ADP-ribosylation is a protein modification process that occurs widely in pathogenic mechanisms, intracellular signaling systems, DNA repair, and cell division. The reaction is catalyzed by ADP-ribosyltransferases which transfer the ADP-ribose moiety of NAD to a target protein with nicotinamide release. This stereospecific reaction is catalyzed by mono- and poly-ADP-ribosyltransferases (mARTs and pARTs). mARTs catalyze the transfer of a single ADP-ribose moiety onto a specific amino acid side chain of a target protein; pARTs (also designated poly-ADP-ribose-polymerases or PARPs), additionally can catalyze the elongation and branching of ADP-ribose units on ADP-ribosylated targets (see <PDOC51059>). The mART subfamily includes many well-known bacterial toxins as well as a number of mammalian and avian ecto-enzymes:

• Clostridium botulinum C3 exoenzyme inactivates the small GTP-binding protein family Rho by ADP-ribosylating asparagine 41, which depolymerizes the actin cytoskeleton [1].
• Clostridium botulinum C2 toxin is composed of the enzyme component C2-I, which ADP-ribosylates actin, and the binding and translocation component C2-II, responsible for the interaction with eukaryotic cell receptors and the following endocytosis [2].
• Clostridium perfringens type E Iota-toxin is an ADP-ribosylating toxin (ADPRT) that ADP-ribosylates actin, which is lethal and dermonecrotic in mammals [3].
• Salmonella typhimurium Mono(ADP-ribosyl)transferase SpvB, a virulence factor.
• Bacillus cereus vegetative insecticidal protein (VIP2), an insect-targeted toxin [4].
• Bacillus cereus Certhrax Toxin, an Anthrax-related ADP-ribosyltransferase. It has two domains, one that binds protective antigen and another that has ADP-ribosyltransferase activity [5].
• Vibrio splendidus Vis toxin [6].
• Pseudomonas syringae type III effector HopU1, a mono-ADP-ribosyltransferase that is injected into plant cells by the type III protein secretion system. Inside the plant cell it suppresses immunity by modifying RNA-binding proteins including the glycine-rich RNA-binding protein GRP7 [7].
• Xanthomonas axonopodis pv. citri (Xac) XopAI, a putative type III effector. Iit has been suggested to be a pathogenicity factor for citrus canker. XopAI uses an altered mART domain to bind its own N-terminal peptide containing a conserved Arg residue [8].
• Mammalian toxin-related ecto-ADP-ribosyltransferases family [Glowacki].

Most known mARTs transfer ADP-ribose onto arginine residues (see <PS01291>). Some enzymatically inactive mART domains, for example, the N-terminal domains of C2 and VIP2 toxins, have acquired a new, protein-binding function.

The mART domain adopts a mixed α/β-fold with a characteristic β-sandwich structure. Each domain core is formed mainly by perpendicular packing of a five-stranded mixed β-sheet against a three-stranded antiparallel β-sheet. The three-stranded sheet is flanked by four consecutive α-helices and the five-stranded sheet by an additional α-helix. A central cleft, which is formed between the four consecutive α-helices and the five-stranded β-sheet and lined by an α-helix and four β-strands, forms the NAD binding pocket (see <PDB:1QS1>). Catalytically active TR mART domains hallmark catalytic residues in the active site. Specifically, (i) a catalytic Arg preceded by an aromatic residue aids in NAD(+) binding and scaffolding of the active site, (ii) a Ser-Thr-Ser motif on a β-strand stabilizes the NAD(+) binding site, (iii) the ADP-ribosyl-turn-turn (ARTT) loop contains a catalytic Glu responsible for the ADP-ribosyltransferase activity and a Gln/Glu two residues upstream that may participate in substrate recognition, and (iv) a “phosphate-nicotinamide” (PN) loop contributes to NAD(+) binding through hydrogen bonds with an Arg and aromatic residues.

The profile we developed covers the entire TR mART core domain.