ADP-ribosylation

ADP-ribosylation is the addition of one or more ADP-ribose moieties to a protein. It is a reversible post-translational modification that is involved in many cellular processes, including cell signaling, DNA repair, gene regulation and apoptosis.Improper ADP-ribosylation has been implicated in some forms of cancer. It is also the basis for the toxicity of bacterial compounds such as cholera toxin, diphtheria toxin, and others. The first suggestion of ADP-ribosylation surfaced during the early 1960s. At this time, Pierre Chambon and coworkers observed the incorporation of ATP into hen liver nuclei extract. After extensive studies on the acid insoluble fraction, several different research laboratories were able to identify ADP-ribose, derived from NAD+, as the incorporated group. Several years later, the enzymes responsible for this incorporation were identified and given the name poly (ADP-ribose) polymerase. Originally, this group was thought to be a linear sequence of ADP-ribose units covalently bonded through a ribose glycosidic bond. It was later reported that branching can occur every 20 to 30 ADP residues. The first appearance of mono-ADP-ribosylation occurred a year later during a study of toxins: corynebacterium diphtheria diphtheria toxin was shown to be dependent on NAD+ in order for it to be completely effective, leading to the discovery of enzymatic conjugation of a single ADP-ribose group by mono-ADP-ribosyl transferase. It was initially thought that ADP-ribosylation was a post translational modification involved solely in gene regulation. However, as more enzymes with the ability to ADP-ribosylate proteins were discovered, the multifunctional nature of ADP-ribosylation became apparent. The first mammalian enzyme with poly-ADP-ribose transferase activity was discovered during the late 1980s. For the next 15 years, it was thought to be the only enzyme capable of adding a chain of ADP-ribose in mammalian cells. During the late 1980s, ADP-ribosyl cyclases, which catalyze the addition of cyclic-ADP-ribose groups to proteins, were discovered. Finally, sirtuins, a family of enzymes that also possess NAD+-dependent deacylation activity, were discovered to also possess mono-ADP-ribosyl transferase activity. The source of ADP-ribose for most enzymes that perform this modification is the redox cofactor NAD+. In this transfer reaction, the N-glycosidic bond of NAD+ that bridges the ADP-ribose molecule and the nicotinamide group is cleaved, followed by nucleophilic attack by the target amino acid side chain. ADP-ribosyltransferases can perform two types of modifications: mono-ADP ribosylation and poly-ADP ribosylation. Mono-ADP ribosyltransferases commonly catalyze the addition of ADP-ribose to arginine side chains using a highly conserved R-S-EXE motif of the enzyme. The reaction proceeds by breaking the bond between nicotinamide and ribose to form an oxonium ion. Next, the arginine side chain of the target protein then acts a nucleophile, attacking the electrophilic carbon adjacent to the oxonium ion. In order for this step to occur, the arginine nucleophile is deprotonated by a glutamate residue on the catalyzing enzyme. Another conserved glutamate residue forms a hydrogen bond with one of the hydroxyl groups on the ribose chain to further facilitate this nucleophilic attack. As a result of the cleavage reaction, nicotinamide is released. The modification can be reversed by ADP-ribosylhydrolases, which cleave the N-glycosidic bond between arginine and ribose to release ADP-ribose and unmodified protein; NAD+ is not restored by the reverse reaction. Poly-(ADP-ribose) polymerases (PARPs) are found mostly in eukaryotes and catalyze the transfer of multiple ADP-ribose molecules to target proteins. As with mono-ADP ribosylation, the source of ADP-ribose is NAD+. PARPs use a catalytic triad of His-Tyr-Glu to facilitate binding of NAD+ and positioning of the end of the existing poly-ADP ribose chain on the target protein; the Glu facilitates catalysis and formation of a (1->2) O-glycosidic linkage between two ribose molecules. There are several other enzymes that recognize poly-ADP ribose chains, hydrolyse them or form branches; over 800 proteins have been annotated to contain the loosely defined poly ADP-ribose binding motif; therefore, in addition to this modification altering target protein conformation and structure, it may also be used as a tag to recruit other proteins or for regulation of the target protein. Many different amino acid side chains have been described as ADP-ribose acceptors. From a chemical perspective, this modification represents protein glycosylation: the transfer of ADP-ribose occurs onto amino acid side chains with a nucleophilic oxygen, nitrogen, or sulfur, resulting in N-, O-, or S-glycosidic linkage to the ribose of the ADP-ribose. Originally, acidic amino acids (glutamate and aspartate) were described as the main sites of ADP-ribosylation. However, many other ADP-ribose acceptor sites such as serine, arginine, cysteine, lysine, diphthamide, phosphoserine, and asparagine have been identified in subsequent works.