Heterotrimeric Protein Phosphatase 2A Research Articles

Influence of point mutations on PR65 conformational adaptability: Insights from molecular simulations and nanoaperture optical tweezers.

PR65 is the HEAT repeat scaffold subunit of the heterotrimeric protein phosphatase 2A (PP2A) and an archetypal tandem repeat protein. Its conformational mechanics plays a crucial role in PP2A function by opening/closing substrate binding/catalysis interface. Using in silico saturation mutagenesis, we identified PR65 "hinge" residues whose substitutions could alter its conformational adaptability and thereby PP2A function, and selected six mutations that were verified to be expressed and soluble. Molecular simulations and nanoaperture optical tweezers revealed consistent results on the specific effects of the mutations on the structure and dynamics of PR65. Two mutants observed in simulations to stabilize extended/open conformations exhibited higher corner frequencies and lower translational scattering in experiments, indicating a shift toward extended conformations, whereas another displayed the opposite features, confirmed by both simulations and experiments. The study highlights the power of single-molecule nanoaperture-based tweezers integrated with in silico approaches for exploring the effect of mutations on protein structure and dynamics.

A small molecule inhibitor of leucine carboxyl methyltransferase-1 inhibits cancer cell survival

Reversible phosphorylation is the basis for signal transduction in eukaryotic cells, and this is tightly controlled by the complex interplay of kinases and phosphatases. Many malignancies are characterized by dysregulation of the delicate protein phosphorylation balance. The targeting of protein phosphatases has been gaining attention as their role in cancer development and progression has been elucidated. The protein phosphatase-2A (PP2A) family of phosphatases are the primary cellular serine/threonine phosphatases. PP2A heterotrimers containing the B55α (PR55α) regulatory subunit have been associated with oncogenic signaling, and B55 subunits are found exclusively in forms of PP2A in which the carboxyl terminus of the catalytic subunit (PP2Ac) is methylated. Methylation of PP2Ac is primarily mediated by leucine carboxyl methyltransferase-1 (LCMT-1). Demethylation is controlled by an esterase, PP2A methylesterase (PME-1). We tested two potential LCMT-1 small molecule inhibitors and found that methyl 4-methyl-2-[(2-methylbenzoyl)amino]-5-[[(3-methylphenyl)amino]carbonyl]-3-thiophenecarboxylate (henceforth referred to as Compound 2) significantly inhibited PP2Ac methylation in vitro (p = 0.0024), and in the MDA-MB-231 breast carcinoma (p = 0.0431) and Rosi melanoma (p = 0.0335) cell lines. Compound 2 significantly reduced survival in HEK-293, HS-5, MDA-MB-231 and Rosi cells; and constrained clonogenic colony formation in MCF7, MDA-MB-231 and Rosi cells. The LCMT-1inhibitor induced G0/G1 cell cycle arrest in Rosi cells (p = 0.0193) and induced apoptosis in MDA-MB-231 cells (p < 0.0001). Increased phosphorylation of the receptor-interacting serine/threonine protein kinase-1 (RIPK1) was detected in MDA-MB-231 (p = 0.0273) and Rosi cells (p = 0.0179) in response to treatment with Compound 2. These data add to the body of evidence pointing to LCMT-1 as an oncogenic target.

Abstract PR002: Loss of protein phosphatase PP2A activity drives AR addiction in prostate cancer

Abstract Reversible protein phosphorylation plays a fundamental role in numerous biological processes and is regulated by a dynamic interplay between protein kinases and phosphatases. Disruption of this balance due to aberrant activation of kinases and inactivation of phosphatases is a hallmark of cancer. The highly conserved protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase with a complex heterotrimeric composition involved in the regulation of numerous critical aspects of cellular function. PP2A is one of the most abundant enzymes, accounting for up to 1% of the total cellular protein in some tissues and represents the majority of serine/threonine phosphatase activities in many tissues. PP2A is comprised of a core dimer consisting of a scaffolding ‘‘A’’ subunit (PP2A-A), and a catalytic ‘‘C’’ subunit (PP2A-C), which form a heterotrimeric complex with one of at least 20 substrate-directing regulatory “B” subunits. PP2A heterotrimerization is regulated through Leucine309 methylation of catalytic C subunit, which influence the binding affinity of the B subunits to the AC dimer and thus the substrate specificity. Loss of the tumor suppressive activity of PP2A is associated with prostate cancer; however, the molecular basis for this loss is not well understood. Here, we report that the leucine carboxy methyltransferase (LCMT1), which methylates the L309 of the catalytic C subunit of the PP2A heterodimer, acts as a suppressor of androgen receptor (AR) addicted prostate cancer (PCa). In a series of tissue microarrays (TMAs) representing 17,747 localized PCa, low methyl-PP2A-C levels were negatively associated with critical parameters of disease progression including high Gleason score, and lymph node metastasis. Biologically, LCMT1 silencing leads to increased AR phosphorylation/activity and enhanced CRPC growth in vivo. Conversely, LCMT1 dependent methyl sensitive PP2A heterotrimers target AR and its critical coactivator MED1 for dephosphorylation through binding of the PP2A-B56α regulatory subunit to two conserved small linear motifs (SLiMs) on MED1, resulting in the eviction of the AR-MED1 complex from the chromatin and loss of target gene expression. Mechanistically, LCMT1 is regulated by PI3K-AKT-mTOR-S6K1-mediated phosphorylation-induced degradation requiring the β-TRCP E3 ubiquitin ligase. Targeted degradation of LCMT1 through this mechanism is associated with acquired resistance to second-generation anti-androgens such as enzalutamide, apalutamide and darolutamide resulting in restored AR-addiction in CRPC cells. Finally, feedforward stabilization of LCMT1 by a small molecule activator of phosphatase (SMAP) results in attenuation of AR signaling and tumor regression in enzalutamide refractory PCa xenografts. Collectively, these results demonstrate that decreased methyl-PP2A-C is a novel prognostic marker for PCa progression and that the loss of LCMT1 is a major determinant of AR-addicted advanced PCa, and support the therapeutic potential of selective small molecules modulators of PP2A for metastatic prostate cancer treatment. Citation Format: Irfan A. Asangani. Loss of protein phosphatase PP2A activity drives AR addiction in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr PR002.

Unraveling the complexity: A comprehensive analysis of the PP2A in cancer and its potential for novel targeted therapies.

Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase considered a potent tumor suppressor that critically regulates diverse cellular processes, including cell cycle progression, apoptosis, or DNA repair. PP2A is typically downregulated in cancers but mechanisms for its inactivation in human cancers are poorly understood. PP2A represents a family of more than 60 phosphatases. According to cellular context, each heterotrimeric PP2A holoenzyme exerts a unique role in cancer, and PP2A isoforms can act either as tumor suppressors or as promoters. Due to wide structural diversity, PP2A has been considered undruggable. However, increasing knowledge predisposes PP2A diversity to therapeutical targeting for the treatment of a broad range of cancer pathologies, including drug resistance or cloaking immune surveillance. In this review, we discuss the regulatory role of PP2A in cancer, its regulation by microRNA and hypoxia, its contribution to therapy resistance development, and the therapeutic potential of direct and indirect targeting, or combinatory administration with other anti-cancer drugs to improve cancer treatment outcomes.

Protein Phosphatase 2A with B' specificity subunits regulates the Hippo-Yorkie signaling axis in the Drosophila eye disc.

Hippo-Yorkie (Hpo-Yki) signaling is central to diverse developmental processes. Although its redeployment has been amply demonstrated, its context-specific regulation remains poorly understood. The Drosophila eye disc is a continuous epithelium folded into two layers, the peripodial epithelium (PE) and the retinal progenitor epithelium. Here, Yki acts in the PE, first to promote PE identity by suppressing retina fate, and subsequently to maintain proper disc morphology. In the latter process, loss of Yki results in the displacement of a portion of the differentiating retinal epithelium onto the PE side. We show that Protein Phosphatase 2A (PP2A) complexes comprising different substrate-specificity B-type subunits govern the Hpo-Yki axis in this context. These include holoenzymes containing the B‴ subunit Cka and those containing the B' subunits Wdb or Wrd. Whereas PP2A(Cka), as part of the STRIPAK complex, is known to regulate Hpo directly, PP2A(Wdb) acts genetically upstream of the antagonistic activities of the Hpo regulators Sav and Rassf. These in vivo data provide the first evidence of PP2A(B') heterotrimer function in Hpo pathway regulation and reveal pathway diversification at distinct developmental times in the same tissue.

A functional interaction between liprin-α1 and B56γ regulatory subunit of protein phosphatase 2A supports tumor cell motility

Scaffold liprin-α1 is required to assemble dynamic plasma membrane-associated platforms (PMAPs) at the front of migrating breast cancer cells, to promote protrusion and invasion. We show that the N-terminal region of liprin-α1 contains an LxxIxE motif interacting with B56 regulatory subunits of serine/threonine protein phosphatase 2A (PP2A). The specific interaction of B56γ with liprin-α1 requires an intact motif, since two point mutations strongly reduce the interaction. B56γ mediates the interaction of liprin-α1 with the heterotrimeric PP2A holoenzyme. Most B56γ protein is recovered in the cytosolic fraction of invasive MDA-MB-231 breast cancer cells, where B56γ is complexed with liprin-α1. While mutation of the short linear motif (SLiM) does not affect localization of liprin-α1 to PMAPs, localization of B56γ at these sites specifically requires liprin-α1. Silencing of B56γ or liprin-α1 inhibits to similar extent cell spreading on extracellular matrix, invasion, motility and lamellipodia dynamics in migrating MDA-MB-231 cells, suggesting that B56γ/PP2A is a novel component of the PMAPs machinery regulating tumor cell motility. In this direction, inhibition of cell spreading by silencing liprin-α1 is not rescued by expression of B56γ binding-defective liprin-α1 mutant. We propose that liprin-α1-mediated recruitment of PP2A via B56γ regulates cell motility by controlling protrusion in migrating MDA-MB-231 cells.

Select Stabilization of a Tumor-Suppressive PP2A Heterotrimer

In cancer, suppression of protein phosphatases, such as protein phosphatase 2A (PP2A), that normally counteract kinases, contributes to aberrant signaling. Leonard et al. recently demonstrated that a novel small-molecule activator of PP2A, DT-061, selectively stabilizes a specific PP2A holoenzyme responsible for dephosphorylating critical oncogenic targets, including MYC. The 3.6-Å cryo-electron microscopy map of the heterotrimer assembly provides insight into the druggable structure of PP2A, guiding future phosphatase therapeutics.

Reduction of protein phosphatase 2A (PP2A) complexity reveals cellular functions and dephosphorylation motifs of the PP2A/B′δ holoenzyme

Protein phosphatase 2A (PP2A) is a large enzyme family responsible for most cellular Ser/Thr dephosphorylation events. PP2A substrate specificity, localization, and regulation by second messengers rely on more than a dozen regulatory subunits (including B/R2, B'/R5, and B″/R3), which form the PP2A heterotrimeric holoenzyme by associating with a dimer comprising scaffolding (A) and catalytic (C) subunits. Because of partial redundancy and high endogenous expression of PP2A holoenzymes, traditional approaches of overexpressing, knocking down, or knocking out PP2A regulatory subunits have yielded only limited insights into their biological roles and substrates. To this end, here we sought to reduce the complexity of cellular PP2A holoenzymes. We used tetracycline-inducible expression of pairs of scaffolding and regulatory subunits with complementary charge-reversal substitutions in their interaction interfaces. For each of the three regulatory subunit families, we engineered A/B charge-swap variants that could bind to one another, but not to endogenous A and B subunits. Because endogenous Aα was targeted by a co-induced shRNA, endogenous B subunits were rapidly degraded, resulting in expression of predominantly a single PP2A heterotrimer composed of the A/B charge-swap pair and the endogenous catalytic subunit. Using B'δ/PPP2R5D, we show that PP2A complexity reduction, but not PP2A overexpression, reveals a role of this holoenzyme in suppression of extracellular signal-regulated kinase signaling and protein kinase A substrate dephosphorylation. When combined with global phosphoproteomics, the PP2A/B'δ reduction approach identified consensus dephosphorylation motifs in its substrates and suggested that residues surrounding the phosphorylation site play roles in PP2A substrate specificity.

RETRACTED: Allosteric Activators of Protein Phosphatase 2A Display Broad Antitumor Activity Mediated by Dephosphorylation of MYBL2

Protein phosphatase 2A (PP2A) enzymes can suppress tumors, but they are often inactivated in human cancers overexpressing inhibitory proteins. Here, we identify a class of small-molecule iHAPs (improved heterocyclic activators of PP2A) that kill leukemia cells by allosterically assembling a specific heterotrimeric PP2A holoenzyme consisting of PPP2R1A (scaffold), PPP2R5E (B56ε, regulatory), and PPP2CA (catalytic) subunits. One compound, iHAP1, activates this complex but does not inhibit dopamine receptor D2, a mediator of neurologic toxicity induced by perphenazine and related neuroleptics. The PP2A complex activated by iHAP1 dephosphorylates the MYBL2 transcription factor on Ser241, causing irreversible arrest of leukemia and other cancer cells in prometaphase. In contrast, SMAPs, a separate class of compounds, activate PP2A holoenzymes containing a different regulatory subunit, do not dephosphorylate MYBL2, and arrest tumor cells in G1 phase. Our findings demonstrate that small molecules can serve as allosteric switches to activate distinct PP2A complexes with unique substrates.

Protein phosphatase PP2A regulates microtubule orientation and dendrite pruning in Drosophila.

Pruning that selectively eliminates inappropriate projections is crucial for sculpting neural circuits during development. During Drosophila metamorphosis, ddaC sensory neurons undergo dendrite-specific pruning in response to the steroid hormone ecdysone. However, the understanding of the molecular mechanisms underlying dendrite pruning remains incomplete. Here, we show that protein phosphatase 2A (PP2A) is required for dendrite pruning. The catalytic (Microtubule star/Mts), scaffolding (PP2A-29B), and two regulatory subunits (Widerborst/Wdb and Twins/Tws) play important roles in dendrite pruning. Functional analyses indicate that PP2A, via Wdb, facilitates the expression of Sox14 and Mical prior to dendrite pruning. Furthermore, PP2A, via Tws, governs the minus-end-out orientation of microtubules (MTs) in the dendrites. Moreover, the levels of Klp10A, a MT depolymerase, increase when PP2A is compromised. Attenuation of Klp10A fully rescues the MT orientation defects in mts or pp2a-29b RNAi ddaC neurons, suggesting that PP2A governs dendritic MT orientation by suppressing Klp10A levels and/or function. Taken together, this study sheds light on a novel function of PP2A in regulating dendrite pruning and dendritic MT polarity in sensory neurons.

Global loss of leucine carboxyl methyltransferase-1 causes severe defects in fetal liver hematopoiesis

Leucine carboxyl methyltransferase-1 (LCMT-1) methylates the C-terminal leucine α-carboxyl group of the catalytic subunits of the protein phosphatase 2A (PP2A) subfamily of protein phosphatases, PP2Ac, PP4c, and PP6c. LCMT-1 differentially regulates the formation and function of a subset of the heterotrimeric complexes that PP2A and PP4 form with their regulatory subunits. Global LCMT-1 knockout causes embryonic lethality in mice, but LCMT-1 function in development is unknown. In this study, we analyzed the effects of global LCMT-1 loss on embryonic development. LCMT-1 knockout causes loss of PP2Ac methylation, indicating that LCMT-1 is the sole PP2Ac methyltransferase. PP2A heterotrimers containing the Bα and Bδ B-type subunits are dramatically reduced in whole embryos, and the steady-state levels of PP2Ac and the PP2A structural A subunit are also down ∼30%. Strikingly, global loss of LCMT-1 causes severe defects in fetal hematopoiesis and usually death by embryonic day 16.5. Fetal livers of homozygous lcmt-1 knockout embryos display hypocellularity, elevated apoptosis, and greatly reduced numbers of hematopoietic stem and progenitor cell-enriched Kit+Lin-Sca1+ cells. The percent cycling cells and mitotic indices of WT and lcmt-1 knockout fetal liver cells are similar, suggesting that hypocellularity may be due to a combination of apoptosis and/or defects in specification, self-renewal, or survival of stem cells. Indicative of a possible intrinsic defect in stem cells, noncompetitive and competitive transplantation experiments reveal that lcmt-1 loss causes a severe multilineage hematopoietic repopulating defect. Therefore, this study reveals a novel role for LCMT-1 as a key player in fetal liver hematopoiesis.

Antiphospholipid antibodies induce thrombosis by PP2A activation via apoER2-Dab2-SHC1 complex formation in endothelium

AbstractIn the antiphospholipid syndrome (APS), antiphospholipid antibody (aPL) recognition of β2 glycoprotein I promotes thrombosis, and preclinical studies indicate that this is due to endothelial nitric oxide synthase (eNOS) antagonism via apolipoprotein E receptor 2 (apoER2)-dependent processes. How apoER2 molecularly links these events is unknown. Here, we show that, in endothelial cells, the apoER2 cytoplasmic tail serves as a scaffold for aPL-induced assembly and activation of the heterotrimeric protein phosphatase 2A (PP2A). Disabled-2 (Dab2) recruitment to the apoER2 NPXY motif promotes the activating L309 methylation of the PP2A catalytic subunit by leucine methyl transferase-1. Concurrently, Src homology domain-containing transforming protein 1 (SHC1) recruits the PP2A scaffolding subunit to the proline-rich apoER2 C terminus along with 2 distinct regulatory PP2A subunits that mediate inhibitory dephosphorylation of Akt and eNOS. In mice, the coupling of these processes in endothelium is demonstrated to underlie aPL-invoked thrombosis. By elucidating these intricacies in the pathogenesis of APS-related thrombosis, numerous potential new therapeutic targets have been identified.

A novel function of twins, B subunit of protein phosphatase 2A, in regulating actin polymerization.

Actin is an important component of the cytoskeleton and its polymerization is delicately regulated by several kinases and phosphatases. Heterotrimeric protein phosphatase 2A (PP2A) is a potent phosphatase that is crucial for cell proliferation, apoptosis, tumorigenesis, signal transduction, cytoskeleton arrangement, and neurodegeneration. To facilitate these varied functions, different regulators determine the different targets of PP2A. Among these regulators of PP2A, the B subunits in particular may be involved in cytoskeleton arrangement. However, little is known about the role of PP2A in actin polymerization in vivo. Using sophisticated fly genetics, we demonstrated a novel function for the fly B subunit, twins, to promote actin polymerization in varied tissue types, suggesting a broad and conserved effect. Furthermore, our genetic data suggest that twins may act upstream of the actin-polymerized-proteins, Moesin and Myosin-light-chain, and downstream of Rho to promote actin polymerization. This work opens a new avenue for exploring the biological functions of a PP2A regulator, twins, in cytoskeleton regulation.

Inhibition of AMP Kinase by the Protein Phosphatase 2A Heterotrimer, PP2APpp2r2d

AMP kinase is a heterotrimeric serine/threonine protein kinase that regulates a number of metabolic processes, including lipid biosynthesis and metabolism. AMP kinase activity is regulated by phosphorylation, and the kinases involved have been uncovered. The particular phosphatases counteracting these kinases remain elusive. Here we discovered that the protein phosphatase 2A heterotrimer, PP2A(Ppp2r2d), regulates the phosphorylation state of AMP kinase by dephosphorylating Thr-172, a residue that activates kinase activity when phosphorylated. Co-immunoprecipitation and co-localization studies indicated that PP2A(Ppp2r2d) directly interacted with AMP kinase. PP2A(Ppp2r2d) dephosphorylated Thr-172 in rat aortic and human vascular smooth muscle cells. A positive correlation existed between decreased phosphorylation, decreased acetyl-CoA carboxylase Acc1 phosphorylation, and sterol response element-binding protein 1c-dependent gene expression. PP2A(Ppp2r2d) protein expression was up-regulated in the aortas of mice fed a high fat diet, and the increased expression correlated with increased blood lipid levels. Finally, we found that the aortas of mice fed a high fat diet had decreased AMP kinase Thr-172 phosphorylation, and contained an Ampk-PP2A(Ppp2r2d) complex. Thus, PP2A(Ppp2r2d) may antagonize the aortic AMP kinase activity necessary for maintaining normal aortic lipid metabolism. Inhibiting PP2A(Ppp2r2d) or activating AMP kinase represents a potential pharmacological treatment for many lipid-related diseases.

Arabidopsis PHOSPHOTYROSYL PHOSPHATASE ACTIVATOR is essential for PROTEIN PHOSPHATASE 2A holoenzyme assembly and plays important roles in hormone signaling, salt stress response, and plant development.

PROTEIN PHOSPHATASE 2A (PP2A) is a major group of serine/threonine protein phosphatases in eukaryotes. It is composed of three subunits: scaffolding subunit A, regulatory subunit B, and catalytic subunit C. Assembly of the PP2A holoenzyme in Arabidopsis (Arabidopsis thaliana) depends on Arabidopsis PHOSPHOTYROSYL PHOSPHATASE ACTIVATOR (AtPTPA). Reduced expression of AtPTPA leads to severe defects in plant development, altered responses to abscisic acid, ethylene, and sodium chloride, and decreased PP2A activity. In particular, AtPTPA deficiency leads to decreased methylation in PP2A-C subunits (PP2Ac). Complete loss of PP2Ac methylation in the suppressor of brassinosteroid insensitive1 mutant leads to 30% reduction of PP2A activity, suggesting that PP2A with a methylated C subunit is more active than PP2A with an unmethylated C subunit. Like AtPTPA, PP2A-A subunits are also required for PP2Ac methylation. The interaction between AtPTPA and PP2Ac is A subunit dependent. In addition, AtPTPA deficiency leads to reduced interactions of B subunits with C subunits, resulting in reduced functional PP2A holoenzyme formation. Thus, AtPTPA is a critical factor for committing the subunit A/subunit C dimer toward PP2A heterotrimer formation.

Abstract 5278: Superoxide mediated selective tyrosine nitration of protein phosphatase 2A-B56δ stabilizes Bcl-2 phosphorylation and its anti-apoptotic activity

Abstract Our work over the years has provided strong evidence that an increase in intracellular superoxide anion provides cancer cells with a survival advantage and promotes tumor chemoresistance. The mechanisms underlying superoxide-mediated pro-survival signaling are poorly understood. Here we report that an increase in intracellular superoxide stabilizes the anti-apoptotic activity of Bcl-2 by mechanisms that involve sustained phosphorylation of Bcl-2 at serine 70. Of note, this effect on Bcl-2 S70 phosphorylation is a function of redox-dependent inactivation of Protein Phosphatase 2A (PP2A), which is targeted to Bcl-2 via its B56 subunit; superoxide induces the release of PP2A-AC catalytic core from Bcl-2-bound B56δ, resulting in sustained phosphorylation of Bcl-2 at S70. Dissociation of the PP2A heterotrimer results from the selective nitration (by peroxynitrite derived from the reaction of superoxide with NO) of a conserved tyrosine residue, which we identify as Y289 of B56δ (B56 δY289). Inhibition of B56δY289 nitration blocked the pro-survival effects of superoxide. Furthermore, we provide evidence for clinical relevance of these findings using primary tissue derived from human lymphoma biopsies. Taken together, these data provide evidence for a novel mechanism in which an increase in intracellular superoxide regulates PP2A function via specific B56 subunit tyrosine nitration, and that PP2A-B56 nitration may be a general mechanism for redox-regulated cellular signaling. Citation Format: Ivan C. C. Low, Thomas Loh, David Virshup, Shazib Pervaiz. Superoxide mediated selective tyrosine nitration of protein phosphatase 2A-B56δ stabilizes Bcl-2 phosphorylation and its anti-apoptotic activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5278. doi:10.1158/1538-7445.AM2014-5278

A Functional Interplay between the Small GTPase Rab11a and Mitochondria-shaping Proteins Regulates Mitochondrial Positioning and Polarization of the Actin Cytoskeleton Downstream of Src Family Kinases

It is believed that mitochondrial dynamics is coordinated with endosomal traffic rates during cytoskeletal remodeling, but the mechanisms involved are largely unknown. The adenovirus early region 4 ORF4 protein (E4orf4) subverts signaling by Src family kinases (SFK) to perturb cellular morphology, membrane traffic, and organellar dynamics and to trigger cell death. Using E4orf4 as a model, we uncovered a functional connection between mitochondria-shaping proteins and the small GTPase Rab11a, a key regulator of polarized transport via recycling endosomes. We found that E4orf4 induced dramatic changes in the morphology of mitochondria along with their mobilization at the vicinity of a polarized actin network typifying E4orf4 action, in a manner controlled by SFK and Rab11a. Mitochondrial remodeling was associated with increased proximity between Rab11a and mitochondrial membranes, changes in fusion-fission dynamics, and mitochondrial relocalization of the fission factor dynamin-related protein 1 (Drp1), which was regulated by the Rab11a effector protein FIP1/RCP. Knockdown of FIP1/RCP or inhibition of Drp1 markedly impaired mitochondrial remodeling and actin assembly, involving Rab11a-mediated mitochondrial dynamics in E4orf4-induced signaling. A similar mobilization of mitochondria near actin-rich structures was mediated by Rab11 and Drp1 in viral Src-transformed cells and contributed to the biogenesis of podosome rosettes. These findings suggest a role for Rab11a in the trafficking of Drp1 to mitochondria upon SFK activation and unravel a novel functional interplay between Rab11a and mitochondria during reshaping of the cell cytoskeleton, which would facilitate mitochondria redistribution near energy-requiring actin-rich structures.

Partial Folding and Unique ATP Function in Protein Phosphatase 2A Latency and Activation

The function of metalloenzyme protein phosphatase 2A (PP2A) relies on proper formation of diverse heterotrimeric PP2A holoenzymes, comprising common scaffold and catalytic (PP2Ac) subunits and a variable regulatory subunit. Holoenzyme assembly is highly regulated by carboxylmethylation of PP2Ac. Our recent study demonstrated that the enzymatic reaction of PP2A methylation is highly responsive to PP2A activation via a direct binding of PP2A methyltransferase to the dynamic PP2A active site (Mol. Cell. 2011, 41(3):331-42). Interestingly, the entire protein fold of PP2Ac is highly dynamic and tends to partially unfold, which renders α4-binding that involves the inner structure of PP2Ac near the active site. α4-binding leads to an allosteric relay of conformational changes that perturbs the scaffold subunit binding site at the opposite surface. This unique mode of α4-binding underlies important mechanisms for stably latency of PP2Ac as well as dissociation of PP2A holoenzymes and recycling of PP2Ac in response to cell signaling. We further showed that PP2A phosphatase activator makes broad contacts with the structural elements surrounding the PP2A active site, which stabilizes the protein fold of apo-PP2Ac required for activation. PTPA-binding also defines a combined ATP-binding pocket that 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 drastically enhances binding of Mg2+ by 10,000-fold, which is crucial for acquisition of pSer/Thr-specific phosphatase activity. Consistent with the activation chaperone function of PTPA and activation-dependent PP2A methylation, PTPA stimulates both PP2A phosphatase activity and methylation. Collectively, our studies revealed a linear pathway for the biogenesis of PP2A holoenzymes, including stable latency, activation, methylation, and holoenzyme assembly, which is elegantly controlled by the protein dynamics and conformational switches of PP2Ac.

Structure of the Ca2+-dependent PP2A heterotrimer and insights into Cdc6 dephosphorylation

The B″/PR72 family of protein phosphatase 2A (PP2A) is an important PP2A family involved in diverse cellular processes, and uniquely regulated by calcium binding to the regulatory subunit. The PR70 subunit in this family interacts with cell division control 6 (Cdc6), a cell cycle regulator important for control of DNA replication. Here, we report crystal structures of the isolated PR72 and the trimeric PR70 holoenzyme at a resolution of 2.1 and 2.4 Å, respectively, and in vitro characterization of Cdc6 dephosphorylation. The holoenzyme structure reveals that one of the PR70 calcium-binding motifs directly contacts the scaffold subunit, resulting in the most compact scaffold subunit conformation among all PP2A holoenzymes. PR70 also binds distinctively to the catalytic subunit near the active site, which is required for PR70 to enhance phosphatase activity toward Cdc6. Our studies provide a structural basis for unique regulation of B″/PR72 holoenzymes by calcium ions, and suggest the mechanisms for precise control of substrate specificity among PP2A holoenzymes.

PP2ACdc55 regulates G1 cyclin stability

Maintaining accurate progression through the cell cycle requires the proper temporal expression and regulation of cyclins. The mammalian D-type cyclins promote G1-S transition. D1 cyclin protein stability is regulated through its ubiquitylation and resulting proteolysis catalyzed by the SCF E3 ubiquitin ligase complex containing the F-box protein, Fbx4. SCF E3-ligase-dependent ubiquitylation of D1 is trigged by an increase in the phosphorylation status of the cyclin. As inhibition of ubiquitin-dependent D1 degradation is seen in many human cancers, we set out to uncover how D-type cyclin phosphorylation is regulated. Here we show that in S. cerevisiae, a heterotrimeric protein phosphatase 2A (PP2ACdc55) containing the mammalian PPP2R2/PR55 B subunit ortholog Cdc55 regulates the stability of the G1 cyclin Cln2 by directly regulating its phosphorylation state. Cells lacking Cdc55 contain drastically reduced Cln2 levels caused by degradation due to cdk-dependent hyperphosphorylation, as a Cln2 mutant unable to be phosphorylated by the yeast cdk Cdc28 is highly stable in cdc55-null cells. Moreover, cdc55-null cells become inviable when the SCFGrr1 activity known to regulate Cln2 levels is eliminated or when Cln2 is overexpressed, indicating a critical relationship between SCF and PP2A functions in regulating cell cycle progression through modulation of G1-S cyclin degradation/stability. In sum, our results indicate that PP2A is absolutely required to maintain G1-S cyclin levels through modulating their phosphorylation status, an event necessary to properly transit through the cell cycle.