathmin on paraffin-embedded tumour sections when specimens were available. Furthermore, a tentative link between loss of heterozygosity (LOH) in the 1p32-1pter region and stathmin overexpression was observed. Our results suggest that stathmin might play a role in breast carcinogenesis and that stathmin-overexpressing tumours may represent a new subtype of breast cancer.

Identification of stathmin as a novel substrate for p38 delta

p38 mitogen-activated protein kinases (MAPK) are a family of kinases that are activated by cellular stresses and inflammatory cytokines. Although there are many similarities shared by the isoforms of p38 (alpha, beta, gamma, and delta), p38 delta differs from the others in some respects such as inhibitor sensitivity and substrate specificity. Utilizing in a solution kinase assay, we identified a novel p38 delta substrate as stathmin. Stathmin is a cytoplasmic protein that was previously reported to be a substrate of several intracellular signaling kinases and has recently been linked to regulation of microtubule dynamics. p38 delta has significantly higher in vitro phosphorylating activity against stathmin than other p38 isoforms or related MAPKs. In transient expression studies, we found that in addition to different stimuli osmotic stress activates p38 delta to phosphorylate stathmin. The sites of phosphorylation were mapped to Ser-25 and Ser-38, both in vitro and in cells.

Serine 16 of Stathmin as a Cytosolic Target for Ca2+/Calmodulin-Dependent Kinase II After CD2 Triggering of Human T Lymphocytes

We investigated specific signaling events initiated after T cell triggering through the costimulatory surface receptors CD2 and CD28 as compared with activation via the Ag receptor (TCR/CD3). We therefore followed the phosphorylation of stathmin, a ubiquitous cytoplasmic phosphoprotein proposed as a general relay integrating diverse intracellular signaling pathways through the combinatorial phosphorylation of serines 16, 25, 38, and 63, the likely physiologic substrates for Ca2+/calmodulin (CaM)-dependent kinases, mitogen-activated protein (MAP) kinase, cyclin-dependent kinases (cdks), and protein kinase A, respectively. We addressed the specific protein kinase systems involved in the CD2 pathway of T cell activation through the analysis of stathmin phosphorylation patterns in exponentially growing Jurkat T cells, as revealed by phosphopeptide mapping. Stimulation via CD2 activated multiple signal transduction pathways, resulting in phosphorylation of distinct sites of stathmin, the combination of which only partially overlaps the CD3- and CD28-induced patterns. The partial redundancy of the three T cell activation pathways was evidenced by the phosphorylation of Ser25 and Ser38, substrates of MAP kinases and of the cdk family kinase(s), respectively. Conversely, the phosphorylation of Ser16 of stathmin was observed in response to both CD2 and CD28 triggering, but not CD3 triggering, with a kinetics compatible with the lasting activation of CaM kinase II in response to CD2 triggering. In vitro, Ser16 of recombinant human stathmin was phosphorylated also by purified CaM kinase II, and in vivo, CaM kinase II activity was indeed stimulated in CD2-triggered Jurkat cells. Altogether, our results favor an association of CaM kinase II activity with costimulatory signals of T lymphocyte activation and phosphorylation of stathmin on Ser16.

Luteinizing hormone-releasing hormone-signal transduction and stathmin phosphorylation in the gonadotrope alphaT3-1 cell line

We have investigated the effects of GnRH (LHRH) and of the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate on stathmin phosphorylation in the gonadotrope alphaT3-1 cell line. Stathmin expression and its phosphorylation were maximal during the exponential phase of cell growth. LHRH stimulated stathmin phosphorylation through a specific receptor in a dose- and time-dependent manner, and TPA induced a similar extensive stathmin phosphorylation. Their effects were inhibited either in PKC-depleted alphaT3-1 cells, or by the PKC inhibitor staurosporine. In the context of the known implication of PKC in LHRH-induced signal transduction, our results show that stathmin phosphorylation is involved in LHRH transduction, either as a result of direct activation of specific PKC isoforms or through a pathway involving kinases downstream to PKC activation.

Microtubule dynamics: if you need a shrink try stathmin/Op18

Recent studies show that stathmin/Op18 may be an important physiological regulator of microtubule dynamics; the activity of stathmin/Op18 is controlled by the actions of several signalling pathways, allowing it to play a central role in coordinating microtubule behaviour.

Stathmin overexpression in 293 cells affects signal transduction and cell growth

Stathmin is a ubiquitous cytoplasmic protein whose phosphorylation state changes markedly in response to extracellular signals, and during the cell cycle. To clarify the function of stathmin, its four phosphorylation sites were mutated to either alanines (4A-stathmin) or glutamates (4E-stathmin). In transfected cells, 4A-stathmin caused a strong G2/M block and also inhibited the responsiveness of a co-transfected fos promoter/ luciferase reporter plasmid to serum stimulation, whereas wild type and 4E-stathmin had relatively minor effects. These results support the idea that stathmin plays a role in multiple cellular processes and indicate that the regulation of the phosphorylation state of stathmin is likely to determine its action.