Sessile plants detect and ward off invading microorganisms with a robust and sophisticated innate immune system in addition to structural physical and chemical barriers . modulate host physiology . Plants have in turn evolved intracellular NOD-like receptors (NLRs) that recognize effectors or effector-mediated changes and mount effector-triggered immunity (ETI) . Recent studies show that protein ADP-ribosylation an important yet less studied posttranslational modification Selumetinib with an emerging role in diverse cellular processes is exploited both by plants to launch effective defense and by bacteria to achieve stealthy attacks to the hosts. Here we summarize the classification and biochemical processes of protein ADP-ribosylation compare the similarities and differences of ADP-ribosylation in plants and animals and discuss the roles of ADP-ribosylation in plant immunity and bacterial pathogenicity. Protein ADP-Ribosylation: Biochemical Classification and Processes ADP-ribosylation is the covalent attachment of ADP-ribose monomer (MAR) or polymer (PAR) derived from nicotinamide adenine dinucleotide (NAD+) to a target protein which is termed mono(ADP-ribosyl)ation (MARylation) or poly(ADP-ribosyl)ation (PARylation) Selumetinib respectively (Fig 1A) . MARylation and PARylation differ not only in the length of the ADP-ribose chain but also in the enzymes catalyzing the reactions and subcellular localization of reactions . MARylation is usually catalyzed by mono(ADP-ribosyl)transferases (ARTs) which were originally discovered as bacterial toxins such as diphtheria toxin and exotoxin Selumetinib A and were classified into the H-Y-E variant H-Y-E and R-S-E groups (H: histidine; Y: tyrosine; E: glutamate; R: arginine; S: serine) based on the conserved motifs in the catalytic domains . In ART the active-site H-Y-E motif is part of the binding pocket for NAD+. The invariant Glu (E) is a key catalytic residue that coordinates the transfer of ADP-ribose to the acceptor site and His (H) facilitates the binding of NAD+. The Glu (E) in R-S-E type ART is also a key catalytic site that is aided by the Arg (R) and Ser (S) residues to position and stabilize NAD+-binding pocket [6 7 ARTs in eukaryotes are classified as secreted or plasma membrane-anchored ectoenzymes (cholera toxin-like ADP-ribosyltransferases [ARTC]) and cytoplasm-localized intracellular enzymes (diphtheria toxin-like ADP-ribosyltransferases [ARTD]) . PARylation is usually catalyzed by poly(ADP-ribosyl) polymerases (PARPs) which are much more prevalent in eukaryotes than in prokaryotes. PARPs catalyze both the initial MARylation and subsequent elongation of the ADP-ribose chain (PARylation) predominantly on glutamate (E) aspartate (D) arginine (R) or lysine (K) residues of an acceptor protein. Interestingly among 17 human PARPs most of them are shown or predicted to be able to catalyze the attachment of MAR to acceptor proteins Rabbit Polyclonal to IRF3. which are functional ARTs and were reclassified as ARTDs recently . PARylation is a reversible process and the covalently attached PAR could be removed by poly(ADP-ribose) glycohydrolases (PARGs) which contain both endo- and exo-glycohydrolase activities (Fig 1A) or by the relatively less-studied ADP-ribosyl hydrolase (ARH). The terminal ADP-ribose or MAR of acceptor proteins can be hydrolyzed by certain macrodomain proteins such as MacroD1 MacroD2 and the terminal ADP-ribose protein glycohydrolase (TARG1) in humans . ADP-ribose released from the hydrolysis of MAR or PAR could be further cleaved to adenosine monophosphate (AMP) and ribose-5-phosphate by nucleoside diphosphate-linked to some moiety-X (Nudix) hydrolases . Protein PARylation regulates a wide range of cellular responses including DNA damage detection and repair chromatin remodeling gene transcription and protein localization and degradation . Compared to PARylation MARylation is less understood in eukaryotes with emerging roles in the regulation of NF-κB signaling gene transcription and unfolded protein response . Fig 1 Protein ADP-ribosylation in plant-bacterium interactions. ADP-Ribosylation: Similarities and Differences in Plants and Animals As in their mammalian counterparts plant PARPs and PARGs are implicated in DNA repair cell cycle genotoxic stress circadian rhythms and gene regulation [11-13]. In contrast to the 17 PARPs in humans the reference plant genome encodes three PARPs (AtPARP1 AtPARP2 and AtPARP3) with the conserved ARTD motif (Fig 1B) [11 12 AtPARP1 bears the highest homology to HsPARP-1 which is the most active.