Arginyltransferase regulates alpha cardiac actin function, myofibril formation and contractility during heart development

Arginylation of ⍺-tubulin at E77 regulates microtubule dynamics via MAP1S

Arginylation is the posttranslational addition of arginine to a protein by arginyltransferase-1 (ATE1). Previous studies have found that ATE1 targets multiple cytoskeletal proteins, and Ate1 deletion causes cytoskeletal defects, including reduced cell motility and adhesion. Some of these defects have been linked to actin arginylation, but the role of other arginylated cytoskeletal proteins has not been studied. Here, we characterize tubulin arginylation and its role in the microtubule cytoskeleton. We identify ATE1-dependent arginylation of ⍺-tubulin at E77. Ate1-/- cells and cells overexpressing non-arginylatable ⍺-tubulinE77A both show a reduced microtubule growth rate and increased microtubule stability. Additionally, they show an increase in the fraction of the stabilizing protein MAP1S associated with microtubules, suggesting that E77 arginylation directly regulates MAP1S binding. Knockdown of Map1s is sufficient to rescue microtubule growth rate and stability to wild-type levels. Together, these results demonstrate a new type of tubulin regulation by posttranslational arginylation, which modulates microtubule growth rate and stability through the microtubule-associated protein, MAP1S.

Targeting human arginyltransferase and post-translational protein arginylation: a pharmacophore-based multilayer screening and molecular dynamics approach to discover novel inhibitors with therapeutic promise

Protein arginylation mediated by arginyltransferase 1 is a crucial regulator of cellular processes in eukaryotes by affecting protein stability, function, and interaction with other macromolecules. This enzyme and its targets are of immense interest for modulating cellular processes in diseased states like obesity and cancer. Despite being an important target molecule, no highly potent drug against this enzyme exists. Therefore, this study focuses on discovering potential inhibitors of human arginyltransferase 1 by computational approaches where screening of over 300,000 compounds from natural and synthetic databases was done using a pharmacophore model based on common features among known inhibitors. The drug-like properties and potential toxicity of the compounds were also assessed in the study to ensure safety and effectiveness. Advanced methods, including molecular simulations and binding free energy calculations, were performed to evaluate the stability and binding efficacy of the most promising candidates. Ultimately, three compounds were identified as potent inhibitors, offering new avenues for developing therapies targeting arginyltransferase 1.

Structural analysis of human ATE1 isoforms and their interactions with Arg-tRNAArg

Posttranslational protein arginylation has been shown as a key regulator of cellular processes in eukaryotes by affecting protein stability, function, and interaction with macromolecules. Thus, the enzyme Arginyltransferase and its targets, are of immense interest to modulate cellular processes in the normal and diseased state. While the study on the effect of this posttranslational modification in mammalian systems gained momentum in the recent times, the detail structures of human ATE1 (hATE1) enzymes has not been investigated so far. Thus, the purpose of this study was to predict the overall structure and the structure function relationship of hATE1 enzyme and its four isoforms. The structure of four ATE1 isoforms were modelled and were docked with 3'end of the Arg-tRNAArg which acts as arginine donor in the arginylation reaction, followed by MD simulation. All the isoforms showed two distinct domains. A compact domain and a somewhat flexible domain as observed in the RMSF plot. A distinct similarity in the overall structure and interacting residues were observed between hATE1-1 and X4 compared to hATE1-2 and 5. While the putative active sites of all the hATE1 isoforms were located at the same pocket, differences were observed in the active site residues across hATE1 isoforms suggesting different substrate specificity. Mining of nsSNPs showed several nsSNPs including cancer associated SNPs with deleterious consequences on hATE1 structure and function. Thus, the current study for the first time shows the structural differences in the mammalian ATE1 isoforms and their possible implications in the function of these proteins.Communicated by Ramaswamy H. Sarma.

Arginyltransferase 1 (ATE1)-mediated proteasomal degradation of viral haemagglutinin protein: a unique host defence mechanism

The extensive protein production in virus-infected cells can disrupt protein homeostasis and activate various proteolytic pathways. These pathways utilize post-translational modifications (PTMs) to drive the ubiquitin-mediated proteasomal degradation of surplus proteins. Protein arginylation is the least explored PTM facilitated by arginyltransferase 1 (ATE1) enzyme. Several studies have provided evidence supporting its importance in multiple physiological processes, including ageing, stress, nerve regeneration, actin formation and embryo development. However, its function in viral pathogenesis is still unexplored. The present work utilizes Newcastle disease virus (NDV) as a model to establish the role of the ATE1 enzyme and its activity in pathogenesis. Our data indicate a rise in levels of N-arginylated cellular proteins in the infected cells. Here, we also explore the haemagglutinin-neuraminidase (HN) protein of NDV as a presumable target for arginylation. The data indicate that the administration of Arg amplifies the arginylation process, resulting in reduced stability of the HN protein. ATE1 enzyme activity inhibition and gene expression knockdown studies were also conducted to analyse modulation in HN protein levels, which further substantiated the findings. Moreover, we also observed Arg addition and probable ubiquitin modification to the HN protein, indicating engagement of the proteasomal degradation machinery. Lastly, we concluded that the enhanced levels of the ATE1 enzyme could transfer the Arg residue to the N-terminus of the HN protein, ultimately driving its proteasomal degradation.

Oligomerization and a distinct tRNA-binding loop are important regulators of human arginyl-transferase function

The arginyl-transferase ATE1 is a tRNA-dependent enzyme that covalently attaches an arginine molecule to a protein substrate. Conserved from yeast to humans, ATE1 deficiency in mice correlates with defects in cardiovascular development and angiogenesis and results in embryonic lethality, while conditional knockouts exhibit reproductive, developmental, and neurological deficiencies. Despite the recent revelation of the tRNA binding mechanism and the catalytic cycle of yeast ATE1, the structure-function relationship of ATE1 in higher organisms is not well understood. In this study, we present the three-dimensional structure of human ATE1 in an apo-state and in complex with its tRNA cofactor and a peptide substrate. In contrast to its yeast counterpart, human ATE1 forms a symmetric homodimer, which dissociates upon binding of a substrate. Furthermore, human ATE1 includes a unique and extended loop that wraps around tRNAArg, creating extensive contacts with the T-arm of the tRNA cofactor. Substituting key residues identified in the substrate binding site of ATE1 abolishes enzymatic activity and results in the accumulation of ATE1 substrates in cells.

Global Analysis of Post-Translational Side-Chain Arginylation Using Pan-Arginylation Antibodies

Arginylation is a post-translational modification mediated by the arginyltransferase 1 (ATE1), which transfers the amino acid arginine to a protein or peptide substrate from a tRNA molecule. Initially, arginylation was thought to occur only on N-terminally exposed acidic residues, and its function was thought to be limited to targeting proteins for degradation. However, more recent data have shown that ATE1 can arginylate side chains of internal acidic residues in a protein without necessarily affecting metabolic stability. This greatly expands the potential targets and functions of arginylation, but tools for studying this process have remained limited. Here, we report the first global screen specifically for side-chain arginylation. We generate and validate "pan-arginylation" antibodies, which are designed to detect side-chain arginylation in any amino acid sequence context. We use these antibodies for immunoaffinity enrichment of side-chain arginylated proteins from wildtype and Ate1 knockout cell lysates. In this way, we identify a limited set of proteins that likely undergo ATE1-dependent side-chain arginylation and that are enriched in specific cellular roles, including translation, splicing, and the cytoskeleton.

Identification of Arginylated Proteins by Mass Spectrometry

Here, we describe the method for the identification of arginylated proteins by mass spectrometry. This method has been originally applied to the identification of N-terminally added Arg on proteins and peptides and then expanded to the side chain modification which has been recently described by our groups. The key steps in this method include the use of the mass spectrometry instruments that can identify peptides with very high pass accuracy (Orbitrap) and apply stringent mass cutoffs during automated data analysis, followed by manual validation of the identified spectra. These methods can be used with both complex and purified protein samples and, to date, constitute the only reliable way to confirm arginylation at a particular site on a protein or peptide.

Assaying the Posttranslational Arginylation of Proteins in Cultured Cells

To evaluate the posttranslational arginylation of proteins in vivo, we describe a protocol for studying the ¹⁴C-Arg incorporation into proteins of cells in culture. The conditions determined for this particular modification contemplate both the biochemical requirements of the enzyme ATE1 and the adjustments that allowed the discrimination between posttranslational arginylation of proteins and de novo synthesis. These conditions are applicable for different cell lines or primary cultures, representing an optimal procedure for the identification and the validation of putative ATE1 substrates.

Protein Arginylation: Milestones of Discovery

Posttranslational modifications have emerged in recent years as the major biological regulators responsible for the orders of magnitude increase in complexity during gene expression and regulation. These "molecular switches" affect nearly every protein in vivo by modulating their structure, activity, molecular interactions, and homeostasis ultimately regulating their functions. While over 350 posttranslational modifications have been described, only a handful of them have been characterized. Until recently, protein arginylation has belonged to the list of obscure, poorly understood posttranslational modifications, before the recent explosion of studies has put arginylation on the map of intracellular metabolic pathways and biological functions. This chapter contains an overview of all the major milestones in the protein arginylation field, from its original discovery in 1963 to this day.

Liquiritin Attenuates Angiotensin II-Induced Cardiomyocyte Hypertrophy via ATE1/TAK1-JNK1/2 Pathway

Objective

To investigate the protective effect and mechanism of liquiritin (LIQ) on cardiomyocyte hypertrophy induced by angiotensin II (Ang II).

Methods

H9c2 cells were pretreated with LIQ before and after Ang II treatment. CCK8 assay was performed to evaluate cell viability. The cell surface area was measured by phalloidin staining. The mRNA expression of atrial and B-type natriuretic peptides (ANP and BNP, respectively) and β-myosin heavy chain (β-MHC) was determined by quantitative reverse transcription-polymerase chain reaction (RT-qPCR); the protein levels of arginyltransferase 1 (ATE1), transforming growth factor beta-activated kinase 1 (TAK1), phos-TAK1, c-Jun N-terminal kinases1/2 (JNK1/2), and phos-JNK1/2 were determined by Western blotting. After constructing the ATE1 overexpression cell models with the pcDNA3.1/ATE1, the abovementioned indicators were tested using the introduced methods.

Results

LIQ at a concentration of ≤30 μM was not cytotoxic to H9c2 cells before exposure to Ang II. The protective effect of LIQ was best observed at 30 μM after Ang II treatment. Phalloidin staining and RT-qPCR results indicated that the deposition of Ang II increased the cell surface area and levels of ANP, BNP, and β-MHC. On the other hand, Western blotting results showed that Ang II increased the ATE1 protein levels and TAK1 and JNK1/2 phosphorylation, which were significantly alleviated after LIQ treatment. LIQ also directly inhibited the ATE1 overexpression in H9c2 cells transfected with pcDNA3.1/ATE1 and further inhibited TAK1 and JNK1/2 phosphorylation.

Conclusion

LIQ can attenuate Ang II-induced cardiomyocyte hypertrophy by regulating the ATE1/TAK1-JNK1/2 pathway.

The Final Maturation State of β-actin Involves N-terminal Acetylation by NAA80, not N-terminal Arginylation by ATE1

Actin is a hallmark protein of the cytoskeleton in eukaryotic cells, affecting a range of cellular functions. Actin dynamics is regulated through a myriad of actin-binding proteins and post-translational modifications. The mammalian actin family consists of six different isoforms, which vary slightly in their N-terminal (Nt) sequences. During and after synthesis, actins undergo an intricate Nt-processing that yields mature actin isoforms. The ubiquitously expressed cytoplasmic β-actin is Nt-acetylated by N-alpha acetyltransferase 80 (NAA80) yielding the Nt-sequence Ac-DDDI-. In addition, β-actin was also reported to be Nt-arginylated by arginyltransferase 1 (ATE1) after further peptidase-mediated processing, yielding RDDI-. To characterize in detail the Nt-processing of actin, we used state-of-the-art proteomics. To estimate the relative cellular levels of Nt-modified proteoforms of actin, we employed NAA80-lacking cells, in which actin was not Nt-acetylated. We found that targeted proteomics is superior to a commercially available antibody previously used to analyze Nt-arginylation of β-actin. Significantly, despite the use of sensitive mass spectrometry-based techniques, we could not confirm the existence of the previously claimed Nt-arginylated β-actin (RDDI-) in either wildtype or NAA80-lacking cells. A very minor level of Nt-arginylation of the initially cleaved β-actin (DDDI-) could be identified, but only in NAA80-lacking cells, not in wildtype cells. We also identified small fractions of cleaved and unmodified β-actin (DDI-) as well as cleaved and Nt-acetylated β-actin (Ac-DDI-). In sum, we show that the multi-step Nt-maturation of β-actin is terminated by NAA80, which Nt-acetylates the exposed Nt-Asp residues, in the virtual absence of previously claimed Nt-arginylation.

Arginylation Regulates G-protein Signaling in the Retina

Arginylation is a post-translational modification mediated by the arginyltransferase (Ate1). We recently showed that conditional deletion of Ate1 in the nervous system leads to increased light-evoked response sensitivities of ON-bipolar cells in the retina, indicating that arginylation regulates the G-protein signaling complexes of those neurons and/or photoreceptors. However, none of the key players in the signaling pathway were previously shown to be arginylated. Here we show that Gαt1, Gβ1, RGS6, and RGS7 are arginylated in the retina and RGS6 and RGS7 protein levels are elevated in Ate1 knockout, suggesting that arginylation plays a direct role in regulating their protein level and the G-protein-mediated responses in the retina.

Availability of Arg, but Not tRNA, Is a Rate-Limiting Factor for Intracellular Arginylation

Protein arginylation, mediated by arginyltransferase ATE1, is a posttranslational modification of emerging biological importance that consists of transfer of the amino acid Arg from tRNA to protein and peptide targets. ATE1 can bind tRNA and exhibits specificity toward particular tRNA types, but its dependence on the availability of the major components of the arginylation reaction has never been explored. Here we investigated key intracellular factors that can potentially regulate arginylation in vivo, including several tRNA types that show strong binding to ATE1, as well as availability of free Arg, in an attempt to identify intracellular rate limiting steps for this enzyme. Our results demonstrate that, while modulation of tRNA levels in cells does not lead to any changes in intracellular arginylation efficiency, availability of free Arg is a potentially rate-limiting factor that facilitates arginylation if added to the cultured cells. Our results broadly outline global pathways that may be involved in the regulation of arginylation in vivo.

Cardiac myosin contraction and mechanotransduction in health and disease

Cardiac myosin is the molecular motor that powers heart contraction by converting chemical energy from ATP hydrolysis into mechanical force. The power output of the heart is tightly regulated to meet the physiological needs of the body. Recent multiscale studies spanning from molecules to tissues have revealed complex regulatory mechanisms that fine-tune cardiac contraction, in which myosin not only generates power output but also plays an active role in its regulation. Thus, myosin is both shaped by and actively involved in shaping its mechanical environment. Moreover, these studies have shown that cardiac myosin-generated tension affects physiological processes beyond muscle contraction. Here, we review these novel regulatory mechanisms, as well as the roles that myosin-based force generation and mechanotransduction play in development and disease. We describe how key intra- and intermolecular interactions contribute to the regulation of myosin-based contractility and the role of mechanical forces in tuning myosin function. We also discuss the emergence of cardiac myosin as a drug target for diseases including heart failure, leading to the discovery of therapeutics that directly tune myosin contractility. Finally, we highlight some of the outstanding questions that must be addressed to better understand myosin's functions and regulation, and we discuss prospects for translating these discoveries into precision medicine therapeutics targeting contractility and mechanotransduction.

ATE1 Inhibits Liver Cancer Progression through RGS5-Mediated Suppression of Wnt/β-Catenin Signaling

Arginyltransferase (ATE1) plays critical roles in many biological functions including cardiovascular development, angiogenesis, adipogenesis, muscle contraction, and metastasis of cancer. However, the role of ATE1 in hepatocellular carcinoma (HCC) remains unknown. In this study, we find that ATE1 plays an essential role in growth and malignancy of liver cancer. ATE1 expression is significantly reduced in human HCC samples compared with normal liver tissue. In addition, low ATE1 expression is correlated with aggressive clinicopathologic features and is an independent poor prognostic factor for overall survival and disease-free survival of patients with HCC. Lentivirus-mediated ATE1 knockdown significantly promoted liver cancer growth, migration, and disease progression in vitro and in vivo. Opposing results were observed when ATE1 was upregulated. Mechanistically, ATE1 accelerated the degradation of β-catenin and inhibited Wnt signaling by regulating turnover of Regulator of G Protein Signaling 5 (RGS5). Loss- and gain-of-function assays confirmed that RGS5 was a key effector of ATE1-mediated regulation of Wnt signaling. Further studies indicated that RGS5 might be involved in regulating the activity of GSK3-β, a crucial component of the cytoplasmic destruction complex. Treatment with a GSK inhibitor (CHIR99021) cooperated with ablation of ATE1 or RGS5 overexpression to promote Wnt/β-catenin signaling, but overexpression of ATE1 or RGS5 knockdown did not reverse the effect of GSK inhibitor. IMPLICATIONS: ATE1 inhibits liver cancer progression by suppressing Wnt/β-catenin signaling and can serve as a potentially valuable prognostic biomarker for HCC.

Arginyltransferase (Ate1) regulates the RGS7 protein level and the sensitivity of light-evoked ON-bipolar responses

Regulator of G-protein signaling 7 (RGS7) is predominately present in the nervous system and is essential for neuronal signaling involving G-proteins. Prior studies in cultured cells showed that RGS7 is regulated via proteasomal degradation, however no protein is known to facilitate proteasomal degradation of RGS7 and it has not been shown whether this regulation affects G-protein signaling in neurons. Here we used a knockout mouse model with conditional deletion of arginyltransferase (Ate1) in the nervous system and found that in retinal ON bipolar cells, where RGS7 modulates a G-protein to signal light increments, deletion of Ate1 raised the level of RGS7. Electroretinographs revealed that lack of Ate1 leads to increased light-evoked response sensitivities of ON-bipolar cells, as well as their downstream neurons. In cultured mouse embryonic fibroblasts (MEF), RGS7 was rapidly degraded via proteasome pathway and this degradation was abolished in Ate1 knockout MEF. Our results indicate that Ate1 regulates RGS7 protein level by facilitating proteasomal degradation of RGS7 and thus affects G-protein signaling in neurons.

R-catcher, a potent molecular tool to unveil the arginylome

Protein arginylation is a critical regulator of a variety of biological processes. The ability to uncover the global arginylation pattern and its associated signaling pathways would enable us to identify novel disease targets. Here, we report the development of a tool able to capture the N-terminal arginylome. This tool, termed R-catcher, is based on the ZZ domain of p62, which was previously shown to bind N-terminally arginylated proteins. Mutating the ZZ domain enhanced its binding specificity and affinity for Nt-Arg. R-catcher pulldown coupled to LC-MS/MS led to the identification of 59 known and putative arginylated proteins. Among these were a subgroup of novel ATE1-dependent arginylated ER proteins that are linked to diverse biological pathways, including cellular senescence and vesicle-mediated transport as well as diseases, such as Amyotrophic Lateral Sclerosis and Alzheimer's disease. This study presents the first molecular tool that allows the unbiased identification of arginylated proteins, thereby unlocking the arginylome and provide a new path to disease biomarker discovery.

Multiple competing RNA structures dynamically control alternative splicing in the human ATE1 gene

The mammalian Ate1 gene encodes an arginyl transferase enzyme with tumor suppressor function that depends on the inclusion of one of the two mutually exclusive exons (MXE), exons 7a and 7b. We report that the molecular mechanism underlying MXE splicing in Ate1 involves five conserved regulatory intronic elements R1-R5, of which R1 and R4 compete for base pairing with R3, while R2 and R5 form an ultra-long-range RNA structure spanning 30 Kb. In minigenes, single and double mutations that disrupt base pairings in R1R3 and R3R4 lead to the loss of MXE splicing, while compensatory triple mutations that restore RNA structure revert splicing to that of the wild type. In the endogenous Ate1 pre-mRNA, blocking the competing base pairings by LNA/DNA mixmers complementary to R3 leads to the loss of MXE splicing, while the disruption of R2R5 interaction changes the ratio of MXE. That is, Ate1 splicing is controlled by two independent, dynamically interacting, and functionally distinct RNA structure modules. Exon 7a becomes more included in response to RNA Pol II slowdown, however it fails to do so when the ultra-long-range R2R5 interaction is disrupted, indicating that exon 7a/7b ratio depends on co-transcriptional RNA folding. In sum, these results demonstrate that splicing is coordinated both in time and in space over very long distances, and that the interaction of these components is mediated by RNA structure.

Posttranslational Arginylation Enzyme Arginyltransferase1 Shows Genetic Interactions With Specific Cellular Pathways in vivo

Arginyltransferase1 (ATE1) is a conserved enzyme in eukaryotes mediating posttranslational arginylation, the addition of an extra arginine to an existing protein. In mammals, the dysregulations of the ATE1 gene (ate1) is shown to be involved in cardiovascular abnormalities, cancer, and aging-related diseases. Although biochemical evidence suggested that arginylation may be involved in stress response and/or protein degradation, the physiological role of ATE1 in vivo has never been systematically determined. This gap of knowledge leads to difficulties for interpreting the involvements of ATE1 in diseases pathogenesis. Since ate1 is highly conserved between human and the unicellular organism Schizosaccharomyces pombe (S. pombe), we take advantage of the gene-knockout library of S. pombe, to investigate the genetic interactions between ate1 and other genes in a systematic and unbiased manner. By this approach, we found that ate1 has a surprisingly small and focused impact size. Among the 3659 tested genes, which covers nearly 75% of the genome of S. pombe, less than 5% of them displayed significant genetic interactions with ate1. Furthermore, these ate1-interacting partners can be grouped into a few discrete clustered categories based on their functions or their physical interactions. These categories include translation/transcription regulation, biosynthesis/metabolism of biomolecules (including histidine), cell morphology and cellular dynamics, response to oxidative or metabolic stress, ribosomal structure and function, and mitochondrial function. Unexpectedly, inconsistent to popular belief, very few genes in the global ubiquitination or degradation pathways showed interactions with ate1. Our results suggested that ATE1 specifically regulates a handful of cellular processes in vivo, which will provide critical mechanistic leads for studying the involvements of ATE1 in normal physiologies as well as in diseased conditions.

Regulation of actin isoforms in cellular and developmental processes

Actin is one of the most abundant and essential intracellular proteins that mediates nearly every form of cellular movement and underlies such key processes as embryogenesis, tissue integrity, cell division and contractility of all types of muscle and non-muscle cells. In mammals, actin is represented by six isoforms, which are encoded by different genes but produce proteins that are 95-99 % identical to each other. The six actin genes have vastly different functions in vivo, and the small amino acid differences between the proteins they encode are rigorously maintained through evolution, but the underlying differences behind this distinction, as well as the importance of specific amino acid sequences for each actin isoform, are not well understood. This review summarizes different levels of actin isoform-specific regulation in cellular and developmental processes, starting with the nuclear actin's role in transcription, and covering the gene-level, mRNA-level, and protein-level regulation, with a special focus on mammalian actins in non-muscle cells.

Arginyltransferase knockdown attenuates cardiac hypertrophy and fibrosis through TAK1-JNK1/2 pathway

Myocardial hypertrophy, an inflammatory condition of cardiac muscles is a maladaptive response of the heart to biomechanical stress, hemodynamic or neurohormonal stimuli. Previous studies indicated that knockout of Arginyltransferase (ATE1) gene in mice and embryos leads to contractile dysfunction, defective cardiovascular development, and impaired angiogenesis. Here we found that in adult rat model, downregulation of ATE1 mitigates cardiac hypertrophic, cardiac fibrosis as well as apoptosis responses in the presence of cardiac stress i.e. renal artery ligation. On contrary, in wild type cells responding to renal artery ligation, there is an increase of cellular ATE1 protein level. Further, we have shown the cardioprotective role of ATE1 silencing is mediated by the interruption of TAK1 activity-dependent JNK1/2 signaling pathway. We propose that ATE1 knockdown in presence of cardiac stress performs a cardioprotective action and the inhibition of its activity may provide a novel approach for the treatment of cardiac hypertrophy.

Quantification of intracellular N-terminal β-actin arginylation

Actin is a ubiquitous, essential, and highly abundant protein in all eukaryotic cells that performs key roles in contractility, adhesion, migration, and leading edge dynamics. The two non-muscle actins, β- and γ-, are ubiquitously present in every cell type and are nearly identical to each other at the amino acid level, but play distinct intracellular roles. The mechanisms regulating this distinction have been the focus of recent interest in the field. Work from our lab has previously shown that β-, but not γ-, actin undergoes N-terminal arginylation on Asp3. While functional evidence suggest that this arginylation may be important to actin's function, progress in these studies so far has been hindered by difficulties in arginylated actin detection, precluding estimations of the abundance of arginylated actin in cells, and its occurrence in different tissues and cell types. The present study represents the first antibody-based quantification of the percentage of arginylated actin in migratory non-muscle cells under different physiological conditions, as well as in different cells and tissues. We find that while the steady-state level of arginylated actin is relatively low, it is consistently present in vivo, and is somewhat more prominent in migratory cells. Inhibition of N-terminal actin acetylation dramatically increases the intracellular actin arginylation level, suggesting that these two modifications may directly compete in vivo. These findings constitute an essential step in our understanding of actin regulation by arginylation, and in uncovering the dynamic interplay of actin's N-terminal modifications in vivo.

Biochemical analysis of protein arginylation

Protein arginylation-enzymatic addition of the amino acid arginine (Arg) to proteins, mediated by arginyltransferase ATE1, has been discovered in 1963, but is still relatively poorly understood. Studies of arginylation present many technical challenges, which arise from the fact that Arg is a regular amino acid that also incorporates into proteins during translation. Thus, in vitro arginylation needs to be conducted in a strictly ribosome-free system, in highly controlled conditions. Identification of arginylated proteins is currently only possible by high precision mass spectrometry, which relies on very high mass accuracy of the instruments, specific ionization patterns during mass fragmentation, as well as multiple stringent steps of automated and manual validation. Below we describe the methods of in vitro arginylation and mass spectrometry analysis of arginylated proteins, developed by our groups during the last 15 years.

Protein arginylation of cytoskeletal proteins in the muscle: modifications modifying function

The cytoskeleton drives many essential processes in normal physiology, and its impairments underlie many diseases, including skeletal myopathies, cancer, and heart failure, that broadly affect developed countries worldwide. Cytoskeleton regulation is a field of investigation of rapidly emerging global importance and a new venue for the development of potential therapies. This review overviews our present understanding of the posttranslational regulation of the muscle cytoskeleton through arginylation, a tRNA-dependent addition of arginine to proteins mediated by arginyltransferase 1. We focus largely on arginylation-dependent regulation of striated muscles, shown to play critical roles in facilitating muscle integrity, contractility, regulation, and strength.

Disruption of embryonic ROCK signaling reproduces the sarcomeric phenotype of hypertrophic cardiomyopathy

Sarcomeric disarray is a hallmark of gene mutations in patients with hypertrophic cardiomyopathy (HCM). However, it is unknown when detrimental sarcomeric changes first occur and whether they originate in the developing embryonic heart. Furthermore, Rho kinase (ROCK) is a serine/threonine protein kinase that is critical for regulating the function of several sarcomeric proteins, and therefore, our aim was to determine whether disruption of ROCK signaling during the earliest stages of heart development would disrupt the integrity of sarcomeres, altering heart development and function. Using a mouse model in which the function of ROCK is specifically disrupted in embryonic cardiomyocytes, we demonstrate a progressive cardiomyopathy that first appeared as sarcomeric disarray during cardiogenesis. This led to abnormalities in the structure of the embryonic ventricular wall and compensatory cardiomyocyte hypertrophy during fetal development. This sarcomeric disruption and hypertrophy persisted throughout adult life, triggering left ventricular concentric hypertrophy with systolic dysfunction, and reactivation of fetal gene expression and cardiac fibrosis, all typical features of HCM. Taken together, our findings establish a mechanism for the developmental origin of the sarcomeric phenotype of HCM and suggest that variants in the ROCK genes or disruption of ROCK signaling could, in part, contribute to its pathogenesis.

Disruption of ROCK activity in embryonic cardiomyocytes revealed a developmental origin for hypertrophic cardiomyopathy.

Actin Post-translational Modifications: The Cinderella of Cytoskeletal Control

Actin is one of the most abundant proteins in eukaryotic cells and the main component of the microfilament system. It plays essential roles in numerous cellular activities, including muscle contraction, maintenance of cell integrity, and motility, as well as transcriptional regulation. Besides interacting with various actin-binding proteins (ABPs), proper actin function is regulated by post-translational modifications (PTMs), such as acetylation, arginylation, oxidation, and others. Here, we explain how actin PTMs can contribute to filament formation and stability, and may have additional actin regulatory functions, which potentially contribute to disease development.

Ate1-mediated posttranslational arginylation affects substrate adhesion and cell migration in Dictyostelium discoideum

The highly conserved enzyme arginyl-tRNA-protein transferase (Ate1) mediates arginylation, a posttranslational modification that is only incompletely understood at its molecular level. To investigate whether arginylation affects actin-dependent processes in a simple model organism, Dictyostelium discoideum, we knocked out the gene encoding Ate1 and characterized the phenotype of ate1-null cells. Visualization of actin cytoskeleton dynamics by live-cell microscopy indicated significant changes in comparison to wild-type cells. Ate1-null cells were almost completely lacking focal actin adhesion sites at the substrate-attached surface and were only weakly adhesive. In two-dimensional chemotaxis assays toward folate or cAMP, the motility of ate1-null cells was increased. However, in three-dimensional chemotaxis involving more confined conditions, the motility of ate1-null cells was significantly reduced. Live-cell imaging showed that GFP-tagged Ate1 rapidly relocates to sites of newly formed actin-rich protrusions. By mass spectrometric analysis, we identified four arginylation sites in the most abundant actin isoform of Dictyostelium, in addition to arginylation sites in other actin isoforms and several actin-binding proteins. In vitro polymerization assays with actin purified from ate1-null cells revealed a diminished polymerization capacity in comparison to wild-type actin. Our data indicate that arginylation plays a crucial role in the regulation of cytoskeletal activities.

Protein arginylation targets alpha synuclein, facilitates normal brain health, and prevents neurodegeneration

Alpha synuclein (α-syn) is a central player in neurodegeneration, but the mechanisms triggering its pathology are not fully understood. Here we found that α-syn is a highly efficient substrate for arginyltransferase ATE1 and is arginylated in vivo by a novel mid-chain mechanism that targets the acidic side chains of E46 and E83. Lack of arginylation leads to increased α-syn aggregation and causes the formation of larger pathological aggregates in neurons, accompanied by impairments in its ability to be cleared via normal degradation pathways. In the mouse brain, lack of arginylation leads to an increase in α-syn’s insoluble fraction, accompanied by behavioral changes characteristic for neurodegenerative pathology. Our data show that lack of arginylation in the brain leads to neurodegeneration, and suggests that α-syn arginylation can be a previously unknown factor that facilitates normal α-syn folding and function in vivo.

Increased Hemodynamic Load in Early Embryonic Stages Alters Myofibril and Mitochondrial Organization in the Myocardium

Normal blood flow is essential for proper heart formation during embryonic development, as abnormal hemodynamic load (blood pressure and shear stress) results in cardiac defects seen in congenital heart disease (CHD). However, the detrimental remodeling processes that relate altered blood flow to cardiac malformation and defects remain unclear. Heart development is a finely orchestrated process with rapid transformations that occur at the tissue, cell, and subcellular levels. Myocardial cells play an essential role in cardiac tissue maturation by aligning in the direction of stretch and increasing the number of contractile units as hemodynamic load increases throughout development. This study elucidates the early effects of altered blood flow on myofibril and mitochondrial configuration in the outflow tract myocardium in vivo. Outflow tract banding was used to increase hemodynamic load in the chicken embryo heart between Hamburger and Hamilton stages 18 and 24 (~24 h during tubular heart stages). 3D focused ion beam scanning electron microscopy analysis determined that increased hemodynamic load induced changes in the developing myocardium, characterized by thicker myofibril bundles that were more disbursed in circumferential orientation, and mitochondria that organized in large clusters around the nucleus. Proteomic mass-spectrometry analysis quantified altered protein composition after banding that is consistent with altered myofibril thin filament assembly and function, and mitochondrial maintenance and organization. Additionally, pathway analysis of the proteomics data identified possible activation of signaling pathways in response to banding, including the renin-angiotensin system (RAS). Imaging and proteomic data combined indicate that myofibril and mitochondrial arrangement in early embryonic stages is a critical developmental process that when disturbed by altered blood flow may contribute to cardiac malformation and defects.

Tampering with springs: phosphorylation of titin affecting the mechanical function of cardiomyocytes

Reversible post-translational modifications of various cardiac proteins regulate the mechanical properties of the cardiomyocytes and thus modulate the contractile performance of the heart. The giant protein titin forms a continuous filament network in the sarcomeres of striated muscle cells, where it determines passive tension development and modulates active contraction. These mechanical properties of titin are altered through post-translational modifications, particularly phosphorylation. Titin contains hundreds of potential phosphorylation sites, the functional relevance of which is only beginning to emerge. Here, we provide a state-of-the-art summary of the phosphorylation sites in titin, with a particular focus on the elastic titin spring segment. We discuss how phosphorylation at specific amino acids can reduce or increase the stretch-induced spring force of titin, depending on where the spring region is phosphorylated. We also review which protein kinases phosphorylate titin and how this phosphorylation affects titin-based passive tension in cardiomyocytes. A comprehensive overview is provided of studies that have measured altered titin phosphorylation and titin-based passive tension in myocardial samples from human heart failure patients and animal models of heart disease. As our understanding of the broader implications of phosphorylation in titin progresses, this knowledge could be used to design targeted interventions aimed at reducing pathologically increased titin stiffness in patients with stiff hearts.

Analyzing N-terminal Arginylation through the Use of Peptide Arrays and Degradation Assays

N^α-terminal arginylation (Nt-arginylation) of proteins is mediated by the Ate1 arginyltransferase (R-transferase), a component of the Arg/N-end rule pathway. This proteolytic system recognizes proteins containing N-terminal degradation signals called N-degrons, polyubiquitylates these proteins, and thereby causes their degradation by the proteasome. The definitively identified ("canonical") residues that are Nt-arginylated by R-transferase are N-terminal Asp, Glu, and (oxidized) Cys. Over the last decade, several publications have suggested (i) that Ate1 can also arginylate non-canonical N-terminal residues; (ii) that Ate1 is capable of arginylating not only α-amino groups of N-terminal residues but also γ-carboxyl groups of internal (non-N-terminal) Asp and Glu; and (iii) that some isoforms of Ate1 are specific for substrates bearing N-terminal Cys residues. In the present study, we employed arrays of immobilized 11-residue peptides and pulse-chase assays to examine the substrate specificity of mouse R-transferase. We show that amino acid sequences immediately downstream of a substrate's canonical (Nt-arginylatable) N-terminal residue, particularly a residue at position 2, can affect the rate of Nt-arginylation by R-transferase and thereby the rate of degradation of a substrate protein. We also show that the four major isoforms of mouse R-transferase have similar Nt-arginylation specificities in vitro, contrary to the claim about the specificity of some Ate1 isoforms for N-terminal Cys. In addition, we found no evidence for a significant activity of the Ate1 R-transferase toward previously invoked non-canonical N-terminal or internal amino acid residues. Together, our results raise technical concerns about earlier studies that invoked non-canonical arginylation specificities of Ate1.

Post-translational protein arginylation in the normal nervous system and in neurodegeneration

Post-translational arginylation of proteins is an important regulator of many physiological pathways in cells. This modification was originally noted in protein degradation during neurodegenerative processes, with an apparently different physiological relevance between central and peripheral nervous system. Subsequent studies have identified a steadily increasing number of proteins and proteolysis-derived polypeptides as arginyltransferase (ATE1) substrates, including β-amyloid, α-synuclein, and TDP43 proteolytic fragments. Arginylation is involved in signaling processes of proteins and polypeptides that are further ubiquitinated and degraded by the proteasome. In addition, it is also implicated in autophagy/lysosomal degradation pathway. Recent studies using mutant mouse strains deficient in ATE1 indicate additional roles of this modification in neuronal physiology. As ATE1 is capable of modifying proteins either at the N-terminus or middle-chain acidic residues, determining which proteins function are modulated by arginylation represents a big challenge. Here, we review studies addressing various roles of ATE1 activity in nervous system function, and suggest future research directions that will clarify the role of post-translational protein arginylation in brain development and various neurological disorders. Arginyltransferase (ATE1), the enzyme responsible for post-translational arginylation, modulates the functions of a wide variety of proteins and polypeptides, and is also involved in the main degradation pathways of intracellular proteins. Regulatory roles of ATE1 have been well defined for certain organs. However, its roles in nervous system development and neurodegenerative processes remain largely unknown, and present exciting opportunities for future research, as discussed in this review.

Assaying the Posttranslational Arginylation of Proteins in Cultured Cells

To evaluate the posttranslational arginylation of proteins in vivo, we describe a protocol for studying the (14)C-Arg incorporation into proteins of cells in culture. The conditions determined for this particular modification contemplate both the biochemical requirements of the enzyme ATE1 and the adjustments that allowed the discrimination between posttranslational arginylation of proteins and de novo synthesis. These conditions are applicable for different cell lines or primary cultures, representing an optimal procedure for the identification and the validation of putative ATE1 substrates.

Identification of Arginylated Proteins by Mass Spectrometry

Here we describe the method for identification of arginylated proteins by mass spectrometry. This method has been originally applied to the identification of N-terminally added Arg on proteins and peptides, and then expanded to identification of side chain arginylation which has been recently described by our groups. The key steps in this method include the use of the mass spectrometry instruments that can identify peptides with very high pass accuracy (Orbitrap) and apply stringent mass cutoffs during automated data analysis, followed by manual validation of the identified spectra. These methods can be used with both complex and purified protein samples and, to date, constitute the only reliable way to confirm arginylation at a particular site on a protein or peptide.

Reduced passive force in skeletal muscles lacking protein arginylation

Arginylation is a posttranslational modification that plays a global role in mammals. Mice lacking the enzyme arginyltransferase in skeletal muscles exhibit reduced contractile forces that have been linked to a reduction in myosin cross-bridge formation. The role of arginylation in passive skeletal myofibril forces has never been investigated. In this study, we used single sarcomere and myofibril measurements and observed that lack of arginylation leads to a pronounced reduction in passive forces in skeletal muscles. Mass spectrometry indicated that skeletal muscle titin, the protein primarily linked to passive force generation, is arginylated on five sites located within the A band, an important area for protein-protein interactions. We propose a mechanism for passive force regulation by arginylation through modulation of protein-protein binding between the titin molecule and the thick filament. Key points are as follows: 1) active and passive forces were decreased in myofibrils and single sarcomeres isolated from muscles lacking arginyl-tRNA-protein transferase (ATE1). 2) Mass spectrometry revealed five sites for arginylation within titin molecules. All sites are located within the A-band portion of titin, an important region for protein-protein interactions. 3) Our data suggest that arginylation of titin is required for proper passive force development in skeletal muscles.

Protein arginylation, a global biological regulator that targets actin cytoskeleton and the muscle

Posttranslational addition of Arg to proteins, mediated by arginyltransferase ATE1 has been first observed in 1963 and remained poorly understood for decades since its original discovery. Recent work demonstrated the global nature of arginylation and its essential role in multiple physiological pathways during embryogenesis and adulthood and identified over a hundred of proteins arginylated in vivo. Among these proteins, the prominent role belongs to the actin cytoskeleton and the muscle, and follow up studies strongly suggests that arginylation constitutes a novel biological regulator of contractility. This review presents an overview of the studies of protein arginylation that led to the discovery of its major role in the muscle.

Protein Arginylation: Over 50 Years of Discovery

Posttranslational modifications have emerged in recent years as the major biological regulators responsible for the orders of magnitude increase in complexity of protein functions. These "molecular switches" affect nearly every protein in vivo by modulating their protein structure, activity, molecular interactions, and homeostasis. While over 350 protein modifications have been described, only a handful of them have been characterized. Until recently, protein arginylation has belonged to the list of obscure, poorly understood posttranslational modifications, before the recent explosion of studies has put arginylation on the map of intracellular metabolic pathways and biological processes. This chapter contains an overview of all the major milestones in the protein arginylation field, from its original discovery in 1963 to this day.

Arginylation of myosin heavy chain regulates skeletal muscle strength

Protein arginylation is a posttranslational modification with an emerging global role in the regulation of actin cytoskeleton. To test the role of arginylation in the skeletal muscle, we generated a mouse model with Ate1 deletion driven by the skeletal muscle-specific creatine kinase (Ckmm) promoter. Ckmm-Ate1 mice were viable and outwardly normal; however, their skeletal muscle strength was significantly reduced in comparison to controls. Mass spectrometry of isolated skeletal myofibrils showed a limited set of proteins, including myosin heavy chain, arginylated on specific sites. Atomic force microscopy measurements of contractile strength in individual myofibrils and isolated myosin filaments from these mice showed a significant reduction of contractile forces, which, in the case of myosin filaments, could be fully rescued by rearginylation with purified Ate1. Our results demonstrate that arginylation regulates force production in muscle and exerts a direct effect on muscle strength through arginylation of myosin.

Loss of ATE1-mediated arginylation leads to impaired platelet myosin phosphorylation, clot retraction, and in vivo thrombosis formation

Protein arginylation by arginyl-transfer RNA protein transferase (ATE1) is emerging as a regulator protein function that is reminiscent of phosphorylation. For example, arginylation of β-actin has been found to regulate lamellipodial formation at the leading edge in fibroblasts. This finding suggests that similar functions of β-actin in other cell types may also require arginylation. Here, we have tested the hypothesis that ATE1 regulates the cytoskeletal dynamics essential for in vivo platelet adhesion and thrombus formation. To test this hypothesis, we generated conditional knockout mice specifically lacking ATE1 in their platelets and in their megakaryocytes and analyzed the role of arginylation during platelet activation. Surprisingly, rather than finding an impairment of the actin cytoskeleton structure and its rearrangement during platelet activation, we observed that the platelet-specific ATE1 knockout led to enhanced clot retraction and in vivo thrombus formation. This effect might be regulated by myosin II contractility since it was accompanied by enhanced phosphorylation of the myosin regulatory light chain on Ser19, which is an event that activates myosin in vivo. Furthermore, ATE1 and myosin co-immunoprecipitate from platelet lysates. This finding suggests that these proteins directly interact within platelets. These results provide the first evidence that arginylation is involved in phosphorylation-dependent protein regulation, and that arginylation affects myosin function in platelets during clot retraction.

Calreticulin-dimerization induced by post-translational arginylation is critical for stress granules scaffolding

Protein arginylation mediated by arginyl-tRNA protein transferase is a post-translational modification that occurs widely in biology, it has been shown to regulate protein and properties and functions. Post-translational arginylation is critical for embryogenesis, cardiovascular development and angiogenesis but the molecular effects of proteins arginylated in vivo are largely unknown. In the present study, we demonstrate that arginylation reduces CRT (calreticulin) thermostability and induces a greater degree of dimerization and oligomerization. R-CRT (arginylated calreticulin) forms disulfide-bridged dimers that are increased in low Ca(2+) conditions at physiological temperatures, a similar condition to the cellular environment that it required for arginylation of CRT. Moreover, R-CRT self-oligomerizes through non-covalent interactions that are enhanced at temperatures above 40 °C, condition that mimics the heat shock treatment where R-CRT is the only isoespecies of CRT that associates in cells to SGs (stress granules). We show that in cells lacking CRT the scaffolding of larger SGs is impaired; the transfection with CRT (hence R-CRT expression) restores SGs assembly whereas the transfection with CRT mutated in Cys146 does not. Thus, R-CRT disulfide-bridged dimers (through Cys146) are essential for the scaffolding of larger SGs under heat shock, although these dimers are not required for R-CRT association to SGs. The alteration in SGs assembly is critical for the normal cellular recover of cells after heat induced stress. We conclude that R-CRT is emerging as a novel protein that has an impact on the regulation of SGs scaffolding and cell survival.

Post-translational modification and regulation of actin

Many of the best-studied actin regulatory proteins use non-covalent means to modulate the properties of actin. Yet, actin is also susceptible to covalent modifications of its amino acids. Recent work is increasingly revealing that actin processing and its covalent modifications regulate important cellular events. In addition, numerous pathogens express enzymes that specifically use actin as a substrate to regulate their hosts' cells. Actin post-translational alterations have been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight specific co-translational and post-translational modifications of actin and discuss the current understanding of the role that these modifications play in regulating actin.

Arginylation-dependent regulation of a proteolytic product of talin is essential for cell-cell adhesion

Talin is a large scaffolding molecule that plays a major role in integrin-dependent cell-matrix adhesion. A role for talin in cell-cell attachment through cadherin has never been demonstrated, however. Here, we identify a novel calpain-dependent proteolytic cleavage of talin that results in the release of a 70-kD C-terminal fragment, which serves as a substrate of posttranslational arginylation. The intracellular levels of this fragment closely correlated with the formation of cell-cell adhesions, and this fragment localized to cadherin-containing cell-cell contacts. Moreover, reintroduction of this fragment rescued the cell-cell adhesion defects in arginyltransferase (Ate1) knockout cells, which normally have a very low level of this fragment. Arginylation of this fragment further enhanced its ability to rescue cell-cell adhesion formation. In addition, arginylation facilitated its turnover, suggesting a dual role of arginylation in its intracellular regulation. Thus, our work identifies a novel proteolytic product of talin that is regulated by arginylation and a new role of talin in cadherin-dependent cell-cell adhesion.

Arginylation regulates myofibrils to maintain heart function and prevent dilated cardiomyopathy

Protein arginylation mediated by arginyltransferase (ATE1) is essential for heart formation during embryogenesis, however its cell-autonomous role in cardiomyocytes and the differentiated heart muscle has never been investigated. To address this question, we generated cardiac muscle-specific Ate1 knockout mice, in which Ate1 deletion was driven by α-myosin heavy chain promoter (αMHC-Ate1 mouse). These mice were initially viable, but developed severe cardiac contractility defects, dilated cardiomyopathy, and thrombosis over time, resulting in high rates of lethality after 6months of age. These symptoms were accompanied by severe ultrastructural defects in cardiac myofibrils, seen in the newborns and far preceding the onset of cardiomyopathy, suggesting that these defects were primary and likely underlay the development of the future heart defects. Several major sarcomeric proteins were arginylated in vivo. Moreover, Ate1 deletion in the hearts resulted in a significant reduction of active and passive myofibril forces, suggesting that arginylation is critical for both myofibril structural integrity and contractility. Thus, arginylation is essential for maintaining the heart function by regulation of the major myofibril proteins and myofibril forces, and its absence in the heart muscle leads to progressive heart failure through cardiomyocyte-specific defects.

Arginylation and methylation double up to regulate nuclear proteins and nuclear architecture in vivo

Protein arginylation and arginine methylation are two posttranslational modifications of emerging importance that involve Arg residues and their modifications. To test a hypothesis that posttranslationally added arginines can be methylated, we used high-precision mass spectrometry and metabolic labeling to find whether posttranslationally added arginines can serve as methylation sites. We identified a number of proteins in vivo, on which posttranslationally added Arg have undergone mono- and dimethylation. This double modification predominantly affects the chromatin-containing nuclear fraction and likely plays an important regulatory role in chromatin-associated proteins. Moreover, inhibition of arginylation and Arg methylation results in a significant reduction of the nucleus size in cultured cells, suggesting changes in chromatin compaction and nuclear architecture. Our findings suggest a functional link between protein regulation by arginylation and methylation that affects nuclear structure in vivo.

Identification of troponin I and actin, alpha cardiac muscle 1 as potential biomarkers for hearts of electrically stimulated chickens

Methods

In this study, proteomics methods have been used to study the effects of different currents and voltages used to stun chickens. Protein profiles of chicken hearts were constructed to detect differences in protein expression and modification. The different voltages studied were 10 V, 40 V and 70 V, while the currents examined were 0.25 A, 0.5 A, and 0.75 A. The profiles obtained from these stunning conditions were compared to the non-stunned (0 A, 0 V) sample.

Results

Proteomics analyses using 2D Platinum ImageMaster 6.0 and Matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry (MALDI-TOF) identified troponin I and alpha cardiac muscle actin 1 in the electrically stimulated heart samples. The overexpression of the proteins was further confirmed at the transcriptional level by Real Time PCR.

Conclusion

The results from MALDI-TOF and Real Time PCR agreed; therefore, this method for identifying biomarkers of electrically stimulated chicken hearts provides a novel approach for differentiation the hearts of increased electrically stimulated chickens from those of non-stunned chickens.

The N-end rule pathway and regulation by proteolysis

The N-end rule relates the regulation of the in vivo half-life of a protein to the identity of its N-terminal residue. Degradation signals (degrons) that are targeted by the N-end rule pathway include a set called N-degrons. The main determinant of an N-degron is a destabilizing N-terminal residue of a protein. In eukaryotes, the N-end rule pathway is a part of the ubiquitin system and consists of two branches, the Ac/N-end rule and the Arg/N-end rule pathways. The Ac/N-end rule pathway targets proteins containing N(α) -terminally acetylated (Nt-acetylated) residues. The Arg/N-end rule pathway recognizes unacetylated N-terminal residues and involves N-terminal arginylation. Together, these branches target for degradation a majority of cellular proteins. For example, more than 80% of human proteins are cotranslationally Nt-acetylated. Thus most proteins harbor a specific degradation signal, termed (Ac)N-degron, from the moment of their birth. Specific N-end rule pathways are also present in prokaryotes and in mitochondria. Enzymes that produce N-degrons include methionine-aminopeptidases, caspases, calpains, Nt-acetylases, Nt-amidases, arginyl-transferases and leucyl-transferases. Regulated degradation of specific proteins by the N-end rule pathway mediates a legion of physiological functions, including the sensing of heme, oxygen, and nitric oxide; selective elimination of misfolded proteins; the regulation of DNA repair, segregation and condensation; the signaling by G proteins; the regulation of peptide import, fat metabolism, viral and bacterial infections, apoptosis, meiosis, spermatogenesis, neurogenesis, and cardiovascular development; and the functioning of adult organs, including the pancreas and the brain. Discovered 25 years ago, this pathway continues to be a fount of biological insights.