Localization of the PP2A B56gamma regulatory subunit at the Golgi complex: possible role in vesicle transport and migration

Developmental and light regulation of tumor suppressor protein PP2A in the retina

Protein phosphatases are a group of universal enzymes that are responsible for the dephosphorylation of various proteins and enzymes in cells. Cellular signal transduction events are largely governed by the phosphorylation of key proteins. The length of cellular response depends on the activation of protein phosphatase that dephosphorylates the phosphate groups to halt a biological response, and fine-tune the defined cellular outcome. Dysregulation of these phosphatase(s) results in various disease phenotypes. The retina is a post-mitotic tissue, and oncogenic tyrosine and serine/ threonine kinase activities are important for retinal neuron survival. Aberrant activation of protein phosphatase(s) may have a negative effect on retinal neurons. In the current study, we characterized tumor suppressor protein phosphatase 2 (PP2A), a major serine/ threonine kinase with a broad substrate specificity. Our data suggest that PP2A is developmentally regulated in the retina, localized predominantly in the inner retina, and expressed in photoreceptor inner segments. Our findings indicate that PKCα and mTOR may serve as PP2A substrates. We found that light regulates PP2A activity. Our studies also suggest that rhodopsin regulates PP2A and its substrate(s) dephosphorylation. PP2A substrate phosphorylation is increased in mice lacking the A-subunit of PP2A. However, there is no accompanying effect on retina structure and function. Together, our findings suggest that controlling the activity of PP2A in the retina may be neuroprotective.

Visualization of subunit interactions and ternary complexes of protein phosphatase 2A in mammalian cells

Protein phosphatase 2A (PP2A) is a ubiquitous phospho-serine/threonine phosphatase that controls many diverse cellular functions. The predominant form of PP2A is a heterotrimeric holoenzyme consisting of a scaffolding A subunit, a variable regulatory B subunit, and a catalytic C subunit. The C subunit also associates with other interacting partners, such as α4, to form non-canonical PP2A complexes. We report visualization of PP2A complexes in mammalian cells. Bimolecular fluorescence complementation (BiFC) analysis of PP2A subunit interactions demonstrates that the B subunit plays a key role in directing the subcellular localization of PP2A, and confirms that the A subunit functions as a scaffold in recruiting the B and C subunits to form a heterotrimeric holoenzyme. BiFC analysis also reveals that α4 promotes formation of the AC core dimer. Furthermore, we demonstrate visualization of specific ABC holoenzymes in cells by combining BiFC and fluorescence resonance energy transfer (BiFC-FRET). Our studies not only provide direct imaging data to support previous biochemical observations on PP2A complexes, but also offer a promising approach for studying the spatiotemporal distribution of individual PP2A complexes in cells.

Identification of dynamic changes in proteins associated with the cellular cytoskeleton after exposure to okadaic acid

Exposure of cells to the diarrhetic shellfish poison, okadaic acid, leads to a dramatic reorganization of cytoskeletal architecture and loss of cell-cell contact. When cells are exposed to high concentrations of okadaic acid (100–500 nM), the morphological rearrangement is followed by apoptotic cell death. Okadaic acid inhibits the broad acting Ser/Thr protein phosphatases 1 and 2A, which results in hyperphosphorylation of a large number of proteins. Some of these hyperphosphorylated proteins are most likely key players in the reorganization of the cell morphology induced by okadaic acid. We wanted to identify these phosphoproteins and searched for them in the cellular lipid rafts, which have been found to contain proteins that regulate cytoskeletal dynamics and cell adhesion. By using stable isotope labeling by amino acids in cell culture cells treated with okadaic acid (400 nM) could be combined with control cells before the isolation of lipid rafts. Protein phosphorylation events and translocations induced by okadaic acid were identified by mass spectrometry. Okadaic acid was shown to regulate the phosphorylation status and location of proteins associated with the actin cytoskeleton, microtubules and cell adhesion structures. A large number of these okadaic acid-regulated proteins have previously also been shown to be similarly regulated prior to cell proliferation and migration. Our results suggest that okadaic acid activates general cell signaling pathways that induce breakdown of the cortical actin cytoskeleton and cell detachment.

Dynamic regulation of extracellular signal-regulated kinase (ERK) by protein phosphatase 2A regulatory subunit B56γ1 in nuclei induces cell migration

Extracellular signal-regulated kinase (ERK) signalling plays a central role in various biological processes, including cell migration, but it remains unknown what factors directly regulate the strength and duration of ERK activation. We found that, among the B56 family of protein phosphatase 2A (PP2A) regulatory subunits, B56γ1 suppressed EGF-induced cell migration on collagen, bound to phosphorylated-ERK, and dephosphorylated ERK, whereas B56α1 and B56β1 did not. B56γ1 was immunolocalized in nuclei. The IER3 protein was immediately highly expressed in response to costimulation of cells with EGF and collagen. Knockdown of IER3 inhibited cell migration and enhanced dephosphorylation of ERK. Analysis of the time course of PP2A-B56γ1 activity following the costimulation showed an immediate loss of phosphatase activity, followed by a rapid increase in activity, and this activity then remained at a stable level that was lower than the original level. Our results indicate that the strength and duration of the nuclear ERK activation signal that is initially induced by ERK kinase (MEK) are determined at least in part by modulation of the phosphatase activity of PP2A-B56γ1 through two independent pathways.

Structure, regulation, and pharmacological modulation of PP2A phosphatases

Protein phosphatases of the type 2A family (PP2A) represent a major fraction of cellular Ser/Thr phosphatase activity in any given human tissue. In this review, we describe how the holoenzymic nature of PP2A and the existence of several distinct PP2A composing subunits allow for the generation of multiple structurally and functionally different PP2A complexes, explaining why PP2A is involved in the regulation of so many diverse cell biological and physiological processes. Moreover, in human disease, most notably in several cancers and Alzheimer's Disease, PP2A expression and/or activity have been found significantly decreased, underscoring its important functions as a major tumor suppressor and tau phosphatase. Hence, several recent preclinical studies have demonstrated that pharmacological restoration of PP2A activity, as well as pharmacological PP2A inhibition, under certain conditions, may be of significant future therapeutic value.

Protein phosphatase 2A affects myofilament contractility in non-failing but not in failing human myocardium

Protein phosphatase (PP) type 2A is a multifunctional serine/threonine phosphatase that is involved in cardiac excitation-contraction coupling. The PP2A core enzyme is a dimer, consisting of a catalytic C and a scaffolding A subunit, which is targeted to several cardiac proteins by a regulatory B subunit. At present, it is controversial whether PP2A and its subunits play a critical role in end-stage human heart failure. Here we report that the application of purified PP2AC significantly increased the Ca2+-sensitivity (ΔpCa50=0.05±0.01) of the contractile apparatus in isolated skinned myocytes of non-failing (NF) hearts. A higher phosphorylation of troponin I (cTnI) was found at protein kinase A sites (Ser23/24) in NF compared to failing myocardium. The basal Ca2+-responsiveness of myofilaments was enhanced in myocytes of ischemic (ICM, ΔpCa50=0.10±0.03) and dilated (DCM, ΔpCa50=0.06±0.04) cardiomyopathy compared to NF. However, in contrast to NF myocytes the treatment with PP2AC did not shift force-pCa relationships in failing myocytes. The higher basal Ca2+-sensitivity in failing myocytes coincided with a reduced protein expression of PP2AC in left ventricular tissue from patients suffering from ICM and DCM (by 50 and 56% compared to NF, respectively). However, PP2A activity was unchanged in failing hearts despite an increase of both total PP and PP1 activity. The expression of PP2AB56α was also decreased by 51 and 62% in ICM and DCM compared to NF, respectively. The phosphorylation of cTnI at Ser23/24 was reduced by 66 and 49% in ICM and DCM compared to NF hearts, respectively. Our results demonstrate that PP2A increases myofilament Ca2+-sensitivity in NF human hearts, most likely via cTnI dephosphorylation. This effect is not present in failing hearts, probably due to the lower baseline cTnI phosphorylation in failing compared to non-failing hearts.

Determinants for Substrate Specificity of Protein Phosphatase 2A

Protein phosphatase 2A- (PP2A-) catalyzed dephosphorylation of target substrate proteins is widespread and critical for cellular function. PP2A is predominantly found as a heterotrimeric complex of a catalytic subunit (C), a scaffolding subunit (A), and one member of 4 families of regulatory subunits (B). Substrate specificity of the holoenzyme complex is determined by the subcellular locale the complex is confined to, selective incorporation of the B subunit, interactions with endogenous inhibitory proteins, and specific intermolecular interactions between PP2A and target substrates. Here, we discuss recent studies that have advanced our understanding of the molecular determinants for PP2A substrate specificity.

A tumor suppressor role for PP2A-B56alpha through negative regulation of c-Myc and other key oncoproteins

Loss or inhibition of the serine/threonine protein phosphatase 2A (PP2A) has revealed a critical tumor suppressor function for PP2A. However, PP2A has also been shown to have important roles in cell cycle progression and survival. Therefore, PP2A is not a typical tumor suppressor. This is most likely due to the fact that PP2A represents a large number of different holoenzymes. Further understanding of PP2A function(s), and especially its tumor suppressor activity, will depend largely on our ability to determine specific targets for these different PP2A holoenzymes and to gain an understanding of how these targets confer tumor suppressor activity or contribute to cell cycle progression and cell survival. Recent work has identified c-Myc as a target of the PP2A holoenzyme, PP2A-B56alpha. This holoenzyme also negatively regulates beta-catenin expression and modulates the anti-apoptotic activity of Bcl2, thus characterizing PP2A-B56alpha as a tumor suppressor PP2A holoenzyme. This review will focus on the role of PP2A-B56alpha in regulating c-Myc and will place this tumor suppressor activity of PP2A within the context of its other tumor suppressor functions. Finally, the mechanism(s) through which PP2A-B56alpha tumor suppressor activity may be lost in cancer will be discussed.

Spatial control of protein phosphatase 2A (de)methylation

Reversible methylation of the protein phosphatase 2A catalytic subunit (PP2A(C)(1)) is an important regulatory mechanism playing a crucial role in the selective recruitment of regulatory B subunits. Here, we investigated the subcellular localization of leucine carboxyl methyltransferase (LCMT1) and protein phosphatase methylesterase (PME-1), the two enzymes catalyzing this process. The results show that PME-1 is predominantly localized in the nucleus and harbors a functional nuclear localization signal, whereas LCMT1 is underrepresented in the nucleus and mainly localizes to the cytoplasm, Golgi region and late endosomes. Indirect immunofluorescence with methylation-sensitive anti-PP2A(C) antibodies revealed a good correlation with the methylation status of PP2A(C), demethylated PP2A(C) being substantially nuclear. Throughout mitosis, demethylated PP2A(C) is associated with the mitotic spindle and during cytokinesis with the cleavage furrow. Overexpression of PME-1, but not of an inactive mutant, results in increased demethylation of PP2A(C) in the nucleus, whereas overexpression of a cytoplasmic PME-1 mutant lacking the NLS results in increased demethylation in the cytoplasm-in all cases, however, without any obvious functional consequences. PME-1 associates with an inactive PP2A population, regardless of its esterase activity or localization. We propose that stabilization of this inactive, nuclear PP2A pool is a major in vivo function of PME-1.

A new paradigm for membrane-organizing and -shaping scaffolds

The clathrin, COPI and COPII scaffolds are paradigm vesicle coats in membrane trafficking. Recent advances in our understanding of the caveolar coat have generated a new paradigm. It represents those membrane coats, where a considerable part of the protein component is lipid modified, and integrated into the cytosolic leaflet of the vesicle membrane by a hairpin-like hydrophobic structure. Such coat proteins are permanently associated with membranes, and form oligomers early after synthesis. These oligomers assemble into a coat that has high affinity for particular lipids, creating lipid microdomains within the membrane. The combined protein-lipid structure should be considered as the scaffold that entraps ligands, either through affinity with the protein or with the lipid component, and that has the ability to shape membranes. Besides scaffolds assembled by caveolins, scaffolds assembled by reticulons and PHB domain-containing proteins such as the reggie/flotillin proteins fit this paradigm.

Anp32e/Cpd1 regulates protein phosphatase 2A activity at synapses during synaptogenesis

Anp32e/Cpd1, a member of the acidic nuclear phosphoprotein (Anp)32 family, is characterized by the presence of an amino terminal domain containing four leucine-rich repeats and a carboxyl-terminal low-compositional complexity acidic region. In previous studies performed to understand the biological role of Anp32e/Cpd1, we showed a predominant presence of Anp32e/Cpd1 in the nucleus. However, when Anp32e/Cpd1 is in the cytoplasm, it co-localizes spatially with protein phosphatase 2A (PP2A) near cell membranes, far from the synapses. In the present work, we show that Anp32e/Cpd1 is also present as a membrane-bound 74/76-kDa protein with a widespread distribution in the brain. We reveal that the expression, synthesis and half-life of this high-molecular-weight form of Anp32e/Cpd1 are spatially and temporally correlated with the cerebellar synaptogenesis period. We demonstrate that synaptic Anp32e/Cpd1 co-localizes, interacts and inhibits PP2A activity, and that phosphorylation of Anp32/Cpd1 is required for the Anp32e-PP2A interaction. Also, subcellular localization was shown with electronic microscopy. Finally, we examine Anp32e/Cpd1 and PP2A distribution in two ataxic mutant models, weaver and staggerer, and show that their co-localization in Purkinje cell dendrites depends on parallel fibre/Purkinje cell contacts. Based on these observations, we propose that Anp32e/Cpd1 mediates synaptogenesis process by modulating PP2A activity.

A B56 regulatory subunit of protein phosphatase 2A localizes to nuclear speckles in cardiomyocytes

Protein phosphatase 2A (PP2A) is widely distributed in heart tissues, yet its precise cellular functions are poorly understood. This study is based on the notion that PP2A action is governed by interactions of the core enzyme with B targeting/regulatory subunits. The subcellular localizations of two B subunits, B56α and B56γ1, were assessed using adenovirus-driven expression of epitope-tagged (hemagglutinin, HA) in cultured neonatal and adult rat ventricular myocytes. Confocal imaging revealed that HA-B56α was excluded from the nucleus and decorated striated structures, whereas HA-B56γ1 was principally found in the nucleus. Precise immunolabeling studies showed that B56γ1 was concentrated in intranuclear structures known as nuclear speckles, macromolecular structures that accumulate transcription and splicing factors. Western blot analyses revealed that overexpression of either B subunit had no effect on the levels of other PP2A subunits in cultured neonatal cardiac cells. However, overexpression of only B56γ1 increased whole cell PP2A activity by 40% when measured in cell extracts. Finally, B56γ1 did not alter global gene expression or expression of hypertrophic gene markers such as α-skeletal actin. However, morphometric analyses of confocal images revealed that B56γ1 alters the dynamic assembly/disassembly process of nuclear speckles in heart cells. These studies provide new insight into mechanisms of PP2A targeting in the subnuclear architecture in cardiomyocytes and into the role of this phosphatase in nuclear signaling.

PP2A: the expected tumor suppressor

PP2A is one of the few serine/threonine-specific phosphatases in the cell, and its complex structure and regulation guarantees its many different functions. Some viruses have chosen to target this enzyme system in order to manage the host cell machinery for their own profit and to program cells into a malignant state. Suppression of PR61/B'gamma, a specific third regulatory subunit of PP2A, can substitute for the viral SV40 protein small t antigen in causing tumorigenic transformation of several human cell lines -- provided that telomerase, SV40 large T antigen and oncogenic Ras are also present. Accumulation of c-Myc seems to be the common denominator.

Protein Phosphatase 2A Activity Associated with Golgi Membranes during the G2/M Phase May Regulate Phosphorylation of ERK2

The extracellular signal-regulated kinase (ERK) 1 and 2 proteins are mitogen-activated protein kinase (MAPK) members that regulate cell proliferation and differentiation. ERK proteins are activated exclusively by MAPK kinase 1 and 2 phosphorylation of threonine and tyrosine residues located within the conserved TXY MAPK activation motif. Although dual phosphorylation of Thr and Tyr residues confers full activation of ERK, in vitro studies suggest that a single phosphorylation on either Thr or Tyr may yield partial ERK activity. Previously, we have demonstrated that phosphorylation of the tyrosine residue (Tyr(P) ERK) may be involved in regulating the Golgi complex structure during the G2 and M phases of the cell cycle (Cha, H., and Shapiro, P. (2001) J. Cell Biol. 153, 1355-1368). In the present study, we examined mechanisms for generating Tyr(P) ERK by determining cell cycle-dependent changes in localized phosphatase activity. Using fractionated nuclei-free cell lysates, we find increased serine/threonine phosphatase activity associated with Golgi-enriched membranes in cells synchronized in the late G2/early M phase as compared with G1 phase cells. The addition of phosphatase inhibitors in combination with immunodepletion assays identified this activity to be related to protein phosphatase 2A (PP2A). The increased activity was accounted for by elevated PP2A association with mitotic Golgi membranes as well as increased catalytic activity after normalization of PP2A protein levels in the phosphatase assays. These data indicate that localized changes in PP2A activity may be involved in regulating proteins involved in Golgi disassembly as cells enter mitosis.