Phosphorylated synthetic peptides as tools for studying protein phosphatases

KEPI plays a negative role in the repression that accompanies translational inhibition guided by the uORF element of human CHOP transcript during stress response

Translation of the downstream coding sequence of some mRNAs may be repressed by the upstream open reading frame (uORF) at their 5'-end. The mechanism underlying this uORF-mediated translational inhibition (uORF-MTI) is not fully understood in vivo. Recently, it was found that zebrafish Endouc or its human orthologue ENDOU (Endouc/ENDOU) plays a positive role in repressing the uORF-MTI of human CHOP (uORF^chop-MTI) during stress by blocking its activity However, the repression of uORF^chop-MTI assisted by an as-yet unidentified negative effector remains to be elucidated. Compared to the upregulated CHOP transcript, we herein report that the kepi (kinase-enhanced PP1 inhibitor) transcript was downregulated in the zebrafish embryos treated with both heat shock and hypoxia. Quantitative RT-PCR also revealed that the level of kepi mRNA was noticeably decreased in both heat-shock-treated and hypoxia-exposed embryos. When kepi mRNA was microinjected into the one-celled embryos from transgenic line huORFZ, the translation of downstream GFP reporter controlled by the uORF^chop-MTI was reduced in the hypoxia-exposed embryos. In contrast, when kepi was knocked down by injection of antisense Morpholino oligonucleotide, the translation of downstream GFP reporter was induced and expressed in the brain and spinal cord of injected embryos in the absence of stress. During normal condition, overexpression of KEPI increased eIF2α phosphorylation, resulting in inducing the translation of uORF-tag mRNA, such as ATF4 and CHOP mRNAs. However, during stress condition, overexpression of KEPI decreased eIF2α phosphorylation, resulting in reducing the GFP reporter and CHOP proteins. This is the first report to demonstrate that KEPI plays a negative role in uORF^chop - mediated translation during ER stress.

Phosphatase and Kinase Substrate Specificity Profiling with Pooled Synthetic Peptides and Mass Spectrometry

Reversible phosphorylation is a pervasive regulatory event in cellular physiology controlled by reciprocal actions of protein kinases and phosphatases. Determining the inherent substrate specificity of kinases and phosphatases is essential for understanding their cellular roles. Synthetic peptides have long served as substrate proxies for defining intrinsic kinase and phosphatase specificities. Here, we describe a high throughput protocol to simultaneously measure specificity constants (k_cat/K_M) of many synthetic peptide substrates in a single pool using label-free quantitative mass spectrometry. The generation of specificity constants from a single pooled reaction provides a rigorous and rapid comparison of substrate variants to help define an enzyme's specificity. Equally applicable to kinases and phosphatases, as well as other enzyme classes, the protocol consists of three general steps: (1) reaction of enzyme with pooled peptide substrates, each ideally with a unique mass and at concentrations well below K_M, (2) analysis of reaction products using liquid chromatography-coupled mass spectrometry (LC-MS), and (3) automated extraction and integration of elution peaks for each substrate/product pair. We incorporate an ionization correction strategy allowing direct calculation of reaction progress, and subsequently k_cat/K_M, from substrate and product peak areas in a single sample, obviating the need for stable isotope labeling. Peptide consumption is minimal, and high peptide purity and accurate concentrations are not required. Access to a high-resolution LC-MS system is the only nonstandard equipment need. We present an analysis pipeline consisting entirely of established open-source software tools, and demonstrate proof of principle with the highly selective cell cycle phosphatase Cdc14 from Saccharomyces cerevisiae.

Evolutionary crossroads of cell signaling: PP1 and PP2A substrate sites in intrinsically disordered regions

Phosphorylation of the hydroxyl group of the amino acids serine and threonine is among the most prevalent post-translational modifications in mammalian cells. Phospho-serine (pSer) and -threonine (pThr) represent a central cornerstone in the cell's toolbox for adaptation to signal input. The true power for the fast modulation of the regulatory pSer/pThr sites arises from the timely attachment, binding and removal of the phosphate. The phosphorylation of serine and threonine by kinases and the binding of pSer/pThr by phosphorylation-dependent scaffold proteins is largely determined by the sequence motif surrounding the phosphorylation site (p-site). The removal of the phosphate is regulated by pSer/pThr-specific phosphatases with the two most prominent ones being PP1 and PP2A. For this family, recent advances brought forward a more complex mechanism for p-site selection. The interaction of regulatory proteins with the substrate protein constitutes a first layer for substrate recognition, but also interactions of the catalytic subunit with the amino acids in close proximity to pSer/pThr contribute to p-site selection. Here, we review the current pieces of evidence for this multi-layered, complex mechanism and hypothesize that, depending on the degree of higher structure surrounding the substrate site, recognition is more strongly influenced by regulatory subunits away from the active site for structured substrate regions, whereas the motif context is of strong relevance with p-sites in disordered regions. The latter makes these amino acid sequences crossroads for signaling and motif strength between kinases, pSer/pThr-binding proteins and phosphatases.

Dissecting the sequence determinants for dephosphorylation by the catalytic subunits of phosphatases PP1 and PP2A

The phosphatases PP1 and PP2A are responsible for the majority of dephosphorylation reactions on phosphoserine (pSer) and phosphothreonine (pThr), and are involved in virtually all cellular processes and numerous diseases. The catalytic subunits exist in cells in form of holoenzymes, which impart substrate specificity. The contribution of the catalytic subunits to the recognition of substrates is unclear. By developing a phosphopeptide library approach and a phosphoproteomic assay, we demonstrate that the specificity of PP1 and PP2A holoenzymes towards pThr and of PP1 for basic motifs adjacent to the phosphorylation site are due to intrinsic properties of the catalytic subunits. Thus, we dissect this amino acid specificity of the catalytic subunits from the contribution of regulatory proteins. Furthermore, our approach enables discovering a role for PP1 as regulator of the GRB-associated-binding protein 2 (GAB2)/14-3-3 complex. Beyond this, we expect that this approach is broadly applicable to detect enzyme-substrate recognition preferences.

The substrate specificity of phosphoprotein phosphatases PP1 and PP2A depends on their catalytic and regulatory subunits. Using proteomics approaches, the authors here provide insights into the sequence specificity of the catalytic subunits and their distinct contributions to PP1 and PP2A selectivity.

Phosphoproteome and drug-response effects mediated by the three protein phosphatase 2A inhibitor proteins CIP2A, SET, and PME-1

Protein phosphatase 2A (PP2A) critically regulates cell signaling and is a human tumor suppressor. PP2A complexes are modulated by proteins such as cancerous inhibitor of protein phosphatase 2A (CIP2A), protein phosphatase methylesterase 1 (PME-1), and SET nuclear proto-oncogene (SET) that often are deregulated in cancers. However, how they impact cellular phosphorylation and how redundant they are in cellular regulation is poorly understood. Here, we conducted a systematic phosphoproteomics screen for phosphotargets modulated by siRNA-mediated depletion of CIP2A, PME-1, and SET (to reactivate PP2A) or the scaffolding A-subunit of PP2A (PPP2R1A) (to inhibit PP2A) in HeLa cells. We identified PP2A-modulated targets in diverse cellular pathways, including kinase signaling, cytoskeleton, RNA splicing, DNA repair, and nuclear lamina. The results indicate nonredundancy among CIP2A, PME-1, and SET in phosphotarget regulation. Notably, PP2A inhibition or reactivation affected largely distinct phosphopeptides, introducing a concept of nonoverlapping phosphatase inhibition- and activation-responsive sites (PIRS and PARS, respectively). This phenomenon is explained by the PPP2R1A inhibition impacting primarily dephosphorylated threonines, whereas PP2A reactivation results in dephosphorylation of clustered and acidophilic sites. Using comprehensive drug-sensitivity screening in PP2A-modulated cells to evaluate the functional impact of PP2A across diverse cellular pathways targeted by these drugs, we found that consistent with global phosphoproteome effects, PP2A modulations broadly affect responses to more than 200 drugs inhibiting a broad spectrum of cancer-relevant targets. These findings advance our understanding of the phosphoproteins, pharmacological responses, and cellular processes regulated by PP2A modulation and may enable the development of combination therapies.

Catalytic Chemoselective O-Phosphorylation of Alcohols

Phosphorylation of alcohols is a fundamentally important reaction in both life science and physical science. Product phosphate monoesters play key roles in living organisms, natural products, pharmaceuticals, and organic materials. Most of the chemical methods to date for synthesizing phosphate monoesters, however, require multistep sequences or are limited to specific types of substrates possibly due to harsh conditions. An alternative way to enable the simple production of phosphate monoesters from highly functionalized precursor alcohols is, thus, highly desired. We report herein a catalytic phosphorylation of alcohols with high functional group tolerance using tetrabutylammonium hydrogen sulfate (TBAHS) and phosphoenolpyruvic acid monopotassium salt (PEP-K) as the catalyst and phosphoryl donor, respectively. This method enables the direct introduction of a nonprotected phosphate group to the hydroxy group of a diverse menu of alcohol substrates, including functionalized small molecules, carbohydrates, and unprotected peptides. Nuclear magnetic resonance, mass spectrometric, and density functional theory analyses suggest that an unprecedented mixed anhydride species, generated from PEP-K and TBAHS, acts as an active phosphoryl donor in this reaction. This operationally simple and chemoselective catalytic phosphorylation allows for the efficient production of densely functionalized O-phosphorylated compounds, which are useful in diverse fields including biology and medicine.

In vivo evidence for a regulatory role of phosphorylation of Arabidopsis Rubisco activase at the Thr78 site

Arabidopsis Rubisco activase (Rca) is phosphorylated at threonine-78 (Thr78) in low light and in the dark, suggesting a potential regulatory role in photosynthesis, but this has not been directly tested. To do so, we transformed an rca-knockdown mutant largely lacking redox regulation with wild-type Rca-β or Rca-β with Thr78-to-Ala (T78A) or Thr78-to-Ser (T78S) site-directed mutations. Interestingly, the T78S mutant was hyperphosphorylated at the Ser78 site relative to Thr78 of the Rca-β wild-type control, as evidenced by immunoblotting with custom antibodies and quantitative mass spectrometry. Moreover, plants expressing the T78S mutation had reduced photosynthesis and quantum efficiency of photosystem II (ϕ_PSII) and reduced growth relative to control plants expressing wild-type Rca-β under all conditions tested. Gene expression was also altered in a manner consistent with reduced growth. In contrast, plants expressing Rca-β with the phospho-null T78A mutation had faster photosynthetic induction kinetics and increased ϕ_PSII relative to Rca-β controls. While expression of the wild-type Rca-β or the T78A mutant fully rescued the slow-growth phenotype of the rca-knockdown mutant grown in a square-wave light regime, the T78A mutants grew faster than the Rca-β control plants at low light (30 µmol photons m^-2 s^-1) and in a fluctuating low-light/high-light environment. Collectively, these results suggest that phosphorylation of Thr78 (or Ser78 in the T78S mutant) plays a negative regulatory role in vivo and provides an explanation for the absence of Ser at position 78 in terrestrial plant species.

LncRNAs-directed PTEN enzymatic switch governs epithelial-mesenchymal transition

Despite the structural conservation of PTEN with dual-specificity phosphatases, there have been no reports regarding the regulatory mechanisms that underlie this potential dual-phosphatase activity. Here, we report that K27-linked polyubiquitination of PTEN at lysines 66 and 80 switches its phosphoinositide/protein tyrosine phosphatase activity to protein serine/threonine phosphatase activity. Mechanistically, high glucose, TGF-β, CTGF, SHH, and IL-6 induce the expression of a long non-coding RNA, GAEA (Glucose Aroused for EMT Activation), which associates with an RNA-binding E3 ligase, MEX3C, and enhances its enzymatic activity, leading to the K27-linked polyubiquitination of PTEN. The MEX3C-catalyzed PTENK27-polyUb activates its protein serine/threonine phosphatase activity and inhibits its phosphatidylinositol/protein tyrosine phosphatase activity. With this altered enzymatic activity, PTENK27-polyUb dephosphorylates the phosphoserine/threonine residues of TWIST1, SNAI1, and YAP1, leading to accumulation of these master regulators of EMT. Animals with genetic inhibition of PTENK27-polyUb, by a single nucleotide mutation generated using CRISPR/Cas9 (PtenK80R/K80R), exhibit inhibition of EMT markers during mammary gland morphogenesis in pregnancy/lactation and during cutaneous wound healing processes. Our findings illustrate an unexpected paradigm in which the lncRNA-dependent switch in PTEN protein serine/threonine phosphatase activity is important for physiological homeostasis and disease development.

From phosphoproteins to phosphoproteomes: a historical account

The first phosphoprotein (casein) was discovered in 1883, yet the enzyme responsible for its phosphorylation was identified only 130 years later, in 2012. In the intervening time, especially in the last decades of the 1900s, it became evident that, far from being an oddity, phosphorylation affects the majority of eukaryotic proteins during their lifespan, and that this reaction is catalysed by the members of a large family of protein kinases, susceptible to a variety of stimuli controlling nearly every aspect of life and death. The aim of this review is to present a historical account of the main steps of this spectacular revolution, which transformed our conception of a biochemical reaction originally held as a sporadic curiosity into the master mechanism governing cell regulation, and, if it is perturbed, causing cell dysregulation.

Temporal Analysis of PP2A Phosphatase Activity During Insulin Stimulation Using a Direct Activity Probe

Protein serine/threonine phosphatases (PSPs) are ubiquitously expressed in mammalian cells. In particular, PP2A accounts for up to 1% of the total protein within cells. Despite clear evidence for the role of PP2A in cellular signaling, there is a lack of information concerning the magnitude and temporal dynamics of PP2A catalytic activity during insulin stimulation. Herein, we describe the development of a direct, fluorescent activity probe capable of reporting on global changes in PP2A enzymatic activity in unfractionated cell lysates. Utilizing this new probe, we profiled the magnitude as well as temporal dynamics of PP2A activity during insulin stimulation of liver hepatocytes. These results provide direct evidence for the rapid response of PP2A catalytic activity to extracellular stimulation, as well as insight into the complex regulation of phosphorylation levels by opposing kinase and phosphatase activities within the cell. This study provides a new tool for investigating the chemical biology of PSPs.

Regulatory roles of phosphorylation in model and pathogenic fungi

Over the past 20 years, considerable advances have been made toward our understanding of how post-translational modifications affect a wide variety of biological processes, including morphology and virulence, in medically important fungi. Phosphorylation stands out as a key molecular switch and regulatory modification that plays a critical role in controlling these processes. In this article, we first provide a comprehensive and up-to-date overview of the regulatory roles that both Ser/Thr and non-Ser/Thr kinases and phosphatases play in model and pathogenic fungi. Next, we discuss the impact of current global approaches that are being used to define the complete set of phosphorylation targets (phosphoproteome) in medically important fungi. Finally, we provide new insights and perspectives into the potential use of key regulatory kinases and phosphatases as targets for the development of novel and more effective antifungal strategies.

Chasing Phosphoarginine Proteins: Development of a Selective Enrichment Method Using a Phosphatase Trap

Arginine phosphorylation is an emerging post-translational protein modification implicated in the bacterial stress response. Although early reports suggested that arginine phosphorylation also occurs in higher eukaryotes, its overall prevalence was never studied using modern mass spectrometry methods, owing to technical difficulties arising from the acid lability of phosphoarginine. As shown recently, the McsB and YwlE proteins from Bacillus subtilis function as a highly specific protein arginine kinase and phosphatase couple, shaping the phosphoarginine proteome. Using a B. subtilis ΔywlE strain as a source for arginine-phosphorylated proteins, we were able to adapt mass spectrometry (MS) protocols to the special chemical properties of the arginine modification. Despite this progress, the analysis of protein arginine phosphorylation in eukaryotes is still challenging, given the great abundance of serine/threonine phosphorylations that would compete with phosphoarginine during the phosphopeptide enrichment procedure, as well as during data-dependent MS acquisition. We thus set out to establish a method for the selective enrichment of arginine-phosphorylated proteins as an initial step in the phosphoproteomic analysis. For this purpose, we developed a substrate-trapping mutant of the YwlE phosphatase that retains binding affinity toward arginine-phosphorylated proteins but cannot hydrolyze the captured substrates. By testing a number of active site substitutions, we identified a YwlE mutant (C9A) that stably binds to arginine-phosphorylated proteins. We further improved the substrate-trapping efficiency by impeding the oligomerization of the phosphatase mutant. The engineered YwlE trap efficiently captured arginine-phosphorylated proteins from complex B. subtilis ΔywlE cell extracts, thus facilitating identification of phosphoarginine sites in the large pool of cellular protein modifications. In conclusion, we present a novel tool for the selective enrichment and subsequent MS analysis of arginine phosphorylation, which is a largely overlooked protein modification that might be important for eukaryotic cell signaling.

Efficient delivery of cell impermeable phosphopeptides by a cyclic peptide amphiphile containing tryptophan and arginine

Phosphopeptides are valuable reagent probes for studying protein-protein and protein-ligand interactions. The cellular delivery of phosphopeptides is challenging because of the presence of the negatively charged phosphate group. The cellular uptake of a number of fluorescent-labeled phosphopeptides, including F'-GpYLPQTV, F'-NEpYTARQ, F'-AEEEIYGEFEAKKKK, F'-PEpYLGLD, F'-pYVNVQN-NH2, and F'-GpYEEI (F' = fluorescein), was evaluated in the presence or absence of a [WR]4, a cyclic peptide containing alternative arginine (R) and tryptophan (W) residues, in human leukemia cells (CCRF-CEM) after 2 h incubation using flow cytometry. [WR]4 improved significantly the cellular uptake of all phosphopeptides. PEpYLGLD is a sequence that mimics the pTyr1246 of ErbB2 that is responsible for binding to the Chk SH2 domain. The cellular uptake of F'-PEpYLGLD was enhanced dramatically by 27-fold in the presence of [WR]4 and was found to be time-dependent. Confocal microscopy of a mixture of F'-PEpYLGLD and [WR]4 in live cells exhibited intracellular localization and significantly higher cellular uptake compared to that of F'-PEpYLGLD alone. Transmission electron microscopy (TEM) and isothermal calorimetry (ITC) were used to study the interaction of PEpYLGLD and [WR]4. TEM results showed that the mixture of PEpYLGLD and [WR]4 formed noncircular nanosized structures with width and height of 125 and 60 nm, respectively. ITC binding studies confirmed the interaction between [WR]4 and PEpYLGLD. The binding isotherm curves, derived from sequential binding models, showed an exothermic interaction driven by entropy. These studies suggest that amphiphilic peptide [WR]4 can be used as a cellular delivery tool of cell-impermeable negatively charged phosphopeptides.

Substrate-dependent metal preference of PPM1H, a cancer-associated protein phosphatase 2C: comparison with other family members

Protein phosphatase 2C (PP2C) family is characterized by requirement of metal cation for phosphatase activity. We previously established that PPM1H is a cancer-associated member of the PP2C family. Here we further characterized the phosphatase activity of PPM1H, focusing on its dependence on metal cation. PPM1H possesses the potential to dephosphorylate p-nitrophenyl phosphate (pNPP), casein and phosphopeptides. Interestingly, PPM1H shows the metal preference that is varied depending on the substrate (substrate-dependent metal preference); PPM1H prefers Mn(2+) when pNPP or phosphopeptides is used as a substrate. Meanwhile, a preference for Mg(2+) is displayed by PPM1H with casein as a substrate. When both cations are added to the reaction, the degree of the effect is always closer to that with Mn(2+) alone, irrespective of the substrate. This preponderance of Mn(2+) is explained by its greater affinity for PPM1H than Mg(2+). From the literature the substrate-dependent metal preference appears to be shared by other PP2Cs. According to the crystal structure, a binuclear metal center of PP2C plays an important role for coordinating the substrate and nucleophilic waters in the active site. Therefore, the differences in the size, preferred geometry and coordination requirements between two metals, in relation to the substrate, may be responsible for this intriguing property.

PKC and Rho in vascular smooth muscle: activation by BOXes and SAH CSF

Cerebral vasospasm (CV) remains a significant cause of delayed neurological deficit and ischemic damage after subarachnoid hemorrhage (SAH), despite intensive research effort. The current lack of an effective therapeutic approach is somewhat due to our lack of understanding regarding the mechanism by which this pathological constriction develops. Recent evidence implicates bilirubin oxidation products (BOXes) in the etiology of CV after SAH: BOXes are found in cerebrospinal fluid from SAH patients with symptomatic or angiographically visible vasospasm (CSFV) but not in CSF from SAH patients with no vasospasm (CSFC). We have previously published research suggesting that the etiology of CV comprises two components: a physiological stimulation to constrict and a pathological failure to relax. Both these components are elicited by CSFV, but not CSFC, and BOXes synthesized in the laboratory potentiate physiological constriction in arterial smooth muscle in vitro, and elicit contraction in pial arteries in vivo. In this paper, we will present our results concerning the action of BOXes on arterial smooth muscle constriction, compared with CSFV. We will also present evidence implicating temporal changes in PKC isoforms and Rho expression in both BOXes- and CSFV-elicited smooth muscle responses.

The Wip1 phosphatase PPM1D dephosphorylates SQ/TQ motifs in checkpoint substrates phosphorylated by PI3K-like kinases

The wild-type p53-induced phosphatase Wip1 (PP2Cdelta or PPM1D) is a member of the protein phosphatase 2C (PP2C) family and controls cell cycle checkpoints in response to DNA damage. p38 MAPK and ATM were identified as physiological substrates of Wip1, and we previously reported a substrate motif that was defined using variants of the p38(180pT 182pY) diphosphorylated peptide, TDDEMpTGpYVAT. However, the substrate recognition motifs for Wip1 have not been fully defined as the sequences surrounding the targeted residues in ATM and p38 MAPK appear to be unrelated. Using a recombinant human Wip1 catalytic domain (rWip1), in this study we measured the kinetic parameters for variants of the ATM(1981pS) phosphopeptide, AFEEGpSQSTTI. We found that rWip1 dephosphorylates phosphoserine and phosphothreonine in the p(S/T)Q motif, which is an essential requirement for substrate recognition. In addition, acidic, hydrophobic, or aromatic amino acids surrounding the p(S/T)Q sequence have a positive influence, while basic amino acids have a negative influence on substrate dephosphorylation. The kinetic constants allow discrimination between true substrates and nonsubstrates of Wip1, and we identified several new putative substrates that include HDM2, SMC1A, ATR, and Wip1 itself. A three-dimensional molecular model of Wip1 with a bound substrate peptide and site-directed mutagenesis analyses suggested that the important residues for ATM(1981pS) substrate recognition are similar but not identical to those for the p38(180pT 182pY) substrate. Results from this study should be useful for predicting new physiological substrates that may be regulated by Wip1 and for developing selective anticancer drugs.

C3G mediated suppression of malignant transformation involves activation of PP2A phosphatases at the subcortical actin cytoskeleton

In previous work, we demonstrated that C3G suppresses Ras oncogenic transformation by a mechanism involving inhibition of ERK phosphorylation. Here we present evidences indicating that this suppression mechanism is mediated, at least in part, by serine/threonine phosphatases of the PP2A family. Thus: (i) ectopic expression of C3G or C3GDeltaCat (mutant lacking the GEF activity) increases specific ERK-associated PP2A phosphatase activities; (ii) C3G and PP2A interact, as demonstrated by immunofluorescence and co-immunoprecipitation experiments; (iii) association between PP2A and MEK or ERK increases in C3G overexpressing cells; (iv) phosphorylated-inactive PP2A level decreases in C3G expressing clones and, most importantly, (v) okadaic acid reverts the inhibitory effect of C3G on ERK phosphorylation. Moreover, C3G interacts with Ksr-1, a scaffold protein of the Ras-ERK pathway that also associates with PP2A. The fraction of C3G involved in transformation suppression is restricted to the subcortical actin cytoskeleton where it interacts with actin. Furthermore, the association between C3G and PP2A remains stable even after cytoskeleton disruption with cytochalasin D, suggesting that the three proteins form a complex at this subcellular compartment. Finally, C3G- and C3GDeltaCat-mediated inhibition of ERK phosphorylation is reverted by incubation with cytochalasin D. We hypothesize that C3G triggers PP2A activation and binding to MEK and ERK at the subcortical actin cytoskeleton, thus favouring ERK dephosphorylation.

Mechanisms of specificity in protein phosphorylation

A typical protein kinase must recognize between one and a few hundred bona fide phosphorylation sites in a background of approximately 700,000 potentially phosphorylatable residues. Multiple mechanisms have evolved that contribute to this exquisite specificity, including the structure of the catalytic site, local and distal interactions between the kinase and substrate, the formation of complexes with scaffolding and adaptor proteins that spatially regulate the kinase, systems-level competition between substrates, and error-correction mechanisms. The responsibility for the recognition of substrates by protein kinases appears to be distributed among a large number of independent, imperfect specificity mechanisms.

Synthesis and evaluation of tripodal peptide analogues for cellular delivery of phosphopeptides

Tripodal peptide analogues were designed on the basis of the phosphotyrosine binding pocket of the Src SH2 domain and assayed for their ability to bind to fluorescein-labeled phosphopeptides. Fluorescence polarization assays showed that a number of amphipathic linear peptide analogues (LPAs), such as LPA4, bind to fluorescein-labeled GpYEEI (F-GpYEEI). LPA4 was evaluated for potential application in cellular delivery of phosphopeptides. Fluorescence microimaging cellular uptake studies with fluorescein-attached LPA4 (F-LPA4) alone or with the mixture of LPA4 and F-GpYEEI in BT-20 cells showed dramatic increase of the fluorescence intensity in cytosol of cells, indicating that LPA4 can function as a delivery tool of F-GpYEEI across the cell membrane. Fluorescent flow cytometry studies showed the cellular uptake of F-LPA4 in an energy-independent pathway and confirmed the cellular uptake of F-GpYEEI in the presence of LPA4. These studies suggest that amphipathic tripodal peptide analogues, such as LPA4, can be used for cellular delivery of phosphopeptides.

Caged O-phosphorothioyl amino acids as building blocks for Fmoc-based solid phase peptide synthesis

The synthesis of 1-(2-nitrophenylethyl) caged O-phosphorothioylserine, -threonine and -tyrosine derivatives is reported. These amino acid building blocks can be directly incorporated into peptides by Fmoc-based solid phase synthesis as their pentafluorophenyl esters or as symmetric anhydrides. Upon irradiation with UV light, the thiophosphate group, representing a hydrolysis resistant phosphate analog, is revealed.

Mechanisms of checkpoint kinase Rad53 inactivation after a double-strand break in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, double-strand breaks (DSBs) activate DNA checkpoint pathways that trigger several responses including a strong G(2)/M arrest. We have previously provided evidence that the phosphatases Ptc2 and Ptc3 of the protein phosphatase 2C type are required for DNA checkpoint inactivation after a DSB and probably dephosphorylate the checkpoint kinase Rad53. In this article we have investigated further the interactions between Ptc2 and Rad53. We showed that forkhead-associated domain 1 (FHA1) of Rad53 interacts with a specific threonine of Ptc2, T376, located outside its catalytic domain in a TXXD motif which constitutes an optimal FHA1 binding sequence in vitro. Mutating T376 abolishes Ptc2 interaction with the Rad53 FHA1 domain and results in adaptation and recovery defects following a DSB. We found that Ckb1 and Ckb2, the regulatory subunits of the protein kinase CK2, are necessary for the in vivo interaction between Ptc2 and the Rad53 FHA1 domain, that Ckb1 binds Ptc2 in vitro and that ckb1Delta and ckb2Delta mutants are defective in adaptation and recovery after a DSB. Our data thus strongly suggest that CK2 is the kinase responsible for the in vivo phosphorylation of Ptc2 T376.

The toxofilin-actin-PP2C complex of Toxoplasma: identification of interacting domains

Toxofilin is a 27 kDa protein isolated from the human protozoan parasite Toxoplasma gondii, which causes toxoplasmosis. Toxofilin binds to G-actin, and in vitro studies have shown that it controls elongation of actin filaments by sequestering actin monomers. Toxofilin affinity for G-actin is controlled by the phosphorylation status of its Ser53, which depends on the activities of a casein kinase II and a type 2C serine/threonine phosphatase (PP2C). To get insights into the functional properties of toxofilin, we undertook a structure-function analysis of the protein using a combination of biochemical techniques. We identified a domain that was sufficient to sequester G-actin and that contains three peptide sequences selectively binding to G-actin. Two of these sequences are similar to sequences present in several G- and F-actin-binding proteins, while the third appears to be specific to toxofilin. Additionally, we identified two toxofilin domains that interact with PP2C, one of which contains the Ser53 substrate. In addition to characterizing the interacting domains of toxofilin with its partners, the present study also provides information on an in vivo-based approach to selectively and competitively disrupt the protein-protein interactions that are important to parasite motility.

Protein phosphatases types 2Cα and 2Cβ in apoptosis

This mini-review highlights the involvement of PP2C (protein phosphatase type 2C) family members α and β in apoptosis. The activity of these isoenzymes can be stimulated by unsaturated fatty acids with special structural features, e.g. oleic acid. Those fatty acids capable of activating PP2Cα and PP2Cβ in vitro induce apoptosis in various cell types as shown here for neurons and endothelial cells. Using RNA interference to reduce the amount of PP2Cα and PP2Cβ results in cells significantly less susceptible to the apoptotic effect of oleic acid. Increased endothelial cell death is considered to be an initial step of atherogenesis. Thus activation of PP2C by physiological unbound (‘free’) unsaturated fatty acids (liberated from lipoproteins) could represent a crucial mechanism in the development of atherosclerosis.

Continuous assay of protein tyrosine phosphatases based on fluorescence resonance energy transfer

An assay method that continuously measures the protein tyrosine phosphatase (PTP)-catalyzed dephosphorylation reaction based on fluorescence resonance energy transfer (FRET) was developed as an improvement of our previously reported discontinuous version [M. Nishikata, K. Suzuki, Y. Yoshimura, Y. Deyama, A. Matsumoto, Biochem. J. 343 (1999) 385-391]. The assay uses oligopeptide substrates that contain (7-methoxycoumarin-4-yl)acetyl (Mca) group as a fluorescence donor and 2,4-dinitrophenyl (DNP) group as a fluorescence acceptor, in addition to a phosphotyrosine residue located between these two groups. In the assay, a PTP solution is added to a buffer solution containing a FRET substrate and chymotrypsin. The PTP-catalyzed dephosphorylation of the substrate and subsequent chymotryptic cleavage of the dephosphorylated substrate results in a disruption of FRET, thereby increasing Mca fluorescence. In this study, we used FRET substrates that are much more susceptible to chymotryptic cleavage after dephosphorylation than the substrate used in our discontinuous assay, thus enabling the continuous assay without significant PTP inactivation by chymotrypsin. The rate of fluorescence increase strictly reflected the rate of dephosphorylation at appropriate chymotrypsin concentrations. Since the continuous assay allows the measurement of initial rate of dephosphorylation reaction, kinetic parameters for the dephosphorylation reactions of FRET substrates by Yersinia, T-cell and LAR PTPs were determined. The continuous assay was compatible with the measurement of very low PTP activity in a crude enzyme preparation and was comparable in sensitivity to assays that use radiolabeled substrates.

Brain PP2A is modified by thiol-disulfide exchange and intermolecular disulfide formation

The regulation of protein phosphatase 2A (PP2A) activity by thiol-disulfide exchange and resulting formation of an intermolecular disulfide was examined following exposure of a rat brain soluble fraction to a biotinylated derivative of the model disulfide HPDP (HPDP-biotin) which would be expected to label reactive protein thiols with a disulfide-linked biotin. The results show that a low concentration (500 microM) of HPDP-biotin produced substantial inhibition of PP2A activity and promoted the binding of the catalytic subunit of PP2A to an immobilized avidin-affinity column. Both the inhibition of PP2A activity and the binding of PP2A to the avidin column were reversed by treatment with the disulfide reducing agent dithiothreitol (DTT). Furthermore, the specific activity of PP2A was up to 7-fold higher in the DTT-eluted fractions from the avidin-affinity column than in the soluble fraction. These findings demonstrate directly that PP2A is susceptible to reversible inhibitory modification by thiol-disulfide exchange and provide mechanistic support for the emerging view that PP2A is an oxidant-sensitive protein phosphatase.

Mouse brain localization of the protein kinase C-enhanced phosphatase 1 inhibitor KEPI (Kinase C-Enhanced PP1 Inhibitor)

We recently identified the protein kinase C-enhanced protein phosphatase 1 (PP1) inhibitor KEPI based on its morphine-induced upregulation in striatum. Regulation of protein serine/threonine dephosphorylation by PP1 can modulate important brain signaling pathways. To improve understanding of KEPI's role in the brain, we have developed anti-KEPI sera in rabbits immunized with a hemocyanin conjugate of KEPI residues 66-80, characterized the specificity that this serum provides, mapped the distribution of immunoreactive KEPI (iKEPI) in mouse brain, rat dorsal root ganglia and striatal cultures and documented KEPI binding to PP1 in vitro. Staining is found in apparently neuronal processes and, often less intensely, in neuronal perikarya in primary cultures and in neurons and neuronal elements from a number of brain regions. iKEPI fiber/terminal patterns are relatively densely distributed in striatum, nucleus accumbens, septum, bed nucleus of the stria terminalis, hippocampus, paraventricular thalamus, ventromedial hypothalamus, interpeduncular nucleus, raphe nuclei, nucleus caudalis of the spinal tract of the trigeminal and dorsal horn of the spinal cord. iKEPI-positive cell bodies lie in the nucleus accumbens, striatum, lateral septal nucleus, granular layer of dentate gyrus, interpeduncular nucleus, dorsal root ganglia and cerebellar vermis. These expression patterns point to possible roles for KEPI in regulating protein dephosphorylation by inhibiting PP1 activities in a number of brain pathways likely to use several different neurotransmitters and to participate in a number of brain functions. Dense KEPI immunoreactivity in nucleus accumbens perikarya, combined with evidence for its regulation by opiates, supports possible roles for KEPI in molecular signal transduction pathways important for drug reward and addiction.

Noonan syndrome-associated SHP2/PTPN11 mutants cause EGF-dependent prolonged GAB1 binding and sustained ERK2/MAPK1 activation

Noonan syndrome is a developmental disorder with dysmorphic facies, short stature, cardiac defects, and skeletal anomalies, which can be caused by missense PTPN11 mutations. PTPN11 encodes Src homology 2 domain-containing tyrosine phosphatase 2 (SHP2 or SHP-2), a protein tyrosine phosphatase that acts in signal transduction downstream to growth factor, hormone, and cytokine receptors. We compared the functional effects of three Noonan syndrome-causative PTPN11 mutations on SHP2's phosphatase activity, interaction with a binding partner, and signal transduction. All SHP2 mutants had significantly increased basal phosphatase activity compared to wild type, but that activity varied significantly between mutants and was further increased after epidermal growth factor stimulation. Cells expressing SHP2 mutants had prolonged extracellular signal-regulated kinase 2 activation, which was ligand-dependent. Binding of SHP2 mutants to Grb2-associated binder-1 was increased and sustained, and tyrosine phosphorylation of both proteins was prolonged. Coexpression of Grb2-associated binder-1-FF, which lacks SHP2 binding motifs, blocked the epidermal growth factor-mediated increase in SHP2's phosphatase activity and resulted in a dramatic reduction of extracellular signal-regulated kinase 2 activation. Taken together, these results document that Noonan syndrome-associated PTPN11 mutations increase SHP2's basal phosphatase activity, with greater activation when residues directly involved in binding at the interface between the N-terminal Src homology 2 and protein tyrosine phosphatase domains are altered. The SHP2 mutants prolonged signal flux through the RAS/mitogen-activated protein kinase (ERK2/MAPK1) pathway in a ligand-dependent manner that required docking through Grb2-associated binder-1 (GAB1), leading to increased cell proliferation.

Effective dephosphorylation of Src substrates by SHP-1

The protein-tyrosine phosphatase SHP-1 is a negative regulator of multiple signal transduction pathways. We observed that SHP-1 effectively antagonized Src-dependent phosphorylations in HEK293 cells. This occurred by dephosphorylation of Src substrates, because Src activity was unaffected in the presence of SHP-1. One reason for efficient dephosphorylation was activation of SHP-1 by Src. Recombinant SHP-1 had elevated activity subsequent to phosphorylation by Src in vitro, and SHP-1 variants with mutated phosphorylation sites in the C terminus, SHP-1 Y538F, and SHP-1 Y538F,Y566F were less active toward Src-generated phosphoproteins in intact cells. A second reason for efficient dephosphorylation is the substrate selectivity of SHP-1. Pull-down experiments with different GST-SHP-1 fusion proteins revealed efficient interaction of Src-generated phosphoproteins with the SHP-1 catalytic domain rather than with the SH2 domains. Phosphopeptides that correspond to good Src substrates were efficiently dephosphorylated by SHP-1 in vitro. Phosphorylated "optimal Src substrate" AEEEIpYGEFEA (where pY is phosphotyrosine) and a phosphopeptide corresponding to a recently identified Src phosphorylation site in p120 catenin, DDLDpY(296)GMMSD, were excellent SHP-1 substrates. Docking of these phosphopeptides into the catalytic domain of SHP-1 by molecular modeling was consistent with the biochemical data and explains the efficient interaction. Acidic residues N-terminal of the phosphotyrosine seem to be of major importance for efficient substrate interaction. Residues C-terminal of the phosphotyrosine probably contribute to the substrate selectivity of SHP-1. We propose that activation of SHP-1 by Src and complementary substrate specificities of SHP-1 and Src may lead to very transient Src signals in the presence of SHP-1.

PP1 control of M phase entry exerted through 14-3-3-regulated Cdc25 dephosphorylation

It has been known for over a decade that inhibition of protein phosphatase 1 (PP1) activity prevents entry into M phase, but the relevant substrate has not been identified. We report here that PP1 is required for dephosphorylation of the Cdc2-directed phosphatase Cdc25 at Ser287 (of Xenopus Cdc25; Ser216 of human Cdc25C), a site that suppresses Cdc25 during interphase. Moreover, PP1 recognizes Cdc25 directly by interacting with a PP1-binding motif in the Cdc25 N-terminus. We have also found that 14-3-3 binding to phospho-Ser287 protects Cdc25 from premature dephosphorylation. Upon entry into M phase, 14-3-3 removal from Cdc25 precedes Ser287 dephosphorylation, suggesting the existence of a phosphatase- independent pathway for 14-3-3 removal from Cdc25. We show here that this dissociation of 14-3-3 from Cdc25 requires the activity of the cyclin-dependent kinase Cdk2, providing a molecular explanation for the previously reported requirement for Cdk2 in promoting mitotic entry. Collectively, our data clarify several steps important for Cdc25 activation and provide new insight into the role of PP1 in Cdc2 activation and mitotic entry.

Protein phosphatase type 2C dephosphorylates BAD

Reversible phosphorylation modulates a cells' susceptibility to apoptosis. The phosphorylation status of BAD, a member of the Bcl-2 protein family, is an important checkpoint governing life-or-death decisions: Phosphorylation of serine residues 112, 136 and 155 on BAD prevents apoptosis. Here we report that BAD is a substrate for PP2C. Ser(155) is involved in heterodimerization with Bcl-X(L). We could demonstrate that PP1, PP2A and PP2C act on this site in vitro. However, only PP2C gives priority to P-Ser(155) compared to P-Ser(112) and P-Ser(136) on BAD. The results indicate that PP2C is an additional factor triggering the pro-apoptotic function of BAD.

Fluorescent peptide probes for high-throughput measurement of protein phosphatases

A homogeneous microplate assay for the serine/threonine protein phosphatases PP1 and PP2A, employing fluorescent-labeled phosphopeptides, has been developed. Phosphopeptides derived from a phosphoacceptor site in myelin basic protein were designed with a cysteine adjacent to the phosphoresidue, allowing site-selective labeling with dyes. The fluorescence emission from the environmentally sensitive fluorophore 7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide was found to be sensitive to the phosphorylation status of an adjacent threonine residue. Upon complete dephosphorylation of the dye-labeled phosphopeptide, a 56% decrease in fluorescence intensity was observed. The change in fluorescence was correlated with the release of inorganic phosphate from the phosphopeptide as measured using the malachite green assay. Conjugation of the fluorophore to the phosphopeptide was found to have no adverse effect on catalysis. A series of four phosphopeptide substrates were developed and characterized to probe PP1 and PP2A activity. The optimum phosphopeptides were then used to determine inhibition parameters for three natural protein phosphatase inhibitors. The use of a peptide-based approach has introduced a degree of specificity not observed with many conventional phosphatase substrates, while retaining the advantages of a real-time homogeneous fluorescence-based format, making the assay ideal for high-density screening.

L1 endocytosis is controlled by a phosphorylation-dephosphorylation cycle stimulated by outside-in signaling by L1

Dynamic regulation of the cell surface expression of adhesion molecules is an important mechanism for controlling neuronal growth cone motility and guidance. Clathrin-mediated vesicular internalization of L1 via the tyrosine-based endocytosis motif YRSL regulates adhesion and signaling by this Ig superfamily molecule. Here, we present evidence that tyrosine-1176 (Y1176) of the YRSL motif is phosphorylated in vivo. The nonreceptor tyrosine kinase (p60src) is implicated in L1-mediated neurite outgrowth, and we find that p60src phosphorylates Y1176 in vitro. Phosphorylation of Y1176 prevents L1 binding to AP-2, an adaptor required for clathrin-mediated internalization of L1. mAb 74-5H7 recognizes the sequence immediately NH2-terminal to the tyrosine-based motif and binds L1 only when Y1176 is dephosphorylated. 74-5H7 identifies a subset of L1 present at points of cell-cell contact and in vesicle-like structures that colocalize with an endocytosis marker. L1-L1 binding or L1 cross-linking induces a rapid increase in 74-5H7 immunoreactivity. Our data suggest a model in which homophilic binding or L1 cross-linking triggers transient dephosphorylation of the YRSL motif that makes L1 available for endocytosis. Thus, the regulation of L1 endocytosis through dephosphorylation of Y1176 is a critical regulatory point of L1-mediated adhesion and signaling.

Effects of PP1/PP2A inhibitor calyculin A on the E-C coupling cascade in murine ventricular myocytes

Calyculin A was used to examine the importance of phosphatases in the modulation of cardiac contractile magnitude in the absence of any neural or humoral stimulation. Protein phosphatase (PP)1 and PP2A activity, twitch contractions, intracellular Ca(2+) concentration ([Ca(2+)](i)) transients, action potentials, membrane currents, and myofilament Ca(2+) sensitivity were measured in isolated mouse ventricular myocytes. Calyculin A (125 nM) inhibited PP1 and PP2A by 50% and 85%, respectively, whereas it doubled the twitch magnitude and increased twitch duration by 50% in field-stimulated cells. Calyculin A-evoked increases in L-type Ca(2+) current (70%) and the resulting [Ca(2+)](i) transient (83%) explain the positive inotropic response. However, increases in twitch and action potential durations did not result from increased myofilament Ca(2+) sensitivity or K(+) current inhibition, respectively. Comparison of the effects of calyculin A and isoproterenol on [Ca(2+)](i) transients and twitch contractions revealed that calyculin A had a much smaller lusitropic effect than the beta-agonist, indicating that calyculin A did not significantly increase sarcoplasmic reticulum Ca(2+) reuptake. Thus while cardiac contractile magnitude is controlled by a steady-state kinase/phosphatase balance, this regulation is not equally operative at all of the steps in the excitation-contraction coupling pathway and may in fact be most important to the regulation of the L-type Ca(2+) channel.

Phosphorylation of bid by casein kinases I and II regulates its cleavage by caspase 8

Bid plays an essential role in Fas-mediated apoptosis of the so-called type II cells. In these cells, following cleavage by caspase 8, the C-terminal fragment of Bid translocates to mitochondria and triggers the release of apoptogenic factors, thereby inducing cell death. Here we report that Bid is phosphorylated by casein kinase I (CKI) and casein kinase II (CKII). Inhibition of CKI and CKII accelerated Fas-mediated apoptosis and Bid cleavage, whereas hyperactivity of the kinases delayed apoptosis. When phosphorylated, Bid was insensitive to caspase 8 cleavage in vitro. Moreover, a mutant of Bid that cannot be phosphorylated was found to be more toxic than wild-type Bid. Together, these data indicate that phosphorylation of Bid represents a new mechanism whereby cells control apoptosis.

Protein phosphatase 1 catalyses the direct hydrolytic cleavage of phosphate monoester in a ternary complex mechanism

The catalytic subunit of the Ser/Thr protein phosphatase 1 (PP1cat) hydrolyses N-acetyl Arg-Arg-Ala-phosphoThr-Val-Ala (K(M) = 3.7 mM) in a reaction that is inhibited competitively by inorganic phosphate (Pi, Ki = 1.6 mM) but unaffected by the product peptide alcohol at concentrations up to 3 mM. The enzyme does not catalyse the incorporation of 18O-label from 18O-labelled water into Pi whether, or not, the product alcohol is present. The dephosphorylated product alcohol of phosphorylated histone. an alternative substrate for the enzyme, serves as a competitive inhibitor for phosphopeptide hydrolysis (Ki = 60 microM) and co-mediates 18O-label exchange into Pi in a concentration-dependent manner (K(M) = 64 microM). These results indicate that hydrolysis occurs through the direct attack of an activated water molecule on the phosphate ester moiety of the substrate in a ternary complex mechanism.

The synthesis of phosphopeptides

Phosphorylation and dephosphorylation are key events in protein expression and regulation and in signal transduction. Phosphopeptides are very useful reagents for the study of these processes, and have been used to great advantage in the study of phosphatase substrate specificity, SH2 domain ligand specificity, and protein-protein interactions. Furthermore, the advent of cell-permeable peptide carriers, such as those from the antennapedia homeodomain and the HIV TAT transcription factor, has allowed the study of intracellular events, thus underscoring the utility of these reagents. In this paper we review methods for the synthesis of phosphopeptides with the emphasis on the preparation of phosphoamino acid building blocks.

Regulation of protein phosphatase-1

Reversible protein phosphorylation is a major regulatory mechanism of intracellular signal transduction. Protein phosphatase 1 (PP1) is one of four major types of serine-threonine phosphatases mediating signaling pathways, but the means by which its activity is modulated has only recently begun to come into focus.

Molecular interactions among protein phosphatase 2A, tau, and microtubules. Implications for the regulation of tau phosphorylation and the development of tauopathies

Hyperphosphorylated forms of the neuronal microtubule (MT)-associated protein tau are major components of Alzheimer's disease paired helical filaments. Previously, we reported that ABalphaC, the dominant brain isoform of protein phosphatase 2A (PP2A), is localized on MTs, binds directly to tau, and is a major tau phosphatase in cells. We now describe direct interactions among tau, PP2A, and MTs at the submolecular level. Using tau deletion mutants, we found that ABalphaC binds a domain on tau that is indistinguishable from its MT-binding domain. ABalphaC binds directly to MTs through a site that encompasses its catalytic subunit and is distinct from its binding site for tau, and ABalphaC and tau bind to different domains on MTs. Specific PP2A isoforms bind to MTs with distinct affinities in vitro, and these interactions differentially inhibit the ability of PP2A to dephosphorylate various substrates, including tau and tubulin. Finally, tubulin assembly decreases PP2A activity in vitro, suggesting that PP2A activity can be modulated by MT dynamics in vivo. Taken together, these findings indicate how structural interactions among ABalphaC, tau, and MTs might control the phosphorylation state of tau. Disruption of these normal interactions could contribute significantly to development of tauopathies such as Alzheimer's disease.

Protein tyrosine phosphatase PTP-S binds to the juxtamembrane region of the hepatocyte growth factor receptor Met

We reported previously that a protein tyrosine phosphatase (PTP) activity is associated with the immunoprecipitated hepatocyte growth factor (HGF) receptor, also known as Met. The activity increased reversibly when Met was stimulated by HGF and decreased when Met was inactivated by PMA. To identify the PTP-binding region, we used deletion mutants of the receptor beta-subunit. The PTP activity did not associate with Tpr-Met, a construct containing residues 1010-1390 of Met fused to Tpr. In contrast, PTP activity was present when the expressed protein contained the full juxtamembrane region (residues 956-1390 of Met) or part of this region (residues 957-1390 or 995-1390), indicating that the PTP-binding region is between residues 995 and 1009. This region includes Tyr1003, a site involved in Met downstream signalling. Incubation of Met immunoprecipitated from GTL-16 cells with an 8-mer phosphopeptide derived from residues 1003-1010 induced a marked decrease in the associated PTP activity, suggesting that the peptide reproduced the PTP-binding region. Mutation of Glu, Asp or Arg at positions -4, -1 or +1 respectively relative to Tyr1003 in a 9-mer peptide (residues 999-1007) abolished the ability of the peptide to decrease the PTP activity associated with Met. Phosphorylation of Tyr1003 was not required for PTP binding, since: (1) both phospho- and dephospho-peptides on a solid bead bound PTP activity from a GTL-16 cell extract, and (2) PTP activity was associated with a Met deletion mutant lacking residues 1-955 in which Tyr1003 had been changed into Phe. In order to partially purify the PTP from the GTL-16 cell extract, an affinity column was prepared using the Met-derived peptide comprising residues 998-1007. Less than 0.1% of the total cellular PTP was retained by the column, and was eluted with low salt concentrations. Using antibodies, this PTP was identified as PTP-S, a soluble PTP present in epithelial cells and fibroblasts.

Effect of citrulline for arginine replacement on the structure and turnover of phosphopeptide substrates of protein phosphatase-1

Phosphorylated and nonphosphorylated forms of a decapeptide corresponding to residues 9 to 18 of glycogen phosphorylase were compared using two-dimensional nuclear magnetic resonance with assignment of both peptides done by the sequential method. Both forms had little secondary structure, but there was evidence for an interaction between arginine-16 and phosphorylated serine at position 14. A change in the chemical shift for the epsilon-nitrogen hydrogen of arginine in position 16 was observed in the spectrum of the phosphorylated peptide and was not evident in a phosphopeptide having citrulline in place of arginine-16. Hydrolysis catalyzed by protein phosphatase-1 was decreased with the citrulline-containing phosphopeptide compared to the arginine-containing phosphopeptide with effects observed on both kcat and Km of the phosphatase reaction. Alkaline phosphatase hydrolyzed these peptides and a di-citrulline peptide equally well. These results are consistent with arginine being favorable in the recognition of substrates by phosphatase-1, possibly recognition as an arginine-phosphoserine complex. As a model study, arginine and two analogs, citrulline and canavanine, were examined for association with inorganic phosphate by nuclear magnetic resonance spectrometry. 31P-NMR measurements showed that arginine and canavanine caused a shift in the phosphate resonance at 20 degreesC. Citrulline caused no change. Changes in chemical shift were measured over the pH range 5-9 with arginine and canavanine both causing a slight decrease in the apparent pKa of inorganic phosphate (DeltapKa approximately 0.15). NaCl, NH4Cl, and guanidine hydrochloride showed little effect on the resonance signal position of inorganic phosphate at pH 6.5, consistent with selectivity for the guanidino group. Temperature (6 degrees, 20 degrees, and 37 degreesC) caused little change in the effect of arginine, but there was some dependency with canavanine, decreasing with temperature. Citrulline caused no change in the chemical shift of phosphate at any temperature. It was concluded that hydrogen bonded complexes were formed between the dianion of phosphate and the protonated form of arginine or canavanine with a bifurcated structure having preference for the omega-hydrogens.

Inhibition of calcineurin by the tyrphostin class of tyrosine kinase inhibitors

Because of their similarity to tyrosine, members of the tyrphostin family of tyrosine kinase inhibitors were tested as possible inhibitors of the protein serine/threonine phosphatase calcineurin. Calcineurin was inhibited by tyrphostins A8 (also designated AG10), A23 (AG18), and A48 (AG112) with p-nitrophenyl phosphate as substrate. The IC50 values estimated with this substrate were 21, 62, and 30 microM for A8, A23, and A48, respectively. Two other tyrphostins, A46 (AG99) and A63 (AG13), did not inhibit calcineurin at concentrations up to 200 microM. Similar inhibition was observed with tyrphostins A8 and A23 using a phosphopeptide substrate (1.0 mM). Tyrphostin A8 showed competitive inhibition against p-nitrophenyl phosphate as the substrate, with an inhibition constant of 18 microM, comparable to the IC50 value. Possible chemical and structural features influencing inhibition are discussed based on a comparison of the structures of the tyrphostins tested.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.