ADP-ribosyl carboxylates

The acid-insoluble fraction of rat liver was analyzed for linkages characteristic of ADP-ribosyl-cysteine, ADP-ribosyl-arginine and ADP-ribosyl-carboxylate. Standard deviations are shown and the number of replicate analyses is shown in parenthesis. 

Mishima, K, et al., ARD 1, a 64-kDa guanine nudeotide-binding protein with a carboxyl-terminal ADP-ribosylation factor domain. J Bid Chem, 1993, 268(12), 8801-7. 

The most common posttranslational modifications, discussed in the following sections, include phosphorylation, sulfation, disulfide formation, N-methylation, O-methylation, S-methylation, N-acetylation, hydroxylation, glycosylation, ADP-ribosylation, prenylation, biotinylation, lipoylation, and phosphopan-tetheine tethering. Many of the posttranslational modifications are proven to be cross talks. Other modifications exist in a smaller extent and include oxidation of methionine, C-methylation, ubiquitylation, carboxylation, and amidation. These topics will not be covered in this chapter which is meant to focus primarily on the recent literature (2005-08). For a more complete coverage of all posttranslational modifications and earlier literature (up to 2005), the reader is referred to Professor Christopher T. Walsh s book Posttranslational Modification of Proteins Expanding Nature s Inventory ... 

This enzyme [EC 2.4.2.30] (also referred to as NAD+ ADP-ribosyltransferase, poly(ADP) polymerase, poly-(adenosine diphosphate ribose) polymerase, and ADP-ribosyltransferase (polymerizing)) catalyzes the reaction of NAD+ with [ADP-D-ribosyl] to produce nicotinamide and [ADP-D-ribosyl]( + i). The ADP-d-ribosyl group of NAD+ is transferred to an acceptor carboxyl group on a histone or on the enzyme itself, and further ADP-ribosyl groups are transferred to the 2 -position of the terminal adenosine moiety, building up a polymer with an average chain length of twenty to thirty units. 

Initial ADP-ribosylation of the y-carboxyl group of glutamate residues (or the carboxyl group of a C-terminal lysine) in the target protein, forming an O-glycoside. 

Fig. 22. Structure of the ADP-ribosyl linkage to the y-carboxylate of glutamate that attaches the poly(ADP-ribose) chain to a protein.

SDS-polyacrylamide gel electrophoresis. The substrate specificity was rather loose with the protein portion ADP-ribosyl histones HI and H2B, peptide fragments of H2B, and nonhistone proteins (a mixture) served as substrates. In contrast, the specificity was very tight with the mono(ADP-ribosyl) portion and the carboxyl ester bond poly- or oligo(ADP-ribosyl) histones were hardly split, and ADP-ribose histone adducts formed chemically through Schiff base reduction [3] or ADP-ribosyl arginine bond formed by avian erythrocyte ADP-ribosyltransferase [14] did not serve as substrate. 

The alkylation-induced changes had demonstrated that important acceptors like histone HI may become nearly exclusively mono(ADP-ribosyl)ated. Mono(ADP-ribosyl)ation of proteins also appears to be of importance in certain diffemtiation processes. When F9 teratocarcinoma cells were incubated with low concentrations of retinoic acid, they differentiated into endoderm cells [22]. Under these conditions, mono(ADPR) conjugates increased considerably. The increase was time dependent, and it was mainly due to a fraction that was neither carboxyl- nor arginine-linked, nor did it correspond to nonenzymic conjugates. However, further experiments are required to determine the nature of these conjugates. 

Conditions for the selective release of ADP-ribose from carboxylate ester linkages to glutamate or aspartate and for release from glycosylic linkages to the guanidinium group of arginine were reported by this laboratory earlier (15). Under conditions whereby ADP-ribosyl-caiboxylate... 

The stracture of the photoproduct is consistent with the notion that Glu-148 could be involved in catalysis. If the gamma methylene group of Glu-148 were in close proximity to C-6 of the nicotinamide ring of NAD in the enzyme-substrate complex, then the carboxyl group of Glu-148 would be within a short radius of C-6, and, depending on the geometry of the complex, might be in direct contact with member atoms of the bond disrupted in the ADP-ribosylation reaction. Also, the fact that the photoproduct was inactive in ADP-ribosylation was consistent with a role of Glu-148 in catalysis. 

Poly(ADP-ribose) polymerase is primarily a nuclear enzyme. The acceptor for the initial ADP-ribose moiety is a glutamate or the carboxyl group of a terminal lysine in the acceptor enzyme, forming an 0-glycoside. This is followed by successive ADP— ribosyl transfer to form poly(ADP-ribose), which may be a linear or branched polymer. 

The a subunit, the gene product (329 amino acids, 36,512) of rpoA, is required for the assembly of the core enzyme and plays a role in promoter recognition. When phage T4 infects E. coli, an Arg residue of the a subunit is modified by ADP-ribosylation which results in a reduction of the affinity of the promoter to the holoenzyme. The carboxyl-terminal domain (CTD, 99 amino acids) is regarded as the contact site for transcription activators, e.g.. 

Fig. 2. The proposed transition-state intermediate of the ADP-ribosyltransferase reaction catalyzed by PT. The carboxylate group of Glu-129 is proposed to interact with the hydrogen of the ribose 2 -hydroxyl group, thereby promoting the ionization of the diol which weakens the N-glycosidic bond by intramolecular stabilization of an oxocarbonium-like intermediate. The His-35-activated cysteine of the Gia protein may simultaneously exert its nucleophilic attack on the weakened N-glycosidic bond to cleave completely the pyridine-ribosyl bond...

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