A metal-dependent form of protein phosphatase 2A

Inhibition of the PP2A activity by the histone chaperone ANP32B is long-range allosterically regulated by respiratory cytochrome c

Repair of injured DNA relies on nucleosome dismantling by histone chaperones and de-phosphorylation events carried out by Protein Phosphatase 2A (PP2A). Typical histone chaperones are the Acidic leucine-rich Nuclear Phosphoprotein 32 family (ANP32) members, e.g. ANP32A, which is also a well-known PP2A inhibitor (a.k.a. I₁PP2A). Here we report the novel interaction between the endogenous family member B—so-called ANP32B—and endogenous cytochrome c in cells undergoing camptothecin-induced DNA damage. Soon after DNA lesions but prior to caspase cascade activation, the hemeprotein translocates to the nucleus to target the Low Complexity Acidic Region (LCAR) of ANP32B; in a similar way, our group recently reported that the hemeprotein targets the acidic domain of SET/Template Activating Factor-Iβ (SET/TAF-Iβ), which is another histone chaperone and PP2A inhibitor (a.k.a. I₂PP2A). The nucleosome assembly activity of ANP32B is indeed unaffected by cytochrome c binding. Like ANP32A, ANP32B inhibits PP2A activity and is thus herein referred to as I₃PP2A. Our data demonstrates that ANP32B-dependent inhibition of PP2A is regulated by respiratory cytochrome c, which induces long-distance allosteric changes in the structured N-terminal domain of ANP32B upon binding to the C-terminal LCAR. In agreement with the reported role of PP2A in the DNA damage response, we propose a model wherein cytochrome c is translocated from the mitochondria into the nucleus upon DNA damage to modulate PP2A activity via its interaction with ANP32B.

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Respiratory cytochrome c interacts with ANP32B under DNA damage in the nucleus.

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Cytochrome c binding to ANP32B LCAR restores ANP32B-mediated PP2A inhibition.

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Cytochrome c emerges as a DNA Damage Response regulator.

Structural basis of PP2A activation by PTPA, an ATP-dependent activation chaperone

Proper activation of protein phosphatase 2A (PP2A) catalytic subunit is central for the complex PP2A regulation and is crucial for broad aspects of cellular function. The crystal structure of PP2A bound to PP2A phosphatase activator (PTPA) and ATPγS reveals that PTPA makes broad contacts with the structural elements surrounding the PP2A active site and the adenine moiety of ATP. PTPA-binding stabilizes the protein fold of apo-PP2A required for activation, and orients ATP phosphoryl groups to bind directly to the PP2A active site. This allows ATP to modulate the metal-binding preferences of the PP2A active site and utilize the PP2A active site for ATP hydrolysis. In vitro, ATP selectively and drastically enhances binding of endogenous catalytic metal ions, which requires ATP hydrolysis and is crucial for acquisition of pSer/Thr-specific phosphatase activity. Furthermore, both PP2A- and ATP-binding are required for PTPA function in cell proliferation and survival. Our results suggest novel mechanisms of PTPA in PP2A activation with structural economy and a unique ATP-binding pocket that could potentially serve as a specific therapeutic target.

Luteinizing hormone receptor activation in ovarian granulosa cells promotes protein kinase A-dependent dephosphorylation of microtubule-associated protein 2D

The actions of LH to induce ovulation and luteinization of preovulatory follicles are mediated principally by activation of cAMP-dependent protein kinase (PKA) in granulosa cells. PKA activity is targeted to specific locations in many cells by A kinase-anchoring proteins (AKAPs). We previously showed that FSH induces expression of microtubule-associated protein (MAP) 2D, an 80-kDa AKAP, in rat granulosa cells, and that MAP2D coimmunoprecipitates with PKA-regulatory subunits in these cells. Here we report a rapid and targeted dephosphorylation of MAP2D at Thr256/Thr259 after treatment with human chorionic gonadotropin, an LH receptor agonist. This event is mimicked by treatment with forskolin or a cAMP analog and is blocked by the PKA inhibitor myristoylated-PKI, indicating a role for cAMP and PKA signaling in phosphoregulation of granulosa cell MAP2D. Furthermore, we show that Thr256/Thr259 dephosphorylation is blocked by the protein phosphatase 2A (PP2A) inhibitor, okadaic acid, and demonstrate interactions between MAP2D and PP2A by coimmunoprecipitation and microcystin-agarose pull-down. We also show that MAP2D interacts with glycogen synthase kinase (GSK) 3beta and is phosphorylated at Thr256/Thr259 by this kinase in the basal state. Increased phosphorylation of GSK3beta at Ser9 and the PP2A B56delta subunit at Ser566 is observed after treatment with human chorionic gonadotropin and appears to result in LH receptor-mediated inhibition of GSK3beta and activation of PP2A, respectively. Taken together, these results show that the phosphorylation status of the AKAP MAP2D is acutely regulated by LH receptor-mediated modulation of kinase and phosphatase activities via PKA.

Mitosis-specific MPM-2 phosphorylation of DNA topoisomerase IIalpha is regulated directly by protein phosphatase 2A

Recent results suggest a role for topoIIalpha (topoisomerase IIalpha) in the fine-tuning of mitotic entry. Mitotic entry is accompanied by the formation of specific phosphoepitopes such as MPM-2 (mitotic protein monoclonal 2) that are believed to control mitotic processes. Surprisingly, the MPM-2 kinase of topoIIalpha was identified as protein kinase CK2, otherwise known as a constitutive interphase kinase. This suggested the existence of alternative pathways for the creation of mitotic phosphoepitopes, different from the classical pathway where the substrate is phosphorylated by a mitotic kinase. In the present paper, we report that topoIIalpha is co-localized with both CK2 and PP2A (protein phosphatase 2A) during interphase. Simultaneous incubation of purified topoIIalpha with CK2 and PP2A had minimal influence on the total phosphorylation levels of topoIIalpha, but resulted in complete disappearance of the MPM-2 phosphoepitope owing to opposite sequence preferences of CK2 and PP2A. Accordingly, short-term exposure of interphase cells to okadaic acid, a selective PP2A inhibitor, was accompanied by the specific appearance of the MPM-2 phosphoepitope on topoIIalpha. During early mitosis, PP2A was translocated from the nucleus, while CK2 remained in the nucleus until pro-metaphase thus permitting the formation of the MPM-2 phosphoepitope. These results underline the importance of protein phosphatases as an alternative way of creating cell-cycle-specific phosphoepitopes.

Crystal structure of a protein phosphatase 2A heterotrimeric holoenzyme

Protein phosphatase 2A (PP2A) is a principal Ser/Thr phosphatase, the deregulation of which is associated with multiple human cancers, Alzheimer's disease and increased susceptibility to pathogen infections. How PP2A is structurally organized and functionally regulated remains unclear. Here we report the crystal structure of an AB'C heterotrimeric PP2A holoenzyme. The structure reveals that the HEAT repeats of the scaffold A subunit form a horseshoe-shaped fold, holding the catalytic C and regulatory B' subunits together on the same side. The regulatory B' subunit forms pseudo-HEAT repeats and interacts with the C subunit near the active site, thereby defining substrate specificity. The methylated carboxy-terminal tail of the C subunit interacts with a highly negatively charged region at the interface between A and B' subunits, suggesting that the C-terminal carboxyl methylation of the C subunit promotes B' subunit recruitment by neutralizing charge repulsion. Together, our structural results establish a crucial foundation for understanding PP2A assembly, substrate recruitment and regulation.

An inactive protein phosphatase 2A population is associated with methylesterase and can be re-activated by the phosphotyrosyl phosphatase activator

We have described recently the purification and cloning of PP2A (protein phosphatase 2A) leucine carboxylmethyltransferase. We studied the purification of a PP2A-specific methylesterase that co-purifies with PP2A and found that it is tightly associated with an inactive dimeric or trimeric form of PP2A. These inactive enzyme forms could be reactivated as Ser/Thr phosphatase by PTPA (phosphotyrosyl phosphatase activator of PP2A). PTPA was described previously by our group as a protein that stimulates the in vitro phosphotyrosyl phosphatase activity of PP2A; however, PP2A-specific methyltransferase could not bring about the activation. The PTPA activation could be distinguished from the Mn2+ stimulation observed with some inactive forms of PP2A, also found associated with PME-1 (phosphatase methylesterase 1). We discuss a potential new function for PME-1 as an enzyme that stabilizes an inactivated pool of PP2A.

A novel and essential mechanism determining specificity and activity of protein phosphatase 2A (PP2A) in vivo

Protein phosphatase 2A (PP2A) is an essential intracellular serine/threonine phosphatase containing a catalytic subunit that possesses the potential to dephosphorylate promiscuously tyrosine-phosphorylated substrates in vitro. How PP2A acquires its intracellular specificity and activity for serine/threonine-phosphorylated substrates is unknown. Here we report a novel and phylogenetically conserved mechanism to generate active phospho-serine/threonine-specific PP2A in vivo. Phosphotyrosyl phosphatase activator (PTPA), a protein of so far unknown intracellular function, is required for the biogenesis of active and specific PP2A. Deletion of the yeast PTPA homologs generated a PP2A catalytic subunit with a conformation different from the wild-type enzyme, as indicated by its altered substrate specificity, reduced protein stability, and metal dependence. Complementation and RNA-interference experiments showed that PTPA fulfills an essential function conserved from yeast to man.

Calcineurin: form and function

Calcineurin is a eukaryotic Ca(2+)- and calmodulin-dependent serine/threonine protein phosphatase. It is a heterodimeric protein consisting of a catalytic subunit calcineurin A, which contains an active site dinuclear metal center, and a tightly associated, myristoylated, Ca(2+)-binding subunit, calcineurin B. The primary sequence of both subunits and heterodimeric quaternary structure is highly conserved from yeast to mammals. As a serine/threonine protein phosphatase, calcineurin participates in a number of cellular processes and Ca(2+)-dependent signal transduction pathways. Calcineurin is potently inhibited by immunosuppressant drugs, cyclosporin A and FK506, in the presence of their respective cytoplasmic immunophilin proteins, cyclophilin and FK506-binding protein. Many studies have used these immunosuppressant drugs and/or modern genetic techniques to disrupt calcineurin in model organisms such as yeast, filamentous fungi, plants, vertebrates, and mammals to explore its biological function. Recent advances regarding calcineurin structure include the determination of its three-dimensional structure. In addition, biochemical and spectroscopic studies are beginning to unravel aspects of the mechanism of phosphate ester hydrolysis including the importance of the dinuclear metal ion cofactor and metal ion redox chemistry, studies which may lead to new calcineurin inhibitors. This review provides a comprehensive examination of the biological roles of calcineurin and reviews aspects related to its structure and catalytic mechanism.

Biochemical characterization of recombinant subunits of type 2A protein phosphatase overexpressed in Pichia pastoris

Methylotrophic yeast Pichia pastoris was used for a medium-scale expression of structural (PR65/A) and catalytic (PP2Ac) subunits of human type 2A protein phosphatase (PP2A). Constructs encoding these subunits, which were designed to introduce eight histidines at their N-termini, were introduced into the KM71 Pichia strain by homologous recombination. Recombinant proteins overproduced after methanol induction were purified from cell-free extracts by anion-exchange chromatography on DEAE-Sepharose, and Ni2+/nitrilotriacetate/agarose. In addition, chromatography on omega-aminohexyl-Sepharose was applied to purify recombinant (r)PR65/A. This purification scheme yielded approximately 5 mg and 100 microg of rPR65/A and rPP2Ac, respectively, from 1 L of the yeast culture. The specific activity of rPP2Ac measured with [32P]phosphorylase a [1.7 micromol.min-1.(mg protein)-1] and its inhibition by okadaic acid (IC50 = 0.66 nM) were similar to PP2A isolated from rabbit skeletal muscle. As demonstrated by immunodetection with methylation state-specific antibodies, recombinant PP2Ac was carboxymethylated at the last C-terminal leucine residue. Recombinant PP2A subunits were able to form a complex as demonstrated both by activity assays in the presence of protamine and by chromatography on protamine-agarose. In summary, P. pastoris provides a convenient heterologous system for the production of recombinant subunits of PP2A.

Regulation of protein phosphorylation in astrocyte cultures by external calcium ions: specific effects on the phosphorylation of glial fibrillary acidic protein (GFAP), vimentin and heat shock protein 27 (HSP27)

The effect of external Ca2+ ([Ca2+]e) on the incorporation of [32P] into total protein, cytoskeletal proteins and the heat shock protein HSP27, was studied in primary cultures of astrocytes from the rat hippocampus. Zero [Ca2+]e increased total 32P-incorporation into astrocyte protein and when this was normalized to 100%, incorporation was significantly increased into glial fibrillary acidic protein (GFAP), vimentin (VIM) and HSP27. The difference in total 32P-incorporation between zero [Ca2+]e and 1 mM [Ca2+]e was reversed by incubation of the cells with the protein phosphatase inhibitor okadaic acid in the range 1-10 nM; higher concentrations of okadaic acid (50-100 nM) further increased total 32P-incorporation. In zero [Ca2+]e the non-specific channel blocker Co2+ (1 mM) decreased total 32P-incorporation by approximately 30%. The results were compared with a previous study [S.T. Wofchuk, R. Rodnight, Age-dependent changes in the regulation by external calcium ions of the phosphorylation of glial fibrillary acidic protein in slices of rat hippocampus, Dev. Brain Res. 85 (1995) 181-186] in which it was shown that in immature hippocampal slices zero [Ca2+]e compared with 1 mM [Ca2+]e increased 32P-incorporation into GFAP without changing total incorporation. The difference between the results for total 32P-incorporation obtained in cultured astrocytes and immature brain tissue was found to be related to the concentration of [Ca2+]e in the medium since in slices concentrations of [Ca2+]e higher than 1 mM progressively decreased total incorporation. The difference may reflect a higher Ca2+-permeability of the plasma membrane in cultured astrocytes and/or to the complex structure of the slice tissue. In two-dimensional electrophoresis HSP27, in contrast to GFAP and VIM, was separated into 3 immunodetectable isoforms only two of which were normally phosphorylated. After labelling in the presence of okadaic acid both immunodetectable and phosphorylated HSP27 focussed as a single polypeptide. Phorbol dibutyrate (1 microM) and zero [Ca2+]e stimulated the phosphorylation of both isoforms, but in the case of zero [Ca2+]e the effect on the more acidic isoform was proportionally greater.

Biochemical characteristics of Eiseniapore, a pore-forming protein in the coelomic fluid of earthworms

The cytolytic protein Eiseniapore (38 kDa) from coelomic fluid of the earthworm Eisenia fetida functionally requires sphingomyelin as revealed by using mammalian erythrocytes and phospholipid vesicles. The effects of ions, glycoproteins and phospholipids were investigated for the two-step Eiseniapore action mode, binding and pore formation in different assays. Eiseniapore lysis is activated by thiol groups but inhibited by metal ions. Eiseniapore binding to target membranes is inhibited by Eiseniapore-regulating factor, vitronectin, heparin and lysophosphatidylcholine. Ca2+ and Mg2+ were found to be not necessary for membrane binding or lytic activity. Sphingomyelin was essential for Eiseniapore-induced leakage of liposomes. We describe a cytolytic protein/toxin in Eiseniapore which differs from the established classification; it can be activated by thiol groups and is inhibited by sphingomyelin. Electron microscopy of erythrocyte membranes confirmed ring-shaped structures (pores) with a central channel with outer (10 nm) and inner (3 nm) diameters as shown previously [Lange, S., Nüssler, F., Kauschke, E., Lutsch, G., Cooper, E.L. & Herrmann, A. (1997) J. Biol. Chem. 272, 20 884-20 892] using artificial membranes. Functional evidence of pore formation by Eiseniapore was revealed as protection of lysis by carbohydrates occurred at an effective diameter above 3 nm. From these results, we suggest a plausible explanation for the mechanism by which components of the earthworm's immune system destroy non-self components.

Effect of substitution inert metal complexes on calcineurin

As a substitute for M(H2O)2+6, Co(NH3)3+6 was found to activate calcineurin with para-nitrophenyl phosphate as substrate. Kinetics for calcineurin catalyzed hydrolysis of para-nitrophenyl phosphate at pH 7.0 with Mn2+, Mg2+, Co2+, and Co(NH3)3+6 were compared. Although kcat and Km were different with the metals, values of kcat/Km were nearly identical for Mn2+ and Mg2+, but lower for Co2+ and Co(NH3)3+6. The concentration of each metal providing half-maximal activation, designated Kact, was evaluated as 15.9 mM for Co(NH3)3+6, compared to Kact = 0.17 mM for Mn2+ and Co2+ and 6.3 mM for Mg2+, respectively. Comparing kcat/Kcat showed that Co(NH3)3+6 was a 170-fold poorer activator of calcineurin than was Mn2+, but only 1.5-fold poorer than Mg2+. Activation by Co(NH3)3+6 indicated that activation of calcineurin by exogenous metal ions can proceed via an outer coordination sphere reaction mechanism with no requirement for the direct coordination of substrate by metal. Because Co(NH3)3+6 was found to support calcineurin activity, the related compound [Co-(ethylenediamine)3]3+ (or Co(en)3+3) was tested as a possible activator. Co(en)3+3 did not support calcineurin activity but did inhibit calcineurin. Co(en)3+3 showed competitive inhibition kinetics with either Mn2+ or pNPP as the varied ligand and the other at a fixed, subsaturating concentration. Inorganic phosphate was used as a known competitive inhibitor to pNPP (B. L. Martin and D. J. Graves, J. Biol. Chem. 261, 14545-14550, 1986) and showed uncompetitive inhibition with Mn2+ as the varied ligand. These patterns are consistent with the mechanism of ligand binding to calcineurin being ordered with metal preceding substrate. Prior formation of a metal-substrate complex was not required for association with calcineurin.

Direct metal analyses of Mn2+-dependent and -independent protein phosphatase 2A from human erythrocytes detect zinc and iron only in the Mn2+-independent one

A Mn2+-dependent protein phosphatase 2A which is composed of a 34 kDa catalytic C' subunit and a 63 kDa regulatory A' subunit, was purified from human erythrocyte cytosol. C' and A' produced V8- and papain-peptide maps identical to those of the 34 kDa catalytic C and the 63 kDa regulatory A subunits of the Mn2+-independent conventional protein phosphatase in human erythrocyte cytosol, respectively. Reconstitution of C'A and CA' revealed that the metal dependency resided in C' and not in A'. In CA, 0.87 +/- 0.12 mol zinc and 0.35 +/- 0.18 mol iron per mol enzyme were detected by atomic absorption spectrophotometry, but manganese, magnesium and cobalt were not detected. None of these metals was detected in C'A'. Pre-incubation of C' with ZnCl2 and FeCl2, but not FeCl3, synergistically stimulated the Mn2+-independent protein phosphatase activity. The protein phosphatase activity of C was unaffected by the same zinc and/or iron treatment. These results suggest that C is a Zn2+- and Fe2+-metalloenzyme and that C' is the apoenzyme.

Effects of microcystins on phosphorylase-a binding to phosphatase-2A: kinetic analysis by surface plasmon resonance biosensor

Cyclic heptapeptide microcystins are a group of hepatoxicants which exert the cytotoxic effects by inhibiting the catalytic activities of phosphatase-2A (PP-2A) and phosphatase-1 (PP-1) and thus disrupt the normal signal transduction pathways. Microcystins interact with PP-2A and PP-1 by a two-step mechanism involving rapid binding and inactivation of protein phosphatase catalytic subunit, followed by a slower covalent interaction. It was proposed that inactivation of PP-2A/PP-1 catalytic activity by microcystins precedes covalent adduct formation. In this study, we used a biosensor based on surface plasmon resonance (SPR) to examine the effects of three microcystins, MCLR, MCRR and MCYR, on the binding between PP-2A and its substrate, phosphorylase-a (PL-a), during the first step of the interaction. The SPR biosensor provides real-time information on the association and dissociation kinetics of PL-a with immobilized PP-2A in the absence and presence of microcystins. It was found that the affinity of PL-a to microcystin-bound PP-2A was four times smaller compared to unbound PP-2A, due to 50% decreases in the association rates and two-fold increases in dissociation rates of PL-a binding to PP-2A. The results suggest that the rapid binding of microcystins to the PP-2A catalytic site leads to the formation of a noncovalent microcystin/PP-2A adduct. While the adduct formation fully inhibits the catalytic activity of PP-2A, it only results in partial inhibition of the substrate binding. The similar effects of the three microcystins on PP-2A suggest that the toxins bind to PP-2A at the same site and cause similar conformational changes. The present work also demonstrates the potential application of biosensor technology in environmental toxicological research.

Structural and enzymatic aspects of rhodopsin phosphorylation

Photoactivated rhodopsin (Rho*) is phosphorylated near the C terminus at multiple sites, predominantly at Ser334, Ser338, and Ser343. We systematically examined the sites of phosphorylation upon flash activation of Rho in rod outer segment (ROS) homogenates. Addition of an inhibitory antibody against rhodopsin kinase (RK) lowered phosphorylation at Ser334, Ser338, and Ser343, without changing the ratio between phosphorylation sites. In contrast, no effect of protein kinase C was detected after stimulation (by a phorbol ester), inhibition (with H7), or reconstitution of protein kinase C with purified ROS membranes. The stoichiometry and the ratio between different phosphorylation sites in purified Rho were also reproduced using RK, purified to apparent homogeneity from ROS or from an insect cell expression system. Thus, we conclude that light-dependent phosphorylation of Rho is mediated primarily by RK. Depalmitoylation of Rho at Cys322 and Cys323 altered the conformation of the C terminus of Rho, as observed by phosphorylation by casein kinase I, but did not affect phosphorylation by RK. The sites of phosphorylation were influenced, however, by the presence of four conserved amino acids at the C terminus of Rho. The accumulation of phosphorylated Ser334 observed in vivo could result from slower dephosphorylation of this site as compared with dephosphorylation of Ser338 and Ser343. These data provide a molecular mechanism for the site-specific phosphorylation of Rho observed in vivo.

Activation of protein phosphatase 1. Formation of a metalloenzyme

The recombinant catalytic subunit of protein phosphatase 1 is produced as an inactive enzyme which can be activated by Mn2+ (Zhang, Z., Bai, G., Deans-Zirattu, S., Browner, M. F., and Lee, E. Y. C. (1992) J. Biol. Chem. 267, 1484-1490). In this report, we have investigated the effects of divalent cations on the activity of recombinant catalytic subunit of protein phosphatase 1. Latent phosphatase 1 can be activated by Co2+ or Mn2+, whereas other metal ions tested including Fe2+, Zn2+, Mg2+, Ca2+, Cu2+, or Ni2+ were not effective or were only weakly effective in activating the enzyme. The Mn(2+)-stimulated activity was susceptible to inactivation by EDTA; however, the Co(2+)-activated phosphatase was stable after dilution and chelation of the Co2+ with excess EDTA. After stable activation of phosphatase 1 using 57Co2+, a stoichiometric amount of 57Co2+ was shown to be tightly bound to phosphatase 1. These findings demonstrate for the first time the generation of a stable metalloenzyme form of phosphatase 1. Fe2+ reversibly deactivated the Co(2+)-stimulated activity, but did not displace the bound Co2+. Interestingly, treatment of the enzyme with a combination of Fe2+ and Zn2+ (but not the individual metal ions) significantly activated phosphatase 1. These results suggest that at least two metal binding sites exist on the enzyme and that protein phosphatase 1 may be an iron/zinc metalloprotein in vivo.