Greatwall phosphorylates an inhibitor of protein phosphatase 2A that is essential for mitosis

Entry into mitosis in eukaryotes requires the activity of cyclin-dependent kinase 1 (Cdk1). Cdk1 is opposed by protein phosphatases in two ways: They inhibit activation of Cdk1 by dephosphorylating the protein kinases Wee1 and Myt1 and the protein phosphatase Cdc25 (key regulators of Cdk1), and they also antagonize Cdk1's own phosphorylation of downstream targets. A particular form of protein phosphatase 2A (PP2A) containing a B55δ subunit (PP2A- B55δ) is the major protein phosphatase that acts on model CDK substrates in Xenopus egg extracts and has antimitotic activity. The activity of PP2A-B55δ is high in interphase and low in mitosis, exactly opposite that of Cdk1. We report that inhibition of PP2A-B55δ results from a small protein, known as α-endosulfine (Ensa), that is phosphorylated in mitosis by the protein kinase Greatwall (Gwl). This converts Ensa into a potent and specific inhibitor of PP2A-B55δ. This pathway represents a previously unknown element in the control of mitosis.     

Bora and the kinase Aurora a cooperatively activate the kinase Plk1 and control mitotic entry

A central question in the study of cell proliferation is, what controls cell-cycle transitions? Although the accumulation of mitotic cyclins drives the transition from the G2 phase to the M phase in embryonic cells, the trigger for mitotic entry in somatic cells remains unknown. We report that the synergistic action of Bora and the kinase Aurora A (Aur-A) controls the G2-M transition. Bora accumulates in the G2 phase and promotes Aur-A-mediated activation of Polo-like kinase 1 (Plk1), leading to the activation of cyclin-dependent kinase 1 and mitotic entry. Mechanistically, Bora interacts with Plk1 and controls the accessibility of its activation loop for phosphorylation and activation by Aur-A. Thus, Bora and Aur-A control mitotic entry, which provides a mechanism for one of the most important yet ill-defined events in the cell cycle.     

Centromeric Aurora-B activation requires TD-60, microtubules, and substrate priming phosphorylation

The chromosome passenger complex (CPC) controls chromosome congression, kinetochore-microtubule attachments, and spindle checkpoint signaling during mitosis. Aurora-B kinase is the catalytic subunit of the CPC. To understand how a single kinase can regulate such diverse events, we have investigated the activation of Aurora-B and describe two distinct activation mechanisms. First, Aurora-B activation in vitro requires two cofactors, telophase disc-60kD (TD-60) and microtubules. TD-60 is critical to localize both the CPC and Haspin kinase activity to centromeres and thus regulates Aurora-B at several levels. Second, Aurora-B substrates can inhibit kinase activation, and this is relieved by phosphorylation of these substrates by the centromeric kinases Plk1 and Haspin. These regulatory mechanisms suggest models for phosphorylation by Aurora-B of centromeric substrates at unaligned chromosomes and merotelic attachments.     

Phosphorylation by cyclin B-Cdk underlies release of mitotic exit activator Cdc14 from the nucleolus

Budding yeast protein phosphatase Cdc14 is sequestered in the nucleolus in an inactive state during interphase by the anchor protein Net1. Upon entry into anaphase, the Cdc14 early anaphase release (FEAR) network initiates dispersal of active Cdc14 throughout the cell. We report that the FEARnetwork promotes phosphorylation of Net1 by cyclin-dependent kinase (Cdk) complexed with cyclin B1 or cyclin B2. These phosphorylations appear to be required for FEAR and sustain the proper timing of late mitotic events. Thus, a regulatory circuit exists to ensure that the arbiter of the mitotic state, Cdk, sets in motion events that culminate in exit from mitosis.     

The protein kinase p90 rsk as an essential mediator of cytostatic factor activity

Persistent activation of p42 mitogen-activated protein kinase (p42 MAPK) during mitosis induces a "cytostatic factor" arrest, the arrest responsible for preventing the parthenogenetic activation of unfertilized eggs. The protein kinase p90 Rsk is a substrate of p42 MAPK; thus, the role of p90 Rsk in p42 MAPK-induced mitotic arrest was examined. Xenopus laevis egg extracts immunodepleted of Rsk lost their capacity to undergo mitotic arrest in response to activation of the Mos-MEK-1-p42 MAPK cascade of protein kinases. Replenishing Rsk-depleted extracts with catalytically competent Rsk protein restored the ability of the extracts to undergo mitotic arrest. Rsk appears to be essential for cytostatic factor arrest.     

Differential phosphorylation of c-Abl in cell cycle determined by cdc2 kinase and phosphatase activity

The product of the c-abl proto-oncogene (c-Abl) is phosphorylated on three sites during interphase and seven additional sites during mitosis. Two interphase and all mitotic c-Abl sites are phosphorylated by cdc2 kinase isolated from either interphase or mitotic cells, with the mitotic cdc2 having an 11-fold higher activity. Inhibition of phosphatases with okadaic acid in interphase cells leads to the phosphorylation of c-Abl mitotic sites, indicating that those sites are preferentially dephosphorylated during interphase. The differential phosphorylation of c-Abl in the cell cycle is therefore determined by an equilibrium between cdc2 kinase and protein phosphatase activities. Treatment of interphase cells with okadaic acid leads to a rounded morphology similar to that observed during mitosis.     

A network of control mediated by regulator of calcium/calmodulin-dependent signaling

Calmodulin (CaM) is a major effector for the intracellular actions of Ca2+ in nearly all cell types. We identified a CaM-binding protein, designated regulator of calmodulin signaling (RCS). G protein-coupled receptor (GPCR)-dependent activation of protein kinase A (PKA) led to phosphorylation of RCS at Ser55 and increased its binding to CaM. Phospho-RCS acted as a competitive inhibitor of CaM-dependent enzymes, including protein phosphatase 2B (PP2B, also called calcineurin). Increasing RCS phosphorylation blocked GPCR- and PP2B-mediated suppression of L-type Ca2+ currents in striatal neurons. Conversely, genetic deletion of RCS significantly increased this modulation. Through a molecular mechanism that amplifies GPCR- and PKA-mediated signaling and attenuates GPCR- and PP2B-mediated signaling, RCS synergistically increases the phosphorylation of key proteins whose phosphorylation is regulated by PKA and PP2B.     

Molecular mimicry regulates ABA signaling by SnRK2 kinases and PP2C phosphatases

Abscisic acid (ABA) is an essential hormone for plants to survive environmental stresses. At the center of the ABA signaling network is a subfamily of type 2C protein phosphatases (PP2Cs), which form exclusive interactions with ABA receptors and subfamily 2 Snfl-related kinase (SnRK2s). Here, we report a SnRK2-PP2C complex structure, which reveals marked similarity in PP2C recognition by SnRK2 and ABA receptors. In the complex, the kinase activation loop docks into the active site of PP2C, while the conserved ABA-sensing tryptophan of PP2C inserts into the kinase catalytic cleft, thus mimicking receptor-PP2C interactions. These structural results provide a simple mechanism that directly couples ABA binding to SnRK2 kinase activation and highlight a new paradigm of kinase-phosphatase regulation through mutual packing of their catalytic sites.     

Requirement of the prolyl isomerase Pin1 for the replication checkpoint

The peptidyl-prolyl isomerase Pin1 has been implicated in regulating cell cycle progression. Pin1 was found to be required for the DNA replication checkpoint in Xenopus laevis. Egg extracts depleted of Pin1 inappropriately transited from the G2 to the M phase of the cell cycle in the presence of the DNA replication inhibitor aphidicolin. This defect in replication checkpoint function was reversed after the addition of recombinant wild-type Pin1, but not an isomerase-inactive mutant, to the depleted extract. Premature mitotic entry in the absence of Pin1 was accompanied by hyperphosphorylation of Cdc25, activation of Cdc2/cyclin B, and generation of epitopes recognized by the mitotic phosphoprotein antibody, MPM-2. Therefore, Pin1 appears to be required for the checkpoint delaying the onset of mitosis in response to incomplete replication.     

Cell cycle regulation of myosin-V by calcium/calmodulin-dependent protein kinase II

Organelle transport by myosin-V is down-regulated during mitosis, presumably by myosin-V phosphorylation. We used mass spectrometry phosphopeptide mapping to show that the tail of myosin-V was phosphorylated in mitotic Xenopus egg extract on a single serine residue localized in the carboxyl-terminal organelle-binding domain. Phosphorylation resulted in the release of the motor from the organelle. The phosphorylation site matched the consensus sequence of calcium/calmodulin-dependent protein kinase II (CaMKII), and inhibitors of CaMKII prevented myosin-V release. The modulation of cargo binding by phosphorylation is likely to represent a general mechanism regulating organelle transport by myosin-V.     

Role of importin-beta in coupling Ran to downstream targets in microtubule assembly

The guanosine triphosphatase Ran stimulates assembly of microtubule asters and spindles in mitotic Xenopus egg extracts. A carboxyl-terminal region of the nuclear-mitotic apparatus protein (NuMA), a nuclear protein required for organizing mitotic spindle poles, mimics Ran's ability to induce asters. This NuMA fragment also specifically interacted with the nuclear transport factor, importin-beta. We show that importin-beta is an inhibitor of microtubule aster assembly in Xenopus egg extracts and that Ran regulates the interaction between importin-beta and NuMA. Importin-beta therefore links NuMA to regulation by Ran. This suggests that similar mechanisms regulate nuclear import during interphase and spindle assembly during mitosis.     

Proteomic screen finds pSer/pThr-binding domain localizing Plk1 to mitotic substrates

We have developed a proteomic approach for identifying phosphopeptide binding domains that modulate kinase-dependent signaling pathways. An immobilized library of partially degenerate phosphopeptides biased toward a particular protein kinase phosphorylation motif is used to isolate phospho-binding domains that bind to proteins phosphorylated by that kinase. Applying this approach to cyclin-dependent kinases (Cdks), we identified the polo-box domain (PBD) of the mitotic kinase polo-like kinase 1 (Plk1) as a specific phosphoserine (pSer) or phosphothreonine (pThr) binding domain and determined its optimal binding motif. This motif is present in known Plk1 substrates such as Cdc25, and an optimal phosphopeptide containing the motif disrupted PBD-substrate binding and localization of the PBD to centrosomes. This finding reveals how Plk1 can localize to specific sites within cells in response to Cdk phosphorylation at those sites and provides a structural mechanism for targeting the Plk1 kinase domain to its substrates.     

Direct activation of the ATM protein kinase by the Mre11/Rad50/Nbs1 complex

The complex containing the Mre11, Rad50, and Nbs1 proteins (MRN) is essential for the cellular response to DNA double-strand breaks, integrating DNA repair with the activation of checkpoint signaling through the protein kinase ATM (ataxia telangiectasia mutated). We demonstrate that MRN stimulates the kinase activity of ATM in vitro toward its substrates p53, Chk2, and histone H2AX. MRN makes multiple contacts with ATM and appears to stimulate ATM activity by facilitating the stable binding of substrates. Phosphorylation of Nbs1 is critical for MRN stimulation of ATM activity toward Chk2, but not p53. Kinase-deficient ATM inhibits wild-type ATM phosphorylation of Chk2, consistent with the dominant-negative effect of kinase-deficient ATM in vivo.     

丙酯草醚对植物细胞有丝分裂的影响

[目的]探讨丙酯草醚的除草机理。[方法]以洋葱根尖为试材,探讨不同时间和不同浓度丙酯草醚对根尖分生细胞有丝分裂的影响。[结果]随着丙酯草醚处理浓度的增加,洋葱根尖有丝分裂指数逐渐下降;在同一浓度下,随着处理时间的延长,有丝分裂指数也呈下降趋势。对不同处理下的洋葱根尖细胞形态观察发现,在低浓度(0.012 5%)丙酯草醚短时间(2 h)处理下,洋葱根尖分生区中仍可见中、后、末期3种分裂状态的细胞。但在超过8 h的高浓度(0.100 0%)丙酯草醚处理下,分裂态细胞明显减少,分生区有明显较多分裂中期的细胞。[结论]丙酯草醚对根尖细胞的分裂有明显抑制作用,并使根尖细胞停滞于有丝分裂中期。     

Abnormal spindle protein, Asp, and the integrity of mitotic centrosomal microtubule organizing centers

The product of the abnormal spindle (asp) gene was found to be an asymmetrically localized component of the centrosome during mitosis, required to focus the poles of the mitotic spindle in vivo. Removing Asp protein function from Drosophila melanogaster embryo extracts, either by mutation or immunodepletion, resulted in loss of their ability to restore microtubule-organizing center activity to salt-stripped centrosome preparations. This was corrected by addition of purified Asp protein. Thus, Asp appears to hold together the microtubule-nucleating gamma-tubulin ring complexes that organize the mitotic centrosome.     

CRACM1 is a plasma membrane protein essential for store-operated Ca2+ entry

Store-operated Ca2+ entry is mediated by Ca2+ release-activated Ca2+ (CRAC) channels following Ca2+ release from intracellular stores. We performed a genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins that inhibit store-operated Ca2+ influx. A secondary patch-clamp screen identified CRACM1 and CRACM2 (CRAC modulators 1 and 2) as modulators of Drosophila CRAC currents. We characterized the human ortholog of CRACM1, a plasma membrane-resident protein encoded by gene FLJ14466. Although overexpression of CRACM1 did not affect CRAC currents, RNAi-mediated knockdown disrupted its activation. CRACM1 could be the CRAC channel itself, a subunit of it, or a component of the CRAC signaling machinery.     

Anaphase onset before complete DNA replication with intact checkpoint responses

Cellular checkpoints prevent mitosis in the presence of stalled replication forks. Whether checkpoints also ensure the completion of DNA replication before mitosis is unknown. Here, we show that in yeast smc5-smc6 mutants, which are related to cohesin and condensin, replication is delayed, most significantly at natural replication-impeding loci like the ribosomal DNA gene cluster. In the absence of Smc5-Smc6, chromosome nondisjunction occurs as a consequence of mitotic entry with unfinished replication despite intact checkpoint responses. Eliminating processes that obstruct replication fork progression restores the temporal uncoupling between replication and segregation in smc5-smc6 mutants. We propose that the completion of replication is not under the surveillance of known checkpoints.