Translational control of ADP-ribosylation in eucaryotic cells

AMPylation and Endoplasmic Reticulum Protein Folding Homeostasis

The endoplasmic reticulum (ER)-localized Hsp70 chaperone, BiP, undergoes a rapid, reversible and inactivating post-translational modification. This covalent modification complements the slower, conventional unfolded protein response (UPR) in matching the supply of active Hsp70 chaperone to the protein folding demand within the ER lumen. Long believed to be ADP-ribosylation, we now know this modification to be AMPylation (adenylylation) of BiP's threonine 518. Here, we review the discovery of the responsible enzyme (the Fic domain-containing protein FICD), the structural and biochemical basis of the inactivating modification and the discovery of FICD's dual role as the enzyme that both AMPylates and deAMPylates BiP. The structural basis of BiP recognition by FICD and recent in vitro insights into oligomeric state-mediated regulation of FICD's antagonistic enzymatic activities are also reviewed, the latter in the context of how such a regulatory system may arise in cells. Last, we consider the physiological significance of BiP AMPylation and speculate on the fitness benefits of this metazoan-specific adaptation.

Post-translational modifications of Hsp70 family proteins: Expanding the chaperone code

Cells must be able to cope with the challenge of folding newly synthesized proteins and refolding those that have become misfolded in the context of a crowded cytosol. One such coping mechanism that has appeared during evolution is the expression of well-conserved molecular chaperones, such as those that are part of the heat shock protein 70 (Hsp70) family of proteins that bind and fold a large proportion of the proteome. Although Hsp70 family chaperones have been extensively examined for the last 50 years, most studies have focused on regulation of Hsp70 activities by altered transcription, co-chaperone "helper" proteins, and ATP binding and hydrolysis. The rise of modern proteomics has uncovered a vast array of post-translational modifications (PTMs) on Hsp70 family proteins that include phosphorylation, acetylation, ubiquitination, AMPylation, and ADP-ribosylation. Similarly to the pattern of histone modifications, the histone code, this complex pattern of chaperone PTMs is now known as the "chaperone code." In this review, we discuss the history of the Hsp70 chaperone code, its currently understood regulation and functions, and thoughts on what the future of research into the chaperone code may entail.

Early Events in the Endoplasmic Reticulum Unfolded Protein Response

The physiological consequences of the unfolded protein response (UPR) are mediated by changes in gene expression. Underlying them are rapid processes involving preexisting components. We review recent insights gained into the regulation of the endoplasmic reticulum (ER) Hsp70 chaperone BiP, whose incorporation into inactive oligomers and reversible AMPylation and de-AMPylation present a first line of response to fluctuating levels of unfolded proteins. BiP activity is tied to the regulation of the UPR transducers by a recently discovered cycle of ER-localized, J protein-mediated formation of a repressive IRE1-BiP complex, whose working we contrast to an alternative model for UPR regulation that relies on direct recognition of unfolded proteins. We conclude with a discussion of mechanisms that repress messenger RNA (mRNA) translation to limit the flux of newly synthesized proteins into the ER, a rapid adaptation that does not rely on new macromolecule biosynthesis.

AMPylation matches BiP activity to client protein load in the endoplasmic reticulum

The endoplasmic reticulum (ER)-localized Hsp70 chaperone BiP affects protein folding homeostasis and the response to ER stress. Reversible inactivating covalent modification of BiP is believed to contribute to the balance between chaperones and unfolded ER proteins, but the nature of this modification has so far been hinted at indirectly. We report that deletion of FICD, a gene encoding an ER-localized AMPylating enzyme, abolished detectable modification of endogenous BiP enhancing ER buffering of unfolded protein stress in mammalian cells, whilst deregulated FICD activity had the opposite effect. In vitro, FICD AMPylated BiP to completion on a single residue, Thr(518). AMPylation increased, in a strictly FICD-dependent manner, as the flux of proteins entering the ER was attenuated in vivo. In vitro, Thr(518) AMPylation enhanced peptide dissociation from BiP 6-fold and abolished stimulation of ATP hydrolysis by J-domain cofactor. These findings expose the molecular basis for covalent inactivation of BiP.

A first line of defense against ER stress

BiP is the predominant DnaK/Hsp70-type chaperone protein in the ER. It is required for folding and assembling newly synthesized ER client proteins, yet having too much BiP inhibits folding. In this issue, Chambers et al. (2012. J. Cell Biol. doi:10.1083/jcb.201202005) report that ADP ribosylation of BiP provides a reversible switch that fine tunes BiP activity according to need.

ADP ribosylation adapts an ER chaperone response to short-term fluctuations in unfolded protein load

Gene expression programs that regulate the abundance of the chaperone BiP adapt the endoplasmic reticulum (ER) to unfolded protein load. However, such programs are slow compared with physiological fluctuations in secreted protein synthesis. While searching for mechanisms that fill this temporal gap in coping with ER stress, we found elevated levels of adenosine diphosphate (ADP)-ribosylated BiP in the inactive pancreas of fasted mice and a rapid decline in this modification in the active fed state. ADP ribosylation mapped to Arg470 and Arg492 in the substrate-binding domain of hamster BiP. Mutations that mimic the negative charge of ADP-ribose destabilized substrate binding and interfered with interdomain allosteric coupling, marking ADP ribosylation as a rapid posttranslational mechanism for reversible inactivation of BiP. A kinetic model showed that buffering fluctuations in unfolded protein load with a recruitable pool of inactive chaperone is an efficient strategy to minimize both aggregation and costly degradation of unfolded proteins.

The dynamic role of GRP78/BiP in the coordination of mRNA translation with protein processing

The role of GRP78/BiP in coordinating endoplasmic reticular (ER) protein processing with mRNA translation was examined in GH3 pituitary cells. ADP-ribosylation of GRP78 and eukaryotic initiation factor (eIF)-2alpha phosphorylation were assessed, respectively, as indices of chaperone inactivation and the inhibition of translational initiation. Inhibition of protein processing by ER stress (ionomycin and dithiothreitol) resulted in GRP78 deribosylation and eIF-2 phosphorylation. Suppression of translation relative to ER protein processing (cycloheximide) produced approximately 50% ADP-ribosylation of GRP78 within 90 min without eIF-2 phosphorylation. ADP-ribosylation was reversed in 90 min by cycloheximide removal in a manner accelerated by ER stressors. Cycloheximide sharply reduced eIF-2 phosphorylation in response to ER stressors for about 30 min; sensitivity returned as GRP78 became increasingly ADP-ribosylated. Reduced sensitivity of eIF-2 to phosphorylation appeared to derive from the accumulation of free, unmodified chaperone as proteins completed processing without replacements. Prolonged (24 h) incubations with cycloheximide resulted in the selective loss of the ADP-ribosylated form of GRP78 and increased sensitivity of eIF-2 phosphorylation in response to ER stressors. Brefeldin A decreased ADP-ribosylation of GRP78 in parallel with increased eIF-2 phosphorylation. The cytoplasmic stressor, arsenite, which inhibits translational initiation through eIF-2 phosphorylation without affecting the ER, also produced ADP-ribosylation of GRP78.

ADP-ribosylation of the molecular chaperone GRP78/BiP

Starvation of mouse hepatoma cells for essential amino acids or glucose results in the ADP-ribosylation of the molecular chaperone BiP/GRP78. Addition of the missing nutrient to the medium reverses the reaction. The signal mediating the response to environmental nutrients involves the translational efficiency. An inhibitor of proteins synthesis, cycloheximide, or reduced temperature, both of which reduce translational efficiency, stimulate the ADP-ribosylation of BiP/GRP78. Inhibition of N-linked glycosylation of proteins results in the overproduction of BiP/GRP78. The over produced protein is not ADP-ribosylated suggesting that this is the functional form of BiP/GRP78. The over produced BiP/GRP78 can, however, be ADP-ribosylated if the cells are starved for an essential amino acid. BiP/GRP78 resides in the lumen of the endoplasmic reticulum where it participates in the assembly of secretory and integral membrane proteins. ADP-ribosylation of BiP/GRP78 during starvation is probably part of a nutritional stress response which conserves limited nutrients by slowing flow through the secretory pathway.

Vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation appears to be a reversible modification of proteins, which occurs in many eukaryotic and prokaryotic organisms. Multiple forms of arginine-specific ADP-ribosyltransferases have been purified and characterized from avian erythrocytes, chicken polymorphonuclear leukocytes and mammalian skeletal muscle. The avian transferases have similar molecular weights of approximately 28 kDa, but differ in physical, regulatory and kinetic properties and subcellular localization. Recently, a 38-kDa rabbit skeletal muscle ADP-ribosyltransferase was purified and cloned. The deduced amino acid sequence contained hydrophobic amino and carboxy termini, consistent with known signal sequences of glycosylphosphatidylinositol (GPI)-anchored proteins. This arginine-specific transferase was present on the surface of mouse myotubes and of NMU cells transfected with the cDNA and was released with phosphatidylinositol-specific phospholipase C. Arginine-specific ADP-ribosyltransferases thus appear to exhibit considerable diversity in their structure, cellular localization, regulation and physiological role.

A novel approach to detect toxin-catalyzed ADP-ribosylation in intact cells: its use to study the action of Pasteurella multocida toxin

Certain microbial toxins are ADP-ribosyltransferases, acting on specific substrate proteins. Although these toxins have been of great utility in studies of cellular regulatory processes, a simple procedure to directly study toxin-catalyzed ADP-ribosylation in intact cells has not been described. Our approach was to use [2-3H]adenine to metabolically label the cellular NAD+ pool. Labeled proteins were then denatured with SDS, resolved by PAGE, and detected by flurography. In this manner, we show that pertussis toxin, after a dose-dependent lag period, [3H]-labeled a 40-kD protein intact cells. Furthermore, incubation of the gel with trichloroacetic acid at 95 degrees C before fluorography caused the release of label from bands other than the pertussis toxin substrate, thus, allowing its selective visualization. The modification of the 40-kD protein was ascribed to ADP-ribosylation of a cysteine residue on the basis of inhibition of labeling by nicotinamide and the release of [3H]ADP-ribose from the labeled protein by mercuric acetate. Cholera toxin catalyzed the [3H]-labeling of a 46-kD protein in the [2-3H]adenine-labeled cells. Pretreatment of the cells with pertussis toxin before the labeling of NAD+ with [2-3H]adenine blocked [2-3H]ADP-ribosylation catalyzed by pertussis toxin, but not that by cholera toxin. Thus, labeling with [2-3H]adenine permits the study of toxin-catalyzed ADP-ribosylation in intact cells. Pasteurella multocida toxin has recently been described as a novel and potent mitogen for Swiss 3T3 cell and acts to stimulate the phospholipase C-mediated hydrolysis of polyphosphoinositides. The basis of the action of the toxin is not known. Using the methodology described here, P. multocida toxin was not found to act by ADP-ribosylation.

ADP-ribosylation in Clostridium difficile toxin-treated cells is not related to cytopathogenicity of toxin B

ADP-ribosylation of a protein in human fibroblasts treated with partially purified Clostridium difficile toxin B was previously reported. Here we show that the same protein was ADP-ribosylated also in human fibroblasts exposed to supernatant from a C. difficile strain producing neither toxin A nor toxin B. Furthermore, in Chinese hamster ovary and in Vero cells, showing toxin B-induced cytopathogenic effect, the protein was not significantly ADP-ribosylated. The results indicate that the ADP-ribosylation is unrelated to the cytopathogenic effect of toxin B. It appears to be caused by another unidentified factor from C. difficile, and the substrate may correspond to a protein modified endogenously in cells exposed to stressful situations. Cellular actin was not ADP-ribosylated by toxin B.

Reversible ADP-ribosylation of the 78 kDa glucose-regulated protein

Starvation of Mouse hepatoma cells for essential amino acids or glucose results in the mono-ADP-ribosylation of the 78 kDa glucose-regulated protein, GRP78. Here we show that the ADP-ribosylated and non-ADP-ribosylated forms of GRP78 are interconvertible during tryptophan starvation and refeeding. In addition, the ADP-ribosylation of GRP78 was shown to be reversible even during nutritional stress. The overexpressed pool of non-ADP-ribosylated GRP78 synthesized during tunicamycin treatment was available for ADP-ribosylation during subsequent amino acid starvation, especially in the absence of tunicamycin. The reversible ADP-ribosylation of GRP78 could be part of a metabolic control mechanism in operation during nutritional stress.

ADP-ribosylation of the 78-kDa glucose-regulated protein during nutritional stress

Starvation of a mouse hepatoma cell line, Hepa, for any essential amino acid results in the mono-ADP-ribosylation of an 80-kDa protein, P80. The ADP-ribose acceptor and its putative precursor were identified in two-dimensional gel patterns and isolated by electroelution. Amino-terminal sequence analysis showed they were the 78-kDa glucose-regulated protein, GRP78. Starvation of Hepa cells for tryptophan or glucose stimulated the relative rate of synthesis, and the ADP-ribosylation of GRP78. Inhibition of N-linked glycosylation by treatment with tunicamycin, 2-deoxy-D-glucose or glucosamine stimulated the synthesis of non-ADP-ribosylated GRP78 up to sixfold with relatively little effect on its ADP-ribosylation. Both forms were identified in mouse liver, lung, heart, kidney, spleen and brain.