Ube2V2 Is a Rosetta Stone Bridging Redox and Ubiquitin Codes, Coordinating DNA Damage Responses

Posttranslational modifications (PTMs) are the lingua franca of cellular communication. Most PTMs are enzyme-orchestrated. However, the reemergence of electrophilic drugs has ushered mining of unconventional/non-enzyme-catalyzed electrophile-signaling pathways. Despite the latest impetus toward harnessing kinetically and functionally privileged cysteines for electrophilic drug design, identifying these sensors remains challenging. Herein, we designed “G-REX”a technique that allows controlled release of reactive electrophiles in vivo. Mitigating toxicity/off-target effects associated with uncontrolled bolus exposure, G-REX tagged <i>first-responding</i> innate cysteines that bind electrophiles under true <i>k</i>_cat/<i>K</i>_m conditions. G-REX identified two allosteric ubiquitin-conjugating proteinsUbe2V1/Ube2V2sharing a novel privileged-sensor-cysteine. This non-enzyme-catalyzed-PTM triggered responses <i>specific to each</i> protein. Thus, G-REX is an unbiased method to identify novel functional cysteines. Contrasting conventional active-site/off-active-site cysteine-modifications that regulate <i>target</i> activity, modification of Ube2V2 allosterically hyperactivated its enzymatically active binding-partner Ube2N, promoting K63-linked client ubiquitination and stimulating H2AX-dependent DNA damage response. This work establishes Ube2V2 as a Rosetta-stone bridging redox and ubiquitin codes to guard genome integrity

Temporally Controlled Targeting of 4‑Hydroxynonenal to Specific Proteins in Living Cells

In-depth chemical understanding of complex biological processes hinges upon the ability to systematically perturb individual systems. However, current approaches to study impacts of biologically relevant reactive small molecules involve bathing of the entire cell or isolated organelle with excess amounts, leading to off-target effects. The resultant lack of biochemical specificity has plagued our understanding of how biological electrophiles mediate signal transduction or regulate responses that confer defense mechanisms to cellular electrophilic stress. Here we introduce a target-specific electrophile delivery platform that will ultimately pave the way to interrogate effects of reactive electrophiles on specific target proteins in cells. The new methodology is demonstrated by photoinducible targeted delivery of 4-hydroxynonenal (HNE) to the proteins Keap1 and PTEN. Covalent conjugation of the HNE-precursor to HaloTag fused to the target proteins enables directed HNE delivery upon photoactivation. The strategy provides proof of concept of selective delivery of reactive electrophiles to individual electrophile-responsive proteins in mammalian cells. It opens a new avenue enabling more precise determination of the pathophysiological consequences of HNE-induced chemical modifications on specific target proteins in cells

Cardiovascular Small Heat Shock Protein HSPB7 Is a Kinetically Privileged Reactive Electrophilic Species (RES) Sensor

Small heat shock protein (sHSP)-B7 (HSPB7) is a muscle-specific member of the non-ATP-dependent sHSPs. The precise role of HSPB7 is enigmatic. Here, we disclose that zebrafish Hspb7 is a kinetically privileged sensor that is able to react rapidly with native reactive electrophilic species (RES), when only substoichiometric amounts of RES are available in proximity to Hspb7 expressed in living cells. Among the two Hspb7-cysteines, this RES sensing is fulfilled by a single cysteine (C117). Purification and characterizations <i>in vitro</i> reveal that the rate for RES adduction is among the most efficient reported for protein-cysteines with native carbonyl-based RES. Covalent-ligand binding is accompanied by structural changes (increase in β-sheet-content), based on circular dichroism analysis. Among the two cysteines, only C117 is conserved across vertebrates; we show that the human ortholog is also capable of RES sensing in cells. Furthermore, a cancer-relevant missense mutation reduces this RES-sensing property. This evolutionarily conserved cysteine-biosensor may play a redox-regulatory role in cardioprotection

Cladribine and Fludarabine Nucleotides Induce Distinct Hexamers Defining a Common Mode of Reversible RNR Inhibition

The enzyme ribonucleotide reductase (RNR) is a major target of anticancer drugs. Until recently, suicide inactivation in which synthetic substrate analogs (nucleoside diphosphates) irreversibly inactivate the RNR-α₂β₂ heterodimeric complex was the only clinically proven inhibition pathway. For instance, this mechanism is deployed by the multifactorial anticancer agent gemcitabine diphosphate. Recently reversible targeting of RNR-α-alone coupled with ligand-induced RNR-α-persistent hexamerization has emerged to be of clinical significance. To date, clofarabine nucleotides are the only known example of this mechanism. Herein, chemoenzymatic syntheses of the active forms of two other drugs, phosphorylated cladribine (ClA) and fludarabine (FlU), allow us to establish that reversible inhibition is common to numerous drugs in clinical use. Enzyme inhibition and fluorescence anisotropy assays show that the di- and triphosphates of the two nucleosides function as reversible (i.e., nonmechanism-based) inhibitors of RNR and interact with the catalytic (C site) and the allosteric activity (A site) sites of RNR-α, respectively. Gel filtration, protease digestion, and FRET assays demonstrate that inhibition is coupled with formation of conformationally diverse hexamers. Studies in 293T cells capable of selectively inducing either wild-type or oligomerization-defective mutant RNR-α overexpression delineate the central role of RNR-α oligomerization in drug activity, and highlight a potential resistance mechanism to these drugs. These data set the stage for new interventions targeting RNR oligomeric regulation

Substoichiometric Hydroxynonenylation of a Single Protein Recapitulates Whole-Cell-Stimulated Antioxidant Response

Lipid-derived electrophiles (LDEs) that can directly modify proteins have emerged as important small-molecule cues in cellular decision-making. However, because these diffusible LDEs can modify many targets [e.g., >700 cysteines are modified by the well-known LDE 4-hydroxynonenal (HNE)], establishing the functional consequences of LDE modification on individual targets remains devilishly difficult. Whether LDE modifications on a single protein are biologically sufficient to activate discrete redox signaling response downstream also remains untested. Herein, using T-REX (targetable reactive electrophiles and oxidants), an approach aimed at selectively flipping a single redox switch in cells at a precise time, we show that a modest level (∼34%) of HNEylation on a single target is sufficient to elicit the pharmaceutically important antioxidant response element (ARE) activation, and the resultant strength of ARE induction recapitulates that observed from whole-cell electrophilic perturbation. These data provide the first evidence that single-target LDE modifications are important individual events in mammalian physiology