PR55��, a Regulatory Subunit of PP2A, Specifically Regulates PP2A-mediated ��-Catenin Dephosphorylation

A central question in Wnt signaling is the regulation of β-catenin phosphorylation and degradation. Multiple kinases, including CKIα and GSK3, are involved in β-catenin phosphorylation. Protein phosphatases such as PP2A and PP1 have been implicated in the regulation of β-catenin. However, which phosphatase dephosphorylates β-catenin in vivo and how the specificity of β-catenin dephosphorylation is regulated are not clear. In this study, we show that PP2A regulates β-catenin phosphorylation and degradation in vivo. We demonstrate that PP2A is required for Wnt/β-catenin signaling in Drosophila. Moreover, we have identified PR55α as the regulatory subunit of PP2A that controls β-catenin phosphorylation and degradation. PR55α, but not the catalytic subunit, PP2Ac, directly interacts with β-catenin. RNA interference knockdown of PR55α elevates β-catenin phosphorylation and decreases Wnt signaling, whereas overexpressing PR55α enhances Wnt signaling. Taken together, our results suggest that PR55α specifically regulates PP2A-mediated β-catenin dephosphorylation and plays an essential role in Wnt signaling.Wnt/β-catenin signaling plays essential roles in development and tumorigenesis (1–3). Our previous work found that β-catenin is sequentially phosphorylated by CKIα⁴ and GSK3 (4), which creates a binding site for β-Trcp (5), leading to degradation via the ubiquitination/proteasome machinery (3). Mutations in β-catenin or APC genes that prevent β-catenin phosphorylation or ubiquitination/degradation lead ultimately to cancer (1, 2).In addition to the involvement of kinases, protein phosphatases, such as PP1, PP2A, and PP2C, are also implicated in Wnt/β-catenin regulation. PP2C and PP1 may regulate dephosphorylation of Axin and play positive roles in Wnt signaling (6, 7). PP2A is a multisubunit enzyme (8–10); it has been reported to play either positive or negative roles in Wnt signaling likely by targeting different components (11–21). Toward the goal of understanding the mechanism of β-catenin phosphorylation, we carried out siRNA screening targeting several major phosphatases, in which we found that PP2A dephosphorylates β-catenin. This is consistent with a recent study where PP2A is shown to dephosphorylate β-catenin in a cell-free system (18).PP2A consists of a catalytic subunit (PP2Ac), a structure subunit (PR65/A), and variable regulatory B subunits (PR/B, PR/B′, PR/B″, or PR/B‴). The substrate specificity of PP2A is thought to be determined by its B subunit (9). By siRNA screening, we further identified that PR55α, a regulatory subunit of PP2A, specifically regulates β-catenin phosphorylation and degradation. Mechanistically, we found that PR55α directly interacts with β-catenin and regulates PP2A-mediated β-catenin dephosphorylation in Wnt signaling.     

Monoubiquitination promotes calpain cleavage of the protein phosphatase 2A (PP2A) regulatory subunit α4, altering PP2A stability and microtubule-associated protein phosphorylation

Multiple neurodegenerative disorders are linked to aberrant phosphorylation of microtubule-associated proteins (MAPs). Protein phosphatase 2A (PP2A) is the major MAP phosphatase; however, little is known about its regulation at microtubules. α4 binds the PP2A catalytic subunit (PP2Ac) and the microtubule-associated E3 ubiquitin ligase MID1, and through unknown mechanisms can both reduce and enhance PP2Ac stability. We show MID1-dependent monoubiquitination of α4 triggers calpain-mediated cleavage and switches α4's activity from protective to destructive, resulting in increased Tau phosphorylation. This regulatory mechanism appears important in MAP-dependent pathologies as levels of cleaved α4 are decreased in Opitz syndrome and increased in Alzheimer disease, disorders characterized by MAP hypophosphorylation and hyperphosphorylation, respectively. These findings indicate that regulated inter-domain cleavage controls the dual functions of α4, and dysregulation of α4 cleavage may contribute to Opitz syndrome and Alzheimer disease.     

PP2A的结构和功能新进展

PP2A是一种丝／苏氨酸磷蛋白磷酸酶,通过可逆性磷酸化使已磷酸化激活的蛋白质脱磷酸,在信号传导中承担负性调节的作用。由一个催化亚基和两个调节亚基构成。：PP2A是一种多功能性酶,底物为众多体内的转录因子和蛋白激酶;酵母,果蝇和小鼠的动物模型的研究中已经发现PP2A在细胞周期调控,形态以及发育中的作用;同时它又在信号转导的级联反应中与其他磷酸化酶和激酶相互作用,构成调节大分子调控下游信号的转导。催化亚基活性主要由转录后水平磷酸化和甲基化的状态调控。     

Microcystins: Synthesis and structure–activity relationship studies toward PP1 and PP2A

Microcystins are highly toxic cyanotoxins responsible for plant, animal and human poisoning. Exposure to microcystins, mainly through drinkable water and contaminated food, is a current world health concern. Although it is quite challenging, the synthesis of these potent cyanotoxins, analogs and derivatives helps to evaluate their toxicological properties and to elucidate their binding mechanisms to their main targets Protein Phosphatase-1 (PP1) and -2A (PP2A). This review focuses on synthetic approaches to prepare microcystins and analogs and compiles structure–activity relationship (SAR) studies that describe the unique features of microcystins that make them so potent.     

Phosphatidic Acid （PA） PP2A Activity and PIN1 Binds PP2AA1 to Regulate Polar Localization

Phospholipase D （PLD） exerts broad biological functions in eukaryotes through regulating downstream effectors by its product, phosphatidic acid （PA）. Protein kinases and phosphatases, such as mammalian target of rapa- mycin （mTOR）, Protein Phosphatase 1 （PP1） and Protein Phosphatase 2C （PP2C）, are PA-binding proteins that execute crucial regulatory functions in both animals and plants. PA participates in many signaling pathways by modulating the enzymatic activity and/or subcellular localization of bound proteins. In this study, we demonstrated that PLD-derived PA interacts with the scaffolding A1 subunit of Protein Phosphatase 2A （PP2A） and regulates PP2A-mediated PIN1 dephos- phorylation in Arabidopsis. Genetic and pharmacological studies showed that both PA and PP2A participate in the regu- lation of auxin distribution. In addition, both the phosphorylation status and polar localization of PIN1 protein were affected by PLD inhibitors, Exogenous PA triggered the membrane accumulation of PP2AA1 and enhanced the PP2A activity at membrane, while PLD inhibition resulted in the reduced endosomal localization and perinuclear aggregation of PP2AA1. These results demonstrate the important role of PLD-derived PA in normal PP2A-mediated PIN dephosphoryl- ation and reveal a novel mechanism, in which PA recruits PP2AA1 to the membrane system and regulates PP2A function on membrane-targeted proteins. As PA and PP2A are conserved among eukaryotes, other organisms might use similar mechanisms to mediate multiple biological processes.     

PPM-1, a PP2Cα/β phosphatase, regulates axon termination and synapse formation in Caenorhabditis elegans

The PHR (Pam/Highwire/RPM-1) proteins are evolutionarily conserved ubiquitin ligases that regulate axon guidance and synapse formation in Caenorhabditis elegans, Drosophila, zebrafish, and mice. In C. elegans, RPM-1 (Regulator of Presynaptic Morphology-1) functions in synapse formation, axon guidance, axon termination, and postsynaptic GLR-1 trafficking. Acting as an E3 ubiquitin ligase, RPM-1 negatively regulates a MAP kinase pathway that includes: dlk-1, mkk-4, and the p38 MAPK, pmk-3. Here we provide evidence that ppm-1, a serine/threonine phosphatase homologous to human PP2Cα(PPM1A) and PP2Cβ(PPM1B) acts as a second negative regulatory mechanism to control the dlk-1 pathway. We show that ppm-1 functions through its phosphatase activity in a parallel genetic pathway with glo-4 and fsn-1 to regulate both synapse formation in the GABAergic motorneurons and axon termination in the mechanosensory neurons. Our transgenic analysis shows that ppm-1 acts downstream of rpm-1 to negatively regulate the DLK-1 pathway, with PPM-1 most likely acting at the level of pmk-3. Our study provides insight into the negative regulatory mechanisms that control the dlk-1 pathway in neurons and demonstrates a new role for the PP2C/PPM phosphatases as regulators of neuronal development.     

SRC激酶和磷酸酶PP1γ2/PP2A的相互作用对小鼠附睾精子成熟及运动的调控

目的 哺乳动物附睾精子成熟、运动能力的获得与维持是保证精子执行正常功能、完成受精的前提和基础,但调控此过程的机制仍未完全阐明.SRC激酶参与小鼠精子获能的调控,Ser/Thr磷酸酶PP1γ2/PP2A是调控小鼠精子成熟、运动性获得的关键酶,但二者是否具有相互作用且这种相互作用是否调控着精子运动并不清楚.为此,本研究探究...     

PP2对大鼠面神经缺血损伤的保护作用

目的:探讨PP2(4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine)在大鼠面神经缺血损伤模型中的保护作用,为进一步探索面神经缺血损伤的治疗方法提供实验依据。方法:实验选取40只雄性SD大鼠,分为四组:假手术组,损伤组,DMSO溶剂对照组和PP2干预组。手术组沿耳廓外上缘行弧形切口,分离暴露出岩动脉,双极电凝灼断。假手术组暴露出岩动脉但不予灼断。PP2干预组和DMSO溶剂对照组于术前30 min分别脑室注射15 μg(总体积10 μL)PP2或相同体积DMSO。于3 d后观察各组大鼠的行为学变化,同时运用免疫共沉淀和免疫印迹的技术分析其中分子机制。结果:(1)PP2干预组的大鼠相对于手术组和DMSO溶剂对照组,瞬目反射和触须动度明显好转(2)大鼠面神经缺血损伤时,损伤组相对于假手术组,NMDAR-PSD95-Src的结合明显增加(P0.05)(3)PP2干预组NR2A-PSD95-Src的结合相对于手术组和DMSO溶剂对照组明显降低(P0.05)。结论:大鼠面神经缺血损伤时,存在NMDAR-PSD95-Src形成的信号通路,PP2可以有效的抑制NMDARPSD95-Src信号通路的形成,从而起到面神经保护作用。     

植物蛋白磷酸酶2C(PP2C)及其在信号转导中的作用

蛋白磷酸酶(protein phosphatase,PP)是蛋白质可逆磷酸化调节机制中的关键酶,蛋白磷酸酶2C(PP2C)是蛋白磷酸酶的一个分支。文章介绍了PP2C的结构及其在信号转导中的研究进展。     

The effect of deleting residue C269 in the β12-β13 loop of protein phosphatase 2A (PP2A)catalytic subunit on the interaction between PP2A and metal ions, especially Mn(2+)

Protein phosphatase 2A (PP2A) is one of the most important Ser/Thr phosphatases in eukaryotic cells. The enzymatic core of PP2A (PP2A(D)) consists of a scaffold subunit (A subunit) and a catalytic subunit (C subunit). When residue Cys269 in the β12-β13 loop of the PP2A C subunit was deleted (ΔC269), the activity and the intrinsic fluorescence intensity of PP2A(D) decreased. Specify the effects of some metal ions on PP2A(D) were also changed. Mn(2+) in particular was an efficient activator of ΔC269 and altered the intrinsic fluorescence spectrum of ΔC269. Remarkably, after pre-treatment of ΔC269 with Mn(2+), the effects of other metal ions showed the same trends as they had on the WT. Molecular dynamics (MD) simulations showed that deletion of Cys269 decreased the polarity of the β12-β13 loop of PP2A Cα. We conclude that deletion of residue Cys269 alters the conformation and activity of PP2A(D) and influences the interaction between PP2A and various metal ions, notably Mn(2+).     

The Bα and Bδ regulatory subunits of PP2A are necessary for assembly of the CaMKIV·PP2A signaling complex

Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is a serine/threonine kinase that is important in synaptic plasticity and T cell maturation. Activation of CaMKIV requires calcium/calmodulin binding and phosphorylation at T200 by CaMK kinase. Our previous work has shown that protein serine/threonine phosphatase 2A (PP2A) forms a complex with CaMKIV and negatively regulates its activity. Here we demonstrate that PP2A tightly regulates T200 phosphorylation of endogenous CaMKIV, but has little effect on the phosphorylation of the ectopically-expressed kinase. This differential regulation of endogenous versus exogenous CaMKIV is due to differences in their ability to associate with PP2A, as exogenous CaMKIV associates poorly with PP2A in comparison to endogenous CaMKIV. The inability of exogenous CaMKIV to associate with PP2A appears to be due to limiting amounts of endogenous PP2A regulatory B subunits, since coexpression of Bα or Bδ causes the recruitment of PP2Ac to ectopic CaMKIV, leading to formation of a CaMKIV·PP2A complex. Together, these data indicate that the B subunits are essential for the interaction of PP2A with CaMKIV.     

PP2A在运动抗阿尔茨海默病进程中的机制研究

Tau蛋白过度磷酸化是AD发病的重要原因,促进脑中p-Tau蛋白的脱磷酸化进程是运动抗AD的重要途径。PP2A是重要的蛋白磷酸酶,对p-Tau蛋白的脱磷酸化有重要作用。有关PP2A介导运动抗AD的Tau蛋白磷酸化机制研究尚不多见。现从Tau蛋白与AD研究、PP2A与AD研究、运动与AD研究、PP2A与运动抗AD研究等方面,系统阐述PP2A在介导运动抗AD进程中的蛋白磷酸化机制,为探明运动抗AD的Tau蛋白途径及运动促进健康的蛋白质修饰机制研究提供参考。     

潘庆民1,于振文2,王月福2

采用盆栽和水泥池栽研究了追氮时期对小麦光合作用、14C同化物运转分配和硝酸还原酶(NR)活性的影响.结果表明,拔节(雌雄蕊原基形成)期较起身(二棱)期追施氮肥,显著提高了小麦开花后的旗叶叶绿素含量和单叶光合速率;灌浆期旗叶14C同化物向籽粒转移比例显著提高,而在营养器官的滞留比例显著降低;旗叶和根系中硝酸还原酶(NR)活性亦显著提高.小麦穗粒数、粒重和产量增加,蛋白质含量提高.     

蛋白磷酸酶PP2A的结构及其肿瘤抑制因子功能

蛋白磷酸酶在细胞的生命活动中起着十分重要的作用,蛋白磷酸酶2A(protein phosphatase 2A, PP2A)作为蛋白磷酸酶家族中十分重要的一员,它几乎与所有真核细胞的生命活动都有密不可分的关系．2006年,PP2A核心酶和全酶晶体结构的陆续破解对于深入了解PP2A自身的结构和亚基之间的相互作用,以及其与结合蛋白作用的机制都有重大的影响．随着PP2A与肿瘤相关性的一系列新研究成果的不断涌现,PP2A在肿瘤发生和细胞迁移中也彰显出十分关键的作用．重点介绍PP2A的组成与结构、催化亚基的特殊修饰、亚基之间的相互作用关系以及PP2A作为一种新的肿瘤抑制因子的生物学功能．     

PP2C相关基因在砂梨休眠进程中的表达分析

对植物蛋白磷酸酶2C(PP2C)相关基因在砂梨Pyrus pyrifolia品系休眠进程中的表达进行分析。结果表明,砂梨PP2C相关基因与李属PP2C基因高度同源。在梨花芽休眠过程中不同PP2C基因调控的作用不同, PP2C-37-1、PP2C-37-2、PP2C-51-1、PP2C-24四个基因与内休眠调控有关,而PP2C-78对于内休眠的解除则有明显作用。PP2C蛋白磷酸酶相关基因注释到植物激素信号转导途径显示,ABA受体PYR/PYL蛋白与PP2C蛋白以及SnRK2(蛋白激酶)蛋白形成ABA信号转导的复合物可以作用于转录因子ABF从而调控梨花芽的休眠。