Glycosylphosphatidylinositol-anchored arginine-specific ADP-ribosyltransferase7.1 (Art7.1) on chicken B cells: the possible role of Art7 in B cell receptor signalling and proliferation

Mechanistic overview of ADP-ribosylation reactions

ADP-ribosylation reactions consist of mono-ADP-ribosylation, poly-ADP-ribosylation and cyclic ADP-ribosylation. These reactions play essential roles in many important physiological and pathophysiological events. The types of chemical linkages, the evolutionarily conserved motif within the enzymes to determine the target specificity, stereochemistry of the ADP-ribosylated products, and the chemical reactions taking place among the enzymes and substrates are discussed.

ADP-Ribosylargininyl reaction of cholix toxin is mediated through diffusible intermediates

Background

Cholix toxin is an ADP-ribosyltransferase found in non-O1/non-O139 strains of Vibrio cholera. The catalytic fragment of cholix toxin was characterized as a diphthamide dependent ADP-ribosyltransferase.

Results

Our studies on the enzymatic activity of cholix toxin catalytic fragment show that the transfer of ADP-ribose to toxin takes place by a predominantly intramolecular mechanism and results in the preferential alkylation of arginine residues proximal to the NAD⁺ binding pocket. Multiple arginine residues, located near the catalytic site and at distal sites, can be the ADP-ribose acceptor in the auto-reaction. Kinetic studies of a model enzyme, M8, showed that a diffusible intermediate preferentially reacted with arginine residues in proximity to the NAD⁺ binding pocket. ADP-ribosylarginine activity of cholix toxin catalytic fragment could also modify exogenous substrates. Auto-ADP-ribosylation of cholix toxin appears to have negatively regulatory effect on ADP-ribosylation of exogenous substrate. However, at the presence of both endogenous and exogenous substrates, ADP-ribosylation of exogenous substrates occurred more efficiently than that of endogenous substrates.

Conclusions

We discovered an ADP-ribosylargininyl activity of cholix toxin catalytic fragment from our studies in auto-ADP-ribosylation, which is mediated through diffusible intermediates. The lifetime of the hypothetical intermediate exceeds recorded and predicted lifetimes for the cognate oxocarbenium ion. Therefore, a diffusible strained form of NAD⁺ intermediate was proposed to react with arginine residues in a proximity dependent manner.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-014-0026-1) contains supplementary material, which is available to authorized users.

Beyond DNA repair, the immunological role of PARP-1 and its siblings

ADP-ribosylation is the addition of one or more (up to some hundreds) ADP-ribose moieties to acceptor proteins. There are two major families of enzymes that catalyse this reaction: extracellular ADP-ribosyl-transferases (ARTs), which are bound to the cell membrane by a glycosylphosphatidylinositol anchor or are secreted, and poly(ADP-ribose)-polymerases (PARPs), which are present in the cell nucleus and/or cytoplasm. Recent findings revealed a wide immunological role for ADP-ribosylating enzymes. ARTs, by sensing extracellular NAD concentration, can act as danger detectors. PARP-1, the prototypical representative of the PARP family, known to protect cells from genomic instability, is involved in the development of inflammatory responses and several forms of cell death. PARP-1 also plays a role in adaptive immunity by modulating the ability of dendritic cells to stimulate T cells or by directly affecting the differentiation and functions of T and B cells. Both PARP-1 and PARP-14 (CoaSt6) knockout mice were described to display reduced T helper type 2 cell differentiation and allergic responses. Our recent findings showed that PARP-1 is involved in the differentiation of Foxp3+ regulatory T (Treg) cells, suggesting a role for PARP-1 in tolerance induction. Also ARTs regulate Treg cell homeostasis by promoting Treg cell apoptosis during inflammatory responses. PARP inhibitors ameliorate immune-mediated diseases in several experimental models, including rheumatoid arthritis, colitis, experimental autoimmune encephalomyelitis and allergy. Together these findings show that ADP-ribosylating enzymes, in particular PARP-1, play a pivotal role in the regulation of immune responses and may represent a good target for new therapeutic approaches in immune-mediated diseases.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

Precursor ion scanning and sequencing of arginine-ADP-ribosylated peptide by mass spectrometry

Arginine (Arg)-specific ADP-ribosylation is one of the posttranslational modifications of proteins and is thought to play an important role in reversibly regulating functions of the target proteins in eukaryotes. However, the physiological target protein has not been established. We examined the fragmentation pattern of both ADP-ribosyl-Arg (ADP-R-Arg) and Arg-ADP-ribosylated peptides by quadrupole tandem mass spectrometry and found a specific cleavage of ADP-R-Arg into N-(ADP-ribosyl)-carbodiimide (ADP-R-carbodiimide) and ornithine. Based on this specific fragmentation pattern, we successfully identified the modification site and sequence of Arg-ADP-ribosylated peptide using a two-step collision and showed that ADP-R-carbodiimide is an excellent marker ion for precursor ion scanning of Arg-ADP-ribosylated peptide. We propose that a combination of the precursor ion scanning with ADP-R-carbodiimide as a marker ion and two-step collision is useful in searching for physiological target proteins of Arg-ADP-ribosylation.