A MAP kinase-dependent spindle assembly checkpoint in Xenopus egg extracts

Distinct, constitutively active MAPK phosphatases function in Xenopus oocytes: implications for p42 MAPK regulation In vivo

Xenopus oocyte maturation requires the phosphorylation and activation of p42 mitogen-activated protein kinase (MAPK). Likewise, the dephosphorylation and inactivation of p42 MAPK are critical for the progression of fertilized eggs out of meiosis and through the first mitotic cell cycle. Whereas the kinase responsible for p42 MAPK activation is well characterized, little is known concerning the phosphatases that inactivate p42 MAPK. We designed a microinjection-based assay to examine the mechanism of p42 MAPK dephosphorylation in intact oocytes. We found that p42 MAPK inactivation is mediated by at least two distinct phosphatases, an unidentified tyrosine phosphatase and a protein phosphatase 2A-like threonine phosphatase. The rates of tyrosine and threonine dephosphorylation were high and remained constant throughout meiosis, indicating that the dramatic changes in p42 MAPK activity seen during meiosis are primarily attributable to changes in MAPK kinase activity. The overall control of p42 MAPK dephosphorylation was shared among four partially rate-determining dephosphorylation reactions, with the initial tyrosine dephosphorylation of p42 MAPK being the most critical of the four. Our findings provide biochemical and kinetic insight into the physiological mechanism of p42 MAPK inactivation.

Inhibition of protein tyrosine phosphatases blocks calcium-induced activation of metaphase II-arrested oocytes of Xenopus laevis

We have studied the effect of a protein tyrosine phosphatases (PTP) inhibitor on calcium-induced activation of Xenopus laevis oocytes arrested at metaphase II. Ammonium molybdate microinjection blocked pronucleus formation following A23187 treatment while cortical granules still underwent exocytosis. Pronuclei still occurred in ammonium molybdate-injected oocytes following 6-DMAP addition. Changes that usually occurred following A23187 exposure were inhibited in the presence of ammonium molybdate in the oocyte: MAPK dephosphorylation, p34(cdc2) rephosphorylation and cyclin B2 and p39(mos) proteolysis. These results suggest that a PTP is involved in the activation of the ubiquitin-dependent degradation machinery.

Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae

The spindle checkpoint arrests cells in mitosis in response to defects in the assembly of the mitotic spindle or errors in chromosome alignment. We determined which spindle defects the checkpoint can detect by examining the interaction of mutations that compromise the checkpoint (mad1, mad2, and mad3) with those that damage various structural components of the spindle. Defects in microtubule polymerization, spindle pole body duplication, microtubule motors, and kinetochore components all activate the MAD-dependent checkpoint. In contrast, the cell cycle arrest caused by mutations that induce DNA damage (cdc13), inactivate the cyclin proteolysis machinery (cdc16 and cdc23), or arrest cells in anaphase (cdc15) is independent of the spindle checkpoint.

Microtubule-based endoplasmic reticulum motility in Xenopus laevis: activation of membrane-associated kinesin during development

The endoplasmic reticulum (ER) in animal cells uses microtubule motor proteins to adopt and maintain its extended, reticular organization. Although the orientation of microtubules in many somatic cell types predicts that the ER should move toward microtubule plus ends, motor-dependent ER motility reconstituted in extracts of Xenopus laevis eggs is exclusively a minus end-directed, cytoplasmic dynein-driven process. We have used Xenopus egg, embryo, and somatic Xenopus tissue culture cell (XTC) extracts to study ER motility during embryonic development in Xenopus by video-enhanced differential interference contrast microscopy. Our results demonstrate that cytoplasmic dynein is the sole motor for microtubule-based ER motility throughout the early stages of development (up to at least the fifth embryonic interphase). When egg-derived ER membranes were incubated in somatic XTC cytosol, however, ER tubules moved in both directions along microtubules. Data from directionality assays suggest that plus end-directed ER tubule extensions contribute approximately 19% of the total microtubule-based ER motility under these conditions. In XTC extracts, the rate of ER tubule extensions toward microtubule plus ends is lower ( approximately 0.4 microm/s) than minus end-directed motility ( approximately 1.3 microm/s), and plus end-directed motility is eliminated by a function-blocking anti-conventional kinesin heavy chain antibody (SUK4). In addition, we provide evidence that the initiation of plus end-directed ER motility in somatic cytosol is likely to occur via activation of membrane-associated kinesin.

Subunits and substrates of the anaphase-promoting complex

The initiation of anaphase and exit from mitosis depend on a ubiquitination complex called the anaphase-promoting complex (APC) or cyclosome. The APC is composed of more than 10 constitutive subunits and associates with additional regulatory factors in mitosis and during the G1 phase of the cell cycle. At the metaphase-anaphase transition the APC ubiquitinates proteins such as Pds1 in budding yeast and Cut2 in fission yeast whose subsequent degradation by the 26S proteasome is essential for the initiation of sister chromatid separation. Later in anaphase and telophase the APC promotes the inactivation of the mitotic cyclin-dependent protein kinase 1 by ubiquitinating its activating subunit cyclin B. The APC also mediates the ubiquitin-dependent proteolysis of several other mitotic regulators, including other protein kinases, APC activators, spindle-associated proteins, and inhibitors of DNA replication.

Dissection of cell division processes in the one cell stage Caenorhabditis elegans embryo by mutational analysis

To identify novel components required for cell division processes in complex eukaryotes, we have undertaken an extensive mutational analysis in the one cell stage Caenorhabditis elegans embryo. The large size and optical properties of this cell permit observation of cell division processes with great detail in live specimens by simple differential interference contrast (DIC) microscopy. We have screened an extensive collection of maternal-effect embryonic lethal mutations on chromosome III with time-lapse DIC video microscopy. Using this assay, we have identified 48 mutations in 34 loci which are required for specific cell division processes in the one cell stage embryo. We show that mutations fall into distinct phenotypic classes which correspond, among others, to the processes of pronuclear migration, rotation of centrosomes and associated pronuclei, spindle assembly, chromosome segregation, anaphase spindle positioning, and cytokinesis. We have further analyzed pronuclear migration mutants by indirect immunofluorescence microscopy using antibodies against tubulin and ZYG-9, a centrosomal marker. This analysis revealed that two pronuclear migration loci are required for generating normal microtubule arrays and four for centrosome separation. All 34 loci have been mapped by deficiencies to distinct regions of chromosome III, thus paving the way for their rapid molecular characterization. Our work contributes to establishing the one cell stage C. elegans embryo as a powerful metazoan model system for dissecting cell division processes.

Nuclear localization of mitogen-activated protein kinase kinase 1 (MKK1) is promoted by serum stimulation and G2-M progression. Requirement for phosphorylation at the activation lip and signaling downstream of MKK

Stimulation of mammalian cells results in subcellular relocalization of Ras pathway enzymes, in which extracellular signal-regulated protein kinases rapidly translocate to nuclei. In this study, we define conditions for nuclear localization of mitogen-activated protein kinase kinase 1 (MKK1) by examining effects of perturbing the nuclear export signal (NES), the regulatory phosphorylation sites Ser218 and Ser222, and a regulatory domain at the N terminus. After disrupting the NES (Delta32-37), nuclear uptake of MKK was enhanced when quiescent cells were activated with serum-phorbol 12-myristate 13-acetate or BXB-Raf-1 cotransfection. Uptake was enhanced by mutation of Ser218 and Ser222 to Glu and Asp, respectively, and blocked by mutation of these residues to Ala, although mutation of Lys97 to Met, which renders MKK catalytically inactive, did not interfere with uptake. Therefore, nuclear uptake of MKK requires incorporation of phosphate or negatively charged residues at the activation lip but not enzyme activity. On the other hand, uptake of an active MKK mutant with disrupted NES (Delta32-51) was elevated in quiescent as well as stimulated cells, and pretreatment of cells with the MKK inhibitor 1,4-diamino-2, 3-dicyano-1,4-bis[2-aminophenylthio]butadiene blocked nuclear uptake. Thus, signaling downstream of MKK is also necessary for translocation. Finally, wild type MKK containing an intact NES translocates to nuclei during mitosis before envelope breakdown. Comparison of mutants with Ser to Glu and Asp or Ala substitutions indicates that Ser phosphorylation is also required for mitotic nuclear uptake of MKK.

Mos positively regulates Xe-Wee1 to lengthen the first mitotic cell cycle of Xenopus

Several key developmental events occur in the first mitotic cell cycle of Xenopus; consequently this cycle has two gap phases and is approximately 60-75 min in length. In contrast, embryonic cycles 2-12 consist only of S and M phases and are 30 min in length. Xe-Wee1 and Mos are translated and degraded in a developmentally regulated manner. Significantly, both proteins are present in the first cell cycle. We showed previously that the expression of nondegradable Mos, during early interphase, delays the onset of M phase in the early embryonic cell cycles. Here we report that Xe-Wee1 is required for the Mos-mediated M-phase delay. We find that Xe-Wee1 tyrosine autophosphorylation positively regulates Xe-Wee1 and is only detected in the first 30 min of the first cell cycle. The level and duration of Xe-Wee1 tyrosine phosphorylation is elevated significantly when the first cell cycle is elongated with nondegradable Mos. Importantly, we show that the tyrosine phosphorylation of Xe-Wee1 is required for the Mos-mediated M-phase delay. These findings indicate that Mos positively regulates Xe-Wee1 to generate the G2 phase in the first cell cycle and establish a direct link between the MAPK signal transduction pathway and Wee1 in vertebrates.

Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery

Whether we consider the division of the simplest unicellular organisms into two daughter cells or the generation of haploid gametes by the most complex eukaryotes, no two processes secure the continuance of life more than the proper replication and segregation of the genetic material. The cell cycle, marked in part by the periodic rise and fall of cyclin-dependent kinase (CDK) activities, is the means by which these two processes are separated. DNA damage and mistakes in chromosome segregation are costly, so nature has further devised elaborate checkpoint mechanisms that halt cell cycle progression, allowing time for repairs or corrections. In this article, we review the mitotic checkpoint mechanism that responds to defects in the chromosome segregation machinery and arrests cells in mitosis prior to anaphase onset. At opposite ends of this pathway are the kinetochore, where many checkpoint proteins reside, and the anaphase-promoting complex (APC), the metaphase-to-interphase transition regulator. Throughout this review we focus on budding yeast but reference parallel processes found in other organisms.

p42-MAP kinase is activated in EGF-stimulated interphase but not in metaphase-arrested HeLa cells

It is known that cellular signals produced in response to an inappropriate spindle formation cause the cell to be arrested at metaphase (M) in the cell cycle. We report here that the 42-kDa isoform of MAPK (ERK2) was tyrosyl-phosphorylated and activated in response to epidermal growth factor (EGF) in interphase but not in M-arrested HeLa cells. However, the basal level of activity of M-arrested cells was higher than that of interphase, although the overall tyrosyl phosphorylation content was small. Further, the EGF receptor and its associated proteins GTPase-activating protein and phospholipase C were phosphorylated in M-arrested cells to a lower extent than they were in interphase. This implies that in spite of its high level of basal activity, the scarcity of MAPK activation in mitosis in response to EGF stems from an early impairment of phosphorylation of the receptor and neighboring proteins. The biological significance of these results underlies the importance of keeping the cell sheltered from extracellular signals when it undergoes division.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Model scenarios for evolution of the eukaryotic cell cycle

Progress through the division cycle of present day eukaryotic cells is controlled by a complex network consisting of (i) cyclin-dependent kinases (CDKs) and their associated cyclins, (ii) kinases and phosphatases that regulate CDK activity, and (iii) stoichiometric inhibitors that sequester cyclin-CDK dimers. Presumably regulation of cell division in the earliest ancestors of eukaryotes was a considerably simpler affair. Nasmyth (1995) recently proposed a mechanism for control of a putative, primordial, eukaryotic cell cycle, based on antagonistic interactions between a cyclin-CDK and the anaphase promoting complex (APC) that labels the cyclin subunit for proteolysis. We recast this idea in mathematical form and show that the model exhibits hysteretic behaviour between alternative steady states: a Gl-like state (APC on, CDK activity low, DNA unreplicated and replication complexes assembled) and an S/M-like state (APC off, CDK activity high, DNA replicated and replication complexes disassembled). In our model, the transition from G1 to S/M ('Start') is driven by cell growth, and the reverse transition ('Finish') is driven by completion of DNA synthesis and proper alignment of chromosomes on the metaphase plate. This simple and effective mechanism for coupling growth and division and for accurately copying and partitioning a genome consisting of numerous chromosomes, each with multiple origins of replication, could represent the core of the eukaryotic cell cycle. Furthermore, we show how other controls could be added to this core and speculate on the reasons why stoichiometric inhibitors and CDK inhibitory phosphorylation might have been appended to the primitive alternation between cyclin accumulation and degradation.

Polo-like kinases: positive regulators of cell division from start to finish

Present in organisms ranging from yeast to man, homologues of the Drosophila Polo kinase control multiple stages of cell division. At the onset of mitosis, Polo-like kinases (Plks) function in centrosome maturation and bipolar spindle formation, and they contribute to the activation of cyclin-dependent kinase (Cdk)1-cyclin B. Subsequently, they are required for the inactivation of Cdk1 and exit from mitosis. In the absence of Plk function, mitotic cyclins fail to be destroyed, indicating that Plks are important regulators of the anaphase-promoting complex/cyclosome (APC/C), a key component of the ubiquitin-dependent proteolytic degradation pathway. Finally, recent evidence implicates Plks in the temporal and spatial coordination of cytokinesis.

Requirement for MAPK activation for normal mitotic progression in Xenopus egg extracts

The p42 mitogen-activated protein kinase (MAPK) is required for progression through meiotic M phase in Xenopus oocytes. This report examines whether it also plays a role in normal mitotic progression. MAPK was transiently activated during mitosis in cycling Xenopus egg extracts after activation of the cyclin-dependent kinase Cdc2-cyclin B. Interference with MAPK activation by immunodepletion of its activator MEK, or by addition of the MEK inhibitor PD98059, caused precocious termination of mitosis and interfered with production of normal mitotic microtubules. Sustained activation of MAPK arrested extracts in mitosis in the absence of active Cdc2-cyclin B. These findings identify a role for MEK and MAPK in maintaining the mitotic state.

Spindle checkpoint protein Xmad1 recruits Xmad2 to unattached kinetochores

The spindle checkpoint prevents the metaphase to anaphase transition in cells containing defects in the mitotic spindle or in chromosome attachment to the spindle. When the checkpoint protein Xmad2 is depleted from Xenopus egg extracts, adding Xmad2 to its endogenous concentration fails to restore the checkpoint, suggesting that other checkpoint component(s) were depleted from the extract through their association with Xmad2. Mass spectrometry provided peptide sequences from an 85-kD protein that coimmunoprecipitates with Xmad2 from egg extracts. This information was used to clone XMAD1, which encodes a homologue of the budding yeast (Saccharomyces cerevisiae) checkpoint protein Mad1. Xmad1 is essential for establishing and maintaining the spindle checkpoint in egg extracts. Like Xmad2, Xmad1 localizes to the nuclear envelope and the nucleus during interphase, and to those kinetochores that are not bound to spindle microtubules during mitosis. Adding an anti-Xmad1 antibody to egg extracts inactivates the checkpoint and prevents Xmad2 from localizing to unbound kinetochores. In the presence of excess Xmad2, neither chromosomes nor Xmad1 are required to activate the spindle checkpoint, suggesting that the physiological role of Xmad1 is to recruit Xmad2 to kinetochores that have not bound microtubules.

MAP kinase inactivation is required only for G2-M phase transition in early embryogenesis cell cycles of the starfishes Marthasterias glacialis and Astropecten aranciacus

Downregulation of MAP kinase is a universal consequence of fertilization in the animal kingdom. Here we show that oocytes of the starfishes Astropecten aranciacus and Marthasterias glacialis complete meiotic maturation and form a pronucleus when treated with 1-methyladenine and then complete DNA replication and arrest at G2 if not fertilized. Release of G2 by fertilization or a variety of parthenogenetic treatments is associated with inactivation of MAP kinase. Prevention of MAP kinase inactivation by microinjection of Ste11-DeltaN, a constitutively active budding yeast MAP kinase kinase kinase, arrests fertilized eggs at G2 in either the first or the second mitotic cell cycle, in a dose-dependent manner. G1 arrest is never observed. Conversely, inactivation of MAP kinase by microinjection of the MAP kinase-specific phosphatase Pyst-1 releases mature starfish oocytes from G2 arrest. The role of MAP kinase in arresting cell cycle at various stages in oocytes of different animal species is discussed.

In vivo and in vitro assessment of mitogen activated protein kinase involvement during quail secondary palate formation

Spatiotemporally regulated cell proliferation and differentiation are crucial for the successful completion of morphogenesis of the vertebrate secondary palate. An understanding of the mechanisms by which these cellular phenomena are regulated during palate development involves the identification of the various signal transduction pathways. In the present study, the presence and activation of mitogen-activated protein (MAP) kinases were investigated during the development of quail secondary palate. The palatal shelves were dissected on days 5-9 of incubation, homogenized, and centrifuged, after which the samples were separated by anion exchange fast protein liquid chromatography. The fractions were analyzed for myelin basic protein (MBP) phosphorylation. In addition, primary cultures of quail palate mesenchymal cells (QPMCs) were treated with epidermal growth factor (EGF) and prepared for MBP phosphorylation assays. A temporally regulated pattern of phosphotransferase activity, characterized by a three-fold increase in phosphotransferase activity toward MBP between days 5 and 8 of incubation, was observed during quail palate development. Western blotting, using MAP kinase antibodies, demonstrated the presence of a 42-kDa isoform between days 5 and 9 of incubation, during which the level of protein remained constant. Antityrosine immunoblotting with 4G10 also detected a 42-kDa protein. Phosphotransferase assays, using either a MAP kinase-specific substrate peptide (S5) or a protein kinase C inhibitor (R3), further confirmed the presence of a MAP kinase in the developing palate of quail. Because diverse biological processes occur concurrently during in vivo palate morphogenesis, the involvement of MAP kinase was explored further in primary cell culture. The data showed that EGF stimulated proliferation and activated 42-kDa MAP kinase in QPMCs. It is suggested that MAP kinase cascade may be involved in growth factor-regulated cell proliferation during morphogenesis of quail secondary palate.

A cell cycle regulated MAP kinase with a possible role in cytokinesis in tobacco cells

Mitogen-activated protein (MAP) kinases have been demonstrated to have a role in meiosis but their involvement in mitotic events is less clear. Using a peptide antibody raised against the tobacco MAP kinase p43(Ntf6) and extracts from synchronized tobacco cell suspension cultures, we show that this kinase is activated specifically during mitosis. Entry into mitosis appears to be necessary for the activation of the kinase, which occurs as a post-translational event. The activation of the kinase occurs in late anaphase/early telophase. The p43(Ntf6) protein shows a transient localization to the cell plate in anaphase cells, in the middle of the two microtubule arrays characteristic of the phragmoplast, a plant-specific structure involved in laying down the new cell wall. The combined data support a role for the MAP kinase p43(Ntf6) in cytokinesis in tobacco cells.

Activation of the MKK/ERK pathway during somatic cell mitosis: direct interactions of active ERK with kinetochores and regulation of the mitotic 3F3/2 phosphoantigen

The mitogen-activated protein (MAP) kinase pathway, which includes extracellular signal-regulated protein kinases 1 and 2 (ERK1, ERK2) and MAP kinase kinases 1 and 2 (MKK1, MKK2), is well-known to be required for cell cycle progression from G1 to S phase, but its role in somatic cell mitosis has not been clearly established. We have examined the regulation of ERK and MKK in mammalian cells during mitosis using antibodies selective for active phosphorylated forms of these enzymes. In NIH 3T3 cells, both ERK and MKK are activated within the nucleus during early prophase; they localize to spindle poles between prophase and anaphase, and to the midbody during cytokinesis. During metaphase, active ERK is localized in the chromosome periphery, in contrast to active MKK, which shows clear chromosome exclusion. Prophase activation and spindle pole localization of active ERK and MKK are also observed in PtK1 cells. Discrete localization of active ERK at kinetochores is apparent by early prophase and during prometaphase with decreased staining on chromosomes aligned at the metaphase plate. The kinetochores of chromosomes displaced from the metaphase plate, or in microtubule-disrupted cells, still react strongly with the active ERK antibody. This pattern resembles that reported for the 3F3/2 monoclonal antibody, which recognizes a phosphoepitope that disappears with kinetochore attachment to the spindles, and has been implicated in the mitotic checkpoint for anaphase onset (Gorbsky and Ricketts, 1993. J. Cell Biol. 122:1311-1321). The 3F3/2 reactivity of kinetochores on isolated chromosomes decreases after dephosphorylation with protein phosphatase, and then increases after subsequent phosphorylation by purified active ERK or active MKK. These results suggest that the MAP kinase pathway has multiple functions during mitosis, helping to promote mitotic entry as well as targeting proteins that mediate mitotic progression in response to kinetochore attachment.

Active MAP kinase in mitosis: localization at kinetochores and association with the motor protein CENP-E

To investigate possible involvement of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 (extracellular signal-regulated kinases) in somatic cell mitosis, we have used indirect immunofluorescence with a highly specific phospho-MAP kinase antibody and found that a portion of the active MAP kinase is localized at kinetochores, asters, and the midbody during mitosis. Although the aster labeling was constant from the time of nuclear envelope breakdown, the kinetochore labeling first appeared at early prometaphase, started to fade during chromosome congression, and then disappeared at midanaphase. At telophase, active MAP kinase localized at the midbody. Based on colocalization and the presence of a MAP kinase consensus phosphorylation site, we identified the kinetochore motor protein CENP-E as a candidate mitotic substrate for MAP kinase. CENP-E was phosphorylated in vitro by MAP kinase on sites that are known to regulate its interactions with microtubules and was found to associate in vivo preferentially with the active MAP kinase during mitosis. Therefore, the presence of active MAP kinase at specific mitotic structures and its interaction with CENP-E suggest that MAP kinase could play a role in mitosis at least in part by altering the ability of CENP-E to mediate interactions between chromosomes and microtubules.

Evidence for MAP kinase activation during mitotic division

MAP kinases have been implicated in the control of a broad spectrum of cellular events in many types of cells. In somatic cells, MAP kinase activation seems to be triggered after exit from a quiescent state (in G0 or G2) only and then inactivated by entry into a proliferative state. In oocytes of various species, a one-time activation of MAP kinase that is apparently not repeated during the succeeding mitotic cycles occurs after meiotic activation. However, several reports suggest that a myelin basic protein (MBP) kinase activity, unrelated to that of maturation promoting factor, can sometimes be detected during mitotic divisions in various types of cells and oocytes. We have reinvestigated this problem in order to determine the origin and the role of MBP kinase that is stimulated at time of mitosis in the fertilized eggs of the sea urchin Paracentrotus lividus. We used anti-ERK1 antibodies or substrates specific for different MAP kinases, and performed in-gel phosphorylation experiments. Our results suggest that an ERK1-like protein was responsible for part of the MBP kinase activity that is stimulated during the first mitotic divisions. Furthermore, we observed that wortmannin, an inhibitor of PI 3-kinase that arrests the fertilized sea urchin eggs at the prometaphase stage, inhibited the inactivation of MAP kinase normally observed when the eggs divide, suggesting a role for PI 3-kinase in the deactivation process of MAP kinase. We also discuss how the activities of MPF and MAP kinase may be interconnected to regulate the first mitotic divisions of the early sea urchin embryo.

Possible role for phosphatidylinositol 3-kinase in regulating meiotic maturation of bovine oocytes in vitro

In this study 2 phosphatidylinositol 3-kinase (PI 3-kinase)-specific inhibitors, wortmannin and 2-[4-Morpholinyl]-8-phenyl-4H-1-benzopyran-4-one (LY294002), were used to investigate whether PI 3-kinase is involved in the signal transduction that leads to bovine oocyte maturation. Bovine follicular oocytes were cultured in vitro for 24 h in a basic medium consisting of tissue culture medium-199 supplemented with LH, FSH, fetal cow serum, Na-pyruvate and gentamicin. The oocytes were then examined for the stage of meiotic progression and degree of cumulus expansion. In Experiment 1, in cumulus-oocyte complexes (COCs), wortmannin, at any level tested (10(-8) M, 10(-7) M or 10(-6) M), had no effect on resumption of meiosis as judged by germinal vesicle breakdown and progression to prometaphase I or metaphase I. However, wortmannin significantly (P < 0.01) decreased the proportion of oocytes developing to metaphase II in a dose-dependent manner. In Experiment 2, when denuded oocytes were cultured with wortmannin at 0, 10(-7) M and 10(-6) M concentrations, the same pattern of response for COCs was observed, with no effect on meiotic resumption and a significant (P < 0.01) decrease in the proportion of oocytes reaching metaphase II. In Experiment 3, half of the recovered COCs were denuded and both denuded and intact COCs were cultured in the presence of 0, 2.5 x 10(-5) M, 5.0 x 10(-5) M and 7.5 x 10(-5) M LY 294002 before being examined for meiotic progression. Whereas LY294002, at any examined level, had no effect on the percentage of oocytes developing to metaphase I, it significantly (P < 0.01) decreased the proportion of metaphase II oocytes when used at 5.0 x 10(-5) or 7.5 x 10(-5) M for both intact COCs and denuded oocytes. In Experiment 4, no significant difference in the degree of cumulus expansion was scored after the COCs were cultured in the presence of wortmannin or LY294002 or in the absence of either treatment. These results provide indirect evidence for a role of PI 3-kinase in the bovine oocyte itself in regulating meiotic progression beyond metaphase I.

Regulation of the cyclin B degradation system by an inhibitor of mitotic proteolysis

The initiation of anaphase and exit from mitosis depend on the anaphase-promoting complex (APC), which mediates the ubiquitin-dependent proteolysis of anaphase-inhibiting proteins and mitotic cyclins. We have analyzed whether protein phosphatases are required for mitotic APC activation. In Xenopus egg extracts APC activation occurs normally in the presence of protein phosphatase 1 inhibitors, suggesting that the anaphase defects caused by protein phosphatase 1 mutation in several organisms are not due to a failure to activate the APC. Contrary to this, the initiation of mitotic cyclin B proteolysis is prevented by inhibitors of protein phosphatase 2A such as okadaic acid. Okadaic acid induces an activity that inhibits cyclin B ubiquitination. We refer to this activity as inhibitor of mitotic proteolysis because it also prevents the degradation of other APC substrates. A similar activity exists in extracts of Xenopus eggs that are arrested at the second meiotic metaphase by the cytostatic factor activity of the protein kinase mos. In Xenopus eggs, the initiation of anaphase II may therefore be prevented by an inhibitor of APC-dependent ubiquitination.

Fizzy is required for activation of the APC/cyclosome in Xenopus egg extracts

The Xenopus homologue of Drosophila Fizzy and budding yeast CDC20 has been characterized. The encoded protein (X-FZY) is a component of a high molecular weight complex distinct from the APC/cyclosome. Antibodies directed against FZY were produced and shown to prevent calmodulin-dependent protein kinase II (CaMKII) from inducing the metaphase to anaphase transition of spindles assembled in vitro in Xenopus egg extracts, and this was associated with suppression of the degradation of mitotic cyclins. The same antibodies suppressed M phase-promoting factor (MPF)-dependent activation of the APC/cyclosome in interphase egg extracts, although they did not appear to alter the pattern or extent of MPF-dependent phosphorylation of APC/cyclosome subunits. As these phosphorylations are thought to be essential for APC/cyclosome activation in eggs and early embryos, we conclude that at least two events are required for MPF to activate the APC/cyclosome, allowing both chromatid segregation and full degradation of mitotic cyclins. The first one, which does not require FZY function, is the phosphorylation of APC/cyclosome subunits. The second one, that requires FZY function (even in the absence of MAD2 protein and when the spindle assembly checkpoint is not activated) is not yet understood at its molecular level.

Mph1, a member of the Mps1-like family of dual specificity protein kinases, is required for the spindle checkpoint in S. pombe

The spindle assembly checkpoint pathway is not essential for normal mitosis but ensures accurate nuclear division by blocking the metaphase to anaphase transition in response to a defective spindle. Here, we report the isolation of a new spindle checkpoint gene, mph1 (Mps1p-like pombe homolog), in the fission yeast Schizosaccharomyces pombe, that is required for checkpoint activation in response to spindle defects. mph1 functions upstream of mad2, a previously characterized component of the spindle checkpoint. Overexpression of mph1, like overexpression of mad2, mimics activation of the checkpoint and imposes a metaphase arrest. mph1 protein shares sequence similarity with Mps1p, a dual specificity kinase that functions in the spindle checkpoint of the budding yeast Saccharomyces cerevisiae. Complementation analysis demonstrates that mph1 and Mps1p are functionally related. They differ in that Mps1p, but not mph1, has an additional essential role in spindle pole body duplication. We propose that mph1 is the MPS1 equivalent in the spindle checkpoint pathway but not in the SPB duplication pathway. Overexpression of mad2 does not require mph1 to impose a metaphase arrest, which indicates a mechanism of spindle checkpoint activation other than mph1/Mps1p kinase-dependent phosphorylation. In the same screen which led to the isolation of mad2 and mph1, we also isolated dph1, a cDNA that encodes a protein 46% identical to an S. cerevisiae SPB duplication protein, Dsk2p. Our initial characterization indicates that S.p. dph1 and S.c. DSK2 are functionally similar. Together these results suggest that the budding and fission yeasts share common elements for SPB duplication, despite differences in SPB structure and the timing of SPB duplication relative to mitotic entry.

The checkpoint protein MAD2 and the mitotic regulator CDC20 form a ternary complex with the anaphase-promoting complex to control anaphase initiation

The spindle assembly checkpoint mechanism delays anaphase initiation until all chromosomes are aligned at the metaphase plate. Activation of the anaphase-promoting complex (APC) by binding of CDC20 and CDH1 is required for exit from mitosis, and APC has been implicated as a target for the checkpoint intervention. We show that the human checkpoint protein hMAD2 prevents activation of APC by forming a hMAD2-CDC20-APC complex. When injected into Xenopus embryos, hMAD2 arrests cells at mitosis with an inactive APC. The recombinant hMAD2 protein exists in two-folded states: a tetramer and a monomer. Both the tetramer and the monomer bind to CDC20, but only the tetramer inhibits activation of APC and blocks cell cycle progression. Thus, hMAD2 binding is not sufficient for inhibition, and a change in hMAD2 structure may play a role in transducing the checkpoint signal. There are at least three different forms of mitotic APC that can be detected in vivo: an inactive hMAD2-CDC20-APC ternary complex present at metaphase, a CDC20-APC binary complex active in degrading specific substrates at anaphase, and a CDH1-APC complex active later in mitosis and in G1. We conclude that the checkpoint-mediated cell cycle arrest involves hMAD2 receiving an upstream signal to inhibit activation of APC.

Mitogen-activated protein kinases (Erk1,2) phosphorylate Lys-Ser-Pro (KSP) repeats in neurofilament proteins NF-H and NF-M

Mammalian neurofilament proteins, particularly midsized (NF-M) and heavy (NF-H) molecular weight neurofilament proteins, are highly phosphorylated in axons. Neurofilament function depends on the state of phosphorylation of the numerous serine/threonine residues in these proteins. Most phosphorylation occurs in the lys-ser-pro (KSP) repeats in the C-terminal tail domains of NF-H and NF-M. In our previous study, cyclin-dependent kinase 5 (cdk5) was shown to phosphorylate specifically the KSPXK repeats in rat NF-H. Because 80% of the repeats are of the KSPXXXK type, it was of interest to determine which kinase phosphorylates these motifs. Using a synthetic KSPXXXK peptide to screen for a specific kinase, we fractionated rat brain extracts by column chromatography and identified extracellular signal-regulated kinase (Erk2) activated by an upstream activator, the mitogen-activated protein kinase kinase MAPKK (MEK), by Western blot analysis, sequence identification, and inhibition by a specific MEK inhibitor (PD 98059). The fraction containing Erk2, as well as bacterially expressed Erk1 and Erk2, phosphorylated all types of KSP motifs in peptides (KSPXK, KSPXXK, KSPXXXK, and KSPXXXXK) derived from NF-M and NF-H. They also phosphorylated an expressed 24 KSPXXXK repeat NF-H polypeptide, an expressed NF-H as well as dephosphorylated native rat NF-H, and NF-M proteins with accompanying decreases in their respective electrophoretic mobilities. A comparative kinetic study of Erk2 and cdk5 phosphorylation of KSPXK and KSPXXXK peptides revealed that, in contrast to cdk5, which phosphorylated only the KSPXK peptide, Erk2 could phosphorylate both. The preferred substrate for Erk2 was KSPXXXK peptide. The MEK inhibitor PD 98059 also inhibited phosphorylation of NF-H, NF-M, and microtubule-associated protein (MAP) in primary rat hippocampal cells and caused a decrease in neurite outgrowth, suggesting that Erk1,2 may play an important role in neurite growth and branching. These data suggest that neuronal Erk1 and Erk2 are capable of phosphorylating serine residues in diverse KSP repeat motifs in NF-M and NF-H.

Cell cycle-regulated processing of HEF1 to multiple protein forms differentially targeted to multiple subcellular compartments

HEF1, p130(Cas), and Efs/Sin constitute a family of multidomain docking proteins that have been implicated in coordinating the regulation of cell adhesion. Each of these proteins contains an SH3 domain, conferring association with focal adhesion kinase; a domain rich in SH2-binding sites, phosphorylated by or associating with a number of oncoproteins, including Abl, Crk, Fyn, and others; and a highly conserved carboxy-terminal domain. In this report, we show that the HEF1 protein is processed in a complex manner, with transfection of a single cDNA resulting in the generation of at least four protein species, p115(HEF1), p105(HEF1), p65(HEF1), and p55(HEF1). We show that p115(HEF1) and p105(HEF1) are different phosphorylation states of the full-length HEF1. p55(HEF1), however, encompasses only the amino-terminal end of the HEF1 coding sequence and arises via cleavage of full-length HEF1 at a caspase consensus site. We find that HEF1 proteins are abundantly expressed in epithelial cells derived from breast and lung tissue in addition to the lymphoid cells in which they have been predominantly studied to date. In MCF-7 cells, we find that expression of the endogenous HEF1 proteins is cell cycle regulated, with p105(HEF1) and p115(HEF1) being rapidly upregulated upon induction of cell growth, whereas p55(HEF1) is produced specifically at mitosis. While p105(HEF1) and p115(HEF1) are predominantly cytoplasmic and localize to focal adhesions, p55(HEF1) unexpectedly is shown to associate with the mitotic spindle. In support of a role at the spindle, two-hybrid library screening with HEF1 identifies the human homolog of the G2/M spindle-regulatory protein Dim1p as a specific interactor with a region of HEF1 encompassed in p55(HEF1). In sum, these data suggest that HEF1 may directly connect morphological control-related signals with cell cycle regulation and thus play a role in pathways leading to the progression of cancer.

PD 98059 prevents establishment of the spindle assembly checkpoint and inhibits the G2-M transition in meiotic but not mitotic cell cycles in Xenopus

Most chemotherapeutic agents block DNA replication, damage DNA, or interfere with chromosome segregation. The existence of checkpoints, which monitor these events, indicates that mechanisms exist to avoid death when essential cellular events are inhibited. A molecular understanding of cellular checkpoints should therefore provide opportunities for the development of inhibitors of checkpoint controls which may increase the potency of chemotherapeutic drugs by inducing catastrophic cell cycle progression. The molecular dissection of cell cycle arrest points is facilitated in the Xenopus egg/oocyte system, in which cell-free systems retain both S/M and spindle assembly checkpoints. Members of the MAP kinase family have been shown to play a role in the induction of G2 to M transition during oocyte maturation and have been implicated in the maintenance of either cytostatic factor- or spindle assembly checkpoint-induced M-phase arrest. Here, we have examined the effects of the inhibitor of MAP kinase kinase activation, PD 98059, on cell cycle progression in Xenopus oocytes and in cell-free extracts. This inhibitor is highly specific for the kinase which activates the classical p42/p44 MAP kinase, having no effect on upstream activators of stress-activated protein kinases. We have found that PD 98059 inhibits oocyte maturation, consistent with a role for p42 MAP kinase as a rate-limiting component in the induction of meiosis, but had no effect on the timing of G2-M transition in cell-free extracts indicating that, unlike meiosis, p42 MAP kinase activation is not limiting for normal mitotic M phase entry. However, we found that cytostatic factor-induced metaphase arrest, as well as the spindle assembly checkpoint, were both abolished in the presence of the drug. These results demonstrate that p42 MAP kinase, and not some other member of the MAP kinase family, is responsible for both CSF- and checkpoint-induced metaphase arrest and suggest that PD 98059 and similar agents may have considerable therapeutic potential for the potentiation of chemotherapeutic regimes.

Mathematical model of the fission yeast cell cycle with checkpoint controls at the G1/S, G2/M and metaphase/anaphase transitions

All events of the fission yeast cell cycle can be orchestrated by fluctuations of a single cyclin-dependent protein kinase, the Cdc13/Cdc2 heterodimer. The G1/S transition is controlled by interactions of Cdc13/Cdc2 and its stoichiometric inhibitor, Rum1. The G2/M transition is regulated by a kinase-phosphatase pair, Wee1 and Cdc25, which determine the phosphorylation state of the Tyr-15 residue of Cdc2. The meta/anaphase transition is controlled by interactions between Cdc13/Cdc2 and the anaphase promoting complex, which labels Cdc13 subunits for proteolysis. We construct a mathematical model of fission yeast growth and division that encompasses all three crucial checkpoint controls. By numerical simulations we show that the model is consistent with a broad selection of cell cycle mutants, and we predict the phenotypes of several multiple-mutant strains that have not yet been constructed.

Phosphorylation of mitogen-activated protein kinase by one-trial and multi-trial classical conditioning

The pathway supporting the conditioned stimulus (CS) is one site of plasticity that has been studied extensively in conditioned Hermissenda. Several signal transduction pathways have been implicated in classical conditioning of this preparation, although the major emphasis has been on protein kinase C. Here we provide evidence for the activation and phosphorylation of a mitogen-activated protein kinase (MAPK) pathway by one-trial and multi-trial conditioning. A one-trial in vitro conditioning procedure consisting of light (CS) paired with the application of 5-HT results in the increased incorporation of 32PO4 into proteins detected with two-dimensional gel electrophoresis. Two of the phosphoproteins have molecular weights of 44 and 42 kDa, consistent with extracellular signal-regulated protein kinases (ERK1 and ERK2). Phosphorylation of the 44 and 42 kDa proteins by one-trial conditioning was inhibited by pretreatment with PD098059, A MEK1 (ERK-Activating kinase) inhibitor. Assays of ERK activity with brain myelin basic protein as a substrate revealed greater ERK activity for the group that received one-trial conditioning compared with an unpaired control group. Western blot analysis of phosphorylated ERK using antibodies recognizing the dually phosphorylated forms of ERK1 and ERK2 showed an increase in phosphorylation after one-trial conditioning compared with unpaired controls. The increased phosphorylation of ERK after one-trial conditioning was blocked by pretreatment with PD098059. Hermissenda that received 10 or 15 conditioning trials showed significant behavioral suppression compared with pseudo-random controls. After conditioning and behavioral testing, the conditioned animals showed significantly greater phosphorylation of ERK compared with the pseudo-random controls. These results show that the ERK-MAPK signaling pathway is activated in Pavlovian conditioning of Hermissenda.

Modeling M-phase control in Xenopus oocyte extracts: the surveillance mechanism for unreplicated DNA

Alternating phases of DNA synthesis and mitosis, during the first 12 cell divisions of frog embryos, are driven by autonomous cytoplasmic oscillations of M-phase promoting factor (MPF). Cell-free extracts of frog eggs provide a convenient preparation for studying the molecular machinery that generates MPF oscillations and the surveillance mechanism that normally prevents entry into mitosis until chromosomal DNA is fully replicated. Early experiments suggested that unreplicated DNA blocks MPF activity by inducing phosphorylation of a crucial tyrosine residue, but recent evidence implicates a stoichiometric inhibitor (an MPF binding protein) as the 'braking' agent. Using a realistic mathematical model of the mitotic control system in frog egg extracts, we suggest that both tyrosine phosphorylation and a stoichiometric inhibitors are involved in the block of MPF by unreplicated DNA. Both pathways operate by raising the cyclin threshold for MPF activation. As a bonus, in the process of analyzing these experiments, we obtain more direct and reliable estimates of the rate constants in the model.

Regulation of cell cycle progression following DNA damage

DNA damage causes an arrest in cell cycle progression. Checkpoints, which monitor the state of the DNA, exist throughout the cycle and negatively regulate cell cycle transitions when damage is detected. The molecular basis of how these checkpoints are activated, and interact with the cell cycle machinery, is just beginning to be understood. Studies in yeast have identified a number of genes involved in a G2 DNA damage checkpoint, while in mammalian cells a G1 checkpoint has been extensively studied.

The regulation of cyclin-dependent kinase inhibitors (CKIs)

Inhibitors of cyclin-dependent kinases (CKIs) play key roles in coordinating cell proliferation and development. They also function to control critical cell cycle transitions and as effectors of checkpoint pathways. The activity of CKIs is tightly controlled through the cell cycle and in response to various signals. Regulation generally affects the levels or availability of the CKIs rather than changing their intrinsic activities. Mechanisms controlling CKI function include the regulation of transcription, translation and proteolysis. In addition some signals appear to induce sequestration of CKIs within the cells, thereby changing their ability to interact with specific targets.

Mitosis and checkpoints that control progression through mitosis in vertebrate somatic cells

During mitosis in vertebrates the sister kinetochores on each replicated chromosome interact with two separating arrays of astral microtubules to form a bipolar spindle that produces and/or directs the forces for chromosome motion. In order to ensure faithful chromosome segregation cells have evolved mechanisms that delay progress into and out of mitosis until certain events are completed. At least two of these mitotic "checkpoint controls" can be identified in vertebrates. The first prevents nuclear envelope breakdown, and thus spindle formation, when the integrity of some nuclear component(s) is compromised. The second prevents chromosome disjunction and exit from mitosis until all of the kinetochores are attached to the spindle.

Mos and the cell cycle

The mos proto-oncogene-encoded serine/threonine protein kinase plays a key cell cycle-regulatory role during meiosis. The Mos protein is required for the activation and stabilisation of M phase-promoting factor MPF. As a component of a large multiprotein complex known as the cytostatic factor (CSF), Mos is involved in causing metaphase II arrest of eggs in vertebrates. Upon expression in somatic cells, Mos causes cell cycle perturbations resulting in cytotoxicity and neoplastic transformation. All the known biological activities of Mos are mediated through activation of the mitogen activated protein (MAP) kinase pathway. Here we discuss the interrelationship between Mos and other cell cycle regulators.

Onset of chromosome segregation at the metaphase to anaphase transition of the cell cycle

Chromosome segregation is one of the most important acts in the life of the cell. Unequal inheritance of chromosomes (aneuploidy) is a cause of a number of disorders, particularly in humans, even though eukaryotic cells can arrest or delay the transition from metaphase to anaphase if an event critical to the completion of metaphase is impaired. In this report, we review recent advances in our knowledge of how the complex process of chromosome segregation is coupled with cell cycle progression, and starts at onset of anaphase with sister chromatids separation of the replicated chromosomes.

Activation of the protein kinase p38 in the spindle assembly checkpoint and mitotic arrest

The mitogen-activated protein kinase (MAPK) superfamily comprises classical MAPK (also called ERK), c-Jun amino-terminal or stress-activated protein kinase (JNK or SAPK), and p38. Although MAPK is essential for meiotic processes in Xenopus oocytes and the spindle assembly checkpoint in Xenopus egg extracts, the role of members of the MAPK superfamily in M phase or the spindle assembly checkpoint during somatic cell cycles has not been elucidated. The kinase p38, but not MAPK or JNK, was activated in mammalian cultured cells when the cells were arrested in M phase by disruption of the spindle with nocodazole. Addition of activated recombinant p38 to Xenopus cell-free extracts caused arrest of the extracts in M phase, and injection of activated p38 into cleaving embryos induced mitotic arrest. Treatment of NIH 3T3 cells with a specific inhibitor of p38 suppressed activation of the checkpoint by nocodazole. Thus, p38 functions as a component of the spindle assembly checkpoint in somatic cell cycles.

Cell cycle arrest in cdc20 mutants of Saccharomyces cerevisiae is independent of Ndc10p and kinetochore function but requires a subset of spindle checkpoint genes

The spindle checkpoint ensures accurate chromosome segregation by inhibiting anaphase onset in response to altered microtubule function and impaired kinetochore function. In this study, we report that the ability of the anti-microtubule drug nocodazole to inhibit cell cycle progression in Saccharomyces cerevisiae depends on the function of the kinetochore protein encoded by NDC10. We examined the role of the spindle checkpoint in the arrest in cdc20 mutants that arrest prior to anaphase with an aberrant spindle. The arrest in cdc20 defective cells is dependent on the BUB2 checkpoint and independent of the BUB1, BUB3, and MAD spindle checkpoint genes. We show that the lesion recognized by Bub2p is not excess microtubules, and the cdc20 arrest is independent of kinetochore function. We show that Cdc20p is not required for cyclin proteolysis at two points in the cell cycle, suggesting that CDC20 is distinct from genes encoding integral proteins of the anaphase promoting complex.

The polo-like kinase Plx1 is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts

Polo-like kinases (Plks), named after the Drosophila gene product polo, have been implicated in the regulation of multiple aspects of mitotic progression, including the activation of the Cdc25 phosphatase, bipolar spindle formation and cytokinesis. Genetic analyses performed in yeast and Drosophila suggest a function for Plks at late stages of mitosis, but biochemical data to support such a function in vertebrate organisms are lacking. Here we have taken advantage of Xenopus egg extracts for exploring the function of Plx1, a Xenopus Plk, during the cell cycle transition from M phase to interphase (I phase). We found that the addition of a catalytically inactive Plx1 mutant to M phase-arrested egg extracts blocked their Ca2+-induced release into interphase. Concomitantly, the proteolytic destruction of several targets of the anaphase-promoting complex and the inactivation of the Cdc2 protein kinase (Cdk1) were prevented. Moreover, the M to I phase transition could be abolished by immunodepletion of Plx1, but was restored upon the addition of recombinant Plx1. These results demonstrate that the exit of egg extracts from M phase arrest requires active Plx1, and they strongly suggest an important role for Plx1 in the activation of the proteolytic machinery that controls the exit from mitosis.

Farnesyl transferase inhibitors cause enhanced mitotic sensitivity to taxol and epothilones

An important class of cellular proteins, which includes members of the p21ras family, undergoes posttranslational farnesylation, a modification required for their partition to membranes. Specific farnesyl transferase inhibitors (FTIs) have been developed that selectively inhibit the processing of these proteins. FTIs have been shown to be potent inhibitors of tumor cell growth in cell culture and in murine models and at doses that cause little toxicity to the animal. These data suggest that these drugs might be useful therapeutic agents. We now report that, when FTI is combined with some cytotoxic antineoplastic drugs, the effects on tumor cells are additive. No interference is noted. Furthermore, FTI and agents that prevent microtubule depolymerization, such as taxol or epothilones, act synergistically to inhibit cell growth. FTI causes increased sensitivity to induction of metaphase block by these agents, suggesting that a farnesylated protein may regulate the mitotic check point. The findings imply that FTI may be a useful agent for the treatment of tumors with wild-type ras that are sensitive to taxanes.

Activation of the p42 mitogen-activated protein kinase pathway inhibits Cdc2 activation and entry into M-phase in cycling Xenopus egg extracts

We have added constitutively active MAP kinase/ERK kinase (MEK), an activator of the mitogen-activated protein kinase (MAPK) signaling pathway, to cycling Xenopus egg extracts at various times during the cell cycle. p42MAPK activation during entry into M-phase arrested the cell cycle in metaphase, as has been shown previously. Unexpectedly, p42MAPK activation during interphase inhibited entry into M-phase. In these interphase-arrested extracts, H1 kinase activity remained low, Cdc2 was tyrosine phosphorylated, and nuclei continued to enlarge. The interphase arrest was overcome by recombinant cyclin B. In other experiments, p42MAPK activation by MEK or by Mos inhibited Cdc2 activation by cyclin B. PD098059, a specific inhibitor of MEK, blocked the effects of MEK(QP) and Mos. Mos-induced activation of p42MAPK did not inhibit DNA replication. These results indicate that, in addition to the established role of p42MAPK activation in M-phase arrest, the inappropriate activation of p42MAPK during interphase prevents normal entry into M-phase.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

Signaling via mitogen-activated protein kinase kinase (MEK1) is required for Golgi fragmentation during mitosis

We have developed an assay using permeabilized cells to monitor fragmentation of the Golgi complex that occurs during mitosis. Golgi stacks, in permeabilized interphase normal rat kidney (NRK) cells, upon incubation with mitotic extracts undergo extensive fragmentation, and the fragmented Golgi membranes are dispersed throughout the cytoplasm. We find that the continued presence of p34cdc2, the mitosis initiation kinase, is not necessary for Golgi fragmentation. Instead, fragmentation depends on cytosolic mitogen-activated protein kinase kinase 1 (MEK1 or MAPKK1). However, the known cytoplasmic substrates for MEK1, ERK1, and ERK2 are not required for this process. Interestingly, we find a Golgi-associated ERK, which we propose as the likely target for MEK1 in Golgi fragmentation.

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.