Protein phosphatase 2A1 is the major enzyme in vertebrate cell extracts that dephosphorylates several physiological substrates for cyclin-dependent protein kinases

Substrate and phosphorylation site selection by phosphoprotein phosphatases

Dynamic protein phosphorylation and dephosphorylation are essential regulatory mechanisms that ensure proper cellular signaling and biological functions. Deregulation of either reaction has been implicated in several human diseases. Here, we focus on the mechanisms that govern the specificity of the dephosphorylation reaction. Most cellular serine/threonine dephosphorylation is catalyzed by 13 highly conserved phosphoprotein phosphatase (PPP) catalytic subunits, which form hundreds of holoenzymes by binding to regulatory and scaffolding subunits. PPP holoenzymes recognize phosphorylation site consensus motifs and interact with short linear motifs (SLiMs) or structural elements distal to the phosphorylation site. We review recent advances in understanding the mechanisms of PPP site-specific dephosphorylation preference and substrate recruitment and highlight examples of their interplay in the regulation of cell division.

Identification of PP2A-B55 targets uncovers regulation of emerin during nuclear envelope reassembly in Drosophila

Mitotic exit requires the dephosphorylation of many proteins whose phosphorylation was needed for mitosis. Protein phosphatase 2A with its B55 regulatory subunit (PP2A-B55) promotes this transition. However, the events and substrates that it regulates are incompletely understood. We used proteomic approaches in Drosophila to identify proteins that interact with and are dephosphorylated by PP2A-B55. Among several candidates, we identified emerin (otefin in Drosophila). Emerin resides in the inner nuclear membrane and interacts with the DNA-binding protein barrier-to-autointegration factor (BAF) via a LEM domain. We found that the phosphorylation of emerin at Ser50 and Ser54 near its LEM domain negatively regulates its association with BAF, lamin and additional emerin in mitosis. We show that dephosphorylation of emerin at these sites by PP2A-B55 determines the timing of nuclear envelope reformation. Genetic experiments indicate that this regulation is required during embryonic development. Phosphoregulation of the emerin-BAF complex formation by PP2A-B55 appears as a key event of mitotic exit that is likely conserved across species.

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

Dephosphorylation in nuclear reassembly after mitosis

In most animal cell types, the interphase nucleus is largely disassembled during mitotic entry. The nuclear envelope breaks down and chromosomes are compacted into separated masses. Chromatin organization is also mostly lost and kinetochores assemble on centromeres. Mitotic protein kinases play several roles in inducing these transformations by phosphorylating multiple effector proteins. In many of these events, the mechanistic consequences of phosphorylation have been characterized. In comparison, how the nucleus reassembles at the end of mitosis is less well understood in mechanistic terms. In recent years, much progress has been made in deciphering how dephosphorylation of several effector proteins promotes nuclear envelope reassembly, chromosome decondensation, kinetochore disassembly and interphase chromatin organization. The precise roles of protein phosphatases in this process, in particular of the PP1 and PP2A groups, are emerging. Moreover, how these enzymes are temporally and spatially regulated to ensure that nuclear reassembly progresses in a coordinated manner has been partly uncovered. This review provides a global view of nuclear reassembly with a focus on the roles of dephosphorylation events. It also identifies important open questions and proposes hypotheses.

Explaining Redundancy in CDK-Mediated Control of the Cell Cycle: Unifying the Continuum and Quantitative Models

In eukaryotes, cyclin-dependent kinases (CDKs) are required for the onset of DNA replication and mitosis, and distinct CDK–cyclin complexes are activated sequentially throughout the cell cycle. It is widely thought that specific complexes are required to traverse a point of commitment to the cell cycle in G1, and to promote S-phase and mitosis, respectively. Thus, according to a popular model that has dominated the field for decades, the inherent specificity of distinct CDK–cyclin complexes for different substrates at each phase of the cell cycle generates the correct order and timing of events. However, the results from the knockouts of genes encoding cyclins and CDKs do not support this model. An alternative “quantitative” model, validated by much recent work, suggests that it is the overall level of CDK activity (with the opposing input of phosphatases) that determines the timing and order of S-phase and mitosis. We take this model further by suggesting that the subdivision of the cell cycle into discrete phases (G0, G1, S, G2, and M) is outdated and problematic. Instead, we revive the “continuum” model of the cell cycle and propose that a combination with the quantitative model better defines a conceptual framework for understanding cell cycle control.

Protein phosphatase 2A holoenzymes regulate leucine-rich repeat kinase 2 phosphorylation and accumulation

LRRK2 is a highly phosphorylated multidomain protein and mutations in the gene encoding LRRK2 are a major genetic determinant of Parkinson's disease (PD). Dephosphorylation at LRRK2's S910/S935/S955/S973 phosphosite cluster is observed in several conditions including in sporadic PD brain, in several disease mutant forms of LRRK2 and after pharmacological LRRK2 kinase inhibition. However, the mechanism of LRRK2 dephosphorylation is poorly understood. We performed a phosphatome-wide reverse genetics screen to identify phosphatases involved in the dephosphorylation of the LRRK2 phosphosite S935. Candidate phosphatases selected from the primary screen were tested in mammalian cells, Xenopus oocytes and in vitro. Effects of PP2A on endogenous LRRK2 phosphorylation were examined via expression modulation with CRISPR/dCas9. Our screening revealed LRRK2 phosphorylation regulators linked to the PP1 and PP2A holoenzyme complexes as well as CDC25 phosphatases. We showed that dephosphorylation induced by different kinase inhibitor triggered relocalisation of phosphatases PP1 and PP2A in LRRK2 subcellular compartments in HEK-293 T cells. We also demonstrated that LRRK2 is an authentic substrate of PP2A both in vitro and in Xenopus oocytes. We singled out the PP2A holoenzyme PPP2CA:PPP2R2 as a powerful phosphoregulator of pS935-LRRK2. Furthermore, we demonstrated that this specific PP2A holoenzyme induces LRRK2 relocalization and triggers LRRK2 ubiquitination, suggesting its involvement in LRRK2 clearance. The identification of the PPP2CA:PPP2R2 complex regulating LRRK2 S910/S935/S955/S973 phosphorylation paves the way for studies refining PD therapeutic strategies that impact LRRK2 phosphorylation.

The adenoviral protein E4orf4: a probing tool to decipher mechanical stress-induced nuclear envelope remodeling in tumor cells

The human adenovirus (Ad) type 2/5 early region 4 (E4) ORF4 protein (E4orf4) exerts a remarkable tumor cell-selective killing activity in mammalian cells. This indicates that E4orf4 can target tumor cell-defining features and is a unique tool to probe cancer cell vulnerabilities. Recently, we found that E4orf4, through an interaction with the polarity protein PAR3, subverts nuclear envelope (NE) remodeling processes in a tumor cell-selective manner. In this Perspective, we outline mechanical signals that modify nuclear dynamics and tumor cell behavior to highlight potential mechanisms for E4orf4's tumoricidal activity. Through an analysis of E4orf4's cellular targets, we define a protein subnetwork that comprises phosphatase systems interconnected to polarity protein hubs, which could contribute to enhanced NE plasticity. We infer that elucidating E4orf4's protein network at a functional level could uncover key mechanisms of NE remodeling that define the tumor cell phenotype.

Antibodies recognizing the C terminus of PP2A catalytic subunit are unsuitable for evaluating PP2A activity and holoenzyme composition

The methyl-esterification of the C-terminal leucine of the protein phosphatase 2A (PP2A) catalytic (C) subunit is essential for the assembly of specific trimeric PP2A holoenzymes, and this region of the C subunit also contains two threonine and tyrosine phosphorylation sites. Most commercial antibodies-including the monoclonal antibody 1D6 that is part of a frequently used, commercial phosphatase assay kit-are directed toward the C terminus of the C subunit, raising questions as to their ability to recognize methylated and phosphorylated forms of the enzyme. Here, we tested several PP2A C antibodies, including monoclonal antibodies 1D6, 7A6, G-4, and 52F8 and the polyclonal antibody 2038 for their ability to specifically detect PP2A in its various modified forms, as well as to coprecipitate regulatory subunits. The tested antibodies preferentially recognized the nonmethylated form of the enzyme, and they did not coimmunoprecipitate trimeric holoenzymes containing the regulatory subunits B or B', an issue that precludes their use to monitor PP2A holoenzyme activity. Furthermore, some of the antibodies also recognized the phosphatase PP4, demonstrating a lack of specificity for PP2A. Together, these findings suggest that reinterpretation of the data generated by using these reagents is required.

Molecular effects of Microcystin-LA in tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is a freshwater phytoplanktivorous fish species reported to accumulate and tolerate large amounts of cyanotoxins such as microcystins (MCs). The present study aimed to investigate molecular responses to the acute exposure of Nile tilapia to the Microcystin-LA analogue (MC-LA). Thus, the specimens were sublethally exposed to 1000 μg kg^-1 of MC-LA for 12, 24, 48, and 96 h. Gene expression of PP1, PP2A, GST, GPX and actin was analyzed by quantitative PCR. The protein abundance profile of PP2A was determined by immunoblotting, while the integrity of its biological function was assessed by a phosphatase enzymatic assay. PP2A activity was significantly and strongly reduced by MC-LA. A resulting feedback mechanism significantly increased PP2A gene expression and protein abundance in all assessed times. However, a recovery of that phosphatase activity was not observed. In this study, the observed increase in GPX gene expression was the only response that could be directly related to the unknown factors associated to the fish survival to such high dose exposure.

Regulation of Cell Division

The challenging task of mitotic cell divisions is to generate two genetically identical daughter cells from a single precursor cell. To accomplish this task, a complex regulatory network evolved, which ensures that all events critical for the duplication of cellular contents and their subsequent segregation occur in the correct order, at specific intervals and with the highest possible fidelity. Transitions between cell cycle stages are triggered by changes in the phosphorylation state and levels of components of the cell cycle machinery. Entry into S-phase and M-phase are mediated by cyclin-dependent kinases (Cdks), serine-threonine kinases that require a regulatory cyclin subunit for their activity. Resetting the system to the interphase state is mediated by protein phosphatases (PPs) that counteract Cdks by dephosphorylating their substrates. To avoid futile cycles of phosphorylation and dephosphorylation, Cdks and PPs must be regulated in a manner such that their activities are mutually exclusive.

Long-term dynamics of multisite phosphorylation

A systematic framework for exploring the long-term dynamics of a reaction network is applied to a minimal model of ERK regulation that distinguishes both monophosphorylated forms and allows for nonzero enzyme processivity. Bistability and oscillations can be observed at high levels of processivity.

Multisite phosphorylation cycles are ubiquitous in cell regulation systems and are studied at multiple levels of complexity, from molecules to organisms, with the ultimate goal of establishing predictive understanding of the effects of genetic and pharmacological perturbations of protein phosphorylation in vivo. Achieving this goal is essentially impossible without mathematical models, which provide a systematic framework for exploring dynamic interactions of multiple network components. Most of the models studied to date do not discriminate between the distinct partially phosphorylated forms and focus on two limiting reaction regimes, distributive and processive, which differ in the number of enzyme–substrate binding events needed for complete phosphorylation or dephosphorylation. Here we use a minimal model of extracellular signal-related kinase regulation to explore the dynamics of a reaction network that includes all essential phosphorylation forms and arbitrary levels of reaction processivity. In addition to bistability, which has been studied extensively in distributive mechanisms, this network can generate periodic oscillations. Both bistability and oscillations can be realized at high levels of reaction processivity. Our work provides a general framework for systematic analysis of dynamics in multisite phosphorylation systems.

Fcp1 phosphatase controls Greatwall kinase to promote PP2A-B55 activation and mitotic progression

During cell division, progression through mitosis is driven by a protein phosphorylation wave. This wave namely depends on an activation-inactivation cycle of cyclin B-dependent kinase (Cdk) 1 while activities of major protein phosphatases, like PP1 and PP2A, appear directly or indirectly repressed by Cdk1. However, how Cdk1 inactivation is coordinated with reactivation of major phosphatases at mitosis exit still lacks substantial knowledge. We show here that activation of PP2A-B55, a major mitosis exit phosphatase, required the phosphatase Fcp1 downstream Cdk1 inactivation in human cells. During mitosis exit, Fcp1 bound Greatwall (Gwl), a Cdk1-stimulated kinase that phosphorylates Ensa/ARPP19 and converts these proteins into potent PP2A-B55 inhibitors during mitosis onset, and dephosphorylated it at Cdk1 phosphorylation sites. Fcp1-catalyzed dephosphorylation drastically reduced Gwl kinase activity towards Ensa/ARPP19 promoting PP2A-B55 activation. Thus, Fcp1 coordinates Cdk1 and Gwl inactivation to derepress PP2A-B55, generating a dephosphorylation switch that drives mitosis progression.

DOI:http://dx.doi.org/10.7554/eLife.10399.001

Cells multiply through a cell division cycle that has distinct phases. In a phase called mitosis, a cell splits its genetic material, which was duplicated in a preceding phase, into two identical sets. Each of these sets will form the genetic material of daughter cells. If this process goes wrong, then cells can die or become cancerous, and so cells have evolved a complex regulatory process to ensure that mitosis begins and ends at the correct time.

For mitosis to begin, an enzyme adds tags called phosphate groups to hundreds of target proteins. These phosphate groups are then removed again to end mitosis. PP2A-B55 is an enzyme that removes these phosphate groups and is needed to complete mitosis, but must remain inactive before this point. This inactivation occurs because a protein called Greatwall activates two other proteins that inhibit PP2A-B55. To reactivate PP2A-B55 at the end of mitosis, Greatwall must be inactivated, but it was not known how cells do this.

Della Monica, Visconti et al. have now investigated this process in human cells. The experiments show that towards the end of mitosis, another enzyme called Fcp1 inactivates Greatwall by removing phosphate groups from it. This allows PP2A-B55 to reactivate.

These studies reveal that Fcp1 is a key factor that is needed to complete mitosis. The next challenge is to determine how Fcp1 activity is regulated at the end of mitosis.

DOI:http://dx.doi.org/10.7554/eLife.10399.002

Interphase H1 phosphorylation: Regulation and functions in chromatin

Many metazoan cell types differentially express multiple non-allelic amino acid sequence variants of histone H1. Although early work revealed that H1 variants, collectively, are phosphorylated during interphase and mitosis, differences between individual H1 variants in the sites they possess for mitotic and interphase phosphorylation have been elucidated only relatively recently. Here, we review current knowledge on the regulation and function of interphase H1 phosphorylation, with a particular emphasis on how differences in interphase phosphorylation among the H1 variants of mammalian cells may enable them to have differential effects on transcription and other chromatin processes.

Protein phosphatase 1 is essential for Greatwall inactivation at mitotic exit

Entry into mitosis is mediated by the phosphorylation of key cell cycle regulators by cyclin-dependent kinase 1 (Cdk1). In Xenopus embryos, the M-phase-promoting activity of Cdk1 is antagonized by protein phosphatase PP2A-B55. Hence, to ensure robust cell cycle transitions, Cdk1 and PP2A-B55 must be regulated so that their activities are mutually exclusive. The mechanism underlying PP2A-B55 inactivation at mitotic entry is well understood: Cdk1-activated Greatwall (Gwl) kinase phosphorylates Ensa/Arpp19, thereby enabling them to bind to and inhibit PP2A-B55. However, the re-activation of PP2A-B55 during mitotic exit, which is essential for cell cycle progression, is less well understood. Here, we identify protein phosphatase PP1 as an essential component of the PP2A-B55 re-activation pathway in Xenopus embryo extracts. PP1 initiates the re-activation of PP2A-B55 by dephosphorylating Gwl. We provide evidence that PP1 targets the auto-phosphorylation site of Gwl, resulting in efficient Gwl inactivation. This step is necessary to facilitate subsequent complete dephosphorylation of Gwl by PP2A-B55. Thus, by identifying PP1 as the phosphatase initiating Gwl inactivation, our study provides the molecular explanation for how Cdk1 inactivation is coupled to PP2A-B55 re-activation at mitotic exit.

The Human Adenovirus Type 5 E4orf4 Protein Targets Two Phosphatase Regulators of the Hippo Signaling Pathway

When expressed alone at high levels, the human adenovirus E4orf4 protein exhibits tumor cell-specific p53-independent toxicity. A major E4orf4 target is the B55 class of PP2A regulatory subunits, and we have shown recently that binding of E4orf4 inhibits PP2A(B55) phosphatase activity in a dose-dependent fashion by preventing access of substrates (M. Z. Mui et al., PLoS Pathog 9:e1003742, 2013, http://dx.doi.org/10.1371/journal.ppat.1003742). While interaction with B55 subunits is essential for toxicity, E4orf4 mutants exist that, despite binding B55 at high levels, are defective in cell killing, suggesting that other essential targets exist. In an attempt to identify additional targets, we undertook a proteomics approach to characterize E4orf4-interacting proteins. Our findings indicated that, in addition to PP2A(B55) subunits, ASPP-PP1 complex subunits were found among the major E4orf4-binding species. Both the PP2A and ASPP-PP1 phosphatases are known to positively regulate effectors of the Hippo signaling pathway, which controls the expression of cell growth/survival genes by dephosphorylating the YAP transcriptional coactivator. We find here that expression of E4orf4 results in hyperphosphorylation of YAP, suggesting that Hippo signaling is affected by E4orf4 interactions with PP2A(B55) and/or ASPP-PP1 phosphatases. Furthermore, knockdown of YAP1 expression was seen to enhance E4orf4 killing, again consistent with a link between E4orf4 toxicity and inhibition of the Hippo pathway. This effect may in fact contribute to the cancer cell specificity of E4orf4 toxicity, as many human cancer cells rely heavily on the Hippo pathway for their enhanced proliferation.

IMPORTANCE

The human adenovirus E4orf4 protein has been known for some time to induce tumor cell-specific death when expressed at high levels; thus, knowledge of its mode of action could be of importance for development of new cancer therapies. Although the B55 form of the phosphatase PP2A has long been known as an essential E4orf4 target, genetic analyses indicated that others must exist. To identify additional E4orf4 targets, we performed, for the first time, a large-scale affinity purification/mass spectrometry analysis of E4orf4 binding partners. Several additional candidates were detected, including key regulators of the Hippo signaling pathway, which enhances cell viability in many cancers, and results of preliminary studies suggested a link between inhibition of Hippo signaling and E4orf4 toxicity.

The Greatwall-PP2A axis in cell cycle control

Cell cycle progression is largely controlled by reversible protein phosphorylation mediated by cyclically activated kinases and phosphatases. It has long been known that cyclin B-Cdk1 activation triggers mitotic entry, and the enzymatic network controlling its activation and inactivation has been well characterized. Much more recently protein phosphatase 2A (PP2A) together with its B55 regulatory subunit has been recognized as the major activity dephosphorylating Cdk1 targets. Moreover, PP2A-B55 activity is high in late M phase and interphase, but low at mitotic entry. A series of discoveries in the fly and frog model systems have uncovered the molecular mechanism mediating this regulation. The Greatwall (Gwl) kinase activates endosulfines, which become specific inhibitors of PP2A-B55. Cdk1-dependent activation of Gwl at mitotic entry leads to PP2A-B55 downregulation, which synergizes with Cdk1 activation to promote the phosphorylated states of several mitotic substrates. Much less is known on the mechanisms inactivating Gwl and endosulfines at mitotic exit. Recent reports show the importance of spatiotemporal regulation of Gwl, endosulfines, and PP2A-B55 for cell cycle progression. The various systems and cell types differ in their dependence on the Gwl-PP2A axis for cell cycle progression. Moreover, this pathway also regulates gene expression in yeast, and this function could be conserved in metazoans.

Identification of the adenovirus E4orf4 protein binding site on the B55α and Cdc55 regulatory subunits of PP2A: Implications for PP2A function, tumor cell killing and viral replication

Adenovirus E4orf4 protein induces the death of human cancer cells and Saccharomyces cerevisiae. Binding of E4orf4 to the B/B55/Cdc55 regulatory subunit of protein phosphatase 2A (PP2A) is required, and such binding inhibits PP2A^B55 activity leading to dose-dependent cell death. We found that E4orf4 binds across the putative substrate binding groove predicted from the crystal structure of B55α such that the substrate p107 can no longer interact with PP2A^B55α. We propose that E4orf4 inhibits PP2A^B55 activity by preventing access of substrates and that at high E4orf4 levels this inhibition results in cell death through the failure to dephosphorylate substrates required for cell cycle progression. However, E4orf4 is expressed at much lower and less toxic levels during a normal adenovirus infection. We suggest that in this context E4orf4 largely serves to recruit novel substrates such as ASF/SF2/SRSF1 to PP2A^B55 to enhance adenovirus replication. Thus E4orf4 toxicity probably represents an artifact of overexpression and does not reflect the evolutionary function of this viral product.

The adenovirus E4orf4 protein when expressed alone at high levels induces the death of human cancer cells but not normal primary cells. It also is toxic in the yeast Saccharomyces cerevisiae, which we have used as a model system in some studies. Toxicity induced by the E4orf4 protein is largely dependent on its ability to associate with the highly conserved B/B55/Cdc55 class of regulatory subunits of protein phosphatase 2A (PP2A), of which the mammalian B55α species is best characterized structurally. We showed previously that binding to B55α appears to inhibit PP2A activity against at least some substrates. In the present study, we mapped the E4orf4 binding site on both yeast Cdc55 and mammalian B55α and propose how such binding may inhibit PP2A activity. The implications of E4orf4 binding on PP2A activity are of significant scientific interest in terms of the process by which PP2A recognizes and dephosphorylates its substrates. We also propose that E4orf4 binding in the context of viral replication serves the quite different function of introducing novel substrates for dephosphorylation by the PP2A holoenzyme.

Protein phosphatase 2A activity is required for functional adherent junctions in endothelial cells

Reversible Ser/Thr phosphorylation of cytoskeletal and adherent junction (AJ) proteins has a critical role in the regulation of endothelial cell (EC) barrier function. We have demonstrated earlier that protein phosphatase 2A (PP2A) activity is important in EC barrier integrity. In the present work, macro- and microvascular EC were examined and we provided further evidence on the significance of PP2A in the maintenance of EC cytoskeleton and barrier function with special focus on the Bα (regulatory) subunit of PP2A. Immunofluorescent staining revealed that the inhibition of PP2A results in changes in the organization of EC cytoskeleton as microtubule dissolution and actin re-arrangement were detected. Depletion of Bα regulatory subunit of PP2A had similar effect on the cytoskeleton structure of the cells. Furthermore, transendothelial electric resistance measurements demonstrated significantly slower barrier recovery of Bα depleted EC after thrombin treatment. AJ proteins, VE-cadherin and β-catenin, were detected along with Bα in pull-down assay. Also, the inhibition of PP2A (by okadaic acid or fostriecin) or depletion of Bα caused β-catenin translocation from the membrane to the cytoplasm in parallel with its phosphorylation on Ser552. In conclusion, our data suggest that the A/Bα/C holoenzyme form of PP2A is essential in EC barrier integrity both in micro- and macrovascular EC.

On the regulation of protein phosphatase 2A and its role in controlling entry into and exit from mitosis

The process of mitosis involves a comprehensive reorganization of the cell: chromosomes condense, the nuclear envelope breaks down, the mitotic spindle is assembled, cells round up and release their ties to the substrate and so on and so forth. This reorganization is triggered by the activation of the protein kinase, Cyclin-Dependent Kinase 1 (CDK1). The end of mitosis is marked by the proteolysis of the cyclin subunit of CDK1, which terminates kinase activity. At this point, the phosphate moieties that altered the properties of hundreds of proteins to bring about the cellular reorganization are removed by protein phosphatases. At least one protein phosphatase, PP2A-B55, is completely shut off in mitosis. Depletion of this particular form of PP2A accelerates entry into mitosis, and blocks exit from mitosis. Control of this phosphatase is achieved by an inhibitor protein (α-endosulfine or ARPP-19) that becomes inhibitory when phosphorylated by a protein kinase called Greatwall, which is itself a substrate of CDK1. Failure to inhibit PP2A-B55 causes arrest of the cell cycle in G2 phase. I will discuss the role of this control mechanism in the control of mitosis.

Protein phosphatase 2a (PP2A) binds within the oligomerization domain of striatin and regulates the phosphorylation and activation of the mammalian Ste20-Like kinase Mst3

Background

Striatin, a putative protein phosphatase 2A (PP2A) B-type regulatory subunit, is a multi-domain scaffolding protein that has recently been linked to several diseases including cerebral cavernous malformation (CCM), which causes symptoms ranging from headaches to stroke. Striatin association with the PP2A A/C (structural subunit/catalytic subunit) heterodimer alters PP2A substrate specificity, but targets and roles of striatin-associated PP2A are not known. In addition to binding the PP2A A/C heterodimer to form a PP2A holoenzyme, striatin associates with cerebral cavernous malformation 3 (CCM3) protein, the mammalian Mps one binder (MOB) homolog, Mob3/phocein, the mammalian sterile 20-like (Mst) kinases, Mst3, Mst4 and STK25, and several other proteins to form a large signaling complex. Little is known about the molecular architecture of the striatin complex and the regulation of these sterile 20-like kinases.

Results

To help define the molecular organization of striatin complexes and to determine whether Mst3 might be negatively regulated by striatin-associated PP2A, a structure-function analysis of striatin was performed. Two distinct regions of striatin are capable of stably binding directly or indirectly to Mob3--one N-terminal, including the coiled-coil domain, and another more C-terminal, including the WD-repeat domain. In addition, striatin residues 191-344 contain determinants necessary for efficient association of Mst3, Mst4, and CCM3. PP2A associates with the coiled-coil domain of striatin, but unlike Mob3 and Mst3, its binding appears to require striatin oligomerization. Deletion of the caveolin-binding domain on striatin abolishes striatin family oligomerization and PP2A binding. Point mutations in striatin that disrupt PP2A association cause hyperphosphorylation and activation of striatin-associated Mst3.

Conclusions

Striatin orchestrates the regulation of Mst3 by PP2A. It binds Mst3 likely as a dimer with CCM3 via residues lying between striatin's calmodulin-binding and WD-domains and recruits the PP2A A/C heterodimer to its coiled-coil/oligomerization domain. Residues outside the previously reported coiled-coil domain of striatin are necessary for its oligomerization. Striatin-associated PP2A is critical for Mst3 dephosphorylation and inactivation. Upon inhibition of PP2A, Mst3 activation appears to involve autophosphorylation of multiple activation loop phosphorylation sites. Mob3 can associate with striatin sequences C-terminal to the Mst3 binding site but also with sequences proximal to striatin-associated PP2A, consistent with a possible role for Mob 3 in the regulation of Mst3 by PP2A.

Phosphatases: providing safe passage through mitotic exit

The mitosis-to-interphase transition involves dramatic cellular reorganization from a state that supports chromosome segregation to a state that complies with all functions of an interphase cell. This process, termed mitotic exit, depends on the removal of mitotic phosphorylations from a broad range of substrates. Mitotic exit regulation involves inactivation of mitotic kinases and activation of counteracting protein phosphatases. The key mitotic exit phosphatase in budding yeast, Cdc14, is now well understood. By contrast, in animal cells, it is now emerging that mitotic exit relies on distinct regulatory networks, including the protein phosphatases PP1 and PP2A.

Regulation of Greatwall kinase during Xenopus oocyte maturation

Greatwall kinase is required for M phase maintenance by inhibiting PP2A. Gwl associates with PP2A in G2 oocytes, but the complex dissociates during M phase (meiosis I). Mutating Lys71 to Met (K71M) generates gain-of-function Gwl kinase activity toward endosulfinethat is sufficient to induce oocyte maturation in the absence of progesterone.

Greatwall kinase has been identified as a key element in M phase initiation and maintenance in Drosophila, Xenopus oocytes/eggs, and mammalian cells. In M phase, Greatwall phosphorylates endosulfine and related proteins that bind to and inhibit protein phosphatase 2A/B55, the principal phosphatase for Cdk-phosphorylated substrates. We show that Greatwall binds active PP2A/B55 in G2 phase oocytes but dissociates from it when progesterone-treated oocytes reach M phase. This dissociation does not require Greatwall kinase activity or phosphorylation at T748 in the presumptive T loop of the kinase. A mutant K71M Greatwall, also known as Scant in Drosophila, induces M phase in the absence of progesterone when expressed in oocytes, despite its reduced stability and elevated degradation by the proteasome. M phase induction by Scant Greatwall requires protein synthesis but is not associated with altered binding or release of PP2A/B55 as compared to wild-type Greatwall. However, in vitro studies with Greatwall proteins purified from interphase cells indicate that Scant, but not wild-type Greatwall, has low but detectable activity against endosulfine. These results demonstrate progesterone-dependent regulation of the PP2A/B55–Greatwall interaction during oocyte maturation and suggest that the cognate Scant Greatwall mutation has sufficient constitutive kinase activity to promote M phase in Xenopus oocytes.

Upregulation of miR-27a contributes to the malignant transformation of human bronchial epithelial cells induced by SV40 small T antigen

The introduction of the Simian virus 40 (SV40) early region, the telomerase catalytic subunit (hTERT) and an oncogenic allele of H-Ras directly transforms primary human cells. SV40 small T antigen (ST), which forms a complex with protein phosphatase 2A (PP2A) and inhibits PP2A activity, is believed to have a critical role in the malignant transformation of human cells. Recent evidence has shown that aberrant microRNA (miRNA) expression patterns are correlated with cancer development. Here, we identified miR-27a as a differentially expressed miRNA in SV40 ST-expressing cells. miR-27a is upregulated in SV40 ST-transformed human bronchial epithelial cells (HBERST). Suppression of miR-27a expression in HBERST cells or lung cancer cell lines (NCI-H226 and SK-MES-1) that exhibited high levels of miR-27a expression lead to cell growth arrested in the G(0)-G(1) phase. In addition, suppression of miR-27a in HBERST cells attenuated the capacity of such cells to grow in an anchorage-independent manner. We also found that suppression of the PP2A B56γ expression resulted in upregulation of miR-27a similar to that achieved by the introduction of ST, indicating that dysregulation of miR-27a expression in ST-expressing cells was mediated by the ST-PP2A interaction. Moreover, we discovered that Fbxw7 gene encoding F-box/WD repeat-containing protein 7 was a potential miR-27a target validated by dual-luciferase reporter system analysis. The inverse correlation between miR-27a expression levels and Fbxw7 protein expression was further confirmed in both cell models and human tumor samples. Fbxw7 regulates cell-cycle progression through the ubiquitin-dependent proteolysis of a set of substrates, including c-Myc, c-Jun, cyclin E1 and Notch 1. Thus, promotion of cell growth arising from the suppression of Fbxw7 by miR-27a overexpression might be responsible for the viral oncoprotein ST-induced malignant transformation. These observations demonstrate that miR-27a functions as an oncogene in human tumorigenesis.

Greatwall kinase, ARPP-19 and protein phosphatase 2A: shifting the mitosis paradigm

Control of entry into mitosis has long been seen in terms of an explosive activation of cyclin-dependent kinase 1, the mitotic driver ensuring the phosphorylation of hundreds of proteins required for cell division. However, if these phosphorylations are maintained during M-phase, they must be removed when cells exit mitosis. It has been surmised that an "antimitotic" phosphatase must be inhibited to allow mitosis entry and activated for returning to interphase. This chapter discusses a series of recent works conducted on Xenopus egg extracts that provide the answers regarding the identity and the regulation of such a phosphatase. PP2A-B55δ is the major phosphatase controlling exit from mitosis; it is negatively regulated by the kinase Greatwall that phosphorylates the small protein ARPP-19 and converts it into a potent PP2A inhibitor. These findings provide a new element of paramount importance in the control of mitosis.

Molecular Cloning of the Genes Encoding the PR55/Bβ/δ Regulatory Subunits for PP-2A and Analysis of Their Functions in Regulating Development of Goldfish, Carassius auratus

The protein phosphatase-2A (PP-2A), one of the major phosphatases in eukaryotes, is a heterotrimer, consisting of a scaffold A subunit, a catalytic C subunit and a regulatory B subunit. Previous studies have shown that besides regulating specific PP-2A activity, various B subunits encoded by more than 16 different genes, may have other functions. To explore the possible roles of the regulatory subunits of PP-2A in vertebrate development, we have cloned the PR55/B family regulatory subunits: β and δ, analyzed their tissue specific and developmental expression patterns in Goldfish ( Carassius auratus). Our results revealed that the full-length cDNA for PR55/Bβ consists of 1940 bp with an open reading frame of 1332 nucleotides coding for a deduced protein of 443 amino acids. The full length PR55/Bδ cDNA is 2163 bp containing an open reading frame of 1347 nucleotides encoding a deduced protein of 448 amino acids. The two isoforms of PR55/B display high levels of sequence identity with their counterparts in other species. The PR55/Bβ mRNA and protein are detected in brain and heart. In contrast, the PR55/Bδ is expressed in all 9 tissues examined at both mRNA and protein levels. During development of goldfish, the mRNAs for PR55/Bβ and PR55/Bδ show distinct patterns. At the protein level, PR55/Bδ is expressed at all developmental stages examined, suggesting its important role in regulating goldfish development. Expression of the PR55/Bδ anti-sense RNA leads to significant downregulation of PR55/Bδ proteins and caused severe abnormality in goldfish trunk and eye development. Together, our results suggested that PR55/Bδ plays an important role in governing normal trunk and eye formation during goldfish development.

Genetic analysis of B55alpha/Cdc55 protein phosphatase 2A subunits: association with the adenovirus E4orf4 protein

The human adenovirus E4orf4 protein is toxic in both human tumor cells and Saccharomyces cerevisiae. Previous studies indicated that most of this toxicity is dependent on an interaction of E4orf4 protein with the B55 class of regulatory subunits of protein phosphatase 2A (PP2A) and in yeast with the B55 homolog Cdc55. We have found previously that E4orf4 inhibits PP2A activity against at least some substrates. In an attempt to understand the mechanism of this inhibition, we used a genetic approach to identify residues in the seven-bladed β-propeller proteins B55α and Cdc55 required for E4orf4 binding. In both cases, amino-terminal polypeptides composed only of blade 1 and at least part of blade 2 were found to bind E4orf4 and overexpression blocked E4orf4 toxicity in yeast. Furthermore, certain amino acid substitutions in blades 1 and 2 within full-length B55α and Cdc55 resulted in loss of E4orf4 binding. Recent mutational analysis has suggested that segments of blades 1 and 2 present on the top face of B55α form part of the "substrate-binding groove." Additionally, these segments are in close proximity to the catalytic C subunit of the PP2A holoenzyme. Thus, our results are consistent with the hypothesis that E4orf4 binding could affect the access of substrates, resulting in the failure to dephosphorylate some PP2A substrates.

A specific form of phospho protein phosphatase 2 regulates anaphase-promoting complex/cyclosome association with spindle poles

In early mitosis, the END (Emi1/NuMA/Dynein-dynactin) network anchors the anaphase-promoting complex/cyclosome (APC/C) to the mitotic spindle and poles. Spindle anchoring restricts APC/C activity, thereby limiting the destruction of spindle-associated cyclin B and ensuring maintenance of spindle integrity. Emi1 binds directly to hypophosphorylated APC/C, linking the APC/C to the spindle via NuMA. However, whether the phosphorylation state of the APC/C is important for its association with the spindle and what kinases and phosphatases are necessary for regulating this event remain unknown. Here, we describe the regulation of APC/C-mitotic spindle pole association by phosphorylation. We find that only hypophosphorylated APC/C associates with microtubule asters, suggesting that phosphatases are important. Indeed, a specific form of PPP2 (CA/R1A/R2B) binds APC/C, and PPP2 activity is necessary for Cdc27 dephosphorylation. Screening by RNA interference, we find that inactivation of CA, R1A, or R2B leads to delocalization of APC/C from spindle poles, early mitotic spindle defects, a failure to congress chromosomes, and decreased levels of cyclin B on the spindle. Consistently, inhibition of cyclin B/Cdk1 activity increased APC/C binding to microtubules. Thus, cyclin B/Cdk1 and PPP2 regulate the dynamic association of APC/C with spindle poles in early mitosis, a step necessary for proper spindle formation.

The M phase kinase Greatwall (Gwl) promotes inactivation of PP2A/B55delta, a phosphatase directed against CDK phosphosites

We have previously shown that Greatwall kinase (Gwl) is required for M phase entry and maintenance in Xenopus egg extracts. Here, we demonstrate that Gwl plays a crucial role in a novel biochemical pathway that inactivates, specifically during M phase, "antimitotic" phosphatases directed against phosphorylations catalyzed by cyclin-dependent kinases (CDKs). A major component of this phosphatase activity is heterotrimeric PP2A containing the B55delta regulatory subunit. Gwl is activated during M phase by Cdk1/cyclin B (MPF), but once activated, Gwl promotes PP2A/B55delta inhibition with no further requirement for MPF. In the absence of Gwl, PP2A/B55delta remains active even when MPF levels are high. The removal of PP2A/B55delta corrects the inability of Gwl-depleted extracts to enter M phase. These findings support the hypothesis that M phase requires not only high levels of MPF function, but also the suppression, through a Gwl-dependent mechanism, of phosphatase(s) that would otherwise remove MPF-driven phosphorylations.

Regulated activity of PP2A-B55 delta is crucial for controlling entry into and exit from mitosis in Xenopus egg extracts

Entry into mitosis depends on the activity of cyclin-dependent kinases (CDKs). Conversely, exit from mitosis occurs when mitotic cyclins are degraded, thereby extinguishing CDK activity. Exit from mitosis must also require mitotic phosphoproteins to revert to their interphase hypophosphorylated forms, but there is a controversy about which phosphatase(s) is/are responsible for dephosphorylating the CDK substrates. We find that PP2A associated with a B55 delta subunit is relatively specific for a model mitotic CDK substrate in Xenopus egg extracts. The phosphatase activity measured by this substrate is regulated during the cell cycle--high in interphase and suppressed during mitosis. Depletion of PP2A-B55 delta (in interphase) from 'cycling' frog egg extracts accelerated their entry into mitosis and kept them indefinitely in mitosis. When PP2A-B55 delta was depleted from mitotic extracts, however, exit from mitosis was hardly delayed, showing that other phosphatase(s) are also required for mitotic exit. Increasing the concentration of PP2A-B55 delta in extracts by adding recombinant enzyme inhibited the entry into mitosis. This form of PP2A seems to be a key regulator of entry into and exit from mitosis.

Involvement of PP2A in viral and cellular transformation

Although the small DNA tumor virus SV40 (simian virus 40) fails to replicate in human cells, understanding how SV40 transforms human and murine cells has and continues to provide important insights into cancer initiation and maintenance. The early region of SV40 encodes two oncoproteins: the large T (LT) and small t (ST) antigens. SV40 LT contributes to murine and human cell transformation in part by inactivating the p53 and retinoblastoma protein tumor suppressor proteins. SV40 ST inhibits the activity of the protein phosphatase 2A (PP2A) family of serine-threonine phosphatases, and this interaction is required for SV40-mediated transformation of human cells. PP2A regulates multiple signaling pathways, suggesting many possible targets important for viral replication and cell transformation. Genetic manipulation of particular PP2A subunits has confirmed a role for specific complexes in transformation, and recent work implicates the perturbation of the phosphatidylinositol 3-kinase/Akt pathway and c-Myc stability in transformation by ST and PP2A. Mutations in PP2A subunits occur at low frequency in human tumors, suggesting that alterations of PP2A signaling play a role in both experimentally induced and spontaneously arising cancers. Unraveling the complexity of PP2A signaling will not only provide further insights into cancer development but may identify novel targets with promise for therapeutic manipulation.

Downregulation of protein phosphatase 2A activity in HeLa cells at the G2-mitosis transition and unscheduled reactivation induced by 12-O-tetradecanoyl phorbol-13-acetate (TPA)

In the cell cycle the transition from G2 phase to cell division (M) is strictly controlled by protein phosphorylation-dephosphorylation reactions effected by several protein kinases and phosphatases. Although much indirect and direct evidence point to a key role of protein phosphatase 2A (PP2A) at the G2/M transition, the control of the enzyme activity prior to and after the transition are not fully clarified. Using synchronized HeLa cells we determined the PP2A activity (i.e. the increment sensitive to inhibition by 2nM okadaic acid) in immunoprecipitates obtained with antibodies raised against a conserved peptide sequence (residues 169-182, Ab(169/182)) of the PP2A catalytic subunit (PP2A C). Two different substrates were offered: the phospho-peptide KR(p)TIRR and histone H1 phosphorylated by means of the cyclin-dependent protein kinase p34(cdc2). The results indicate that in HeLa cells the specific activity of PP2A towards both substrates goes through a minimum in late G2 phase and stays low until metaphase. Treatment of G2 cells with TPA (10(-7) M) caused a reactivation of the downregulated PP2A activity within 20 min, i.e. the same time frame within which TPA was shown earlier to block HeLa cells at the transition from G2 to mitosis [Kinzel et al., 1988. Cancer Res. 48, 1759-1762]. Activation of PP2A was also induced by TPA in mitotic cells. The low activity of PP2A in mitotic cells was accompanied by a strong reaction of mitotic PP2A C with anti-P-Tyr antibodies in Western blots, which was reversed by treatment of mitotic cells with TPA. The results suggest that the activity of cellular PP2A requires downregulation for the transition from G2 phase to mitosis. Unscheduled reactivation of PP2A induced by TPA in late G2 phase appears to inhibit the progress into mitosis.

Identification of specific PP2A complexes involved in human cell transformation

The SV40 small t antigen (ST) interacts with the serine-threonine protein phosphatase 2A (PP2A). To investigate the role of this interaction in transformation, we suppressed the expression of the PP2A B56gamma subunit in human embryonic kidney (HEK) epithelial cells expressing SV40 large T antigen, hTERT, and H-RAS. Suppression of PP2A B56gamma expression inhibited PP2A-specific phosphatase activity similar to that achieved by ST and conferred the ability to grow in an anchorage-independent fashion and to form tumors. Overexpression of PP2A B56gamma3 in tumorigenic HEK cells expressing ST or human lung cancer cell lines partially reversed the tumorigenicity of these cells. These observations identify specific PP2A complexes involved in human cell transformation.

Cyclin G2 associates with protein phosphatase 2A catalytic and regulatory B' subunits in active complexes and induces nuclear aberrations and a G1/S phase cell cycle arrest

Cyclin G2, together with cyclin G1 and cyclin I, defines a novel cyclin family expressed in terminally differentiated tissues including brain and muscle. Cyclin G2 expression is up-regulated as cells undergo cell cycle arrest or apoptosis in response to inhibitory stimuli independent of p53 (Horne, M., Donaldson, K., Goolsby, G., Tran, D., Mulheisen, M., Hell, J. and Wahl, A. (1997) J. Biol. Chem. 272, 12650-12661). We tested the hypothesis that cyclin G2 may be a negative regulator of cell cycle progression and found that ectopic expression of cyclin G2 induces the formation of aberrant nuclei and cell cycle arrest in HEK293 and Chinese hamster ovary cells. Cyclin G2 is primarily partitioned to a detergent-resistant compartment, suggesting an association with cytoskeletal elements. We determined that cyclin G2 and its homolog cyclin G1 directly interact with the catalytic subunit of protein phosphatase 2A (PP2A). An okadaic acid-sensitive (<2 nm) phosphatase activity coprecipitates with endogenous and ectopic cyclin G2. We found that cyclin G2 also associates with various PP2A B' regulatory subunits, as previously shown for cyclin G1. The PP2A/A subunit is not detectable in cyclin G2-PP2A-B'-C complexes. Notably, cyclin G2 colocalizes with both PP2A/C and B' subunits in detergent-resistant cellular compartments, suggesting that these complexes form in living cells. The ability of cyclin G2 to inhibit cell cycle progression correlates with its ability to bind PP2A/B' and C subunits. Together, our findings suggest that cyclin G2-PP2A complexes inhibit cell cycle progression.

A novel cAMP-stimulated pathway in protein phosphatase 2A activation

Elevated cAMP in NRK-52E and L6 cells causes a marked reduction in the phosphorylation of numerous phosphoproteins, as detected initially with phosphoserine-specific antibodies. Here, we show that elevation of cAMP in NRK cells by forskolin/3-isobutyl-1-methylxanthine (IBMX) treatment decreased phosphorylation of substrates for different protein kinases, pointing to a common protein phosphatase as a target for cAMP-dependent regulation. Forskolin/IBMX treatment completely dephosphorylated a selective protein phosphatase 2A (PP2A) substrate, elongation factor-2 (EF-2), at its Ca(2+) calmodulin-dependent kinase site, and decreased phosphorylation of substrates for cyclin-dependent kinases, including retinoblastoma (Rb) protein. As reported before, forskolin/IBMX also decreased phosphorylation of a protein kinase C substrate, the Na,K-ATPase. The cAMP-stimulated dephosphorylation was blocked by the protein phosphatases 1 (PP1) and PP2A inhibitor okadaic acid at concentrations selective for PP2A but was not blocked by tautomycin at concentrations selective for PP1. The data implicate PP2A as a cAMP-activated phosphatase. Contrary to expectation, we found evidence that cAMP-dependent activation of PP2A did not depend on protein kinase A (PKA). Pretreatment of cells with the PKA inhibitor H89 abolished PKA activity measured in cell extracts and significantly decreased cAMP-activated phosphorylation of a known PKA substrate, ARPP-19, in cells, but failed to block the cAMP-stimulated dephosphorylation of EF-2, Rb, and other proteins. This novel pathway of PP2A activation, acting on the time scale of minutes, represents yet another example of a cAMP-mediated, PKA-independent signaling mechanism. Because PP2A is active toward a variety of endogenous substrates, cAMP-stimulated dephosphorylation may have complicated the interpretation of many prior studies.

Protein phosphatases 1 and 2A transiently associate with myosin during the peak rate of secretion from mast cells

Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II, are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5-fold) in these proteins. Phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the same changes, at a time coincident with its slower rate of secretion. Coimmunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements, and led to increased phosphorylation of the myosin heavy and light chains at protein kinase C-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A, is required for mast cell secretion.

The role of adenovirus E4orf4 protein in viral replication and cell killing

It has only been within the last few years that insights have been gained into the remarkable diversity of functions of the adenovirus early transcription region 4 (E4) products. The polypeptide encoded by E4 open reading frame 4 (E4orf4) has emerged as an enigmatic product. Although it accomplishes certain functions that propel viral replication, it has also been shown to be highly toxic, an effect that could dampen the infectious cycle, but that also might serve to facilitate release of viral progeny. When expressed alone, E4orf4 induces a novel form of p53-independent apoptosis in cancer cells but not in normal human cells, thus making it of potential use in cancer gene therapy. In addition, knowledge of its mechanism of action, especially with regard to its interaction with protein phosphatase 2A (PP2A), could provide insights to develop new small molecule anti-cancer drugs. Thus future studies on E4orf4 should be both informative and potentially valuable therapeutically. In this study we review the current status of knowledge on E4orf4.

The linkage between beta1 integrin and the actin cytoskeleton is differentially regulated by tyrosine and serine/threonine phosphorylation of beta1 integrin in normal and cancerous human breast cells

BACKGROUND

Structural requirements for the beta1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts. In this study, we examined the reason for the reduced surface expression of beta1 integrin in human breast cancer MCF-7 cells compared to normal human breast epithelial (HBE) cells, both of which adhered to collagen type IV.

RESULTS

The beta1 integrin immunoprecipitates from either HBE or MCF-7 cells involved alpha-actinin while actin coprecipitated with beta1 integrin from HBE cells but not from MCF-7 cells. Immunoblotting using the anti-phosphotyrosine (PY) antibody indicated the phosphorylation of beta1 integrin at least at tyrosine in both cells. Dephosphorylation of beta1 integrin from HBE cells by protein tyrosine phosphatase (PTP), but not by protein serine/threonine phosphatase (PP), caused dissociation of actin from beta1 integrin, although dephosphorylation of it from MCF-7 cells by either PTP or PP caused association of the two proteins. In MCF-7 cells beta1 integrin coprecipitated doublet of proteins having the Ca2+/calmodulin-dependent protein kinase (CaMK) II activity that was susceptible to KN-62, a specific inhibitor of CaMKII.

CONCLUSION

The results suggest that beta1 integrin is tyrosine phosphorylated and links with actin via alpha-actinin in HBE cells but prevented from linking with actin in MCF-7 cells by phosphorylation at both tyrosine and serine/threonine of beta1 integrin which forms a complex with alpha-actinin and CaMKII. Thus the linkage formation of beta1 integrin with actin may be differentially regulated by its tyrosine and serine/threonine phosphorylation in normal HBE cells and breast cancer MCF-7 cells.

Toxicity of human adenovirus E4orf4 protein in Saccharomyces cerevisiae results from interactions with the Cdc55 regulatory B subunit of PP2A

The E4orf4 protein of human adenovirus induces p53-independent apoptosis, a process that may promote cell death and viral spread. When expressed alone, E4orf4 kills transformed cells but not normal human cells. The only clear target of E4orf4 in mammalian cells is the Balpha (B55) subunit of protein phosphatase 2A (PP2A), a member of one of three classes of regulatory B subunits. Here we report the effects of E4orf4 in Saccharomyces cerevisiae, which encodes two PP2A regulatory B subunits, CDC55 and RTS1, that share homology with mammalian B and B' subunits, respectively. E4orf4 expression was found to be toxic in yeast, resulting in the accumulation of cells in G2/M phase that failed to grow upon removal of E4orf4. E4orf4-expressing yeast also displayed an elongated cell morphology similar to cdc55 deletion strains. E4orf4 required CDC55 to elicit its effect, whereas RTS1 was dispensable. The recruitment of the PP2A holoenzyme by E4orf4 was entirely dependent on Cdc55. These studies indicate that E4orf4-induced apoptosis in mammalian cells and cell death in yeast require functional interactions with B-type subunits of PP2A. However, some inhibition of growth by E4orf4 was observed in the cdc55 strain and with an E4orf4 mutant that fails to interact with Cdc55, indicating that E4orf4 may possess a second Cdc55-independent function affecting cell growth.

Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling

Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases, minimally containing a well conserved catalytic subunit, the activity of which is highly regulated. Regulation is accomplished mainly by members of a family of regulatory subunits, which determine the substrate specificity, (sub)cellular localization and catalytic activity of the PP2A holoenzymes. Moreover, the catalytic subunit is subject to two types of post-translational modification, phosphorylation and methylation, which are also thought to be important regulatory devices. The regulatory ability of PTPA (PTPase activator), originally identified as a protein stimulating the phosphotyrosine phosphatase activity of PP2A, will also be discussed, alongside the other regulatory inputs. The use of specific PP2A inhibitors and molecular genetics in yeast, Drosophila and mice has revealed roles for PP2A in cell cycle regulation, cell morphology and development. PP2A also plays a prominent role in the regulation of specific signal transduction cascades, as witnessed by its presence in a number of macromolecular signalling modules, where it is often found in association with other phosphatases and kinases. Additionally, PP2A interacts with a substantial number of other cellular and viral proteins, which are PP2A substrates, target PP2A to different subcellular compartments or affect enzyme activity. Finally, the de-regulation of PP2A in some specific pathologies will be touched upon.

Type 2A protein phosphatase, the complex regulator of numerous signaling pathways

Type 2A protein phosphatase (PP2A) comprises a diverse family of phosphoserine- and phosphothreonine-specific enzymes ubiquitously expressed in eukaryotic cells. Common to all forms of PP2A is a catalytic subunit (PP2Ac) which can form two distinct complexes, one with a structural subunit termed PR65/A and another with an alpha4 protein. The PR65/A-PP2Ac dimer may further associate with a regulatory subunit and form a trimeric holoenzyme. To date, three distinct families of regulatory subunits, which control substrate selectivity and phosphatase activity and target PP2A holoenzymes to their substrates, have been identified. Other molecular mechanisms that regulate PP2Ac function include phosphorylation, carboxyl methylation, inhibition by intracellular protein inhibitors (I(1)(PP2A) and I(2)(PP2A)), and stimulation by ceramide. PP2A dephosphorylates many proteins in vitro, but in vivo protein kinases and transcription factors appear to represent two major sets of substrates. Several natural compounds can inhibit PP2A activity and are used to study its function. Mutations in genes encoding PR65/A subunits have been identified in several different human cancers and the PP2A inhibitor, termed fostriecin, is being tested as an anticancer drug. Thus, a more thorough understanding of PP2A structure and function may lead to the development of novel strategies against human diseases.

Up-regulation of serine/threonine protein phosphatase type 2A regulatory subunits during methylprednisolone-induced differentiation of leukaemic HL-60 cells

Serine/threonine protein phosphatase 2A (PP2A) may play a role in leukaemic cell differentiation of the HL-60 myeloid leukaemic cell-line after methylprednisolone induction. We have investigated the specific enzyme activity and expression of catalytic and regulatory subunits of PP2A. The resulting specific enzyme activity and immunoblots showed an increase in enzyme activity and the expression of regulatory subunits after methylprednisolone treatment. There was no change in the expression of PP2A catalytic subunits. It is suggested that the effect of methylprednisolone on leukaemic differentiation may be the result of PP2A upregulation.

Adenovirus E4orf4 protein interacts with both Balpha and B' subunits of protein phosphatase 2A, but E4orf4-induced apoptosis is mediated only by the interaction with Balpha

Adenovirus E4orf4 protein is a multifunctional viral regulator, which is involved in down regulation of virally-modulated signal transduction, in control of alternative splicing of viral mRNAs, and in induction of apoptosis in transformed cells. It has been previously shown that E4orf4 interacts with protein phosphatase 2A through the phosphatase Balpha subunit. It was further shown that PP2A is required for performing the various E4orf4 functions. We report here that E4orf4 interacts with multiple isoforms of the PP2A-B' subunit, as well as with Balpha. We map the interaction sites of the B subunits on E4orf4 and show that they overlap but are not identical. We identify a dominant negative E4orf4 mutant, which disrupts the PP2A holoenzyme. We show that induction of apoptosis by E4orf4, which we previously reported to require the interaction with Balpha, is not affected by the interaction with B'. Our results suggest that the interaction of E4orf4 with various PP2A subpopulations may mediate the different E4orf4 functions.