Protein phosphatase type-1 and glycogen bind to a domain in the skeletal muscle regulatory subunit containing conserved hydrophobic sequence motif

Molecular Pathogenesis of Familial Wolff-Parkinson-White Syndrome

Familial Wolff-Parkinson-White (WPW) syndrome is an autosomal dominant inherited disease and consists of a small percentage of WPW syndrome which exhibits ventricular pre-excitation by development of accessory atrioventricular pathway. A series of mutations in PRKAG2 gene encoding gamma2 subunit of 5'AMP-activated protein kinase (AMPK) has been identified as the cause of familial WPW syndrome. AMPK is one of the most important metabolic regulators of carbohydrates and lipids in many types of tissues including cardiac and skeletal muscles. Patients and animals with the mutation in PRKAG2 gene exhibit aberrant atrioventricular conduction associated with cardiac glycogen overload. Recent studies have revealed "novel" significance of canonical pathways leading to glycogen synthesis and provided us profound insights into molecular mechanism of the regulation of glycogen metabolism by AMPK. This review focuses on the molecular basis of the pathogenesis of cardiac abnormality due to PRKAG2 mutation and will provide current overviews of the mechanism of glycogen regulation by AMPK. J. Med. Invest. 65:1-8, February, 2018.

Protein Scaffolds Control Localized Protein Kinase Cζ Activity

Atypical protein kinase C (aPKC) isozymes modulate insulin signaling and cell polarity, but how their activity is controlled in cells is not well understood. These enzymes are constitutively phosphorylated, insensitive to second messengers, and have relatively low activity. Here we show that protein scaffolds not only localize but also differentially control the catalytic activity of the aPKC PKCζ, thus promoting activity toward localized substrates and restricting activity toward global substrates. Using cellular substrate readouts and scaffolded activity reporters in live cell imaging, we show that PKCζ has highly localized and differentially controlled activity on the scaffolds p62 and Par6. Both scaffolds tether aPKC in an active conformation as assessed through pharmacological inhibition of basal activity, monitored using a genetically encoded reporter for PKC activity. However, binding to Par6 is of higher affinity and is more effective in locking PKCζ in an active conformation. FRET-based translocation assays reveal that insulin promotes the association of both p62 and aPKC with the insulin-regulated scaffold IRS-1. Using the aPKC substrate MARK2 as another readout for activity, we show that overexpression of IRS-1 reduces the phosphorylation of MARK2 and enhances its plasma membrane localization, indicating sequestration of aPKC by IRS-1 away from MARK2. These results are consistent with scaffolds serving as allosteric activators of aPKCs, tethering them in an active conformation near specific substrates. Thus, signaling of these intrinsically low activity kinases is kept at a minimum in the absence of scaffolding interactions, which position the enzymes for stoichiometric phosphorylation of substrates co-localized on the same protein scaffold.

Phosphorylation-dependent autoinhibition of myosin light chain phosphatase accounts for Ca2+ sensitization force of smooth muscle contraction

The reversible regulation of myosin light chain phosphatase (MLCP) in response to agonist stimulation and cAMP/cGMP signals plays an important role in the regulation of smooth muscle (SM) tone. Here, we investigated the mechanism underlying the inhibition of MLCP induced by the phosphorylation of myosin phosphatase targeting subunit (MYPT1), a regulatory subunit of MLCP, at Thr-696 and Thr-853 using glutathione S-transferase (GST)-MYPT1 fragments having the inhibitory phosphorylation sites. GST-MYPT1 fragments, including only Thr-696 and only Thr-853, inhibited purified MLCP (IC(50) = 1.6 and 60 nm, respectively) when they were phosphorylated with RhoA-dependent kinase (ROCK). The activities of isolated catalytic subunits of type 1 and type 2A phosphatases (PP1 and PP2A) were insensitive to either fragment. Phospho-GST-MYPT1 fragments docked directly at the active site of MLCP, and this was blocked by a PP1/PP2A inhibitor microcystin (MC)-LR or by mutation of the active sites in PP1. GST-MYPT1 fragments induced a contraction of beta-escin-permeabilized ileum SM at constant pCa 6.3 (EC(50) = 2 microm), which was eliminated by Ala substitution of the fragment at Thr-696 or by ROCK inhibitors or 8Br-cGMP. GST-MYPT1-(697-880) was 5-times less potent than fragments including Thr-696. Relaxation induced by 8Br-cGMP was not affected by Ala substitution at Ser-695, a known phosphorylation site for protein kinase A/G. Thus, GST-MYPT1 fragments are phosphorylated by ROCK in permeabilized SM and mimic agonist-induced inhibition and cGMP-induced activation of MLCP. We propose a model in which MYPT1 phosphorylation at Thr-696 and Thr-853 causes an autoinhibition of MLCP that accounts for Ca(2+) sensitization of smooth muscle force.

A prevalent variant in PPP1R3A impairs glycogen synthesis and reduces muscle glycogen content in humans and mice

BACKGROUND

Stored glycogen is an important source of energy for skeletal muscle. Human genetic disorders primarily affecting skeletal muscle glycogen turnover are well-recognised, but rare. We previously reported that a frameshift/premature stop mutation in PPP1R3A, the gene encoding RGL, a key regulator of muscle glycogen metabolism, was present in 1.36% of participants from a population of white individuals in the UK. However, the functional implications of the mutation were not known. The objective of this study was to characterise the molecular and physiological consequences of this genetic variant.

METHODS AND FINDINGS

In this study we found a similar prevalence of the variant in an independent UK white population of 744 participants (1.46%) and, using in vivo (13)C magnetic resonance spectroscopy studies, demonstrate that human carriers (n = 6) of the variant have low basal (65% lower, p = 0.002) and postprandial muscle glycogen levels. Mice engineered to express the equivalent mutation had similarly decreased muscle glycogen levels (40% lower in heterozygous knock-in mice, p < 0.05). In muscle tissue from these mice, failure of the truncated mutant to bind glycogen and colocalize with glycogen synthase (GS) decreased GS and increased glycogen phosphorylase activity states, which account for the decreased glycogen content.

CONCLUSIONS

Thus, PPP1R3A C1984DeltaAG (stop codon 668) is, to our knowledge, the first prevalent mutation described that directly impairs glycogen synthesis and decreases glycogen levels in human skeletal muscle. The fact that it is present in approximately 1 in 70 UK whites increases the potential biomedical relevance of these observations.

Molecular basis for the modulation of native T-type Ca2+ channels in vivo by Ca2+/calmodulin-dependent protein kinase II

Ang II receptor activation increases cytosolic Ca2+ levels to enhance the synthesis and secretion of aldosterone, a recently identified early pathogenic stimulus that adversely influences cardiovascular homeostasis. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a downstream effector of the Ang II-elicited signaling cascade that serves as a key intracellular Ca2+ sensor to feedback-regulate Ca2+ entry through voltage-gated Ca2+ channels. However, the molecular mechanism(s) by which CaMKII regulates these important physiological targets to increase Ca2+ entry remain unresolved. We show here that CaMKII forms a signaling complex with alpha1H T-type Ca2+ channels, directly interacting with the intracellular loop connecting domains II and III of the channel pore (II-III loop). Activation of the kinase mediated the phosphorylation of Ser1198 in the II-III loop and the positive feedback regulation of channel gating both in intact cells in situ and in cells of the native adrenal zona glomerulosa stimulated by Ang II in vivo. These data define the molecular basis for the in vivo modulation of native T-type Ca2+ channels by CaMKII and suggest that the disruption of this signaling complex in the zona glomerulosa may provide a new therapeutic approach to limit aldosterone production and cardiovascular disease progression.

A homogeneous assay for relative affinity of binding proteins using a green fluorescent protein tag and membrane disk

When the association between a ligand immobilized on a membrane disk and a fluorescence-labeled analyte was monitored with a fluorescent microplate reader, the time-dependent increase in fluorescence intensity of the reaction mixture was observed. A novel assay system for the specific interaction based on this phenomenon was designated the homogeneous assay for fluorescence concentrated on membrane (HAFCOM). In this study, streptococcal protein G (SpG) and glycogen-binding subunit R5 of protein phosphatase 1 (PPP1R5) tagged by green fluorescent protein (GFP) were used as the fluorescence-labeled analytes, and the affinity change caused by various amino acid substitutions was measured with HAFCOM. From the site-directed mutagenesis of SpG and PPP1R5, it was clarified that (i) the association rate constant of the Lys454Pro/Glu456Gln mutant of SpG to goat immunoglobulin G was almost equivalent to that of the wild-type but its dissociation rate constant was about 2.7 times that of the wild-type and (ii) the amino acid substitutions of Phe180 in PPP1R5 reduced glycogen-binding by 30-50%. Since HAFCOM using the GFP-tagged analyte requires no special chemicals and instruments, this system can easily and economically assay the specific interaction between target protein and ligand.

Glycogen- and PP1c-targeting Subunit GM Is Phosphorylated at Ser48 by Sarcoplasmic Reticulum-bound Ca2+-Calmodulin Protein Kinase in Rabbit Fast Twitch Skeletal Muscle

Multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaMKII) is a Ser/Thr protein kinase uniformly distributed within the sarcoplasmic reticulum (SR) of skeletal muscle. In fast twitch muscle, no specific substrates of CaMKII have yet been identified in nonjunctional SR. Previous electron microscopy data showed that glycogen particles containing glycogen synthase (GS) associate with SR at the I band level. Furthermore, recent evidence implicates CaMKII in regulation of glucose and glycogen metabolism. Here, we demonstrate that the glycogen- and protein phosphatase 1-targeting subunit, also known as G(M), selectively localizes to the SR membranes of rabbit skeletal muscle and that G(M) and GS co-localize at the level of the I band. We further show that G(M), GS, and PP1c assemble in a structural complex that selectively localizes to nonjunctional SR and that G(M) is phosphorylated by SR-bound CaMKII and dephosphorylated by PP1c. On the other hand, no evidence for a structural interaction between G(M) and CaMKII was obtained. Using His-tagged G(M) recombinant fragments and site-directed mutagenesis, we demonstrate that the target of CaMKII is Ser(48). Taken together, these data suggest that SR-bound CaMKII participates in the regulation of GS activity through changes in the phosphorylation state of G(M). Based on these findings, we propose that SR-bound CaMKII participates in the regulation of glycogen metabolism, under physiological conditions involving repetitive raises elevations of [Ca(2+)](i).

A novel transmembrane Ser/Thr kinase complexes with protein phosphatase-1 and inhibitor-2

Protein kinases and protein phosphatases exert coordinated control over many essential cellular processes. Here, we describe the cloning and characterization of a novel human transmembrane protein KPI-2 (Kinase/Phosphatase/Inhibitor-2) that was identified by yeast two-hybrid using protein phosphatase inhibitor-2 (Inh2) as bait. KPI-2 mRNA was predominantly expressed in skeletal muscle. KPI-2 is a 1503-residue protein with two predicted transmembrane helices at the N terminus, a kinase domain, followed by a C-terminal domain. The transmembrane helices were sufficient for targeting proteins to the membrane. KPI-2 kinase domain has about 60% identity with its closest relative, a tyrosine kinase. However, it only exhibited serine/threonine kinase activity in autophosphorylation reactions or with added substrates. KPI-2 kinase domain phosphorylated protein phosphatase-1 (PP1C) at Thr(320), which attenuated PP1C activity. KPI-2 C-terminal domain directly associated with PP1C, and this required a VTF motif. Inh2 associated with KPI-2 C-terminal domain with and without PP1C. Thus, KPI-2 is a kinase with sites to associate with PP1C and Inh2 to form a regulatory complex that is localized to membranes.

Neurabins recruit protein phosphatase-1 and inhibitor-2 to the actin cytoskeleton

Inhibitor-2 (I-2) bound protein phosphatase-1 (PP1) and several PP1-binding proteins from rat brain extracts, including the actin-binding proteins, neurabin I and neurabin II. Neurabins from rat brain lysates were sedimented by I-2 and its structural homologue, I-4. The central domain of both neurabins bound PP1 and I-2, and mutation of a conserved PP1-binding motif abolished neurabin binding to both proteins. Microcystin-LR, a PP1 inhibitor, also attenuated I-2 binding to neurabins. Immunoprecipitation of neurabin I established its association with PP1 and I-2 in HEK293T cells and suggested that PP1 mediated I-2 binding to neurabins. The C terminus of I-2, although not required for PP1 binding, facilitated PP1 recruitment by neurabins, which also targeted I-2 to polymerized F-actin. Mutations that attenuated PP1 binding to I-2 and neurabin I suggested distinct and overlapping sites for these two proteins on the PP1 catalytic subunit. Immunocytochemistry in epithelial cells and cultured hippocampal neurons showed that endogenous neurabin II and I-2 colocalized at actin-rich structures, consistent with the ability of neurabins to target the PP1.I-2 complex to actin cytoskeleton and regulate cell morphology.

Human skeletal muscle expresses a glycogen-targeting subunit of PP1 that is identical to the insulin-sensitive glycogen-targeting subunit G(L) of liver

Insulin has been previously shown to regulate the expression of the hepatic glycogen-targeting subunit, G(L), of protein phosphatase 1 (PP1) and is believed to control the activity of the PP1-G(L) complex by modulation of the level of phosphorylase a, which allosterically inhibits the activity of PP1-G(L). These mechanisms contribute to the ability of insulin to increase hepatic glycogen synthesis. Human G(L) shows >88% amino acid identity to its rat and mouse homologs, with complete conservation of the phosphorylase a binding site. G(L) is highly expressed in the liver and present at appreciable levels in heart tissue of all three species. Surprisingly, G(L) is highly expressed in human skeletal muscle while only being detected at very low levels in rat, mouse, and rabbit skeletal muscle. The amino acid sequence of G(L) predicted from the cDNA is identical in human liver and skeletal muscle and encoded by a gene on chromosome 8 at p23.1. The species-specific difference in the level of expression of G(L) mRNA and protein in skeletal muscle has important implications for understanding the mechanisms by which insulin regulates glycogen synthesis in human skeletal muscle and for questions regarding whether rodents are appropriate models for this purpose.

Protein phosphatase 1 – targeted in many directions

Protein phosphatase 1 (PP1) is a major eukaryotic protein serine/threonine phosphatase that regulates an enormous variety of cellular functions through the interaction of its catalytic subunit (PP1c) with over fifty different established or putative regulatory subunits. Most of these target PP1c to specific subcellular locations and interact with a small hydrophobic groove on the surface of PP1c through a short conserved binding motif – the RVxF motif – which is often preceded by further basic residues. Weaker interactions may subsequently enhance binding and modulate PP1 activity/specificity in a variety of ways. Several putative targeting subunits do not possess an RVxF motif but nevertheless interact with the same region of PP1c. In addition, several ‘modulator’ proteins bind to PP1c but do not possess a domain targeting them to a specific location. Most are potent inhibitors of PP1c and possess at least two sites for interaction with PP1c, one of which is identical or similar to the RVxF motif.Regulation of PP1c in response to extracellular and intracellular signals occurs mostly through changes in the levels, conformation or phosphorylation status of targeting subunits. Understanding of the mode of action of PP1c complexes may facilitate development of drugs that target particular PP1c complexes and thereby modulate the phosphorylation state of a very limited subset of proteins.

Glycogen-targeting subunits and glucokinase differentially affect pathways of glycogen metabolism and their regulation in hepatocytes

Overexpression of the glucose-phosphorylating enzyme glucokinase (GK) or members of the family of glycogen-targeting subunits of protein phosphatase-1 increases hepatic glucose disposal and glycogen synthesis. This study was undertaken to evaluate the functional properties of a novel, truncated glycogen-targeting subunit derived from the skeletal muscle isoform G(M)/R(Gl) and to compare pathways of glycogen metabolism and their regulation in cells with overexpressed targeting subunits and GK. When overexpressed in hepatocytes, truncated G(M)/R(Gl) (G(M)DeltaC) was approximately twice as potent as full-length G(M)/R(Gl) in stimulation of glycogen synthesis, but clearly less potent than GK or two other native glycogen-targeting subunits, G(L) and PTG. We also found that cells with overexpressed G(M)DeltaC are unique in that glycogen was efficiently degraded in response to lowering of media glucose concentrations, stimulation with forskolin, or a combination of both maneuvers, whereas cells with overexpressed G(L), PTG, or GK exhibited impairment in one or both of these glycogenolytic signaling pathways. (2)H NMR analysis of purified glycogen revealed that hepatocytes with overexpressed GK synthesized a larger portion of their glycogen from triose phosphates and a smaller portion from tricarboxylic acid cycle intermediates than cells with overexpressed glycogen-targeting subunits. Additional evidence for activation of distinct pathways of glycogen synthesis by GK and targeting subunits is provided by the additive effect of co-overexpression of the two types of proteins upon glycogen synthesis and a much larger stimulation of glucose utilization, glucose transport, and lactate production elicited by GK. We conclude that overexpression of the novel targeting subunit G(M)DeltaC confers unique regulation of glycogen metabolism. Furthermore, targeting subunits and GK stimulate glycogen synthesis by distinct pathways.

Reversal of diet-induced glucose intolerance by hepatic expression of a variant glycogen-targeting subunit of protein phosphatase-1

Glycogen-targeting subunits of protein phosphatase-1 facilitate interaction of the phosphatase with enzymes of glycogen metabolism. Expression of one family member, PTG, in the liver of normal rats improves glucose tolerance without affecting other plasma variables but leaves animals unable to reduce hepatic glycogen stores in response to fasting. In the current study, we have tested whether expression of other targeting subunit isoforms, such as the liver isoform G(L), the muscle isoform G(M)/R(Gl), or a truncated version of G(M)/R(Gl) termed G(M)DeltaC in liver ameliorates glucose intolerance in rats fed on a high fat diet (HF). HF animals overexpressing G(M)DeltaC, but not G(L) or G(M)/R(Gl), exhibited a decline in blood glucose of 35-44 mg/dl relative to control HF animals during an oral glucose tolerance test (OGTT) such that levels were indistinguishable from those of normal rats fed on standard chow at all but one time point. Hepatic glycogen levels were 2.1-2.4-fold greater in G(L)- and G(M)DeltaC-overexpressing HF rats compared with control HF animals following OGTT. In a second set of studies on fed and 20-h fasted HF animals, G(M)DeltaC-overexpressing rats lowered their liver glycogen levels by 57% (from 402 +/- 54 to 173 +/- 27 microg of glycogen/mg of protein) in the fasted versus fed states compared with only 44% in G(L)-overexpressing animals (from 740 +/- 35 to 413 +/- 141 microg of glycogen/mg of protein). Since the OGTT studies were performed on 20-h fasted rats, this meant that G(M)DeltaC-overexpressing rats synthesized much more glycogen than G(L)-overexpressing HF rats during the OGTT (419 versus 117 microg of glycogen/mg of protein, respectively), helping to explain why G(M)DeltaC preferentially enhanced glucose clearance. We conclude that G(M)DeltaC has a unique combination of glycogenic potency and responsiveness to glycogenolytic signals that allows it to be used to lower blood glucose levels in diabetes.

Phosphorylation and microtubule association of the Opitz syndrome protein mid-1 is regulated by protein phosphatase 2A via binding to the regulatory subunit alpha 4

Opitz syndrome (OS) is a human genetic disease characterized by deformities such as cleft palate that are attributable to defects in embryonic development at the midline. Gene mapping has identified OS mutations within a protein called Mid1. Wild-type Mid1 predominantly colocalizes with microtubules, in contrast to mutant versions of Mid1 that appear clustered in the cytosol. Using yeast two-hybrid screening, we found that the alpha4-subunit of protein phosphatases 2A/4/6 binds Mid1. Epitope-tagged alpha4 coimmunoprecipitated endogenous or coexpressed Mid1 from COS7 cells, and this required only the conserved C-terminal region of alpha4. Localization of Mid1 and alpha4 was influenced by one another in transiently transfected cells. Mid1 could recruit alpha4 onto microtubules, and high levels of alpha4 could displace Mid1 into the cytosol. Metabolic (32)P labeling of cells showed that Mid1 is a phosphoprotein, and coexpression of full-length alpha4 decreased Mid1 phosphorylation, indicative of a functional interaction. Association of green fluorescent protein-Mid1 with microtubules in living cells was perturbed by inhibitors of MAP kinase activation. The conclusion is that Mid1 association with microtubules, which seems important for normal midline development, is regulated by dynamic phosphorylation involving MAP kinase and protein phosphatase that is targeted specifically to Mid1 by alpha4. Human birth defects may result from environmental or genetic disruption of this regulatory cycle.

Overexpression of the glycogen targeting (G(M)) subunit of protein phosphatase-1

The G(M) glycogen-targeting subunit of protein phosphatase-1 (PP1) is believed to be involved in dephosphorylation of the enzymes of glycogen metabolism. To assess the roles of G(M) on glycogen metabolism, we created site-directed G(M) mutants and overexpressed them in Chinese hamster ovary (CHO) cells expressing human insulin receptor. Overexpressed G(M) recruited glycogen synthase as well as PP1 to the glycogen pellet, and upregulated basal glycogen synthase activity. Overexpressed G(M)-67A (Ser-67 replaced with alanine) exhibited decreased sensitivity to suppression of glycogen synthase activity by forskolin, while overexpression of G(M)-48A (Ser-48 replaced with alanine) preserved glycogen synthase activation in response to insulin. These observations indicate that in CHO cells overexpressing G(M); (1) G(M) translocates glycogen synthase to the glycogen pellet and affected basal glycogen synthase, (2) Ser-67 might be involved in the suppression of glycogen synthase activity by glycogenolytic agents, and (3) Ser-48 might not commit to activation of glycogen synthase by insulin.

Glycogen synthase association with the striated muscle glycogen-targeting subunit of protein phosphatase-1. Synthase activation involves scaffolding regulated by beta-adrenergic signaling

Glycogen-binding subunits for protein phosphatase-1 (PP1) target the PP1 catalytic subunit (PP1C) to glycogen particles, where the enzymes glycogen synthase and glycogen phosphorylase are concentrated. Here we identify sites within the striated muscle glycogen-binding subunit (G(M)) that mediate direct binding to glycogen synthase. Both PP1C and glycogen synthase were coimmunoprecipitated with a full-length FLAG-tagged G(M) transiently expressed in COS7 cells or C2C12 myotubes. Deletion and mutational analysis of a glutathione S-transferase (GST) fusion of the N-terminal domain of G(M) (residues 1-240) identified two putative sites for binding to glycogen synthase, one of which is the WXNXGXNYX(I/L) motif that is conserved among the family of PP1 glycogen-binding subunits. Either deletion of this motif or Ala substitution of Asn-228 in this motif disrupted the binding of glycogen synthase. Expression of full-length FLAG-G(M) in cells increased the activity of endogenous glycogen synthase, but protein disabled in either PP1 binding or glycogen synthase binding did not produce synthase activation. The results show that efficient activation of glycogen synthase requires a scaffold function of G(M) that involves simultaneous binding of both PP1C and glycogen synthase. Isoproterenol and forskolin treatment of cells decreased glycogen synthase binding to FLAG-G(M), thereby limiting synthase activation by PP1. This response was insensitive to inhibition by H-89, therefore probably not involving cAMP-dependent protein kinase, but did require inclusion of microcystin-LR during cell lysis, implying that phosphorylation was modulating binding of glycogen synthase. Phosphorylation control of binding to a scaffold site on the G(M) subunit of PP1 offers a new mechanism for regulation of muscle glycogen synthase in response to beta-adrenergic signals.

Insulin-stimulated phosphorylation of the protein phosphatase-1 striated muscle glycogen-targeting subunit and activation of glycogen synthase

Protein phosphatase-1 (PP-1) in heart and skeletal muscle binds to a glycogen-targeting subunit (G(M)) in the sarcoplasmic reticulum. Phosphorylation of G(M) has been postulated to govern activity of PP1 in response to adrenaline and insulin. In this study, we used biochemical assays and G(M) expression in living cells to examine the effects of insulin on the phosphorylation of G(M), and the binding of PP-1 to G(M). We also assayed glycogen synthase activation in cells expressing wild type G(M) and G(M) mutated at the phosphorylation sites. In biochemical assays kinase(s) prepared from insulin-stimulated Chinese hamster ovary (CHO-IR) cells and C2C12 myotubes phosphorylated a glutathione S-transferase (GST) fusion protein, GST-G(M)(1-240), at both site 1 (Ser(48)) and site 2 (Ser(67)). Phosphorylation of both sites was dependent on activation of the mitogen-activated protein kinase pathway, involving in particular ribosomal protein S6 kinase. Full-length G(M) was expressed in CHO-IR cells and metabolic (32)P labeling at sites 1 and 2 was increased by insulin treatment. The G(M) expressed in CHO-IR cells or in C2C12 myotubes co-immunoprecipitated endogenous PP-1, and association was transiently lost following treatment of the cells with insulin. In contrast PP-1 binding to G(M)(S67T), a version of G(M) not phosphorylated at site 2, was unaffected by insulin treatment. Expression of G(M) increased basal activity of endogenous glycogen synthase in CHO-IR cells. Insulin stimulated glycogen synthase activity the same extent in cells expressing wild type G(M) or G(M) mutated to eliminate phosphorylation site 1 and/or site 2. Phosphorylation of G(M) is stimulated by insulin, but this phosphorylation is not involved in insulin control of glycogen metabolism. We speculate that other functions of G(M) at the sarcoplasmic reticulum membrane might be affected by insulin.

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.

Structure of the human glycogen-associated protein phosphatase 1 regulatory subunit hGM: homology modeling revealed an (alpha/beta)8-barrel-like fold in the multidomain protein

Protein phosphatase 1 (PP1) is widely distributed among tissues and species and acts as a regulator of many important cellular processes. By targeting the catalytic part of PP1 (PP1C) toward particular loci and substrates, regulatory subunits constitute key elements conferring specificity to the holoenzyme. Here, we report the identification of an (alpha/beta)8-barrel-like structure within the N-ter stretch of the human PP1 regulatory subunit hGM, which is part of the family of diverse proteins associated with glycogen metabolism. Protein homology modeling gave rise to a three-dimensional (3D) model for the 381 N-ter residue stretch of hGM, based on sequence similarity with Streptomyces olivochromogenes xylose isomerase, identified by using FASTA. The alignment was subsequently extended by using hydrophobic cluster analysis. The homology-derived model includes the putative glycogen binding area located within the 142-230 domain of hGM as well as a structural characterization of the PP1C interacting domain (segment 51-67). Refinement of the latter by molecular dynamics afforded a topology that is in agreement with previous X-ray studies (Egloff et al., 1997). Finite difference Poisson-Boltzmann calculations performed on the interacting domains of PP1C and hGM confirm the complementarity of the local electrostatic potentials of the two partners. This work highlights the presence of a conserved fold among distant species (mammalian, Caenorhabditis elegans, yeast) and, thus, emphasizes the involvement of PP1 in crucial basic cellular functions.

Importance of the beta12-beta13 loop in protein phosphatase-1 catalytic subunit for inhibition by toxins and mammalian protein inhibitors

Type-1 protein serine/threonine phosphatases (PP1) are uniquely inhibited by the mammalian proteins, inhibitor-1 (I-1), inhibitor-2 (I-2), and nuclear inhibitor of PP1 (NIPP-1). In addition, several natural compounds inhibit both PP1 and the type-2 phosphatase, PP2A. Deletion of C-terminal sequences that included the beta12-beta13 loop attenuated the inhibition of the resulting PP1alpha catalytic core by I-1, I-2, NIPP-1, and several toxins, including tautomycin, microcystin-LR, calyculin A, and okadaic acid. Substitution of C-terminal sequences from the PP2A catalytic subunit produced a chimeric enzyme, CRHM2, that was inhibited by toxins with dose-response characteristics of PP1 and not PP2A. However, CRHM2 was insensitive to the PP1-specific inhibitors, I-1, I-2, and NIPP-1. The anticancer compound, fostriecin, differed from other phosphatase inhibitors in that it inhibited wild-type PP1alpha, the PP1alpha catalytic core, and CRHM2 with identical IC(50). Binding of wild-type and mutant phosphatases to immobilized microcystin-LR, NIPP-1, and I-2 established that the beta12-beta13 loop was essential for the association of PP1 with toxins and the protein inhibitors. These studies point to the importance of the beta12-beta13 loop structure and conformation for the control of PP1 functions by toxins and endogenous proteins.

A conserved domain for glycogen binding in protein phosphatase-1 targeting subunits

The skeletal muscle glycogen-binding subunit (GM) of protein phosphatase-1 (PP1) is the founding member of a family of proteins that tether the PP1 catalytic subunit (PP1C) to glycogen and promote the dephosphorylation of glycogen synthase. A hydrophobic sequence (called here the VFV motif) is conserved among GM, the liver subunit GL, and the widely expressed subunits, PTG, R5 and U5. This study analyzed the role of this VFV motif in binding to glycogen and PP1C. Glutathione S-transferase (GST) fusions with the N-terminal domain of GM (GST-GM(1-240)) and with the full length R5 protein (GST-R5) both bound to glycogen in a co-sedimentation assay. In contrast, GST itself did not bind to glycogen. A single residue substitution in GST-GM(1-240), F155A, reduced glycogen binding by 40%. Double residue substitutions V150A/F155A and F155A/V159A resulted in greater reductions (60-70%) in glycogen binding, showing these hydrophobic residues influenced the protein-glycogen interaction. The wild type and V150A/ F155A fusion proteins were digested by trypsin into the same sized fragments at the same rate. Furthermore, the wild type and mutated GST-GM proteins as well as GST-R5 bound equivalent amounts of PP1C, in either pull-down or far-Western assays. These results demonstrated retention of overall tertiary structure by the mutated fusion proteins, and indicated that glycogen and PP1C binding are independent of one another. A 68 residue segment of R5 encompassing the VFV motif was sufficient to produce glycogen binding when fused to GST. This motif, that is in bacterial and fungal starch metabolizing enzymes, probably has been conserved during evolution as a functional domain for binding glycogen and starch.

The structure and mechanism of protein phosphatases: insights into catalysis and regulation

Eukaryotic protein phosphatases are structurally and functionally diverse enzymes that are represented by three distinct gene families. Two of these, the PPP and PPM families, dephosphorylate phosphoserine and phosphothreonine residues, whereas the protein tyrosine phosphatases (PTPs) dephosphorylate phosphotyrosine amino acids. A subfamily of the PTPs, the dual-specificity phosphatases, dephosphorylate all three phosphoamino acids. Within each family, the catalytic domains are highly conserved, with functional diversity endowed by regulatory domains and subunits. The protein Ser/Thr phosphatases are metalloenzymes and dephosphorylate their substrates in a single reaction step using a metal-activated nucleophilic water molecule. In contrast, the PTPs catalyze dephosphorylation by use of a cysteinyl-phosphate enzyme intermediate. The crystal structures of a number of protein phosphatases have been determined, enabling us to understand their catalytic mechanisms and the basis for substrate recognition and to begin to provide insights into molecular mechanisms of protein phosphatase regulation.

A protein phosphatase-1-binding motif identified by the panning of a random peptide display library

An unusually large number of regulatory or targeting proteins that bind to the catalytic subunit of protein phosphatase-1 have been recently reported. This can be explained by their possession of a common protein motif that interacts with a binding site on protein phosphatase-1. The existence of such a motif was established by the panning of a random peptide library in which peptide sequences are displayed on the Escherichia coli bacterial flagellin protein for bacteria that bound to protein phosphatase-1. There were 79 isolates containing 46 unique sequences with the conserved motif VXF or VXW, where X was most frequently His or Arg. In addition, this sequence was commonly preceded by 2-5 basic residues and followed by 1 acidic residue. This study demonstrates that binding to protein phosphatase-1 can be conferred to a protein by the presentation of a peptide motif on a surface loop. This binding motif is found in a number of protein phosphatase-1-binding proteins.

B cell receptor-associated protein alpha4 displays rapamycin-sensitive binding directly to the catalytic subunit of protein phosphatase 2A

Recently, TAP42 was isolated as a high copy suppressor of sit4-, a yeast phosphatase related to protein phosphatase 2A (PP2A). TAP42 is related to the murine alpha4 protein, which was discovered independently by its association with Ig-alpha in the B cell receptor complex. Herein we show that a glutathione S-transferase (GST)-alpha4 fusion protein bound the catalytic subunit (C) of human PP2A from monomeric or multimeric preparations of PP2A in a "pull-down" assay. In an overlay assay, the GST-alpha4 protein bound to the phosphorylated and unphosphorylated forms of C that were separated in two-dimensional gels and immobilized on filters. The results show direct and exclusive binding of alpha4 to C. This is unusual because all known regulatory B subunits, or tumor virus antigens, bind stably only to the AC dimer of PP2A. The alpha4-C form of PP2A had an increased activity ratio compared with the AC form of PP2A when myelin basic protein phosphorylated by mitogen-activated protein kinase and phosphorylase a were used as substrates. Recombinant alpha4 cleaved from GST was phosphorylated by p56(lck) tyrosine kinase and protein kinase C. A FLAG-tagged alpha4 expressed in COS7 cells was recovered as a protein containing phosphoserine and coimmunoprecipitated with the C but not the A subunit of PP2A. Treatment of cells with rapamycin prevented the association of PP2A with FLAG-alpha4. The results reveal a novel heterodimer alpha4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.

Characterization of the interaction between DARPP-32 and protein phosphatase 1 (PP-1): DARPP-32 peptides antagonize the interaction of PP-1 with binding proteins

The catalytic subunit of PP-1 (PP-1C) is potently inhibited (IC50, approximately 1 nM) by DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, M(r) 32,000), inhibitor-1, and inhibitor-2. The NH2-terminal 50 amino acid residues of DARPP-32 and inhibitor-1 are similar, and phosphorylation of a common threonine residue (Thr-34/Thr-35) is necessary for inhibition of PP-1C. We have characterized further the interaction between DARPP-32 and PP-1C. Using synthetic peptides derived from the NH2-terminal region of DARPP-32, residues 6-11, RKKIQF, have been shown to be required for inhibition of PP-1C. Peptides containing this motif were able to antagonize the inhibition of PP-1C by phospho-DARPP-32 and phosphoinhibitor-1. The inhibition of PP-1C by inhibitor-2, but not by okadaic acid, microcystin, or calyculin A, was also attentuated by these antagonist peptides. These results together with results from other studies support a model in which two subdomains of phospho-DARPP-32 interact with PP-1C. The region encompassing phospho-Thr-34 appears to interact with the active site of the enzyme blocking enzyme activity. The region encompassing the RKKIQF motif binds to a domain of PP-1C removed from the active site. Amino acid sequence analysis indicates that basic and hydrophobic features of the RKKIQF motif are conserved in the binding domains of certain PP-1C targeting proteins, suggesting that interaction of inhibitor proteins and targeting proteins may be mutually exclusive.