Reversible ADP-ribosylation of the 78 kDa glucose-regulated protein

Post-translational modifications of Hsp70 family proteins: Expanding the chaperone code

Cells must be able to cope with the challenge of folding newly synthesized proteins and refolding those that have become misfolded in the context of a crowded cytosol. One such coping mechanism that has appeared during evolution is the expression of well-conserved molecular chaperones, such as those that are part of the heat shock protein 70 (Hsp70) family of proteins that bind and fold a large proportion of the proteome. Although Hsp70 family chaperones have been extensively examined for the last 50 years, most studies have focused on regulation of Hsp70 activities by altered transcription, co-chaperone "helper" proteins, and ATP binding and hydrolysis. The rise of modern proteomics has uncovered a vast array of post-translational modifications (PTMs) on Hsp70 family proteins that include phosphorylation, acetylation, ubiquitination, AMPylation, and ADP-ribosylation. Similarly to the pattern of histone modifications, the histone code, this complex pattern of chaperone PTMs is now known as the "chaperone code." In this review, we discuss the history of the Hsp70 chaperone code, its currently understood regulation and functions, and thoughts on what the future of research into the chaperone code may entail.

Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges

: Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.

Unfolded protein response-regulated Drosophila Fic (dFic) protein reversibly AMPylates BiP chaperone during endoplasmic reticulum homeostasis

Drosophila Fic (dFic) mediates AMPylation, a covalent attachment of adenosine monophosphate (AMP) from ATP to hydroxyl side chains of protein substrates. Here, we identified the endoplasmic reticulum (ER) chaperone BiP as a substrate for dFic and mapped the modification site to Thr-366 within the ATPase domain. The level of AMPylated BiP in Drosophila S2 cells is high during homeostasis, whereas the level of AMPylated BiP decreases upon the accumulation of misfolded proteins in the ER. Both dFic and BiP are transcriptionally activated upon ER stress, supporting the role of dFic in the unfolded protein response pathway. The inactive conformation of BiP is the preferred substrate for dFic, thus endorsing a model whereby AMPylation regulates the function of BiP as a chaperone, allowing acute activation of BiP by deAMPylation during an ER stress response. These findings not only present the first substrate of eukaryotic AMPylator but also provide a target for regulating the unfolded protein response, an emerging avenue for cancer therapy.

Side chain specificity of ADP-ribosylation by a sirtuin

Endogenous mono-ADP-ribosylation in eukaryotes is involved in regulating protein synthesis, signal transduction, cytoskeletal integrity, and cell proliferation, although few cellular ADP-ribosyltransferases have been identified. The sirtuins constitute a highly conserved family of protein deacetylases, and several family members have also been reported to perform protein ADP-ribosylation. We characterized the ADP-ribosylation reaction of the nuclear sirtuin homolog Trypanosoma brucei SIR2-related protein 1 (TbSIR2RP1) on both acetylated and unacetylated substrates. We demonstrated that an acetylated substrate is not required for ADP-ribosylation to occur, indicating that the reaction performed by TbSIR2RP1 is a genuine enzymatic reaction and not a side reaction of deacetylation. Biochemical and MS data showed that arginine is the major ADP-ribose acceptor for unacetylated substrates, whereas arginine does not appear to be the major ADP-ribose acceptor in reactions with acetylated histone H1.1. We performed combined ab initio quantum mechanical/molecular mechanical molecular dynamics simulations, which indicated that sirtuin ADP-ribosylation at arginine is energetically feasible, and involves a concerted mechanism with a highly dissociative transition state. In comparison with the corresponding nicotinamide cleavage in the deacetylation reaction, the simulations suggest that sirtuin ADP-ribosylation would be several orders slower but less sensitive to nicotinamide inhibition, which is consistent with experimental results. These results suggest that TbSIR2RP1 can perform ADP-ribosylation using two distinct mechanisms, depending on whether or not the substrate is acetylated.

Structure-based mechanism of ADP-ribosylation by sirtuins

Sirtuins comprise a family of enzymes found in all organisms, where they play a role in diverse processes including transcriptional silencing, aging, regulation of transcription, and metabolism. The predominant reaction catalyzed by these enzymes is NAD(+)-dependent lysine deacetylation, although some sirtuins exhibit a weaker ADP-ribosyltransferase activity. Although the Sir2 deacetylation mechanism is well established, much less is known about the Sir2 ADP-ribosylation reaction. We have studied the ADP-ribosylation activity of a bacterial sirtuin, Sir2Tm, and show that acetylated peptides containing arginine or lysine 2 residues C-terminal to the acetyl lysine, the +2 position, are preferentially ADP-ribosylated at the +2 residue. A structure of Sir2Tm bound to the acetylated +2 arginine peptide shows how this arginine could enter the active site and react with a deacetylation reaction intermediate to yield an ADP-ribosylated peptide. The new biochemical and structural studies presented here provide mechanistic insights into the Sir2 ADP-ribosylation reaction and will aid in identifying substrates of this reaction.

Curcumin downregulates H19 gene transcription in tumor cells

Curcumin (diferuloymethane), a natural compound used in traditional medicine, exerts an antiproliferative effect on various tumor cell lines by an incompletely understood mechanism. It has been shown that low doses of curcumin downregulate DNA topoisomerase II alpha (TOP2A) which is upregulated in many malignances. The activity of TOP2A is required for RNA polymerase II transcription on chromatin templates. Recently, it has been reported that CTCF, a multifunctional transcription factor, recruits the largest subunit of RNA polymerase II (LS Pol II) to its target sites genome-wide. This recruitment of LS Pol II is more pronounced in proliferating cells than in fully differentiated cells. As expression of imprinted genes is often altered in tumors, we investigated the potential effect of curcumin treatment on transcription of the imprinted H19 gene, located distally from the CTCF binding site, in human tumor cell lines HCT 116, SW 620, HeLa, Cal 27, Hep-2 and Detroit 562. Transcription of TOP2A and concomitantly H19 was supressed in all tumor cell lines tested. Monoallelic IGF2 expression was maintained in curcumin-treated cancer cells, indicating the involvement of mechanism/s other than disturbance of CTCF insulator function at the IGF2/H19 locus. Curcumin did not alter H19 gene transcription in primary cell cultures derived from normal human tissues.

Nicotinamide adenine dinucleotide: beyond a redox coenzyme

ADP-ribosylation using nicotinamide adenine dinucleotide (NAD+) is an important type of enzymatic reaction that affects many biological processes. A brief introductory review is given here to various ADP-ribosyltransferases, including poly(ADP-ribose) polymerase (PARPs), mono(ADP-ribosyl)-transferases (ARTs), NAD(+)-dependent deacetylases (sirtuins), tRNA 2'-phosphotransferases, and ADP-ribosyl cyclases (CD38 and CD157). Focus is given to the enzymatic reactions, mechanisms, structures, and biological functions.

Nuclear ADP-ribosylation reactions in mammalian cells: where are we today and where are we going?

Since poly-ADP ribose was discovered over 40 years ago, there has been significant progress in research into the biology of mono- and poly-ADP-ribosylation reactions. During the last decade, it became clear that ADP-ribosylation reactions play important roles in a wide range of physiological and pathophysiological processes, including inter- and intracellular signaling, transcriptional regulation, DNA repair pathways and maintenance of genomic stability, telomere dynamics, cell differentiation and proliferation, and necrosis and apoptosis. ADP-ribosylation reactions are phylogenetically ancient and can be classified into four major groups: mono-ADP-ribosylation, poly-ADP-ribosylation, ADP-ribose cyclization, and formation of O-acetyl-ADP-ribose. In the human genome, more than 30 different genes coding for enzymes associated with distinct ADP-ribosylation activities have been identified. This review highlights the recent advances in the rapidly growing field of nuclear mono-ADP-ribosylation and poly-ADP-ribosylation reactions and the distinct ADP-ribosylating enzyme families involved in these processes, including the proposed family of novel poly-ADP-ribose polymerase-like mono-ADP-ribose transferases and the potential mono-ADP-ribosylation activities of the sirtuin family of NAD(+)-dependent histone deacetylases. A special focus is placed on the known roles of distinct mono- and poly-ADP-ribosylation reactions in physiological processes, such as mitosis, cellular differentiation and proliferation, telomere dynamics, and aging, as well as "programmed necrosis" (i.e., high-mobility-group protein B1 release) and apoptosis (i.e., apoptosis-inducing factor shuttling). The proposed molecular mechanisms involved in these processes, such as signaling, chromatin modification (i.e., "histone code"), and remodeling of chromatin structure (i.e., DNA damage response, transcriptional regulation, and insulator function), are described. A potential cross talk between nuclear ADP-ribosylation processes and other NAD(+)-dependent pathways is discussed.

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

Conventional and functional proteomics using large format two-dimensional gel electrophoresis 24 hours after controlled cortical impact in postnatal day 17 rats

Conventional and functional proteomics have significant potential to expand our understanding of traumatic brain injury (TBI) but have not yet been used. The purpose of the present study was to examine global hippocampal protein changes in postnatal day (PND) 17 immature rats 24 h after moderate controlled cortical impact (CCI). Silver nitrate stains or protein kinase B (PKB) phosphoprotein substrate antibodies were used to evaluate high abundance or PKB pathway signal transduction proteins representing conventional and functional proteomic approaches, respectively. Isoelectric focusing was performed over a nonlinear pH range of 3-10 with immobilized pH gradients (IPG strips) using supernatant from the most soluble cellular protein fraction of hippocampal tissue protein lysates from six paired sham and injured PND 17 rats. Approximately 1,500 proteins were found in each silver stained gel with 40% matching of proteins. Of these 600 proteins, 52% showed a twofold, 20% a fivefold, and 10% a 10-fold decrease or increase. Spot matching with existing protein databases revealed changes in important cytoskeletal and cell signalling proteins. PKB substrate protein phosphorylation was best seen in large format two-dimensional blots and known substrates of PKB such as glucose transporter proteins 3 and 4 and forkhead transcription factors, identified based upon molecular mass and charge, showed altered phosphorylation 24 h after injury. These results suggest that combined conventional and functional proteomic approaches are powerful, complementary and synergistic tools revealing multiple protein changes and posttranslational protein modifications that allow for more specific and comprehensive functional assessments after pediatric TBI.

BGP-15, a nicotinic amidoxime derivate protecting heart from ischemia reperfusion injury through modulation of poly(ADP-ribose) polymerase

The protective effect of O-(3-piperidino-2-hydroxy-1-propyl)nicotinic amidoxime (BGP-15) against ischemia-reperfusion-induced injury was studied in the Langendorff heart perfusion system. To understand the molecular mechanism of the cardioprotection, the effect of BGP-15 on ischemic-reperfusion-induced reactive oxygen species (ROS) formation, lipid peroxidation single-strand DNA break formation, NAD(+) catabolism, and endogenous ADP-ribosylation reactions were investigated. These studies showed that BGP-15 significantly decreased leakage of lactate dehydrogenase, creatine kinase, and aspartate aminotransferase in reperfused hearts, and reduced the rate of NAD(+) catabolism. In addition, BGP-15 dramatically decreased the ischemia-reperfusion-induced self-ADP-ribosylation of nuclear poly(ADP-ribose) polymerase(PARP) and the mono-ADP-ribosylation of an endoplasmic reticulum chaperone GRP78. These data raise the possibility that BGP-15 may have a direct inhibitory effect on PARP. This hypothesis was tested on isolated enzyme, and kinetic analysis showed a mixed-type (noncompetitive) inhibition with a K(i) = 57 +/- 6 microM. Furthermore, BGP-15 decreased levels of ROS, lipid peroxidation, and single-strand DNA breaks in reperfused hearts. These data suggest that PARP may be an important molecular target of BGP-15 and that BGP-15 decreases ROS levels and cell injury during ischemia-reperfusion in the heart by inhibiting PARP activity.

Enhanced ADP-ribosylation and its diminution by lipoamide after ischemia-reperfusion in perfused rat heart

Poly-ADP-ribose polymerase (PARP) is considered to play an important role in oxidative cell damage. We assumed that ischemia-reperfusion resulting from the increasing reactive oxygen species (ROS) can lead to the activation of endogenous mono- and poly-ADP-ribosylation reactions and that the reduction of ROS level by lipoamide, a less known antioxidant, can reverse these unfavorable processes. Experiments were performed on isolated Langendorff hearts subjected to 60-min ischemia followed by reperfusion. ROS, malondialdehyde, deoxyribonucleic acid (DNA) breaks, and NAD+ content were assayed in the hearts, and the ADP-ribosylation of cytoplasmic and nuclear proteins were determined by Western blot assay. Ischemia-reperfusion caused a moderate (30.2 +/- 8%) increase in ROS production determined by the dihydrorhodamine 123 method and significantly increased the malondialdehyde production (from < 1 to 23 +/- 2.7 nmol/ml), DNA damage (undamaged DNA decreased from 71 +/- 7% to 23.1 +/- 5%), and NAD+ catabolism. In addition, ischemia-reperfusion activated the mono-ADP-ribosylation of GRP78 and the self-ADP-ribosylation of the nuclear PARP. The perfusion of hearts with lipoamide significantly decreased the ischemia-reperfusion-induced cell membrane damage determined by enzyme release (LDH, CK, and GOT), decreased the ROS production, reduced the malondialdehyde production to 5.5 +/- 2.4 nmol/ml, abolished DNA damage, and reduced NAD+ catabolism. The ischemia-reperfusion-induced activation of poly- and mono-ADP-ribosylation reactions were also reverted by lipoamide. In isolated rat heart mitochondria, dihydrolipoamide was found to be a better antioxidant than dihydrolipoic acid. Ischemia-reperfusion by ROS overproduction and increasing DNA breaks activates PARP leading to accelerated NAD+ catabolism, impaired energy metabolism, and cell damage. Lipoamide by reducing ROS levels halts PARP activation and membrane damage and improves the recovery of postischemic myocardium.

Role and regulation of the ER chaperone BiP

BiP, an HSP70 molecular chaperone located in the lumen of the endoplasmic reticulum (ER), binds newly-synthesized proteins as they are translocated into the ER and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery, as well as playing a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome. BiP is an abundant protein under all growth conditions, but its synthesis is markedly induced under conditions that lead to the accumulation of unfolded polypeptides in the ER. This attribute provides a marker for disease states that result from misfolding of secretory and transmembrane proteins.

Role of reactive oxygen species and poly-ADP-ribose polymerase in the development of AZT-induced cardiomyopathy in rat

The short term cardiac side-effects of AZT (3'-azido-3'-deoxythymidine, zidovudine) was studied in rats to understand the biochemical events contributing to the development of AZT-induced cardiomyopathy. Developing rats were treated with AZT (50 mg/kg/day) for 2 wk and the structural and functional changes were monitored in the cardiac muscle. AZT treatment provoked a surprisingly fast appearance of cardiac malfunctions in developing animals characterized by prolonged RR, PR and QT intervals and J point depression. Electron microscopy showed abnormal mitochondrial structure but the cardiomyocyte had normal myofibers. The AZT treatment of rats significantly increased ROS and peroxynitrite formation in heart tissues as determined by the oxidation of nonfluorescent dihydrorhodamine123 and dichlorodihydro-fluorescein diacetate (H2DCFDA) to fluorescent dyes, and induced single-strand DNA breaks. Lipid peroxidation and oxidation of cellular proteins determined from protein carbonyl content were increased as a consequence of AZT treatment. Activation of the nuclear poly-ADP-ribose polymerase and the accelerated NAD+ catabolism were also observed in AZT-treated animals. Western blot analysis showed that mono-ADP-ribosylation of glucose regulated protein (GRP78/BIP) was enhanced by AZT treatment, that process inactivates GRP78. In this way moderate decrease in the activity of respiratory complexes was detected in the heart of AZT-treated animals indicating a damaged mitochondrial energy production. There was a significant decrease in creatine phosphate concentration resulting in a decrease in creatine phosphate/creatine ratio from 2.08 to 0.58. ATP level remained close to normal but the total extractable ADP increased with 45%. The calculated free ATP/ADP ratio decreased from 340 to 94 in the heart of AZT-treated rats as a consequence of increased free ADP concentration. It was assumed that the increased free ADP in AZT-treated cardiomyocyte may help cells to compensate the defective ATP production in damaged mitochondria by activating the ATP synthesis in undamaged mitochondria. Southern blot analysis did not show decreased quantity of mtDNA deriving from AZT-treated rat hearts indicating that under our experimental conditions AZT-induced heart abnormalities are not the direct consequence of the mtDNA depletion. These data show that ROS-mediated oxidative damages, activated ADP-ribosylation reactions and accelerated NAD+ catabolism play basic roles in the development of AZT-induced cardiomyopathy in our animal model and indicated that these ROS-mediated processes can be important factors in the development of myopathy and cardiomyopathy in zidovudine-treated AIDS patients.

The dynamic role of GRP78/BiP in the coordination of mRNA translation with protein processing

The role of GRP78/BiP in coordinating endoplasmic reticular (ER) protein processing with mRNA translation was examined in GH3 pituitary cells. ADP-ribosylation of GRP78 and eukaryotic initiation factor (eIF)-2alpha phosphorylation were assessed, respectively, as indices of chaperone inactivation and the inhibition of translational initiation. Inhibition of protein processing by ER stress (ionomycin and dithiothreitol) resulted in GRP78 deribosylation and eIF-2 phosphorylation. Suppression of translation relative to ER protein processing (cycloheximide) produced approximately 50% ADP-ribosylation of GRP78 within 90 min without eIF-2 phosphorylation. ADP-ribosylation was reversed in 90 min by cycloheximide removal in a manner accelerated by ER stressors. Cycloheximide sharply reduced eIF-2 phosphorylation in response to ER stressors for about 30 min; sensitivity returned as GRP78 became increasingly ADP-ribosylated. Reduced sensitivity of eIF-2 to phosphorylation appeared to derive from the accumulation of free, unmodified chaperone as proteins completed processing without replacements. Prolonged (24 h) incubations with cycloheximide resulted in the selective loss of the ADP-ribosylated form of GRP78 and increased sensitivity of eIF-2 phosphorylation in response to ER stressors. Brefeldin A decreased ADP-ribosylation of GRP78 in parallel with increased eIF-2 phosphorylation. The cytoplasmic stressor, arsenite, which inhibits translational initiation through eIF-2 phosphorylation without affecting the ER, also produced ADP-ribosylation of GRP78.

Effects of long-term alcohol intake on ADP ribosylation in rat liver plasma membranes

Mono-ADP-ribosylation, In which the ADP-ribosyl moiety is transferred from NAD to an acceptor protein, is one of the important posttranslational modifications of cellular proteins. Because mounting evidence suggests significant biological roles of this reaction in transmembrane signal transduction and other cell metabolic reactions, we assessed how long-term alcohol intake alters toxin catalyzed- and endogenous mono-ADP-ribosylation in the liver of a rat model. We first found that thiol-preactivated cholera toxin-catalyzed ADP-ribosylation of the alpha-subunit of the stimulatory GTP-binding protein was enhanced after long-term alcohol intake. Unexpectedly, but interestingly, this enhancement was not accompanied by a concomitant increase of cholera toxin-catalyzed stimulation of the adenylate cyclase activity. We also found that long-term alcohol intake remarkably enhanced endogenous mono-ADP-ribosylation of a 58 kDa protein in plasma membranes. Thus, long-term alcohol intake stimulated endogenous, as well as, toxin-catalyzed mono-ADP-ribosylations. Characterization of the 58 kDa protein may uncover pathophysiological roles of this interesting phenomenon in alcohol-induced liver damage.

The purification of a cysteine-dependent NAD+ glycohydrolase activity from bovine erythrocytes and evidence that it exhibits a novel ADP-ribosyltransferase activity

An NAD+:cysteine ADP-ribosyltransferase activity was purified from bovine erythrocytes on the assumption that, like pertussis toxin, the enzyme would exhibit a cysteine-dependent NAD+ glycohydrolase activity. A three-step purification procedure was developed involving (1) precipitation with 40% (NH4)2SO4, (2) binding to a cysteine-Sepharose affinity column, and (3) binding to an NAD+ affinity column. PAGE showed a single band of M(r) 45,000. The enzyme had been purified 47,000-fold and had a specific activity of 1900 nmol nicotinamide released/min per mg. A study of the kinetic properties of this enzyme showed saturation kinetics for cysteine (Km = 4.0 mM). The ability of this enzyme to ADP-ribosylate protein was investigated using re-sealed inverted bovine erythrocyte ghosts. Incubation of the purified enzyme with erythrocyte ghosts and [adenylate-32P]NAD+ led to the enhanced dose-dependent labelling of several proteins, a doublet of high M(r) and proteins of M(r) 60,000, 55,000 and 29,000, identified by autoradiography of separated proteins on SDS/PAGE. The enzyme-catalysed labelling of the major component at M(r) 55,000 was blocked by pre-treatment of the erythrocyte ghosts with N-ethymaleimide, a sulphydryl alkylating agent, and the label was released by mercuric ion, but not by hydroxylamine. These experiments suggested that a cysteine residue on the target protein had been mono-ADP-ribosylated. This supposition was further supported by identification of the mercf1p4ion-released radiolabelled product as ADP-ribose by HPLC, and the observation that free ADP-ribose was unable to modify the membrane target protein directly.

Two-dimensional electrophoretic mapping of hepatic and renal stress proteins

Cellular stress proteins and molecular chaperones are responsive to a variety of stressors and therefore comprise an ideal set of proteins with the potential to be used as biomarkers of chemical toxicity. We have investigated the expression of a group of well established heat shock and glucose-regulated proteins in the rat liver and kidney using large-scale two-dimensional protein electrophoresis and computerized image analysis. Our goal was to determine the level of their expression in unstressed target tissues and map their coordinate positions on conventional format two-dimensional electrophoresis (2-DE) gels. All the proteins studied, except for Hsp25 (heat-shock protein) whose expression fell below the level of analyzable detection, were constitutively expressed in liver and kidney. With the exception of Hsp70, all the stress proteins analyzed were constitutively more abundant in the liver than the kidney. Comparison of the sum total of all stress protein abundances revealed a nearly threefold higher level of expression in the liver than the kidney. Our results suggest that this group of proteins has significant responsibilities in normal, unstressed cells, due to their constitutive abundance. Correspondingly, the 2-DE stress protein pattern established in this study may be very useful in toxicologic screening as well as describing a broad range of molecular effects of xenobiotic exposure.

Chemical modifications of a recombinant bovine stress-inducible 70 kDa heat-shock protein (Hsp70) mimics Hsp70 isoforms from tissues

A cDNA clone for the stress-inducible 70 kDa heat-shock protein (Hsp70) has been isolated from a bovine skeletal-muscle cDNA library. This mRNA encodes a protein with a calculated molecular mass of 70250 Da. The cDNA has one continuous open reading frame capable of encoding a 641-amino-acid protein. Expression of this cDNA in a bacterial expression system produced a protein with a mobility identical with that of the inducible Hsp70 protein from bovine skeletal muscle as determined by SDS/PAGE. Two-dimensional gel electrophoresis demonstrated this protein to have focusing properties identical with that of a minor isoform from bovine skeletal muscle. Upon carbamylation of this bacterially expressed protein, a train of charged proteins with charge differences of -1 were produced. These carbamylated proteins were shown to have similar focusing mobilities to the Hsp70 isoforms isolated from bovine skeletal muscle. These results demonstrate the identification of a skeletal-muscle inducible Hsp70 gene and suggest that the presence of multiple Hsp70 isoforms may be the product of post-translational modifications to the Hsp70 proteins.

BiP (GRP78), an essential hsp70 resident protein in the endoplasmic reticulum

BiP is a constitutively-expressed resident protein of the endoplasmic reticulum (ER) of all eucaryotic cells, and belongs to the highly conserved hsp70 protein family. In the ER, BiP is involved in polypeptide translocation, protein folding and presumably protein degradation as well. These functions are essential to cell viability, as has been shown for yeast. In this review, I will summarize the structural features of hsp70 proteins and focus on those experiments which revealed the biological function of BiP.

ADP-ribosylation of the molecular chaperone GRP78/BiP

Starvation of mouse hepatoma cells for essential amino acids or glucose results in the ADP-ribosylation of the molecular chaperone BiP/GRP78. Addition of the missing nutrient to the medium reverses the reaction. The signal mediating the response to environmental nutrients involves the translational efficiency. An inhibitor of proteins synthesis, cycloheximide, or reduced temperature, both of which reduce translational efficiency, stimulate the ADP-ribosylation of BiP/GRP78. Inhibition of N-linked glycosylation of proteins results in the overproduction of BiP/GRP78. The over produced protein is not ADP-ribosylated suggesting that this is the functional form of BiP/GRP78. The over produced BiP/GRP78 can, however, be ADP-ribosylated if the cells are starved for an essential amino acid. BiP/GRP78 resides in the lumen of the endoplasmic reticulum where it participates in the assembly of secretory and integral membrane proteins. ADP-ribosylation of BiP/GRP78 during starvation is probably part of a nutritional stress response which conserves limited nutrients by slowing flow through the secretory pathway.

Vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation appears to be a reversible modification of proteins, which occurs in many eukaryotic and prokaryotic organisms. Multiple forms of arginine-specific ADP-ribosyltransferases have been purified and characterized from avian erythrocytes, chicken polymorphonuclear leukocytes and mammalian skeletal muscle. The avian transferases have similar molecular weights of approximately 28 kDa, but differ in physical, regulatory and kinetic properties and subcellular localization. Recently, a 38-kDa rabbit skeletal muscle ADP-ribosyltransferase was purified and cloned. The deduced amino acid sequence contained hydrophobic amino and carboxy termini, consistent with known signal sequences of glycosylphosphatidylinositol (GPI)-anchored proteins. This arginine-specific transferase was present on the surface of mouse myotubes and of NMU cells transfected with the cDNA and was released with phosphatidylinositol-specific phospholipase C. Arginine-specific ADP-ribosyltransferases thus appear to exhibit considerable diversity in their structure, cellular localization, regulation and physiological role.

Stimulation of mono-ADP ribosylation in rat liver plasma membranes after long-term alcohol intake

ADP ribosylation is considered one of the important covalent modifications of cellular proteins catalyzed by ADP ribosyltransferase, which transfers ADP ribose moiety of NAD to an acceptor protein. Because a growing body of evidence has suggested significant biological roles for mono-ADP ribosylations in transmembrane signal transduction and other cell metabolism, how alcohol intake alters them is of interest. Cholera toxin and pertussis toxin have been widely used as probes to investigate the roles of GTP-binding proteins (G-proteins) in the transduction of hormonal and sensory signals. We first tested effects of long-term alcohol intake on these toxin-catalyzed ADP ribosylations of G-proteins in rat liver plasma membranes. Treatment of rat liver plasma membrane with [32P]NAD and thiol-preactivated cholera toxin resulted in the labeling of a 44-kD band, most likely an alpha-subunit of the stimulatory GTP-binding protein, the extent of which was much greater in alcohol-fed rats than in pair-fed controls. Analogous experiments with pertussis toxin also demonstrated enhancement of toxin-catalyzed ADP ribosylation of the inhibitory GTP-binding protein after long-term alcohol intake. More interesting was that long-term alcohol intake remarkably stimulated endogenous mono-ADP ribosylation of a 58-kD protein in a GTP-dependent manner. In vitro, ethanol (50 mmol/L) or a single load of ethanol (3 gm/kg) did not stimulate the reaction. Thus long-term alcohol intake stimulated both toxin-catalyzed and endogenous mono-ADP ribosylations of proteins in rat liver plasma membranes. Pursuit of alcohol interaction with mono-ADP ribosylation may provide an interesting approach to the study of alcohol's effects on the liver.

Role of ADP-ribosylation in endothelial signal transduction and prostacyclin production

ADP-ribosylation of proteins by the enzymatic transfer of ADP-ribose from NAD has been implicated in a number of biological processes. We report that inhibitors of ADP-ribosylation, most notably the novel inhibitor of arginine specific cellular mono(ADP-ribosyl) transferase, meta-iodobenzylguanidine (MIBG) as well as nicotinamide, L-arginine methyl ester (LAME) and guanyltyramine, inhibit histamine-induced endothelial production of inositol phosphates, release of arachidonic acid and production of prostacyclin (PGI2). Those same responses were unaffected by MIBG when triggered by thrombin or leukotriene C4. These findings suggest that ADP-ribosylation serves a role in histamine-induced production of prostacyclin and imply differences in transduction pathways employed by the different agonists.

Nitric oxide stimulates the ADP-ribosylation of a 41-kDa cytosolic protein in Dictyostelium discoideum

Nitric oxide-releasing compounds were shown to activate an ADP-ribosyltransferase activity in the cytosol of Dictyostelium discoideum. The enzyme ADP-ribosylated a cytosolic protein of approximately 41 kDa, p41. Neither cGMP nor GTP and its analogues affected this ADP-ribosylation. p41 differs from other substrates ADP-ribosylated by cholera, pertussis, or diphtheria toxins. Treatment of ADP-ribosylated p41 with snake venom phosphodiesterase released adenosine 5'-monophosphate, indicating a mono-ADP-ribose-protein linkage. This linkage was stable to neutral hydroxylamine but was sensitive to mercury ions and iodomethane, suggesting an attachment to a cysteine residue. Treatment of intact cells with nitric oxide-releasing compounds appeared to stimulate the ADP-ribosylation of p41 and this modification was reversible.

Mammalian stress response: induction of the glucose-regulated protein family

The glucose-regulated protein family consists of a set of stress-inducible proteins localized in the endoplasmic reticulum. Since their discovery in 1977, significant advances in our understanding of their structure, function and regulation have been made. Recent findings concerning the physiological roles played by the glucose-regulated proteins, and their regulations at the transcriptional, post-transcriptional, translational and post-translational levels are summarized.

Interactions of liver Grp78 and Escherichia coli recombinant Grp78 with ATP: multiple species and disaggregation

The hamster gene encoding the 78-kDa glucose-regulated protein (Grp78) was expressed in Escherichia coli as a fusion protein with glutathione S-transferase. After induction with isopropyl beta-D-thiogalactopyranoside, the recombinant Grp78 was purified to homogeneity by affinity column chromatography of the fusion protein followed by thrombin cleavage. The purified recombinant protein was compared with liver Grp78 for its ability to interact with ATP. Like liver Grp78, the recombinant protein contained a weak ATPase activity and a Ca(2+)-stimulated autophosphorylation activity. However, unlike liver Grp78, in which the autophosphorylation reaction is stimulated less than 50% by CaCl2, the reaction with the recombinant Grp78 was stimulated about 15-fold in the presence of Ca2+. Although the liver protein showed at least four isoforms after two-dimensional gel electrophoresis, the recombinant Grp78 had one major species corresponding to the most basic form seen in liver. Both the liver Grp78 and the recombinant protein existed primarily as monomers and dimers. A small amount of oligomers was also present in the liver Grp78. When either protein was incubated with ATP, there was a conversion of the higher molecular weight species to the monomeric form.

Calcium-dependent autophosphorylation of the glucose-regulated protein, Grp78

The characteristics of phosphorylation of the 78-kDa glucose-regulated protein (Grp78), also known as the immunoglobulin heavy chain binding protein, were studied in vitro and in vivo. The purified protein from either calf liver or bovine kidney cells (MDBK) could be phosphorylated in vitro with [gamma-32P]ATP, in a reaction that is stimulated by Ca2+ and inhibited by the Ca(2+)-chelator ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA). In the presence of EGTA, excess Ca2+ increased the rate of phosphorylation about 18-fold. Based on EGTA/Ca2+ titrations, the optimal Ca2+ concentration for phosphorylation was estimated to be 10-50 microM. Other divalent cations such as Mg2+, Mn2+, and Zn2+ were found to be inhibitory as was the Ca2+ antagonist lanthanum (La3+). The in vivo phosphorylation of Grp78 was studied in MDBK cells labeled with 32Pi. In the presence of inducers of Grp78 synthesis, such as ionomycin, tunicamycin, or 2-deoxyglucose, there was a large increase in the level of Grp78 in the cells but a decrease in the amount of phosphorylated protein. Two-dimensional gel analysis of Grp78 purified from bovine liver and MDBK cells identified at least four isoforms. After in vivo and in vitro phosphorylation of Grp78 all the acidic isoforms contained radioactivity but not the most basic isoform. Phosphoamino acid analysis of Grp78 showed that serine and threonine were phosphorylated in vivo and only threonine was phosphorylated in vitro.