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.     

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.     

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.     

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.     

cdc2 gene expression at the G1 to S transition in human T lymphocytes

The product of the cdc2 gene, designated p34cdc2, is a serine-threonine protein kinase that controls entry of eukaryotic cells into mitosis. Freshly isolated human T lymphocytes (G0 phase) were found to have very low amounts of p34cdc2 and cdc2 messenger RNA. Expression of cdc2 increased 18 to 24 hours after exposure of T cells to phytohemagglutinin, coincident with the G1 to S transition. Antisense oligodeoxynucleotides could reduce the increase in cdc2 expression and inhibited DNA synthesis, but had no effect on several early and mid-G1 events, including blastogenesis and expression of interleukin-2 receptors, transferrin receptors, c-myb, and c-myc. Induction of cdc2 required prior induction of c-myb and c-myc. These results suggest that cdc2 induction is part of an orderly sequence of events that occurs at the G1 to S transition in T cells.     

Calpain II involvement in mitosis

Mitotic spindle disassembly requires major structural alterations in the associated cytoskeletal proteins and mitosis is known to be associated with Ca2+-sequestering phenomena and calcium transients. To examine the possible involvement of a ubiquitous Ca2+-activated protease, calpain II, in the mitotic process, synchronized PtK1 cells were monitored by immunofluorescence for the relocation of calpain II. The plasma membrane was the predominant location of calpain II in interphase. However, as mitosis progressed, calpain II relocated to (i) an association with mitotic chromosomes, (ii) a perinuclear location in anaphase, and (iii) a mid-body location in telophase. Microinjection of calpain II near the nucleus of a PtK1 cell promoted the onset of metaphase. Injection of calpain II at late metaphase promoted a precocious disassembly of the mitotic spindle and the onset of anaphase. These data suggest that calpain II is involved in mitosis.     

Chromosomal and nucleolar RNA synthesis in root tips during mitosis

Comparative rates of RNA synthesis in chromatin and nucleolar fractions during mitosis in root-tip cells of Allium and Nigella were studied by pulse-labeling of cells with tritiated cytidine. Although the rate of RNA synthesis decreases in the condensing chromosomes during prophase, it remains normal in the nucleolar fraction as long as nucleoli are maintained. RNA synthesis stops in mitotic cells lacking distinct nucleoli. In the late telophase or very early interphase cells, RNA synthesis resumes at a faster rate in the pronucleolar bodies than in the chromatin.     

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。     

A cell cycle phosphoproteome of the yeast centrosome

Centrosomes organize the bipolar mitotic spindle, and centrosomal defects cause chromosome instability. Protein phosphorylation modulates centrosome function, and we provide a comprehensive map of phosphorylation on intact yeast centrosomes (18 proteins). Mass spectrometry was used to identify 297 phosphorylation sites on centrosomes from different cell cycle stages. We observed different modes of phosphoregulation via specific protein kinases, phosphorylation site clustering, and conserved phosphorylated residues. Mutating all eight cyclin-dependent kinase (Cdk)-directed sites within the core component, Spc42, resulted in lethality and reduced centrosomal assembly. Alternatively, mutation of one conserved Cdk site within γ-tubulin (Tub4-S360D) caused mitotic delay and aberrant anaphase spindle elongation. Our work establishes the extent and complexity of this prominent posttranslational modification in centrosome biology and provides specific examples of phosphorylation control in centrosome function.     

Efficient persistence of extrachromosomal KSHV DNA mediated by latency-associated nuclear antigen

Primary effusion lymphoma (PEL) cells harbor Kaposi's sarcoma-associated herpesvirus (KSHV) episomes and express a KSHV-encoded latency-associated nuclear antigen (LANA). In PEL cells, LANA and KSHV DNA colocalized in dots in interphase nuclei and along mitotic chromosomes. In the absence of KSHV DNA, LANA was diffusely distributed in the nucleus or on mitotic chromosomes. In lymphoblasts, LANA was necessary and sufficient for the persistence of episomes containing a specific KSHV DNA fragment. Furthermore, LANA colocalized with the artificial KSHV DNA episomes in nuclei and along mitotic chromosomes. These results support a model in which LANA tethers KSHV DNA to chromosomes during mitosis to enable the efficient segregation of KSHV episomes to progeny cells.     

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.     

保卫细胞微管骨架参与蛋白丝氨酸/苏氨酸磷酸化调节的气孔运动

【目的】探讨在气孔运动的信息传递通路中,保卫细胞微管骨架与蛋白丝氨酸/苏氨酸磷酸化两种因素之间是否存在相互作用,进而深入了解气孔运动机理。【方法】以拟南芥野生型及GFP-α-tubulin-6植株为材料,利用药理学试验、激光共聚焦扫描显微镜观察、免疫印迹等细胞学及生物化学方法研究丝氨酸/苏氨酸蛋白磷酸化对保卫细胞气孔运动及微管骨架的影响。【结果】丝氨酸/苏氨酸蛋白激酶抑制剂星型孢菌素（staurosporine,STS）能促进光照下气孔开放,微管特异性抑制剂长春花碱（vinblastine）和微管稳定剂紫杉醇（taxol）分别减弱和增强其作用;1/2A型磷酸酶抑制剂冈田酸（okadaic acid,OA）、花萼海绵诱癌素A（calyculin A,CalA）抑制光照诱导的气孔开放,微管特异性药物长春花碱和紫杉醇又能分别增强和削弱该抑制作用。激光共聚焦扫描显微镜下观察,两种磷酸酶抑制剂OA和CalA处理后,正常开放气孔保卫细胞中整齐有序的辐射状微管数量显著减少,变为以交错网状及解聚态为主;蛋白激酶抑制剂STS处理则增加了辐射状微管的比例。微管特异性药物紫杉醇、长春花碱分别与以上3种抑制剂共同处理,同样能够在一定程度上改变其对保卫细胞微管骨架组织排布的影响。提取保卫细胞原生质体总蛋白进行免疫印迹检测,55 kD分子量处的微管蛋白发生明显的蛋白丝氨酸磷酸化。【结论】蛋白丝氨酸/苏氨酸磷酸化可以通过调节保卫细胞微管骨架的动态排布进行气孔运动的信息传递。     

The substrate of Greatwall kinase, Arpp19, controls mitosis by inhibiting protein phosphatase 2A

Initiation and maintenance of mitosis require the activation of protein kinase cyclin B-Cdc2 and the inhibition of protein phosphatase 2A (PP2A), which, respectively, phosphorylate and dephosphorylate mitotic substrates. The protein kinase Greatwall (Gwl) is required to maintain mitosis through PP2A inhibition. We describe how Gwl activation results in PP2A inhibition. We identified cyclic adenosine monophosphate-regulated phosphoprotein 19 (Arpp19) and α-Endosulfine as two substrates of Gwl that, when phosphorylated by this kinase, associate with and inhibit PP2A, thus promoting mitotic entry. Conversely, in the absence of Gwl activity, Arpp19 and α-Endosulfine are dephosphorylated and lose their capacity to bind and inhibit PP2A. Although both proteins can inhibit PP2A, endogenous Arpp19, but not α-Endosulfine, is responsible for PP2A inhibition at mitotic entry in Xenopus egg extracts.     

Nuclear reassembly excludes large macromolecules

Interphase nucleus and cytoplasm are distinct compartments, whose soluble macromolecular contents mix when the nuclear envelope disassembles at mitosis. To determine how their interphase identities are reestablished, fibroblasts were loaded with fluorescent dextrans and then allowed to divide. Large dextrans (molecular weight of 40,000 or more) were excluded from condensed mitotic chromosomes and from newly formed, postmitotic interphase nuclei. Therefore, postmitotic reassembly of the nucleus as a compartment distinct from cytoplasm occurs by exclusion not only of organelles but also of soluble macromolecules. This might occur by exclusion of macromolecules from condensed chromatin throughout mitosis and completion of nuclear envelope assembly before nuclear expansion.     

Activation of the DNA replication checkpoint through RNA synthesis by primase

When DNA replication is inhibited during the synthesis (S) phase of the cell cycle, a signaling pathway (checkpoint) is activated that serves to prevent mitosis from initiating before completion of replication. This replication checkpoint acts by down-regulating the activity of the mitotic inducer cdc2-cyclin B. Here, we report the relation between chromatin structure and induction of the replication checkpoint. Chromatin was competent to initiate a checkpoint response only after the DNA was unwound and DNA polymerase alpha had been loaded. Checkpoint induction did not require new DNA synthesis on the unwound template strand but did require RNA primer synthesis by primase. These findings identify the RNA portion of the primer as an important component of the signal that activates the replication checkpoint.     

Visualization of a Ran-GTP gradient in interphase and mitotic Xenopus egg extracts

The small guanosine triphosphatase Ran is loaded with guanosine triphosphate (GTP) by the chromatin-bound guanine nucleotide exchange factor RCC1 and releases import cargoes in the nucleus during interphase. In mitosis, Ran-GTP promotes spindle assembly around chromosomes by locally discharging cargoes that regulate microtubule dynamics and organization. We used fluorescence resonance energy transfer-based biosensors to visualize gradients of Ran-GTP and liberated cargoes around chromosomes in mitotic Xenopus egg extracts. Both gradients were required to assemble and maintain spindle structure. During interphase, Ran-GTP was highly enriched in the nucleoplasm, and a steep concentration difference between nuclear and cytoplasmic Ran-GTP was established, providing evidence for a Ran-GTP gradient surrounding chromosomes throughout the cell cycle.     

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.     

利用洋葱根尖微核技术对洗涤剂诱变效应的研究

根据微核技术原理,用6种不同浓度梯度的3种洗涤剂处理洋葱根尖细胞,对其遗传毒理学效应进行研究.结果表明:3种洗涤剂对洋葱根尖细胞都有不同程度的遗传损伤,其中分裂间期主要表现为微核、双核、核出芽以及核破裂;分裂期主要表现为染色体断片、粘连、滞后以及桥等遗传损伤效应.统计数据显示:用不同洗涤剂处理过的洋葱根尖细胞中,间期微核数、分裂期染色体桥、断片的细胞数以及有丝分裂指数均明显高于对照组,存在显著性差异.说明洗涤剂在一定程度上具有遗传毒性,其中洗衣粉的毒性最大,肥皂次之,香皂最弱.PI值显示洗涤剂在高浓度时均对水环境造成一定程度的污染.同时证明洋葱敏感度高,可以作为微核技术新的供试材料.