Characterisation of a novel glycosylphosphatidylinositol-anchored mono-ADP-ribosyltransferase isoform in ovary cells

ARTC1-mediated VAPB ADP-ribosylation regulates calcium homeostasis

Mono-ADP-ribosylation (MARylation) is a post-translational modification that regulates a variety of biological processes, including DNA damage repair, cell proliferation, metabolism, and stress and immune responses. In mammals, MARylation is mainly catalyzed by ADP-ribosyltransferases (ARTs), which consist of two groups: ART cholera toxin-like (ARTCs) and ART diphtheria toxin-like (ARTDs, also known as PARPs). The human ARTC (hARTC) family is composed of four members: two active mono-ADP-ARTs (hARTC1 and hARTC5) and two enzymatically inactive enzymes (hARTC3 and hARTC4). In this study, we systematically examined the homology, expression, and localization pattern of the hARTC family, with a particular focus on hARTC1. Our results showed that hARTC3 interacted with hARTC1 and promoted the enzymatic activity of hARTC1 by stabilizing hARTC1. We also identified vesicle-associated membrane protein-associated protein B (VAPB) as a new target of hARTC1 and pinpointed Arg50 of VAPB as the ADP-ribosylation site. Furthermore, we demonstrated that knockdown of hARTC1 impaired intracellular calcium homeostasis, highlighting the functional importance of hARTC1-mediated VAPB Arg50 ADP-ribosylation in regulating calcium homeostasis. In summary, our study identified a new target of hARTC1 in the endoplasmic reticulum and suggested that ARTC1 plays a role in regulating calcium signaling.

Overview of the mammalian ADP-ribosyl-transferases clostridia toxin-like (ARTCs) family

Mono-ADP-ribosylation is a reversible post-translational protein modification that modulates the function of proteins involved in different cellular processes, including signal transduction, protein transport, transcription, cell cycle regulation, DNA repair and apoptosis. In mammals, mono-ADP-ribosylation is mainly catalyzed by members of two different classes of enzymes: ARTCs and ARTDs. The human ARTC family is composed of four structurally related ecto-mono-ARTs, expressed at the cell surface or secreted into the extracellular compartment that are either active mono-ARTs (hARTC1, hARTC5) or inactive proteins (hARTC3, hARTC4). The human ARTD enzyme family consists of 17 multidomain proteins that can be divided on the basis of their catalytic activity into polymerases (ARTD1-6), mono-ART (ARTD7-17), and the inactive ARTD13. In recent years, ADP-ribosylation was intensively studied, and research was dominated by studies focusing on the role of this modification and its implication on various cellular processes. The aim of this review is to provide a general overview of the ARTC enzymes. In the following sections, we will report the mono-ADP-ribosylation reactions that are catalysed by the active ARTC enzymes, with a particular focus on hARTC1 that recently has been intensively studied with the discovery of new targets and functions.

ARTC1-mediated ADP-ribosylation of GRP78/BiP: a new player in endoplasmic-reticulum stress responses

Protein mono-ADP-ribosylation is a reversible post-translational modification of cellular proteins. This scheme of amino-acid modification is used not only by bacterial toxins to attack host cells, but also by endogenous ADP-ribosyltransferases (ARTs) in mammalian cells. These latter ARTs include members of three different families of proteins: the well characterised arginine-specific ecto-enzymes (ARTCs), two sirtuins, and some members of the poly(ADP-ribose) polymerase (PARP/ARTD) family. In the present study, we demonstrate that human ARTC1 is localised to the endoplasmic reticulum (ER), in contrast to the previously characterised ARTC proteins, which are typical GPI-anchored ecto-enzymes. Moreover, using the "macro domain" cognitive binding module to identify ADP-ribosylated proteins, we show here that the ER luminal chaperone GRP78/BiP (glucose-regulated protein of 78 kDa/immunoglobulin heavy-chain-binding protein) is a cellular target of human ARTC1 and hamster ARTC2. We further developed a procedure to visualise ADP-ribosylated proteins using immunofluorescence. With this approach, in cells overexpressing ARTC1, we detected staining of the ER that co-localises with GRP78/BiP, thus confirming that this modification occurs in living cells. In line with the key role of GRP78/BiP in the ER stress response system, we provide evidence here that ARTC1 is activated during the ER stress response, which results in acute ADP-ribosylation of GRP78/BiP paralleling translational inhibition. Thus, this identification of ARTC1 as a regulator of GRP78/BiP defines a novel, previously unsuspected, player in GRP78-mediated ER stress responses.

PARP16/ARTD15 is a novel endoplasmic-reticulum-associated mono-ADP-ribosyltransferase that interacts with, and modifies karyopherin-ß1

Background

Protein mono-ADP-ribosylation is a reversible post-translational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. The latter include members of three different families of proteins: the well characterized arginine-specific ecto-enzymes ARTCs, two sirtuins and, more recently, novel members of the poly(ADP-ribose) polymerase (PARP/ARTD) family that have been suggested to act as cellular mono-ADP-ribosyltransferases. Here, we report on the characterisation of human ARTD15, the only known ARTD family member with a putative C-terminal transmembrane domain.

Methodology/Principal Findings

Immunofluorescence and electron microscopy were performed to characterise the sub-cellular localisation of ARTD15, which was found to be associated with membranes of the nuclear envelope and endoplasmic reticulum. The orientation of ARTD15 was determined using protease protection assay, and is shown to be a tail-anchored protein with a cytosolic catalytic domain. Importantly, by combining immunoprecipitation with mass spectrometry and using cell lysates from cells over-expressing FLAG-ARTD15, we have identified karyopherin-ß1, a component of the nuclear trafficking machinery, as a molecular partner of ARTD15. Finally, we demonstrate that ARTD15 is a mono-ADP-ribosyltransferase able to induce the ADP-ribosylation of karyopherin-ß1, thus defining the first substrate for this enzyme.

Conclusions/Significance

Our data reveal that ARTD15 is a novel ADP-ribosyltransferase enzyme with a new intracellular location. Finally, the identification of karyopherin-ß1 as a target of ARTD15-mediated ADP-ribosylation, hints at a novel regulatory mechanism of karyopherin-ß1 functions.