NAD<sup>+</sup>-Dependent Enzymes at the Endoplasmic Reticulum

The post-translational modifications of proteins by mono- and poly-ADP-ribosylation involve the cleavage of βNAD⁺, with the release of its nicotinamide moiety, accompanied by the transfer of a single (mono) or several (poly) ADP-ribose molecules from βNAD⁺ to a specific amino-acid residue of various cellular proteins. Thus, both mono- and poly-ADP-ribosylation are NAD⁺-consuming reactions. ADP-ribosylation reactions have been reported to have important roles in the nucleus, and in mitochondrial activity. Distinct subcellular NAD⁺ pools have been identified, not only in the nucleus and the mitochondria, but also in the endoplasmic reticulum and peroxisomes. Recent reports have shed new light on the correlation between NAD⁺-dependent ADP-ribosylation reactions and the endoplasmic reticulum. We have demonstrated that ARTD15/PARP16 is a novel mono-ADP-ribosyltransferase with a new intracellular location, as it is associated with the endoplasmic reticulum. The endoplasmic reticulum is a membranous network of tubules, vesicles, and cisternae that are interconnected in the cytoplasm of eukaryotic cells. This intracellular compartment is responsible for many cellular functions, including facilitation of protein folding and assembly, biosynthesis of lipids, storage of intracellular Ca²⁺, and transport of proteins. ARTD15 might have a role in both the nucleo-cytoplasmic shuttling, through importinβ1 mono-ADP-ribosylation, and in the unfolded protein response through its ability to ADP-ribosylate two components of this pathway: PERK and IRE1. This review summarizes our present knowledge of the enzymes and targets involved in ADP-ribosylation reactions, with special regard to the novel regulatory reactions that occurs at the level of the endoplasmic reticulum, and that can affect the function of this organelle.