Cyclin B interaction with microtubule-associated protein 4 (MAP4) targets p34cdc2 kinase to microtubules and is a potential regulator of M-phase microtubule dynamics

Separation of tubulin subunits under nondenaturing conditions

The dissociation and separation of the tubulin alpha- and beta-subunits have been achieved by binding alpha-subunits to an immunoadsorbent gel and selectively inducing release of free beta-subunits. The immunoadsorbent gel was prepared by coupling the monoclonal antibody YL1/2 to Sepharose 4B which specifically recognizes the C-terminal end of tyrosinated alpha-subunits. Extensive tubulin subunit dissociation and separation occurred in Tris buffer at neutral pH but was greatly enhanced at basic pHs (8. 0-8.5). The binding of colchicine to heterodimeric tubulin resulted in a marked protection against dissociation. The dissociation of tubulin subunits was accompanied by loss of colchicine binding capacity, and ability to polymerize into microtubules. As shown by circular dichroism, loss of functional properties was not due to extensive denaturation of tubulin, as tubulin retained most of its secondary structure. Neither of the separated alpha- or beta-subunits was able to bind colchicine, but functional tubulin that was able to bind colchicine could be reconstituted from the dissociated subunits by changing the buffer to a neutral mixture of Tris and Pipes. The yield of reconstitution, as estimated from kinetic measurements of colchicine binding capacity, amounted to about 25%. Such a yield can probably be improved with minor changes in experimental conditions. The quantitative dissociation of tubulin into separated "native" alpha- and beta-subunits should provide a powerful tool for further studies on the properties of the individual tubulin subunits and the structure-function relationships of the tubulins.

The endogenous and cell cycle-dependent phosphorylation of tau protein in living cells: implications for Alzheimer's disease

In Alzheimer's disease the neuronal microtubule-associated protein tau becomes highly phosphorylated, loses its binding properties, and aggregates into paired helical filaments. There is increasing evidence that the events leading to this hyperphosphorylation are related to mitotic mechanisms. Hence, we have analyzed the physiological phosphorylation of endogenous tau protein in metabolically labeled human neuroblastoma cells and in Chinese hamster ovary cells stably transfected with tau. In nonsynchronized cultures the phosphorylation pattern was remarkably similar in both cell lines, suggesting a similar balance of kinases and phosphatases with respect to tau. Using phosphopeptide mapping and sequencing we identified 17 phosphorylation sites comprising 80-90% of the total phosphate incorporated. Most of these are in SP or TP motifs, except S214 and S262. Since phosphorylation of microtubule-associated proteins increases during mitosis, concomitant with increased microtubule dynamics, we analyzed cells mitotically arrested with nocodazole. This revealed that S214 is a prominent phosphorylation site in metaphase, but not in interphase. Phosphorylation of this residue strongly decreases the tau-microtubule interaction in vitro, suppresses microtubule assembly, and may be a key factor in the observed detachment of tau from microtubules during mitosis. Since S214 is also phosphorylated in Alzheimer's disease tau, our results support the view that reactivation of the cell cycle machinery is involved in tau hyperphosphorylation.

Endomitosis of Human Megakaryocytes Are Due to Abortive Mitosis

During megakaryocyte differentiation, the promegakaryoblast (immature megakaryocyte) increases its ploidy to a 2x DNA content by a poorly understood process called endomitosis. This leads to the formation of a giant cell, the megakaryocyte (MK), which subsequently gives rise to platelets. In this report, we show that endomitotis of human MKs is due to abortive mitosis. Human MKs were obtained by a two-step purification of CD34+ blood or marrow precursors followed by in vitro culture in the presence of MK growth factors. Microcoscopic examination shows that a large number of centrosomes (up to 32) and centrioles are present in polyploid MKs. After nocodazole treatment, more than 20% of the MK are blocked in a typical pseudo-metaphase. Both spontaneous and nocodazole-induced endomitosis are associated with a breakdown of the nuclear envelope and possess a complex mitotic spindle composed of several asters. Spindle microtubules radiate from each aster, creating a spherical structure. At metaphase, expression of the kinetochore phosphoepitope recognized by the 3F3/2 antibody is lost, and the sister chromatides segregate moving toward the spindle poles. After limited segregation, the chromosomes decondense and the nuclear envelope reforms in the absence of cytokinesis, isolating all chromosomes in a single nucleus. It has been proposed that endomitosis could be due to an abnormal CDK1 activity or an absence of cyclin B1. Our results show that cyclin B1 can be detected in all MKs, including those with a ploidy of 8N or more. The cyclin B1 staining colocalizes with the mitotic spindle. Using flow cytometry, the level of cyclin B1 increased until 8N, but remained identical in 16N and 32N MKs. Cell sorting was used to separate the MKs into a 2N/4N and >4N population. Both cyclin B1 and CDK1 could be detected in the endomitotic polyploid MKs using Western blot analysis, and a histone H1 kinase activity was associated with immunoprecipitated cyclin B1. We conclude that endomitosis of human MKs is due to abortive mitosis, possibly due to alterations in the regulation of mitotic exit.

Protein phosphatase 4 is an essential enzyme required for organisation of microtubules at centrosomes in Drosophila embryos

The protein serine/threonine phosphatase 4 (PP4), which localises to centrosomes/spindle pole bodies in human cells, is shown to exhibit a similar localisation in Drosophila cells and embryos and possess a highly conserved (91% identical) amino acid sequence from humans to invertebrates. A homozygous Drosophila melanogaster strain mutant in the PP4 gene at 19C1-2 has been produced using P element mutagenesis. This strain, termed centrosomes minus microtubules (cmm), has reduced amounts of PP4 mRNA, approximately 25% of normal PP4 protein in early embryos and exhibits a semi-lethal phenotype with only 10% viability in certain conditions. Reversion mutagenesis shows that the phenotype is due to the presence of the P element in the PP4 mRNA. In early cmm embryos, nuclear divisions become asynchronous and large regions containing centrosomes with no well defined radiating microtubules are visible. In such areas, most nuclei arrest during mitosis with condensed DNA, and mitotic spindle microtubules are either absent, or aberrant and unconnected to the centrosome. A reduction in the staining of gamma-tubulin at centrosomes in cmm embryos suggests a conformational change or relocation of this protein, which is known to be essential for initiation of microtubule growth. These findings indicate that PP4 is required for nucleation, growth and/or stabilisation of microtubules at centrosomes/spindle pole bodies.

Endomitosis of Human Megakaryocytes Are Due to Abortive Mitosis

During megakaryocyte differentiation, the promegakaryoblast (immature megakaryocyte) increases its ploidy to a 2x DNA content by a poorly understood process called endomitosis. This leads to the formation of a giant cell, the megakaryocyte (MK), which subsequently gives rise to platelets. In this report, we show that endomitotis of human MKs is due to abortive mitosis. Human MKs were obtained by a two-step purification of CD34+ blood or marrow precursors followed by in vitro culture in the presence of MK growth factors. Microcoscopic examination shows that a large number of centrosomes (up to 32) and centrioles are present in polyploid MKs. After nocodazole treatment, more than 20% of the MK are blocked in a typical pseudo-metaphase. Both spontaneous and nocodazole-induced endomitosis are associated with a breakdown of the nuclear envelope and possess a complex mitotic spindle composed of several asters. Spindle microtubules radiate from each aster, creating a spherical structure. At metaphase, expression of the kinetochore phosphoepitope recognized by the 3F3/2 antibody is lost, and the sister chromatides segregate moving toward the spindle poles. After limited segregation, the chromosomes decondense and the nuclear envelope reforms in the absence of cytokinesis, isolating all chromosomes in a single nucleus. It has been proposed that endomitosis could be due to an abnormal CDK1 activity or an absence of cyclin B1. Our results show that cyclin B1 can be detected in all MKs, including those with a ploidy of 8N or more. The cyclin B1 staining colocalizes with the mitotic spindle. Using flow cytometry, the level of cyclin B1 increased until 8N, but remained identical in 16N and 32N MKs. Cell sorting was used to separate the MKs into a 2N/4N and >4N population. Both cyclin B1 and CDK1 could be detected in the endomitotic polyploid MKs using Western blot analysis, and a histone H1 kinase activity was associated with immunoprecipitated cyclin B1. We conclude that endomitosis of human MKs is due to abortive mitosis, possibly due to alterations in the regulation of mitotic exit.

In vivo regulation of the entry into M-phase: initial activation and nuclear translocation of cyclin B/Cdc2

The cyclin B/Cdc2 complex, Cdc2 kinase governs M-phase. Although the intracomplex modification for its activation in vitro has been described extensively, its regulation in vivo is not so well explained so far. In this article, we will focus on the intracellular regulation of the cyclin B/Cdc2 activity, in particular, how it is initially activated in vivo, how its nuclear translocation is executed specifically at the onset of M-phase, and how the activation and the nuclear translocation are coordinated in the cell. These concerted regulations may determine the appropriate timing for the initiation of M-phase.

The plant cell cycle: conserved and unique features in mitotic control

Somatic plant cells can use a hormone checkpoint in late G2 phase. Here cytokinin stimulates removal of phosphotyrosine from p34cdc2 kinase and concurrently capacity for activation of the kinase by Cdc25 phosphatase declines while activity of the kinase increases and cells enter mitosis. Processes unique to plant mitosis are driven by the mitotically active kinase since the enzyme taken from plant cells in metaphase, when injected, can disassemble the preprophase band microtubules that form in G2 phase at the site of the future cross wall. This action is specific, since microtubules are not depolymerised when in interphase cytoplasmic array, or spindle, or phragmoplast. Plant metaphase kinase acts as MPF by accelerating chromosome condensation and nuclear envelope breakdown.

Cyclin B2-null mice develop normally and are fertile whereas cyclin B1-null mice die in utero

Two B-type cyclins, B1 and B2, have been identified in mammals. Proliferating cells express both cyclins, which bind to and activate p34(cdc2). To test whether the two B-type cyclins have distinct roles, we generated lines of transgenic mice, one lacking cyclin B1 and the other lacking cyclin B2. Cyclin B1 proved to be an essential gene; no homozygous B1-null pups were born. In contrast, nullizygous B2 mice developed normally and did not display any obvious abnormalities. Both male and female cyclin B2-null mice were fertile, which was unexpected in view of the high levels and distinct patterns of expression of cyclin B2 during spermatogenesis. We show that the expression of cyclin B1 overlaps the expression of cyclin B2 in the mature testis, but not vice versa. Cyclin B1 can be found both on intracellular membranes and free in the cytoplasm, in contrast to cyclin B2, which is membrane-associated. These observations suggest that cyclin B1 may compensate for the loss of cyclin B2 in the mutant mice, and implies that cyclin B1 is capable of targeting the p34(cdc2) kinase to the essential substrates of cyclin B2.

Purification of a WD repeat protein, EMAP, that promotes microtubule dynamics through an inhibition of rescue

The major microtubule-associated protein in echinoderms is a 77-kDa, WD repeat protein, called EMAP. EMAP-related proteins have been identified in sea urchins, starfish, sanddollars, and humans. We describe the purification of sea urchin EMAP and demonstrate that EMAP binding to microtubules is saturable at a molar ratio of 1 mol of EMAP to 3 mol of tubulin dimer. Unlike MAP-2, MAP-4, or tau proteins, EMAP binding to microtubules is not lost by cleavage of tubulin with subtilisin. In addition to binding to the microtubule polymer, EMAP binds to tubulin dimers in a 1:1 molar ratio. The abundance of EMAP in the egg suggests that it could function to regulate microtubule assembly. To test this hypothesis, we examined the effects of EMAP on the dynamic instability of microtubules nucleated from axoneme fragments as monitored by video-enhanced differential interference contrast microscopy. Addition of 2.2 microM EMAP to 21 microM tubulin results in a slight increase in the elongation and shortening velocities at the microtubule plus ends but not at the minus ends. Significantly, EMAP inhibits the frequency of rescue 8-fold without producing a change in the frequency of catastrophe. These results indicate that EMAP, unlike brain microtubule-associated proteins, promotes microtubule dynamics.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

A propagated wave of MPF activation accompanies surface contraction waves at first mitosis in Xenopus

During the period of mitosis, two surface contraction waves (SCWs) progress from the animal to vegetal poles of the Xenopus egg. It has been shown that these SCWs occur in parallel with the activation of MPF and with its subsequent inactivation in the animal and vegetal hemispheres, suggesting that they are responses to propagated waves of MPF activity across the egg. We have analysed the mechanism of MPF regulation in different regions of the egg in detail in relation to SCW progression. The distributions of histone HI kinase activity and of Cdc2 and cyclin B (the catalytic and regulatory subunits of MPF) were followed by dissection of intact eggs following freezing and in cultured fragments separated by ligation. Cdc2 was found to be distributed evenly throughout the egg cytoplasm. Loss of phosphorylated (inactive) forms of Cdc2 coincided spatially with the wave of MPF activation, while cyclin B2 accumulation occurred in parallel in animal and vegetal regions. In ligated vegetal pole fragments no MPF activation or Cdc2 dephosphorylation were detectable. A wave of cyclin B destruction that occurred in concert with the second SCW was also blocked. Taken together these results indicate that the triggering mechanism for MPF activation requires components specific to the animal cytoplasm, acting via Cdc2 dephosphorylation, and that MPF activation subsequently propagates autocatalytically across the egg. SCW progression in the vegetal hemisphere was followed directly by time-lapse videomicroscopy of subcortical mitochondrial islands. The first SCW traversed the vegetal pole at the time of MPF activation in this region. Like MPF activation and inactivation, SCWs were blocked in the vegetal region by ligation. These observations reinforce the hypothesis that the first SCW is a direct consequence of the MPF activation wave. It may reflect depolymerisation of the subcortical microtubule network since it coincided exactly with the arrest of the microtubule-dependent movement of ‘cortical rotation’ and was related in direction in most eggs. The cyclin B destruction wave and associated cortical contraction of the second SCW may be localised downstream consequences of the MPF activation wave, or they may propagate independently from the animal cytoplasm.

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.

Regulation of microtubule dynamics by extracellular signals: cAMP-dependent protein kinase switches off the activity of oncoprotein 18 in intact cells

Oncoprotein 18 (Op18, also termed p19, 19K, metablastin, stathmin, and prosolin) is a recently identified regulator of microtubule (MT) dynamics. Op18 is a target for both cell cycle and cell surface receptor-coupled kinase systems, and phosphorylation of Op18 on specific combinations of sites has been shown to switch off its MT-destabilizing activity. Here we show that induced expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) results in a dramatic increase in cellular MT polymer content concomitant with phosphorylation and partial degradation of Op18. That PKA may regulate the MT system by downregulation of Op18 activity was evaluated by a genetic system allowing conditional co-expression of PKA and a series of kinase target site-deficient mutants of Op18. The results show that phosphorylation of Op18 on two specific sites, Ser-16 and Ser-63, is necessary and sufficient for PKA to switch off Op18 activity in intact cells. The regulatory importance of dual phosphorylation on Ser-16 and Ser-63 of Op18 was reproduced by in vitro assays. These results suggest a simple model where PKA phosphorylation downregulates the MT-destabilizing activity of Op18, which in turn promotes increased tubulin polymerization. Hence, the present study shows that Op18 has the potential to regulate the MT system in response to external signals such as cAMP-linked agonists.

Phosphorylation of microtubule-associated proteins from the ovaries of hemipteran insects by MPF and MAP kinase: possible roles in the regulation of microtubules during oogenesis

Nutritive tubes that link the developing oocytes to the nurse cells in ovarioles of hemipteran insects contain extensive arrays of microtubules. These are established, then later depolymerised, by developmentally regulated processes. Breakdown of the microtubules corresponds with the activation of M-phase promoting factor (MPF) and mitogen-activated protein kinase (MAP kinase), later in oogenesis, as the oocytes proceed to arrest at the first meiotic metaphase [Lane and Stebbings, Roux's Arch Dev Biol 205:150-159 (1995)]. The mechanisms that lead to the breakdown of nutritive tube microtubules are unknown. Here, we have investigated the possibility that the insect ovarian microtubules are regulated by MPF- or MAP kinase-dependent phosphorylation, focusing upon the prominent high molecular weight microtubule-associated protein (HMW MAP) enriched in this system, which is a potential target for protein kinase activity in vivo. We have purified the prominent HMW MAPs from the ovaries of two species of hemipterans, and have shown them to be substrates in vitro for the activities of MPF and MAP kinase. However, although the catalytic component of MPF (p34cdc2) is present within microtubule-rich portions of hemipteran ovarioles, we have found that neither this protein nor its regulatory partner (cyclin B) co-purify with microtubules during taxol-mediated microtubule isolation.

A measure of the mitotic index: studies of the abundance and half-life of p34cdc2 in cultured cells and normal and neoplastic tissues

BACKGROUND

The cdc2 gene encodes a protein kinase, p34cdc2, that is essential for mitosis, and is present at high levels in dividing cells. Classical studies of the levels of this protein in dividing and resting cells used antibodies that cross-react with other members of the CDK family, in particular with CDK2. We have therefore re-examined the abundance of p34cdc2 in a variety of tissues and cell lines, using a highly specific, epitope-mapped monoclonal antibody that does not react with CDK2.

RESULTS

We observed high levels of p34cdc2 in proliferating cells, especially those in neoplastic tissues. Cells that have withdrawn from the cell cycle have low or undetectable levels. At the end of mitosis, the level of p34cdc2 declines, with simple first-order kinetics, with a half-life which is never less than 6h and is more typically about 18h. The persistence of p34cdc2 after the last cell division is comparable to that of PCNA, a commonly used marker of proliferation.

CONCLUSIONS

The immunochemical detection of p34cdc2 provides an accurate, reliable and meaningful measure of the proliferative activity of cells in tissues. We suggest that p34cdc2 should be considered as the most authentic molecular marker of the mitotic index.

MAP4 is the in vivo substrate for CDC2 kinase in HeLa cells: identification of an M-phase specific and a cell cycle-independent phosphorylation site in MAP4

We reported previously that cdc2 kinase decreased the microtubule-stabilizing ability of a major HeLa cell microtubule-associated protein, MAP4, by phosphorylation in vitro [Ookata, K., et al. (1995) J. Cell Biol. 128, 849-862]. An important question raised by this study is whether MAP4 is indeed phosphorylated by cdc2 kinase at mitosis in vivo. We present here evidence that cdc2 kinase is the major M-phase MAP4 kinase, and, further, we identify two phosphorylation sites within the proline-rich domain of MAP4. Metabolic 32P labeling showed the increased phosphorylation of MAP4 at mitosis. A specific inhibitor of cdc2 kinase, butyrolactone I, inhibited phosphorylation of MAP4 both in mitotic HeLa cells and in the mitotic HeLa cell extract. The phosphopeptide map analysis revealed the high similarity of in vivo labeled mitotic MAP4 to that phosphorylated by cdc2 kinase in vitro. Ser-696 and Ser-787, both of which lie within SPXK consensus sequences for cdc2 kinase, were identified as phosphorylation sites in the proline-rich region of MAP4 in vivo and in vitro. Immunoblotting with antibodies that recognize the phosphorylation state of Ser-696 or Ser-787 showed that Ser-787 in the SPSK sequence was specifically phosphorylated at mitosis while Ser-696 in the SPEK sequence was phosphorylated both at mitosis and in interphase. These results suggest that cdc2 kinase directly regulates microtubule dynamics at mitosis through phosphorylation of MAP4 at a number of sites, including Ser-787.

Coupling cell division and cell death to microtubule dynamics

The mitotic spindle is a self-organizing structure that is constructed primarily from microtubules. Among the most important spindle microtubules are those that bind to kinetochores and form the fibers along which chromosomes move. Chemotherapeutics such as taxol and the vinca alkaloids perturb kinetochore-microtubule attachment and disrupt chromosome segregation. This activates a checkpoint pathway that delays cell cycle progression and induces programmed cell death. Recent work has identified at least four mammalian spindle assembly checkpoint proteins.

Conserved and novel regulators of the plant cell cycle

Cell division is highly regulated, both spatially and temporally, during plant development. Recent evidence implicates cyclin-dependent kinases (cdks) and their associated proteins as the principal temporal regulators of cell division. It is now known that plants contain an extended family of cdks, some of which appear to be unique to this group. Positive rate-limiting regulators of cell proliferation and growth include mitotic or B-type cyclins whose transcription is restricted to the G2 and M phases. Current research suggests that MYB-related transcription factors may be responsible for this restriction. Cdk-interacting proteins, such as cdk inhibitors and suc1 homologues, have been isolated using yeast two-hybrid approaches.

Plant CDC2 is not only targeted to the pre-prophase band, but also co-localizes with the spindle, phragmoplast, and chromosomes

A polyclonal antiserum against the p34cdc2 homologue of Arabidopsis thaliana, CDC2aAt, was used in parallel with a polyclonal antiserum against the PSTAIRE motif to study the subcellular localization of CDC2 during the cell cycle of isolated root tip cells of Medicago sativa. During interphase, CDC2 was located in the nucleus and in the cytoplasm. The cytoplasmic localization persisted during the complete cell cycle, whereas the nuclear signal disappeared at nuclear envelope breakdown. At the beginning of anaphase, the anti-CDC2aAt antibody transiently co-localized with condensed chromosomes. The chromosomal co-localization disappeared as anaphase continued and remained excluded from the separated chromosomes until cytokinesis, when CDC2 re-located to the newly forming nuclei. We also observed a co-localization of CDC2 with three microtubular structures, the pre-prophase band, the spindle, and the phragmoplast.

XMAP310: a Xenopus rescue-promoting factor localized to the mitotic spindle

To understand the role of microtubule-associated proteins (MAPs) in the regulation of microtubule (MT) dynamics we have characterized MAPs prepared from Xenopus laevis eggs (Andersen, S.S.L., B. Buendia, J.E. Domínguez, A. Sawyer, and E. Karsenti. 1994. J. Cell Biol. 127:1289-1299). Here we report on the purification and characterization of a 310-kD MAP (XMAP310) that localizes to the nucleus in interphase and to mitotic spindle MTs in mitosis. XMAP310 is present in eggs, oocytes, a Xenopus tissue culture cell line, testis, and brain. We have purified XMAP310 to homogeneity from egg extracts. The purified protein cross-links pure MTs. Analysis of the effect of this protein on MT dynamics by time-lapse video microscopy has shown that it increases the rescue frequency 5-10-fold and decreases the shrinkage rate twofold. It has no effect on the growth rate or the catastrophe frequency. Microsequencing data suggest that XMAP230 and XMAP310 are novel MAPs. Although the three Xenopus MAPs characterized so far, XMAP215 (Vasquez, R.J., D.L. Gard, and L. Cassimeris. 1994. J. Cell Biol. 127:985-993), XMAP230, and XMAP310 are localized to the mitotic spindle, they have distinct effects on MT dynamics. While XMAP215 promotes rapid MT growth, XMAP230 decreases the catastrophe frequency and XMAP310 increases the rescue frequency. This may have important implications for the regulation of MT dynamics during spindle morphogenesis and chromosome segregation.

Phosphorylation states of microtubule-associated protein 2 (MAP2) determine the regulatory role of MAP2 in microtubule dynamics

Phosphorylation-dependent regulation of microtubule-stabilizing activities of microtubule-associated protein 2 (MAP2) was examined using optical microscopy. MAP2, purified from mammalian brain, was phosphorylated by either cAMP-dependent protein kinase (PKA) or cyclin B-dependent cdc2 kinase. Using PKA, 15 mol of phosphoryl groups was incorporated per mole of MAP2, but about 70% of the phosphates was distributed to the projection region. Using cdc2 kinase, 7-10 mol of phosphoryl groups was incorporated per mole of MAP2, and more than 60% of the phosphates was distributed to the microtubule-binding region. Both types of phosphorylation similarly reduced binding activity of MAP2 onto microtubules. Direct observation of individual microtubules using dark-field microscopy showed that interconversion between the polymerization phase and the depolymerization phase was repeated in both unphosphorylated and PKA-phosphorylated MAP2. In cdc2 kinase-phosphorylated MAP2, however, the phase transition from depolymerization to polymerization occurred with difficulty, with the result being that the half-life of individual microtubules was as short as in the absence of MAP2. Examination of spontaneous polymerization of microtubules using dark-field microscopy showed that the microtubule-nucleating activity of MAP2 was reduced by PKA-dependent phosphorylation and was completely abolished by cdc2 kinase-dependent phosphorylation. These observations show that cdc2 kinase-dependent phosphorylation inhibits both the microtubule-stabilizing activity and the microtubule-nucleating activity of MAP2, while PKA-dependent phosphorylation affects only the microtubule-nucleating activity of MAP2.

Mitotic chromatin regulates phosphorylation of Stathmin/Op18

Meiotic and mitotic spindles are required for the even segregation of duplicated chromosomes to the two daughter cells. The mechanism of spindle assembly is not fully understood, but two have been proposed that are not mutually exclusive. The 'search and capture' model suggests that dynamic microtubules become progressively captured and stabilized by the kinetochores on chromosomes, leading to spindle assembly. The 'local stabilization' model proposes that chromosomes change the state of the cytoplasm around them, making it more favourable to microtubule polymerization. It has been shown that Stathmin/Op18 inhibits microtubule polymerization in vitro by interaction with tubulin, and that overexpression in tissue culture cells of non-phosphorylatable mutants of Stathmin/Op18 prevents the assembly of mitotic spindles. We have used Xenopus egg extracts and magnetic chromatin beads to show that mitotic chromatin induces phosphorylation of Stathmin/Op18. We have also shown that Stathmin/Op18 is one of the factors regulated by mitotic chromatin that governs preferential microtubule growth around chromosomes during spindle assembly.

Chinese hamster protein homologous to human putative protein kinase KIAA0204 is associated with nuclei, microtubules and centrosomes in CHO-K1 cells

Monoclonal antibody raised against a preparation of loach fish sperm centrosomes was used for screening of cDNA expressing library of Chinese hamster CHO-K1 cells. Two positive clones appeared to encode 628 amino acid protein fragment that was 72% identical to human KIAA0204 protein, i.e. putative protein kinase. Polyclonal antibodies raised against products of cDNA expression in E. coli recognized 210-kDa polypeptide in CHO-K1 cells and immunostained nuclear speckles, centrosomes and microtubules in these cells. The 210-kDa polypeptide (named MAK-L) co-sedimented with exogenous microtubules. Thus, one more protein kinase seems to be associated with the microtubule network in vertebrate cells.

CDK1 inactivation regulates anaphase spindle dynamics and cytokinesis in vivo

Through association with CDK1, cyclin B accumulation and destruction govern the G2/M/G1 transitions in eukaryotic cells. To identify CDK1 inactivation-dependent events during late mitosis, we expressed a nondestructible form of cyclin B (cyclin BDelta90) by microinjecting its mRNA into prometaphase normal rat kidney cells. The injection inhibited chromosome decondensation and nuclear envelope formation. Chromosome disjunction occurred normally, but anaphase-like movement persisted until the chromosomes reached the cell periphery, whereupon they often somersaulted and returned to the cell center. Injection of rhodamine-tubulin showed that this movement occurred in the absence of a central anaphase spindle. In 82% of cells cytokinesis was inhibited; the remainder split themselves into two parts in a process reminiscent of Dictyostelium cytofission. In all cells injected, F-actin and myosin II were diffusely localized with no detectable organization at the equator. Our results suggest that a primary effect of CDK1 inactivation is on spindle dynamics that regulate chromosome movement and cytokinesis. Prolonged CDK1 activity may prevent cytokinesis through inhibiting midzone microtubule formation, the behavior of proteins such as TD60, or through the phosphorylation of myosin II regulatory light chain.

Microtubule-entrained kinase activities associated with the cortical cytoskeleton during cytokinesis

Research over the past few years has demonstrated the central role of protein phosphorylation in regulating mitosis and the cell cycle. However, little is known about how the mechanisms regulating the entry into mitosis contribute to the positional and temporal regulation of the actomyosin-based contractile ring formed during cytokinesis. Recent studies implicate p34cdc2 as a negative regulator of myosin II activity, suggesting a link between the mitotic cycle and cytokinesis. In an effort to study the relationship between protein phosphorylation and cytokinesis, we examined the in vivo and in vitro phosphorylation of actin-associated cortical cytoskeletal (CSK) proteins in an isolated model of the sea urchin egg cortex. Examination of cortices derived from eggs or zygotes labeled with 32P-orthophosphate reveals a number of cortex-associated phosphorylated proteins, including polypeptides of 20, 43 and 66 kDa. These three major phosphoproteins are also detected when isolated cortices are incubated with [32P]ATP in vitro, suggesting that the kinases that phosphorylate these substrates are also specifically associated with the cortex. The kinase activities in vivo and in vitro are stimulated by fertilization and display cell cycle-dependent activities. Gel autophosphorylation assays, kinase assays and immunoblot analysis reveal the presence of p34cdc2 as well as members of the mitogen-activated protein kinase family, whose activities in the CSK peak at cell division. Nocodazole, which inhibits microtubule formation and thus blocks cytokinesis, significantly delays the time of peak cortical protein phosphorylation as well as the peak in whole-cell histone H1 kinase activity. These results suggest that a key element regulating cortical contraction during cytokinesis is the timing of protein kinase activities associated with the cortical cytoskeleton that is in turn regulated by the mitotic apparatus.

Mitosis-specific phosphorylation of gar2, a fission yeast nucleolar protein structurally related to nucleolin

The nucleolar protein gar2 of fission yeast is structurally related to the multifunctional nucleolar protein nucleolin from vertebrates and has been shown to be implicated in production of 18S rRNA. gar2 contains several potential casein kinase 2 (CK2) phosphorylation sites and a single putative p34(cdc2 )phosphorylation site in the consensus S50PKK. Here, we show that, like nucleolin, gar2 is phosphorylated in vitro by both highly purified CK2 from CHO cells and p34(cdc2 )from starfish oocytes. Moreover, the substitution of alanine for the N-terminal serine 50 abolishes phosphorylation by p34(cdc2 )in vitro. We also provide evidence that gar2 is phosphorylated in vitro by a p13(suc1)-Sepharose-bound kinase from Schizosaccharomyces pombe extracts that displays cell cycle-regulated activity similar to that of the p34(cdc2(kinase. In vivo 32P labeling of cells indicates that gar2 is a phosphoprotein and that incorporation of phosphate on residue 50 occurs specifically at mitosis. Taken together, these results lead us to propose that gar2 is likely to be an in vivo substrate for the mitotic p34(cdc2 )kinase. However, this posttranslational modification of the gar2 protein does not appear to be essential for normal production of 18S rRNA.

Changes in microtubule structures during the first cell cycle of physiologically polyspermic newt eggs

The unfertilized egg of the newt, Cynops pyrrhogaster, has a second meiotic spindle at the animal pole and numerous cortical cytasters. After physiologically polyspermic fertilization, all sperm nuclei incorporated into the egg develop sperm asters, and the cortical cytasters change into bundles of cortical microtubules. The size of the sperm asters in the animal hemisphere is approximately 5.6-fold larger than that in the vegetal hemisphere. Only one sperm nucleus moves toward the center of the animal hemisphere to form a zygote nucleus with the egg nucleus. This movement is inhibited by nocodazole, but not by cytochalasin B. The centrosome in the zygote nucleus divides into two parts to form a bipolar spindle for the first cleavage synchronously with the nuclear cycle, but centrosomes of accessory sperm nuclei in the vegetal hemisphere remained to form monopolar interphase asters and subsequently degenerate around the first cleavage stage. The size of sperm asters in monospermically fertilized Xenopus eggs was approximately 37-fold larger than those in Cynops eggs. Since sperm asters that formed in polyspermically fertilized Xenopus eggs exclude each other, the formation of a zygote nucleus is inhibited. Cynops sperm nuclei form larger asters in Xenopus eggs, whereas Xenopus sperm nuclei form smaller asters in Cynops eggs compared with those in homologous eggs. Since there was no significant difference in the concentration of monomeric tubulin between those eggs, the size of sperm asters is probably regulated by a component(s) in egg cytoplasm. Smaller asters in physiologically polyspermic newt eggs might be useful for selecting only one sperm nucleus to move toward the egg nucleus.

The Drosophila gene abnormal spindle encodes a novel microtubule-associated protein that associates with the polar regions of the mitotic spindle

abnormal spindle, a gene required for normal spindle structure and function in Drosophila melanogaster, lies immediately adjacent the gene tolloid at 96A/B. It encodes a 220-kD polypeptide with a predicted pI of 10.8. The recessive mutant allele asp1 directs the synthesis of a COOH terminally truncated or internally deleted peptide of approximately 124 kD. Wild-type Asp protein copurifies with microtubules and is not released by salt concentrations known to dissociate most other microtubule-associated proteins. The bacterially expressed NH2-terminal 512-amino acid peptide, which has a number of potential phosphorylation sites for p34(cdc2) and MAP kinases, strongly binds to microtubules. The central 579-amino acid segment of the molecule contains one short motif homologous to sequences in a number of actin bundling proteins and a second motif present at the calmodulin binding sites of several proteins. Immunofluorescence studies show that the wild-type Asp protein is localized to the polar regions of the spindle immediately surrounding the centrosome. These findings are discussed in relation to the known spindle abnormalities in asp mutants.

The association of tubulin carboxypeptidase activity with microtubules in brain extracts is modulated by phosphorylation/dephosphorylation processes

Tubulin carboxypeptidase, the enzyme which releases the COOH terminal tyrosine from the alpha-chain of tubulin, remains associated with microtubules through several cycles of assembly/disassembly (Arce CA, Barra HS: FEBS Lett 157: 75-78, 1983). Here, we present evidence indicating that in rat brain extract the carboxypeptidase/microtubules association is regulated by the relative activities of endogenous protein kinase(s) and phosphatase(s) which seem to determine the phosphorylation state of the enzyme (or another entity) and in some way the affinity of the enzyme for microtubules. The presence of 2.5 mM ATP during the in vitro microtubule formation resulted in a low recovery of carboxypeptidase activity in the microtubule fraction. This ATP-induced effect was not due to alteration of the enzyme activity or to inhibition of microtubule assembly but to a decrease of the association of the enzyme with microtubules. We found that the ATP-induced effect was not mediated by modifications on the microtubules but, presumably, on the enzyme molecule. The non-hydrolyzable ATP analogue, AMP-PCP, did not reproduce the effect of ATP. The inclusion of phosphatase inhibitors in the homogenization buffer also led to a decrease in the amount of tubulin carboxypeptidase associated with microtubules. Finally, we found that, in concordance with the mechanism hypothesized, the magnitude of the carboxypeptidase/microtubule association correlated well with the different incubation conditions created to favor maximal, minimal or intermediate protein phosphorylation states.

A role for mitogen-activated protein kinase in the spindle assembly checkpoint in XTC cells

The spindle assembly checkpoint prevents cells whose spindles are defective or chromosomes are misaligned from initiating anaphase and leaving mitosis. Studies of Xenopus egg extracts have implicated the Erk2 mitogen-activated protein kinase (MAP kinase) in this checkpoint. Other studies have suggested that MAP kinases might be important for normal mitotic progression. Here we have investigated whether MAP kinase function is required for mitotic progression or the spindle assembly checkpoint in vivo in Xenopus tadpole cells (XTC). We determined that Erk1 and/or Erk2 are present in the mitotic spindle during prometaphase and metaphase, consistent with the idea that MAP kinase might regulate or monitor the status of the spindle. Next, we microinjected purified recombinant XCL100, a Xenopus MAP kinase phosphatase, into XTC cells in various stages of mitosis to interfere with MAP kinase activation. We found that mitotic progression was unaffected by the phosphatase. However, XCL100 rendered the cells unable to remain arrested in mitosis after treatment with nocodazole. Cells injected with phosphatase at prometaphase or metaphase exited mitosis in the presence of nocodazole-the chromosomes decondensed and the nuclear envelope re-formed-whereas cells injected with buffer or a catalytically inactive XCL100 mutant protein remained arrested in mitosis. Coinjection of constitutively active MAP kinase kinase-1, which opposes XCL100's effects on MAP kinase, antagonized the effects of XCL100. Since the only known targets of MAP kinase kinase-1 are Erk1 and Erk2, these findings argue that MAP kinase function is required for the spindle assembly checkpoint in XTC cells.

Inhibitors of protein kinases and phosphatases alter root morphology and disorganize cortical microtubules

To investigate molecular mechanisms controlling plant morphogenesis, we examined the morphology of primary roots of Arabidopsis thaliana and the organization of cortical microtubules in response to inhibitors of serine/threonine protein phosphatases and kinases. We found that cantharidin, an inhibitor of types 1 and 2A protein phosphatases, as previously reported for okadaic acid and calyculin A (R.D. Smith, J.E. Wilson, J.C. Walker, T.I. Baskin [1994] Planta 194: 516-524), inhibited elongation and stimulated radial expansion. Of the protein kinase inhibitors tested, chelerythrine, 6-dimethylaminopurine, H-89, K252a, ML-9, and staurosporine all inhibited elongation, but only staurosporine appreciably stimulated radial expansion. To determine the basis for the root swelling, we examined cortical microtubules in semithin sections of material embedded in butyl-methyl-methacrylate. Chelerythrine and 100 nM okadaic acid, which inhibited elongation without causing swelling, did not change the appearance of cortical arrays, but calyculin A, cantharidin, and staurosporine, which caused swelling, disorganized cortical microtubules. The stability of the microtubules in the aberrant arrays was not detectably different from those in control arrays, as judged by similar sensitivity to depolymerization by cold or oryzalin. These results identify protein phosphorylation and dephosphorylation as requirements in one or more steps that organize the cortical array of microtubules.

Microinjection of anti-alpha-tubulin antibody (DM1A) inhibits progesterone-induced meiotic maturation and deranges the microtubule array in follicle-enclosed oocytes of the frog, Rana pipiens

Microinjection of anti-alpha-tubulin (Dm1A) inhibited progesterone-induced meiotic maturation in large follicle-enclosed oocytes of the frog, Rana pipiens. DM1A (46 nl; 10 mg/ml) injection significantly increased the ED50 value for progesterone as determined by germinal vesicle dissolution (GVD) bioassay. By contrast, low doses of microinjected DM1A (46 nl; 2.5 mg/ml), anti-actin (clone KJ43A), anti-cytokeratin (C-11), anti-intermediate filament antibody (IFA), generic IgG (46 nl; 20 mg/ml) or sodium azide (46 nl; 1 mg/ml), an antibody preservative, were without inhibitory effect in this bioassay. Microinjected, affinity-purified DM1A (46 nl; 7.5 mg/ml) was also inhibitory, but preabsorption with pure tubulin prior to injection significantly reduced the inhibitory effect. DM1A injection had no effect on centrifugation-induced germinal vesicle migration (GVM). Previous work indicated that drugs (e.g. demecolcine and nocodazole), which destabilise microtubules, enhance both centrifugation-induced GVM and progesterone-induced GVD in Rana oocytes. Taking these results together, it is suggested that DM1A injection may have differential effects on microtubules in this cell. Thus, while the majority of microtubules were apparently depolymerised by DM1A (46 nl; 10 mg/ml) injection, a small subpopulation appeared to be stabilised as bundles. Confocal immunofluorescence microscopy of follicle-enclosed oocytes after DM1A injection revealed a major loss of microtubules throughout the cell; however, apparent sparse bundles of microtubules arranged in an approximately 600 microns shell were associated with the injectate region 24 h post-injection. By contrast, control follicle-enclosed oocytes topically labelled with DM1A post-fixation had extensive microtubule arrays similar to those previously reported in Xenopus oocytes. Intracellular recording after DM1A injection and progesterone treatment yielded an intermediate membrane potential (Vm = -31.8 mV) compared with control (immature) DM1A-injected cells (Vm = -44.7 mV) or potassium balanced salt solution (KBS)-injected cells matured with progesterone (Vm = -13.9 mV). These results suggest that DM1A injection does not completely inhibit electrophysiological changes initiated by progesterone. Working hypotheses are proposed that suggest a role for microtubules in the action of progesterone which normally lifts the prophase I block in the Rana follicle-enclosed oocyte.

Overexpression of full- or partial-length MAP4 stabilizes microtubules and alters cell growth

To investigate the in vivo functions of MAP4, a microtubule-associated protein expressed almost ubiquitously in vertebrate cells, we prepared stably transfected clonal mouse Ltk- cell lines expressing full-length MAP4 (L-MAP4 cells) or its MT-binding domain (L-MTB cells). Although transfectants showed no dramatic defect in morphology, organellar distribution, or level of MT polymer, as compared to naive Ltk- cells or L-MOCK cells (transfected with vector alone), MTs in L-MAP4 and L-MTB cells showed greater stability than those in control cells, as monitored by the level of post-translationally detyrosinated alpha-tubulin and by a quantitative nocodazole-resistance assay. In vivo, the MT-binding domain of MAP4 stabilized MTs less potently than full-length MAP4, in contrast to the equivalent efficacy demonstrated in studies of in vitro MT polymerization (Aizawa et al. (1991), J. Biol. Chem. 266, 9841–9846), L-MAP4 and L-MTB cells grew significantly more slowly than control cells; this growth inhibition was not due to mitotic arrest or cell death. L-MAP4 and L-MTB cells also exhibited greater tolerance to the MT-depolymerizing agent, nocodazole, but not to the MT-polymerizing agent, Taxol. Our results demonstrate that MAP4 and its MT-binding domain are capable of MT stabilization in vivo, and that increasing the intracellular level of MAP4 affects cell growth parameters.

LK6, a short lived protein kinase in Drosophila that can associate with microtubules and centrosomes

A number of polyclonal mouse sera were raised against Drosophila proteins that bound to microtubules in vitro (Kellogg et al. (1989) J. Cell Biol. 109, 2977–2991). Some of these sera recognised centrosomes in vivo, and we have been using these to screen expression libraries to isolate cDNAs encoding these putative centrosomal microtubule-associated proteins. Here we report the cloning of one such cDNA that encodes a novel serine/threonine protein kinase called LK6. The protein appears to exist in two forms: an abundant 185 kDa form and a rarer approximately 220 kDa form that interacts with microtubules. At least some of the LK6 protein is located in centrosomes at all stages of the cell cycle in fly embryos. Interestingly, the protein contains a PEST-like sequence and is rapidly turned over in vivo. Constitutive overexpression of LK6 is deleterious to flies and causes defects in microtubule organisation in both eggs and early embryos, whereas constitutive overexpression of a mutant form containing a point mutation that severely impairs the kinase activity is without effect. These findings suggest that LK6 may play a role in regulating microtubule function.

Microtubule assembly in clarified Xenopus egg extracts

Crude cytoplasmic extracts made from Xenopus eggs have proven to be uniquely useful in the studies of the mechanism of spindle microtubule assembly dynamics and chromosome movement during progression through the cell cycle. We examined microtubule dynamic instability in the Xenopus system using video-enhanced differential interference contrast microscopy (VE-DIC), which required high-speed centrifugation in order to clarify crude Xenopus extracts of refractile particles. Surprisingly, the resultant clarified, undiluted extracts exhibited virtually no microtubule catastrophe, even in the presence of high MPF (cyclin B/p34cdc2 kinase) activity and mitogen-activated protein (MAP) kinase activity, a down-stream kinase also implicated in regulating microtubule dynamics. Microtubule elongation occurred at plus ends, and interphase microtubules grew at 17-30 microns/min while metaphase [meiotic, myelin basic protein kinase activity which is diagnostic for cytostatic factor (CSF)-arrested] microtubules grew at about 10 microns/min. Plus-end shortening rates for both interphase and metaphase extracts were > 50 microns/min. Addition of okadaic acid, a protein phosphatase inhibitor known to activate MAP kinase activity and cause an increase in microtubule turnover in extracts made from sea urchin eggs, had no effect on microtubule catastrophe in either interphase or metaphase Xenopus extracts. In addition, the microtubules assembled in interphase extracts were less sensitive to dilution than those in metaphase. This study is the first to describe the dynamic instability of microtubules in Xenopus extracts without the addition of exogenous tubulins or other buffer contaminants.

Plant cyclins: a unified nomenclature for plant A-, B- and D-type cyclins based on sequence organization

The comparative analysis of a large number of plant cyclins of the A/B family has recently revealed that plants possess two distinct B-type groups and three distinct A-type groups of cyclins. Despite earlier uncertainties, this large-scale comparative analysis has allowed an unequivocal definition of plant cyclins into either A or B classes. We present here the most important results obtained in this study, and extend them to the case of plant D-type cyclins, in which three groups are identified. For each of the plant cyclin groups, consensus sequences have been established and a new, rational, plant-wide naming system is proposed in accordance with the guidelines of the Commission on Plant Gene Nomenclature. This nomenclature is based on the animal system indicating cyclin classes by an upper-case roman letter, and distinct groups within these classes by an arabic numeral suffix. The naming of plant cyclin classes is chosen to indicate homology to their closest animal class. The revised nomenclature of all described plant cyclins is presented, with their classification into groups CycA1, CycA2, CycA3, CycB1, CycB2, CycD1, CycD2 and CycD3.

MHP1, an essential gene in Saccharomyces cerevisiae required for microtubule function

The gene for a microtubule-associated protein (MAP), termed MHP1 (MAP-Homologous Protein 1), was isolated from Saccharomyces cerevisiae by expression cloning using antibodies specific for the Drosophila 205K MAP. MHP1 encodes an essential protein of 1,398 amino acids that contains near its COOH-terminal end a sequence homologous to the microtubule-binding domain of MAP2, MAP4, and tau. While total disruptions are lethal, NH2-terminal deletion mutations of MHP1 are viable, and the expression of the COOH-terminal two-thirds of the protein is sufficient for vegetative growth. Nonviable deletion-disruption mutations of MHP1 can be partially complemented by the expression of the Drosophila 205K MAP. Mhp1p binds to microtubules in vitro, and it is the COOH-terminal region containing the tau-homologous motif that mediates microtubule binding. Antibodies directed against a COOH-terminal peptide of Mhp1p decorate cytoplasmic microtubules and mitotic spindles as revealed by immunofluorescence microscopy. The overexpression of an NH2-terminal deletion mutation of MHP1 results in an accumulation of large-budded cells with short spindles and disturbed nuclear migration. In asynchronously growing cells that overexpress MHP1 from a multicopy plasmid, the length and number of cytoplasmic microtubules is increased and the proportion of mitotic cells is decreased, while haploid cells in which the expression of MHP1 has been silenced exhibit few microtubules. These results suggest that MHP1 is essential for the formation and/or stabilization of microtubules.

Cell cycle-dependent phosphorylation of the 77 kDa echinoderm microtubule-associated protein (EMAP) in vivo and association with the p34cdc2 kinase

The most abundant microtubule-associated protein in sea urchin eggs and embryos is the 77 kDa echinoderm microtubule-associated protein (EMAP). EMAP localizes to the mitotic spindle as well as the interphase microtubule array and is a likely target for a cell cycle-activated kinase. To determine if EMAP is phosphorylated in vivo, sea urchin eggs and embryos were metabolically labeled with 32PO4 and a monospecific antiserum was used to immunoprecipitate EMAP from 32P-labeled eggs and embryos. In this study, we demonstrate that the 77 kDa EMAP is phosphorylated in vivo by two distinct mechanisms. In the unfertilized egg, EMAP is constitutively phosphorylated on at least five serine residues. During the first cleavage division following fertilization, EMAP is phosphorylated with a cell cycle-dependent time course. As the embryo enters mitosis, EMAP phosphorylation increases, and as the embryo exits mitosis, phosphorylation decreases. During mitosis, EMAP is phosphorylated on 10 serine residues and two-dimensional phosphopeptide mapping reveals a mitosis-specific site of phosphorylation. At all stages of the cell cycle, a 33 kDa polypeptide copurifies with the 77 kDa EMAP, regardless of phosphorylation state. Antibodies against the cdc2 kinase were used to demonstrate that the 33 kDa polypeptide is the p34cdc2 kinase. The p34cdc2 kinase copurifies with the mitotic apparatus and immunostaining indicates that the p34cdc2 kinase is concentrated at the spindle poles. Models for the interaction of the p34cdc2 kinase and the 77 kDa EMAP are presented.

Wild-type p53 negatively regulates the expression of a microtubule-associated protein

The product of the p53 tumor suppressor gene has a well-documented activity as a transcriptional activator, and several studies indicate that this function is at least in part essential for the ability of p53 to suppress cellular proliferation. However, there is growing evidence that some activities of wild-type p53 may be independent of its trans-activation function; in fact, recent investigations have indicated that the transcriptional repression function of p53, rather than its trans-activation function, may be influential in p53-mediated apoptosis. The focus of this study has been on the identification of genes that exhibit decreased expression during p53-dependent apoptosis, and therefore represent potential p53-repressed genes influential in programmed cell death. This report identifies the gene encoding the microtubule-associated protein MAP4 as one whose mRNA and protein expression decrease in cells following induction of wild-type p53. Importantly, decreased MAP4 expression following p53 induction can be inhibited by molecules that prevent p53-mediated transcriptional repression and apoptosis, such as the adenovirus E1B-19K protein and the Wilms tumor gene product WT1. Additionally, overexpression of MAP4 in cells induced to undergo p53-dependent apoptosis significantly delays this process, indicating that the negative regulation of this gene by p53 may be influential in the rapid progression of apoptosis.

Immortal, non-tumourigenic mouse mammary outgrowths express high levels of cyclin B1 and activation of cyclin B1/cdc2 kinase

Neoplastic transformation of mouse mammary epithelial cells is the result of several identifiable phenotypic changes which presumably require sequential genetic alterations. In our model system, mammary cells progress from a mortal state (virgin duct) to several morphologically distinct intermediate states. The intermediate states are distinct cell populations that are phenotypically identified as immortal, non-tumourigenic (i.e. EL11), weakly tumourigenic ductal/alveolar hyperplasia (i.e. EL12) and moderately tumourigenic alveolar hyperplasiaa (i.e. TM12) to invasive tumours (i.e. EL12T/TM12T). We have studied the changes in total cyclin A and B1 levels, cyclin A and B1 complexed to cdc2, cyclin B1cdc2 kinase activity and cyclin D proteins in EL11 and EL12 immortalized outgrowth lines. Results revealed increased levels in total cyclin B1 (> 5-fold), cyclin B1/cdc2 (3-4-fold) and cyclin B1/cdc2 kinase activity (2-3.5-fold) in EL11 and EL12 phenotypes when compared to control mammary gland (virgin). No changes in the levels of total cyclin A or cycln A associated to cdc2 were observed. Cyclin D1, D2 and D3 protein levels were low in the EL11 immortal ductal outgrowth. Exposure to hormones via a pituitary isograft stimulated the synthesis of cyclin D1 and D2 but not D3 associated to cdk4 as well as total cdk4 proteins. Bromodeoxyuridine (BrdUrd) labelling indices showed marked increases in immortal ductal outgrowths (EL11 and EL12) when compared to virgin, suggesting that epithelial cells are cycling in these cell populations. Even in the presence of hormone stimulation, EL11 outgrowths were not tumourigenic, suggesting that other events are necessary to drive the cells to a tumourigenic phenotype. The results suggest that increased levels of cyclin B1 and cyclin B1-cdc2 kinase activities are early events and may be an important marker for the immortalized phenotype.

Sequential dephosphorylation of p34(cdc2) on Thr-14 and Tyr-15 at the prophase/metaphase transition

The G2-M transition of the cell cycle is triggered by the p34(cdc2)/cyclin B kinase. During the prophase/metaphase transition, the inactive, Thr-14/Tyr-15 phosphorylated form of p34(cdc2) (TP-YP) is modified to an active, Thr-14/Tyr-15 dephosphorylated form (T-Y) by the cdc25 dual-specificity phosphatase. Using highly synchronized starfish oocytes as a cellular model, we show that dephosphorylation in vivo and in vitro occurs in two steps: Thr-14 dephosphorylation precedes Tyr-15 dephosphorylation. The transient intermediate form (T-YP), which can be obtained in vitro by treatment of TP-YP by protein phosphatase 2A, displays low but significant kinase activity. These results raise the possibility that the intermediate form T-YP may be involved in the autocatalytic amplification of the p34(cdc2)/cyclin B complex through phosphorylation/activation of the cdc25 phosphatase and phosphorylation/inactivation of the wee1 kinase.

Microtubule dynamics at the G2/M transition: abrupt breakdown of cytoplasmic microtubules at nuclear envelope breakdown and implications for spindle morphogenesis

We recently developed a direct fluorescence ratio assay (Zhai, Y., and G.G. Borisy. 1994. J. Cell Sci. 107:881-890) to quantify microtubule (MT) polymer in order to determine if net MT depolymerization occurred upon anaphase onset as the spindle was disassembled. Our results showed no net decrease in polymer, indicating that the disassembly of kinetochore MTs was balanced by assembly of midbody and astral MTs. Thus, the mitosis-interphase transition occurs by a redistribution of tubulin among different classes of MTs at essentially constant polymer level. We now examine the reverse process, the interphase-mitosis transition. Specifically, we quantitated both the level of MT polymer and the dynamics of MTs during the G2/M transition using the fluorescence ratio assay and a fluorescence photoactivation approach, respectively. Prophase cells before nuclear envelope breakdown (NEB) had high levels of MT polymer (62%) similar to that previously reported for random interphase populations (68%). However, prophase cells just after NEB had significantly reduced levels (23%) which recovered as MT attachments to chromosomes were made (prometaphase, 47%; metaphase, 56%). The abrupt reorganization of MTs at NEB was corroborated by anti-tubulin immunofluorescence staining using a variety of fixation protocols. Sensitivity to nocodazole also increased at NEB. Photoactivation analyses of MT dynamics showed a similar abrupt change at NEB, basal rates of MT turnover (pre-NEB) increased post-NEB and then became slower later in mitosis. Our results indicate that the interphase-mitosis (G2/M) transition of the MT array does not occur by a simple redistribution of tubulin at constant polymer level as the mitosis-interphase (M/G1) transition. Rather, an abrupt decrease in MT polymer level and increase in MT dynamics occurs tightly correlated with NEB. A subsequent increase in MT polymer level and decrease in MT dynamics occurs correlated with chromosome attachment. These results carry implications for understanding spindle morphogenesis. They indicate that changes in MT dynamics may cause the steady-state MT polymer level in mitotic cells to be lower than in interphase. We propose that tension exerted on the kMTs may lead to their lengthening and thereby lead to an increase in the MT polymer level as chromosomes attach to the spindle.

The conserved mitotic kinase polo is regulated by phosphorylation and has preferred microtubule-associated substrates in Drosophila embryo extracts

The Drosophila gene polo encodes a protein kinase required for progression through mitosis. Wild-type polo protein migrates as a tight doublet of 67 kDa which is converted to a single band by phosphatase treatment, which also inactivates the kinase. We have determined putative polo substrates in a cell-free system derived from mutant embryos. Exogenous polo protein kinase phosphorylates proteins of sizes 220 kDa, 85 kDa and 54 kDa, to a greater extent when added to extracts of polo(1)-derived embryos compared with extracts of wild-type embryos, which in both cases have been subject to mild heat treatment to inactivate endogenous kinases. Proteins of the same size are predominantly phosphorylated by the endogenous kinases present in wild-type extracts, and are either not phosphorylated or are poorly phosphorylated in extracts of polo(1)-derived embryos. We show that a specific monoclonal antibody to beta-tubulin precipitates the phosphorylated 54 kDa protein together with an associated 85 kDa protein also phosphorylated by polo protein kinase. Moreover polo binds to an 85 kDa protein which is enriched in microtubule preparations. We discuss the extent to which these in vitro phosphorylation results reflect the effects of mutations in polo on microtubule behaviour during the mitotic cycle.

Chicken microtubule-associated protein 4 (MAP4): a novel member of the MAP4 family

Chicken gizzard smooth muscle has often been used as a source of proteins of the contractile and cytoskeletal apparatus. In the present study, we isolated a hitherto unknown doublet of proteins, with apparent molecular weights of 200 kDa, from embryonic chicken gizzard and showed its association with the microtubules (MTs) and by immunofluorescence staining of cultured cells. Immunoblot analysis also revealed the ubiquitous expression of this protein in all embryonic chicken tissues examined. Molecular cloning techniques allowed its identification as the chicken homologue of the microtubule-associated protein 4 (MAP4), known from mammalian species, and revealed approximately 90% of its amino acid sequence. MAP4 is the major MAP of non-neuronal tissues and cross-species comparisons clearly demonstrated its highly conserved overall structure, consisting of a basic C-terminal MT-binding region and an acidic N-terminal projection domain of unknown function. Despite these conserved features, overall sequence homologies to its mammalian counterparts are rather low and focused to distinct regions of the molecule. Among these are a conserved 18-amino acid motif, which is known to mediate binding to MTs and a part of the MT-binding domain known as the proline-rich region, which is thought to be the regulatory domain of MAP4. The N-terminal 59 amino acids are a conserved and unique feature of the MAP4 sequence and might be an indication that MAP4 performs other functions besides the enhancement of MT assembly.

The pool of map kinase associated with microtubules is small but constitutively active

Mitogen-activated protein kinase (MAPK) is activated by many kinds of stimuli and plays an important role in integrating signal transduction cascades. MAPK is present abundantly in brain, where we have studied its association with microtubules. Immunofluorescence of primary hippocampal neurons revealed that MAPK staining co-localized with microtubules and biochemical analyses showed that MAPK co-purified with microtubules. Approximately 4% of MAPK in cytosolic extracts was associated with microtubules, where it was associated with both tubulin and microtubule-associated proteins (MAPs) fractions. Further fractionation of MAPs suggested that a portion of MAPK is associated with MAP2. An association with MAP2 was also demonstrated by co-immunoprecipitation and in vitro binding experiments. A similar association was shown for the juvenile MAP2 isoform, MAP2C. The pool of MAPK associated with microtubules had a higher activity relative to the nonassociated pool in both brain and proliferating PC12 cells. Although MAPK was activated by nerve growth factor in PC12 cells, the activity of microtubule-associated MAPK did not further increase. These results raise the possibility that microtubule-associated MAPK operates through constitutive phosphorylation activity to regulate microtubule function in neurons.

High molecular weight microtubule-associated proteins contain O-linked-N-acetylglucosamine

We have examined the post-translational modification of high molecular weight microtubule-associated proteins (MAPs) have shown that MAP1, MAP2, and MAP4 are glycosylated. The presence of carbohydrate residues on these proteins was indicated by labeling with biotin hydrazide following periodate oxidation, a specific and well established method for detecting saccharide moieties on proteins. Both MAP2 and MAP4 were also labeled in vitro by UDP-[3H]galactose in the presence of galactosyltransferase. Labeling by galactosyltransferase indicated that MAP2 and MAP4 contained terminal nonreducing GlcNAc residues, and they appeared to be O-linked to the proteins as shown by their sensitivity to beta-elimination. Chromatographic analysis showed that the GlcNAc residues were directly linked to the proteins as monosaccharides. Thus, we have added MAP2 and MAP4 to the list of intracellular O-GlcNAc-modified proteins, which includes other cytoskeletal proteins such as cytokeratins 8, 13, and 18 and neurofilament proteins NF-L and NF-M. We further characterized the O-GlcNAc modification of MAP2, and stoichiometric analysis indicated that nearly 10% of the MAP2 isolated from rat brain is modified by O-GlcNAc. However, this estimate is thought to reflect the minimal level of O-GlcNAc modification present on MAP2. We have also shown that both the O-GlcNAc and biotin hydrazide-reactive carbohydrate moieties are located on the projection domain of MAP2. Three O-GlcNAc-containing peaks were observed following fast protein liquid chromatography of a tryptic digest of MAP2, suggesting that multiple modification sites exist. The specific modification sites and functional significance of the O-GlcNAc glycosylation on the high Mr MAPs remain to be determined.

Phosphorylation of microtubule-associated proteins MAP2 and MAP4 by the protein kinase p110mark. Phosphorylation sites and regulation of microtubule dynamics

The phosphorylation of microtubule-associated proteins (MAPs) is thought to be a key factor in the regulation of microtubule stability. We have shown recently that a novel protein kinase, termed p110 microtubule-affinity regulating kinase ("MARK"), phosphorylates microtubule-associated protein tau at the KXGS motifs in the region of internal repeats and causes the detachment of tau from microtubules (Drewes, G., Trinczek, B., Illenberger, S., Biernat, J., Schmitt-Ulms, G., Meyer, H.E., Mandelkow, E.-M., and Mandelkow, E. (1995) J. Biol. Chem. 270, 7679-7688). Here we show that p110mark phosphorylates analogous KXGS sites in the microtubule binding domains of the neuronal MAP2 and the ubiquitous MAP4. Phosphorylation in vitro leads to the dissociation of MAP2 and MAP4 from microtubules and to a pronounced increase in dynamic instability. Thus, the phosphorylation of the repeated motifs in the microtubule binding domains of MAPs by p110mark might provide a mechanism for the regulation of microtubule dynamics in cells.

The human CAS protein which is homologous to the CSE1 yeast chromosome segregation gene product is associated with microtubules and mitotic spindle

Human CAS cDNA contains a 971-aa open reading frame that is homologous to the essential yeast gene CSE1. CSE1 is involved in chromosome segregation and is necessary for B-type cyclin degradation in mitosis. Using antibodies to CAS, it was shown that CAS levels are high in proliferating and low in nonproliferating cells. Here we describe the distribution of CAS in cells and tissues analyzed with antibodies against CAS. CAS is an approximately 100-kDa protein present in the cytoplasm of proliferating cells at levels between 2 x 10(5) and 1 x 10(6) molecules per cell. The intracellular distribution of CAS resembles that of tubulin. In interphase cells, anti-CAS antibody shows microtubule-like patterns and in mitotic cells it labels the mitotic spindle. CAS is removed from microtubules by mild detergent treatment (cytoskeleton preparations) and in vincristine- or taxol-treated cells. CAS is diffusely distributed in the cytoplasm with only traces present in tubulin paracrystals or bundles. Thus, CAS appears to be associated with but not to be an integral part of microtubules. Immunohistochemical staining of frozen tissues shows elevated amounts of CAS in proliferating cells such as testicular spermatogonia and cells in the basal layer cells of the colon. CAS was also concentrated in the respiratory epithelium of the trachea and in axons and Purkinje cells in the cerebellum. These cells contain many microtubules. The cellular location of CAS is consistent with an important role in cell division as well as in ciliary movement and vesicular transport.

Association of mitogen-activated protein kinases with microtubules in mouse macrophages

Taxol, a microtubule-binding diterpene, mimics many effects of lipopolysaccharide (LPS) on mouse macrophages. The LPS-mimetic effects of taxol appear to be under the same genetic control as responses to LPS itself. Thus we have postulated a role for microtubule-associated proteins (MAP) in the response of macrophages to LPS. Stimulation of macrophages by LPS quickly induces the activation of mitogen-activated protein kinases (MAPK). MAPK are generally considered cytosolic enzymes. Herein we report that much of the LPS-activatable pool of MAPK in primary mouse peritoneal macrophages is microtubule associated. By immunofluorescence, MAPK were localized to colchicine- and nocodazole-disruptible filaments. From both mouse brain and RAW 264.7 macrophages, MAPK could be coisolated with polymerized tubulin. Fractionation of primary macrophages into cytosol-, microfilament-, microtubule-, and intermediated filament-rich extracts revealed that approximately 10% of MAPK but none of MAPK kinase (MEK1A and MEK2) was microtubule bound. Exposure of macrophages to LPS did not change the proportion of MAPK bound to microtubules, but preferentially activated the microtubule-associated pool. These findings confirm the prediction that LPS activates a kinase bound to microtubules. Together with LPS-mimetic actions of taxol and the shared genetic control of responses to LPS and taxol, these results support the hypothesis that a major LPS-signaling pathway in mouse macrophages may involve activation of one or more microtubule-associated kinases.

A muscle-specific variant of microtubule-associated protein 4 (MAP4) is required in myogenesis

Microtubule-associated protein 4 (MAP4) transcripts vary in different mouse tissues, with striated muscle (skeletal and cardiac) expressing 8- and 9-kb transcripts preferentially to the more widely distributed 5.5- and 6.5-kb transcripts (West, R. W., Tenbarge, K. M. and Olmsted, J. B. (1991). J. Biol. Chem. 266, 21886–21896). Cloning of the sequence unique to the muscle transcripts demonstrated that these mRNAs vary from the more ubiquitous ones by a single 3.2-kb coding region insertion within the projection domain of MAP4. During differentiation of the myogenic cell line, C2C12, muscle-specific MAP4 transcripts appear within 24 hours of growth in differentiation medium, and a larger MAP4 isotype (350 X 10(3) Mr) accumulates to high levels by 48 hours of differentiation. In situ hybridization analyses of transcript distribution in mouse embryos demonstrated that muscle-specific transcripts appear early in myogenesis. To block the expression of the muscle-specific MAP4, stable lines of C2C12 were generated bearing an antisense construct with the muscle-specific MAP4 sequence. Myoblast growth was unaffected whereas myotube formation was severely perturbed. Fusion occurred in the absence of the muscle MAP4 isotype, but the multinucleate syncytia were short and apolar, microtubules were disorganized and normal anisotropic myofibrils were absent. The patterns of expression of the muscle-specific transcripts and the antisense experiments indicated that this unique structural form of MAP4 plays a critical role in the formation and maintenance of muscle.