Endogenous ADP-ribosylation of the G protein beta subunit prevents the inhibition of type 1 adenylyl cyclase

PARP10 Mediates Mono-ADP-Ribosylation of Aurora-A Regulating G2/M Transition of the Cell Cycle

Intracellular mono-ADP-ribosyltransferases (mono-ARTs) catalyze the covalent attachment of a single ADP-ribose molecule to protein substrates, thus regulating their functions. PARP10 is a soluble mono-ART involved in the modulation of intracellular signaling, metabolism and apoptosis. PARP10 also participates in the regulation of the G1- and S-phase of the cell cycle. However, the role of this enzyme in G2/M progression is not defined. In this study, we found that genetic ablation, protein depletion and pharmacological inhibition of PARP10 cause a delay in the G2/M transition of the cell cycle. Moreover, we found that the mitotic kinase Aurora-A, a previously identified PARP10 substrate, is actively mono-ADP-ribosylated (MARylated) during G2/M transition in a PARP10-dependent manner. Notably, we showed that PARP10-mediated MARylation of Aurora-A enhances the activity of the kinase in vitro. Consistent with an impairment in the endogenous activity of Aurora-A, cells lacking PARP10 show a decreased localization of the kinase on the centrosomes and mitotic spindle during G2/M progression. Taken together, our data provide the first evidence of a direct role played by PARP10 in the progression of G2 and mitosis, an event that is strictly correlated to the endogenous MARylation of Aurora-A, thus proposing a novel mechanism for the modulation of Aurora-A kinase activity.

PKD-dependent PARP12-catalyzed mono-ADP-ribosylation of Golgin-97 is required for E-cadherin transport from Golgi to plasma membrane

Adenosine diphosphate (ADP)-ribosylation is a posttranslational modification involved in key regulatory events catalyzed by ADP-ribosyltransferases (ARTs). Substrate identification and localization of the mono-ADP-ribosyltransferase PARP12 at the trans-Golgi network (TGN) hinted at the involvement of ARTs in intracellular traffic. We find that Golgin-97, a TGN protein required for the formation and transport of a specific class of basolateral cargoes (e.g., E-cadherin and vesicular stomatitis virus G protein [VSVG]), is a PARP12 substrate. PARP12 targets an acidic cluster in the Golgin-97 coiled-coil domain essential for function. Its mutation or PARP12 depletion, delays E-cadherin and VSVG export and leads to a defect in carrier fission, hence in transport, with consequent accumulation of cargoes in a trans-Golgi/Rab11-positive intermediate compartment. In contrast, PARP12 does not control the Golgin-245-dependent traffic of cargoes such as tumor necrosis factor alpha (TNFα). Thus, the transport of different basolateral proteins to the plasma membrane is differentially regulated by Golgin-97 mono-ADP-ribosylation by PARP12. This identifies a selective regulatory mechanism acting on the transport of Golgin-97- vs. Golgin-245-dependent cargoes. Of note, PARP12 enzymatic activity, and consequently Golgin-97 mono-ADP-ribosylation, depends on the activation of protein kinase D (PKD) at the TGN during traffic. PARP12 is directly phosphorylated by PKD, and this is essential to stimulate PARP12 catalytic activity. PARP12 is therefore a component of the PKD-driven regulatory cascade that selectively controls a major branch of the basolateral transport pathway. We propose that through this mechanism, PARP12 contributes to the maintenance of E-cadherin-mediated cell polarity and cell-cell junctions.

Targeting ADP-ribosylation as an antimicrobial strategy

ADP-ribosylation (ADPr) is an ancient reversible modification of cellular macromolecules controlling major biological processes as diverse as DNA damage repair, transcriptional regulation, intracellular transport, immune and stress responses, cell survival and proliferation. Furthermore, enzymatic reactions of ADPr are central in the pathogenesis of many human diseases, including infectious conditions. By providing a review of ADPr signalling in bacterial systems, we highlight the relevance of this chemical modification in the pathogenesis of human diseases depending on host-pathogen interactions. The post-antibiotic era has raised the need to find alternative approaches to antibiotic administration, as major pathogens becoming resistant to antibiotics. An in-depth understanding of ADPr reactions provides the rationale for designing novel antimicrobial strategies for treatment of infectious diseases. In addition, the understanding of mechanisms of ADPr by bacterial virulence factors offers important hints to improve our knowledge on cellular processes regulated by eukaryotic homologous enzymes, which are often involved in the pathogenesis of human diseases.

Mono-ADP-Ribosylation Catalyzed by Arginine-Specific ADP-Ribosyltransferases

Methods are described for determination of arginine-specific mono-ADP-ribosyltransferase activity of purified proteins and intact cells by monitoring the transfer of ADP-ribose from NAD⁺ to a model substrate, e.g., arginine, agmatine, and peptide (human neutrophil peptide-1 [HNP1]), and for the nonenzymatic hydrolysis of ADP-ribose-arginine to ornithine, a noncoded amino acid. In addition, preparation of purified ADP-ribosylarginine is included as a control substrate for ADP-ribosylation reactions.

ARTC1-mediated ADP-ribosylation of GRP78/BiP: a new player in endoplasmic-reticulum stress responses

Protein mono-ADP-ribosylation is a reversible post-translational modification of cellular proteins. This scheme of amino-acid modification is used not only by bacterial toxins to attack host cells, but also by endogenous ADP-ribosyltransferases (ARTs) in mammalian cells. These latter ARTs include members of three different families of proteins: the well characterised arginine-specific ecto-enzymes (ARTCs), two sirtuins, and some members of the poly(ADP-ribose) polymerase (PARP/ARTD) family. In the present study, we demonstrate that human ARTC1 is localised to the endoplasmic reticulum (ER), in contrast to the previously characterised ARTC proteins, which are typical GPI-anchored ecto-enzymes. Moreover, using the "macro domain" cognitive binding module to identify ADP-ribosylated proteins, we show here that the ER luminal chaperone GRP78/BiP (glucose-regulated protein of 78 kDa/immunoglobulin heavy-chain-binding protein) is a cellular target of human ARTC1 and hamster ARTC2. We further developed a procedure to visualise ADP-ribosylated proteins using immunofluorescence. With this approach, in cells overexpressing ARTC1, we detected staining of the ER that co-localises with GRP78/BiP, thus confirming that this modification occurs in living cells. In line with the key role of GRP78/BiP in the ER stress response system, we provide evidence here that ARTC1 is activated during the ER stress response, which results in acute ADP-ribosylation of GRP78/BiP paralleling translational inhibition. Thus, this identification of ARTC1 as a regulator of GRP78/BiP defines a novel, previously unsuspected, player in GRP78-mediated ER stress responses.

New PARP targets for cancer therapy

Poly(ADP-ribose) polymerases (PARPs) modify target proteins post-translationally with poly(ADP-ribose) (PAR) or mono(ADP-ribose) (MAR) using NAD(+) as substrate. The best-studied PARPs generate PAR modifications and include PARP1 and the tankyrase PARP5A, both of which are targets for cancer therapy with inhibitors in either clinical trials or preclinical development. There are 15 additional PARPs, most of which modify proteins with MAR, and their biology is less well understood. Recent data identify potentially cancer-relevant functions for these PARPs, which indicates that we need to understand more about these PARPs to effectively target them.

Recognition of mono-ADP-ribosylated ARTD10 substrates by ARTD8 macrodomains

ADP-ribosyltransferases (ARTs) catalyze the transfer of ADP-ribose from NAD(+) onto substrates. Some ARTs generate in an iterative process ADP-ribose polymers that serve as adaptors for distinct protein domains. Other ARTs, exemplified by ARTD10, function as mono-ADP-ribosyltransferases, but it has been unclear whether this modification occurs in cells and how it is read. We observed that ARTD10 colocalized with ARTD8 and defined its macrodomains 2 and 3 as readers of mono-ADP-ribosylation both in vitro and in cells. The crystal structures of these two ARTD8 macrodomains and isothermal titration calorimetry confirmed their interaction with ADP-ribose. These macrodomains recognized mono-ADP-ribosylated ARTD10, but not poly-ADP-ribosylated ARTD1. This distinguished them from the macrodomain of macroH2A1.1, which interacted with poly- but not mono-ADP-ribosylated substrates. Moreover, Ran, an ARTD10 substrate, was also read by ARTD8 macrodomains. This identifies readers of mono-ADP-ribosylated proteins, defines their structures, and demonstrates the presence of this modification in cells.

PARP16/ARTD15 is a novel endoplasmic-reticulum-associated mono-ADP-ribosyltransferase that interacts with, and modifies karyopherin-ß1

Background

Protein mono-ADP-ribosylation is a reversible post-translational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. The latter include members of three different families of proteins: the well characterized arginine-specific ecto-enzymes ARTCs, two sirtuins and, more recently, novel members of the poly(ADP-ribose) polymerase (PARP/ARTD) family that have been suggested to act as cellular mono-ADP-ribosyltransferases. Here, we report on the characterisation of human ARTD15, the only known ARTD family member with a putative C-terminal transmembrane domain.

Methodology/Principal Findings

Immunofluorescence and electron microscopy were performed to characterise the sub-cellular localisation of ARTD15, which was found to be associated with membranes of the nuclear envelope and endoplasmic reticulum. The orientation of ARTD15 was determined using protease protection assay, and is shown to be a tail-anchored protein with a cytosolic catalytic domain. Importantly, by combining immunoprecipitation with mass spectrometry and using cell lysates from cells over-expressing FLAG-ARTD15, we have identified karyopherin-ß1, a component of the nuclear trafficking machinery, as a molecular partner of ARTD15. Finally, we demonstrate that ARTD15 is a mono-ADP-ribosyltransferase able to induce the ADP-ribosylation of karyopherin-ß1, thus defining the first substrate for this enzyme.

Conclusions/Significance

Our data reveal that ARTD15 is a novel ADP-ribosyltransferase enzyme with a new intracellular location. Finally, the identification of karyopherin-ß1 as a target of ARTD15-mediated ADP-ribosylation, hints at a novel regulatory mechanism of karyopherin-ß1 functions.

Molecular and biological characterization of Streptococcal SpyA-mediated ADP-ribosylation of intermediate filament protein vimentin

The Gram-positive bacterial pathogen Streptococcus pyogenes produces a C3 family ADP-ribosyltransferase designated SpyA (S. pyogenes ADP-ribosyltransferase). Our laboratory has identified a number of eukaryotic protein targets for SpyA, prominent among which are the cytoskeletal proteins actin and vimentin. Because vimentin is an unusual target for modification by bacterial ADP-ribosyltransferases, we quantitatively compared the activity of SpyA on vimentin and actin. Vimentin was the preferred substrate for SpyA (k(cat), 58.5 ± 3.4 min(-1)) relative to actin (k(cat), 10.1 ± 0.6 min(-1)), and vimentin was modified at a rate 9.48 ± 1.95-fold greater than actin. We employed tandem mass spectrometry analysis to identify sites of ADP-ribosylation on vimentin. The primary sites of modification were Arg-44 and -49 in the head domain, with several additional secondary sites identified. Because the primary sites are located in a domain of vimentin known to be important for the regulation of polymerization by phosphorylation, we investigated the effects of SpyA activity on vimentin polymerization, utilizing an in vitro NaCl-induced filamentation assay. SpyA inhibited vimentin filamentation, whereas a catalytic site mutant of SpyA had no effect. Additionally, we demonstrated that expression of SpyA in HeLa cells resulted in collapse of the vimentin cytoskeleton, whereas expression in RAW 264.7 cells impeded vimentin reorganization upon stimulation of this macrophage-like cell line with LPS. We conclude that SpyA modification of vimentin occurs in an important regulatory region of the head domain and has significant functional effects on vimentin assembly.

Identification and partial characterization of Rhizopus nigricans Gβ proteins and their expression in the presence of progesterone

The mammalian steroid hormone progesterone actuates a signalling pathway in the zygomycete Rhizopus nigricans which includes heterotrimeric G proteins. To investigate the possibility that the Gβ subunit of these proteins is involved in the signalling, a cDNA library from R. nigricans exposed to progesterone was prepared and a sequence coding for a Gβ subunit was searched for. Using degenerate primers, two sequences, RnGPB1 and RnGPB2, were identified that exhibited a high degree of identity with those for Gβ from other filamentous fungi, but not from yeast. The presence of more than one Gβ subunit is very rare among the fungi, and it has been to date reported only for Rhizopus oryzae. We have shown that progesterone increases the expression of RnGPB1, but has no influence on the expression of RnGPB2. Therefore, our studies imply the involvement of Gβ subunit 1 in the response of R. nigricans to progesterone. Moreover, the Gβ subunit is subjected to endogenous ADP-ribosylation in the presence of NAD, which could be important in some, as yet unknown, cell process. Article from a special issue on steroids and microorganisms.

Endogenous protein mono-ADP-ribosylation in Arabidopsis thaliana

Protein mono-ADP-ribosylation post-translationally transfers the ADP-ribose moiety from the β-NAD+ donor to various protein acceptors. This type of modification has been widely characterized and shown to regulate protein activities in animals, yeast and prokaryotes, but has never been reported in plants. In this study, using [³²P]NAD+ as the substrate, ADP-ribosylated proteins in Arabidopsis were investigated. One protein substrate of 32 kDa in adult rosette leaves was found to be radiolabeled. Heat treatment, protease sensitivity and nucleotide derivative competition assays suggested a covalent reaction of NAD+ with the 32 kDa protein. [carbonyl-¹⁴C]NAD+ could not label the 32 kDa protein, confirming that the modification was ADP-ribosylation. Poly (ADP-ribose) polymerase inhibitor failed to suppress the reaction, but chemicals that destroy mono-ADP-ribosylation on specific amino acid residues could break up the linkage, suggesting that the reaction was not a poly-ADP-ribosylation but rather a mono-ADP-ribosylation. This modification mainly existed in leaves and was enhanced by oxidative stresses. In young seedlings, two more protein substrates with the size of 45 kDa and over 130 kDa, respectively, were observed in addition to the 32 kDa protein, indicating that different proteins were modified at different developmental stages. Although the substrate proteins remain to be identified, this is the first report on the characterization of endogenously mono-ADP-ribosylated proteins in plants.

Third intracellular loop of glucagon like-peptide-1 receptor is coupled with endogenous mono-ADP-ribosyltransferase - novel type of receptor regulation?

Our previous studies revealed the main role of the third intracellular loop (IC(3)) of glucagon-like peptide-1 receptor (GLP-1 receptor), in G-protein activation, where the presence or absence of agonist and the receptor phosphorylation seemed to be the only regulatory mechanisms. In order to further study the signaling mechanisms of GLP-1 receptor, we investigated the effect of the third intracellular loop-derived peptide on endogenous mono-ADP-ribosyltransferase mediated mono-ADP-ribosylation of G-proteins β subunit in CHO cells. Results showed an inhibitory effect of IC(3) peptide on mono-ADP-ribosylation of β subunit, obviously via the mechanism of competitive inhibition. Excluding the activity of this inhibitory mechanism via pertussis toxin-sensitive G proteins, the direct functional coupling of IC(3) of GLP-1 receptor and endogenous mono-ADP-ribosyltransferase was confirmed. We suggest that this arginine specific enzymatic posttranslational modification of third intracellular loop of GLP-1 receptor might represent a possible novel mechanism of receptor activity regulation and the pharmacological potential in treatment of diabetes mellitus type 2.

Characterisation of a novel glycosylphosphatidylinositol-anchored mono-ADP-ribosyltransferase isoform in ovary cells

The mammalian mono-ADP-ribosyltransferases are a family of enzymes related to bacterial toxins that can catalyse both intracellular and extracellular mono-ADP-ribosylation of target proteins involved in different cellular processes, such as cell migration, signalling and inflammation. Here, we report the molecular cloning and functional characterisation of a novel glycosylphosphatidylinositol (GPI)-anchored mono-ADP-ribosyltransferase isoform from Chinese hamster ovary (CHO) cells (cARTC2.1) that has both NAD-glycohydrolase and arginine-specific ADP-ribosyltransferase activities. cARTC2.1 has the R-S-EXE active-site motif that is typical of arginine-specific ADP-ribosyltransferases, with Glu209 as the predicted catalytic amino acid. When over-expressed in CHO cells, the E209G single point mutant of cARTC2.1 cannot hydrolyse NAD(+), although it retains low arginine-specific ADP-ribosyltransferase activity. This ADP-ribosyltransferase activity was abolished only with an additional mutation in the R-S-EXE active-site motif, with both of the glutamate residues of the EKE sequence of cARTC2.1 mutated to glycine (E207/209G). These glutamate-mutated proteins localise to the plasma membrane, as does wild-type cARTC2.1. Thus, the partial or total loss of enzymatic activity of cARTC2.1 that arises from these mutations does not affect its cellular localisation. Importantly, an endogenous ADP-ribosyltransferase is indeed expressed and active in a subset of CHO cells, while a similar activity cannot be detected in ovarian cancer cells. With respect to this endogenous ecto-ART activity, we have identified two cell populations: ART-positive and ART-negative CHO cells. The subset of ART-positive cells, which represented 5% of the total cells, is tightly maintained in the CHO cell population.

The macro domain protein family: structure, functions, and their potential therapeutic implications

Macro domains are ancient, highly evolutionarily conserved domains that are widely distributed throughout all kingdoms of life. The 'macro fold' is roughly 25kDa in size and is composed of a mixed α-β fold with similarity to the P loop-containing nucleotide triphosphate hydrolases. They function as binding modules for metabolites of NAD(+), including poly(ADP-ribose) (PAR), which is synthesized by PAR polymerases (PARPs). Although there is a high degree of sequence similarity within this family, particularly for residues that might be involved in catalysis or substrates binding, it is likely that the sequence variation that does exist among macro domains is responsible for the specificity of function of individual proteins. Recent findings have indicated that macro domain proteins are functionally promiscuous and are implicated in the regulation of diverse biological functions, such as DNA repair, chromatin remodeling and transcriptional regulation. Significant advances in the field of macro domain have occurred in the past few years, including biological insights and the discovery of novel signaling pathways. To provide a framework for understanding these recent findings, this review will provide a comprehensive overview of the known and proposed biochemical, cellular and physiological roles of the macro domain family. Recent data that indicate a critical role of macro domain regulation for the proper progression of cellular differentiation programs will be discussed. In addition, the effect of dysregulated expression of macro domain proteins will be considered in the processes of tumorigenesis and bacterial pathogenesis. Finally, a series of observations will be highlighted that should be addressed in future efforts to develop macro domains as effective therapeutic targets.

Compartmentation of NAD+-dependent signalling

NAD(+) plays central roles in energy metabolism as redox carrier. Recent research has identified important signalling functions of NAD(+) that involve its consumption. Although NAD(+) is synthesized mainly in the cytosol, nucleus and mitochondria, it has been detected also in vesicular and extracellular compartments. Three protein families that consume NAD(+) in signalling reactions have been characterized on a molecular level: ADP-ribosyltransferases (ARTs), Sirtuins (SIRTs), and NAD(+) glycohydrolases (NADases). Members of these families serve important regulatory functions in various cellular compartments, e.g., by linking the cellular energy state to gene expression in the nucleus, by regulating nitrogen metabolism in mitochondria, and by sensing tissue damage in the extracellular compartment. Distinct NAD(+) pools may be crucial for these processes. Here, we review the current knowledge about the compartmentation and biochemistry of NAD(+)-converting enzymes that control NAD(+) signalling.

Mono-ADP-ribosylation of the G protein betagamma dimer is modulated by hormones and inhibited by Arf6

Mono-ADP-ribosylation is a reversible post-translational modification that can modulate the functions of target proteins. We have previously demonstrated that the β subunit of heterotrimeric G proteins is endogenously mono-ADP-ribosylated, and once modified, the βγ dimer is inactive toward its effector enzymes. To better understand the physiological relevance of this post-translational modification, we have studied its hormonal regulation. Here, we report that Gβ subunit mono-ADP-ribosylation is differentially modulated by G protein-coupled receptors. In intact cells, hormone stimulation of the thrombin receptor induces Gβ subunit mono-ADP-ribosylation, which can affect G protein signaling. Conversely, hormone stimulation of the gonadotropin-releasing hormone receptor (GnRHR) inhibits Gβ subunit mono-ADP-ribosylation. We also provide the first demonstration that activation of the GnRHR can activate the ADP-ribosylation factor Arf6, which in turn inhibits Gβ subunit mono-ADP-ribosylation. Indeed, removal of Arf6 from purified plasma membranes results in loss of GnRHR-mediated inhibition of Gβ subunit mono-ADP-ribosylation, which is fully restored by re-addition of purified, myristoylated Arf6. We show that Arf6 acts as a competitive inhibitor of the endogenous ADP-ribosyltransferase and is itself modified by this enzyme. These data provide further understanding of the mechanisms that regulate endogenous ADP-ribosylation of the Gβ subunit, and they demonstrate a novel role for Arf6 in hormone regulation of Gβ subunit mono-ADP-ribosylation.

ADP-ribosylation of arginine

Arginine adenosine-5′-diphosphoribosylation (ADP-ribosylation) is an enzyme-catalyzed, potentially reversible posttranslational modification, in which the ADP-ribose moiety is transferred from NAD⁺ to the guanidino moiety of arginine. At 540 Da, ADP-ribose has the size of approximately five amino acid residues. In contrast to arginine, which, at neutral pH, is positively charged, ADP-ribose carries two negatively charged phosphate moieties. Arginine ADP-ribosylation, thus, causes a notable change in size and chemical property at the ADP-ribosylation site of the target protein. Often, this causes steric interference of the interaction of the target protein with binding partners, e.g. toxin-catalyzed ADP-ribosylation of actin at R177 sterically blocks actin polymerization. In case of the nucleotide-gated P2X7 ion channel, ADP-ribosylation at R125 in the vicinity of the ligand-binding site causes channel gating. Arginine-specific ADP-ribosyltransferases (ARTs) carry a characteristic R-S-EXE motif that distinguishes these enzymes from structurally related enzymes which catalyze ADP-ribosylation of other amino acid side chains, DNA, or small molecules. Arginine-specific ADP-ribosylation can be inhibited by small molecule arginine analogues such as agmatine or meta-iodobenzylguanidine (MIBG), which themselves can serve as targets for arginine-specific ARTs. ADP-ribosylarginine specific hydrolases (ARHs) can restore target protein function by hydrolytic removal of the entire ADP-ribose moiety. In some cases, ADP-ribosylarginine is processed into secondary posttranslational modifications, e.g. phosphoribosylarginine or ornithine. This review summarizes current knowledge on arginine-specific ADP-ribosylation, focussing on the methods available for its detection, its biological consequences, and the enzymes responsible for this modification and its reversal, and discusses future perspectives for research in this field.

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.

Cloning, expression, purification and crystallization as well as X-ray fluorescence and preliminary X-ray diffraction analyses of human ADP-ribosylhydrolase 1

Human ADP-ribosylhydrolase 1 (hARH1, ADPRH) cleaves the glycosidic bond of ADP-ribose attached to an Arg residue of a protein. hARH1 has been cloned, expressed heterologously in Escherichia coli, purified and crystallized in complex with K(+) and ADP. The orthorhombic crystals contained one monomer per asymmetric unit, exhibited a solvent content of 43% and diffracted X-rays to a resolution of 1.9 A. A prerequisite for obtaining well diffracting crystals was the performance of X-ray fluorescence analysis on poorly diffracting apo hARH1 crystals, which revealed the presence of trace amounts of K(+) in the crystal. Adding K-ADP to the crystallization cocktail then resulted in a crystal of different morphology and with dramatically improved diffraction properties.

Combining affinity purification by ADP-ribose-binding macro domains with mass spectrometry to define the mammalian ADP-ribosyl proteome

Mono-ADP-ribosylation is a reversible posttranslational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. For the latter, both the enzymes and their targets have largely remained elusive, mainly due to the lack of specific techniques to study this reaction. The recent discovery of the macro domain, a protein module that interacts selectively with ADP-ribose, prompted us to investigate whether this interaction can be extended to the identification of ADP-ribosylated proteins. Here, we report that macro domains can indeed be used as selective baits for high-affinity purification of mono-ADP-ribosylated proteins, which can then be identified by mass spectrometry. Using this approach, we have identified a series of cellular targets of ADP-ribosylation reactions catalyzed by cellular ADP-ribosyltransferases and toxins. These proteins include most of the known targets of ADP-ribosylation, indicating the validity of this method, and a large number of other proteins, which now need to be individually validated. This represents an important step toward the discovery of new ADP-ribosyltransferase targets and an understanding of the physiological role and the pharmacological potential of this protein modification.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

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.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

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.

Physiological relevance of the endogenous mono(ADP-ribosyl)ation of cellular proteins

The mono(ADP-ribosyl)ation reaction is a post-translational modification that is catalysed by both bacterial toxins and eukaryotic enzymes, and that results in the transfer of ADP-ribose from betaNAD+ to various acceptor proteins. In mammals, both intracellular and extracellular reactions have been described; the latter are due to glycosylphosphatidylinositol-anchored or secreted enzymes that are able to modify their targets, which include the purinergic receptor P2X7, the defensins and the integrins. Intracellular mono(ADP-ribosyl)ation modifies proteins that have roles in cell signalling and metabolism, such as the chaperone GRP78/BiP, the beta-subunit of heterotrimeric G-proteins and glutamate dehydrogenase. The molecular identification of the intracellular enzymes, however, is still missing. A better molecular understanding of this reaction will help in the full definition of its role in cell physiology and pathology.

Novel features of amphiphilic peptide Mas7 in signalling via heterotrimeric G-proteins

Amphiphilic peptide Mas7, a structural analogue of mastoparan is a known activator of heterotrimeric Gi-proteins and its downstream effectors. This study investigated the functional interaction of Mas7 with a plasma membrane protein from CHO cells, the endogenous mono-ADP-ribosyltransferase. The substrate of endogenous mono-ADP-ribosyltransferase was the ADP-ribosylated protein with a molecular mass of 36 kDa, which corresponded to the beta subunit of heterotrimeric G-proteins. The effect of Mas7 on endogenous mono-ADP-ribosyltransferase activity was in the micromolar range with a maximal activation of 205% over the basal. In pertussis treated plasma membranes, it was found that the effect of Mas7 on endogenous mono-ADP-ribosyltransferase was partially blocked, which suggests the involvement of G-proteins, such as Gi or G0. In addition, an immunoassay was developed for the visualization of interaction between the a subunit and the betagamma dimer of G-protein on a Ni-NTA support. The physical interaction was tested of Mas7 with the heterotrimeric G-protein alphai2 subunit, which was overexpressed together with beta1gamma2-His6 subunits in sf9 cells. An interaction between Gi2 heterotrimer and Mas7 was not observed, which was not in accordance with previously reported results of mastoparan obtained for Gi-proteins from bovine brain. In conclusion, the signal is mediated from Mas7 to endogenous mono-ADP-ribosyltransferase via pertussis sensitive G-proteins. Furthermore, it is hypothesized that Gi2 G-proteins are not involved in the process.

Active poly(ADPribose) metabolism in DNAase- and salt-resistant rat testis chromatin with high transcriptional activity/competence

A chromatin fraction, named pP fraction, was prepared from rat testis nuclei, which had been digested with nuclease in order to separate soluble and insoluble chromatin. This fraction resembled nuclear matrix as it was highly resistant to DNAase digestion, had a high content of proteins compared to the low DNA percentage, and a noticeable transcriptional activity. Moreover, poly(ADPribosyl)ation system (i.e., poly(ADPR)polymerase, poly(ADPribose), and acceptor proteins) was still present at high levels. In order to study whether it might be identified as the protein support surrounding chromatin loops, this pP fraction was further analyzed after 3 M NaCl extraction. The 3 M NaCl extract and the highly insoluble pellet, named Nuclear Matrix Pellet, were characterized as it regards DNA, newly synthesized RNA and proteins. Furthermore, poly(ADPribose) metabolism was analyzed by measuring both poly(ADPribose) polymerase and poly(ADPribose) glycohydrolase activities, poly(ADPribose) distribution and by identifying protein acceptors. The final pellet had features of nuclear matrix containing less than 10% DNA and high percentage of proteins; 28% of newly synthesized RNA was still associated with this fraction. Long and branched polyADPribose were found in the nuclear matrix-like pellet, although ADPribose acceptors (mainly H1 and core histones) appeared to be modified mostly with short ADPribose oligomers. Longest and branched polymers were retained on the top of protein gel, likely bound to automodified poly(ADPribose) polymerase.

Functional aspects of protein mono-ADP-ribosylation

Mono-ADP-ribosylation is the enzymatic transfer of ADP-ribose from NAD(+) to acceptor proteins. It is catalysed by cellular ADP-ribosyltransferases and certain bacterial toxins. There are two subclasses of cellular enzymes: the ectoenzymes that modify targets such as integrins, defensin and other cell surface molecules; and the intracellular enzymes that act on proteins involved in cell signalling and metabolism, such as the beta-subunit of heterotrimeric G proteins, GRP78/BiP and elongation factor 2. The genes that encode the ectoenzymes have been cloned and their protein products are well characterized, yet little is known about the intracellular ADP-ribosyltransferases, which may be part of a novel protein family with an important role in regulating cell function. ADP-ribosylation usually leads to protein inactivation, providing a mechanism to inhibit protein functions in both physiological and pathological conditions.

Different role of intracellular loops of glucagon-like peptide-1 receptor in G-protein coupling

Previous studies revealed the importance of the third intracellular loop of glucagon-like peptide-1 receptor (GLP-1R) in coupling to G(s) and G(i1) proteins. In order to further study the signaling mechanisms of GLP-1R, we tested three peptides, corresponding to the sequences of the first (IC(1)), the second (IC(2)), and the third (IC(3)) intracellular loop of GLP-1R, for their interactions with heterotrimeric G-proteins of different types (G(alphas), G(alphao), G(alphai1), and G(alpha11) plus G(beta1gamma2)) overexpressed in sf9 cells. IC(3) peptide powerfully stimulates all types of tested G-proteins, whereas IC(1) and IC(2) peptides show differential effects on G-proteins. Both IC(1) and IC(2) peptides activate G(s) and cooperate with IC(3) peptide in its stimulation. G(o) is not affected by IC(1) and IC(2). G(i1) and G(11) are not affected by IC(1), but are activated by IC(2), which in activation cooperates with IC(3). We suggest that GLP-1R is not coupled only to G(s) and G(i1), as shown previously, but also to G(o) and G(11). IC(3) loop is the main switch that mediates signaling via GLP-1R to G-proteins, while IC(1) and IC(2) loops are important in discrimination between different types of G-proteins.

Mono-ADP-ribosylation: a tool for modulating immune response and cell signaling

Mono-ADP-ribosylation is a posttranslational modification of cellular proteins that has the potential to regulate various cell functions. This reaction consists of the enzymatic transfer of ADP-ribose to specific acceptor amino acid residues (predominantly arginine and cysteine). The best-known cellular ADP-ribosyltransferases (the enzymes that catalyze this reaction) are the seven ectoenzymes, members of the ART family. Recently, ADP-ribosylated human neutrophil-derived peptide (HNP-1, an antimicrobial peptide secreted by immune cells) has been identified in the bronchoalveolar lavage fluid from individuals who smoke cigarettes. This demonstrates that ADP-ribosylation of HNP-1 occurs in vivo. In vitro experiments have indicated that ART-1, an enzyme also present in the airway epithelium, specifically modifies Arg(14) of the HNP-1, causing the loss of the peptide's antimicrobial and cytotoxic activity, while preserving its chemotactic activity. From a functional point of view, these data support a role of ADP-ribosylation in the innate immune response. Additional functions proposed for the ADP-ribosylation reaction involve the intracellular ADP-ribosyltransferases, which are molecularly unrelated to the ARTs and intervene in cell signaling and metabolism cascades. The growing understanding of the biological roles of protein and peptide ADP-ribosylation represents a powerful tool for novel pharmacological interventions.

Endogenous mono-ADP-ribosylation of the free Gbetagamma prevents stimulation of phosphoinositide 3-kinase-gamma and phospholipase C-beta2 and is activated by G-protein-coupled receptors

We have recently demonstrated that the beta subunit of the heterotrimeric G-proteins is endogenously mono-ADP-ribosylated in intact cells. The modified betagamma heterodimer loses its ability to inhibit calmodulin-stimulated type 1 adenylate cyclase and, remarkably, is de-ADP-ribosylated by a cytosolic hydrolase that completes an ADP-/de-ADP-ribosylation cycle of potential physiological relevance. In the present study, we show that this ADP-ribosylation might indeed be a general mechanism for termination of betagamma signalling, since the ADP-ribosylated betagamma subunit is also unable to activate both phosphoinositide 3-kinase-gamma and phospholipase C-beta2. Moreover, we show that beta subunit ADP-ribosylation is induced by G-protein-coupled receptor activation, since hormone stimulation of Chinese-hamster ovary plasma membranes leads to increases in beta subunit labelling. This occurs when betagamma is in its active heterodimeric conformation, since full inhibition of this modification can be achieved by binding of GDP-alphai3 to the betagamma heterodimer. Taken together, these findings delineate a pathway that arises from the activation of a G-protein-coupled receptor and leads to the inhibition of betagamma activity through its reversible mono-ADP-ribosylation.

An extracellular monoADP-ribosyl transferase activity in Entamoeba histolytica trophozoites

Due to the important role of monoADP-ribosyl transferases in physiological and pathological events, we investigated whether the protozoan parasite Entamoeba histolytica had monoADP-ribosyl transferase activity. Reactions were initiated using ameba-free medium as the source of both enzyme and ADP-ribosylation substrate(s) and [32P]NAD+ as source of ADP-ribose. Proteins were analyzed by electrophoresis, and [32P]-labeled proteins were detected by autoradiography. Using the crude extracellular medium, a major labeled product of Mr 37.000 was observed. The yield of this product was reduced markedly using medium from Brefeldin A-treated trophozoites, indicating that the extracellular monoADP-ribosyl transferase and/or its substrate depended on vesicular transport. The labeling of the 37-kDa substrate was dependent on reaction time, temperature, pH, and the ratio of unlabeled NAD+ to [32P]NAD+. After two purification steps, several new substrates were observed, perhaps due to their enrichment. The reaction measured ADP-ribosylation since [14C-carbonyl]NAD+ was not incorporated into ameba substrates and a 75-fold molar excess of ADP-ribose caused no detectable inhibition of the monoADP-ribosyl transferase reaction. On the basis of sensitivity to NH2OH, the extracellular monoADP-ribosyl transferase of E. histolytica may be an arginine-specific enzyme. These results demonstrate the existence in E. histolytica of at least one extracellular monoADP-ribosyl transferase, whose localization depends upon a secretion process.

Inhibition of epithelial chloride secretion by butyrate: role of reduced adenylyl cyclase expression and activity

Butyrate and other short-chain fatty acids (SCFAs) are found at high concentrations in the colonic lumen and affect multiple epithelial cell functions. To better understand how SCFAs regulate ion transport, we investigated the effects of SCFAs on Cl(-) secretion in human colonic epithelial cell line T(84). Butyrate inhibited Cl(-) secretory responses to prostaglandin E(2), forskolin, and cholera toxin. Other SCFAs were less effective or inactive. Reduced secretion was associated with decreased synthesis of the second messenger cAMP rather than increased degradation. Expression and activity of adenylyl cyclase were decreased by butyrate, whereas phosphodiesterase activity was unaffected and phosphodiesterase inhibition did not reverse the effects of butyrate on Cl(-) secretion. Furthermore, butyrate decreased expression of the basolateral Na-K-2Cl cotransporter, indicating that it might modulate the secretory capacity of the cells. However, butyrate did not affect secretory responses to the calcium-dependent secretagogue carbachol, cAMP analogs, or uroguanylin, indicating that normal secretory responses to adequate levels of second messengers in butyrate-treated T(84) cells are possible. These results show that butyrate affects several aspects of epithelial Cl(-) secretion, including second messenger generation and expression of key ion transporters. However, these effects may not all be equally important in determining Cl(-) secretion in response to physiologically relevant secretagogues.

Structural and functional characterization of human NAD kinase

NADP is essential for biosynthetic pathways, energy, and signal transduction. Its synthesis is catalyzed by NAD kinase. Very little is known about the structure, function, and regulation of this enzyme from multicellular organisms. We identified a human NAD kinase cDNA and the corresponding gene using available database information. A cDNA was amplified from a human fibroblast cDNA library and functionally overexpressed in Escherichia coli. The obtained cDNA, slightly different from that deposited in the database, encodes a protein of 49 kDa. The gene is expressed in most human tissues, but not in skeletal muscle. Human NAD kinase differs considerably from that of prokaryotes by subunit molecular mass (49 kDa vs 30-35 kDa). The catalytically active homotetramer is highly selective for its substrates, NAD and ATP. It did not phosphorylate the nicotinic acid derivative of NAD (NAAD) suggesting that the potent calcium-mobilizing pyridine nucleotide NAADP is synthesized by an alternative route.

Regulation of glutamate dehydrogenase by reversible ADP-ribosylation in mitochondria

Mitochondrial ADP-ribosylation leads to modification of two proteins of approximately 26 and 53 kDA: The nature of these proteins and, hence, the physiological consequences of their modification have remained unknown. Here, a 55 kDa protein, glutamate dehydrogenase (GDH), was established as a specific acceptor for enzymatic, cysteine-specific ADP-ribosylation in mitochondria. The modified protein was isolated from the mitochondrial preparation and identified as GDH by N-terminal sequencing and mass spectrometric analyses of tryptic digests. Incubation of human hepatoma cells with [14C]adenine demonstrated the occurrence of the modification in vivo. Purified GDH was ADP-ribosylated in a cysteine residue in the presence of the mitochondrial activity that transferred the ADP-ribose from NAD+ onto the acceptor site. ADP- ribosylation of GDH led to substantial inhibition of its catalytic activity. The stoichiometry between incorporated ADP-ribose and GDH subunits suggests that modification of one subunit per catalytically active homohexamer causes the inactivation of the enzyme. Isolated, ADP-ribosylated GDH was reactivated by an Mg2+-dependent mitochondrial ADP-ribosylcysteine hydrolase. GDH, a highly regulated enzyme, is the first mitochondrial protein identified whose activity may be modulated by ADP-ribosylation.

The G protein beta subunit is a determinant in the coupling of Gs to the beta 1-adrenergic and A2a adenosine receptors

The signaling specificity of five purified G protein betagamma dimers, beta(1)gamma(2), beta(2)gamma(2), beta(3)gamma(2), beta(4)gamma(2), and beta(5)gamma(2), was explored by reconstituting them with G(s) alpha and receptors or effectors in the adenylyl cyclase cascade. The ability of the five betagamma dimers to support receptor-alpha-betagamma interactions was examined using membranes expressing the beta(1)-adrenergic or A2a adenosine receptors. These receptors discriminated among the defined heterotrimers based solely on the beta isoform. The beta(4)gamma(2) dimer demonstrated the highest coupling efficiency to either receptor. The beta(5)gamma(2) dimer coupled poorly to each receptor, with EC(50) values 40-200-fold higher than those observed with beta(4)gamma(2). Strikingly, whereas the EC(50) of the beta(1)gamma(2) dimer at the beta(1)-adrenergic receptor was similar to beta(4)gamma(2), its EC(50) was 20-fold higher at the A2a adenosine receptor. Inhibition of adenylyl cyclase type I (AC1) and stimulation of type II (AC2) by the betagamma dimers were measured. betagamma dimers containing Gbeta(1-4) were able to stimulate AC2 similarly, and beta(5)gamma(2) was much less potent. beta(1)gamma(2), beta(2)gamma(2), and beta(4)gamma(2) inhibited AC1 equally; beta(3)gamma(2) was 10-fold less effective, and beta(5)gamma(2) had no effect. These data argue that the beta isoform in the betagamma dimer can determine the specificity of signaling at both receptors and effectors.

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.