Aminopeptidases trim Xaa-Pro proteins, initiating their degradation by the Pro/N-degron pathway

N-degron pathways

An N-degron is a degradation signal whose main determinant is a "destabilizing" N-terminal residue of a protein. Specific N-degrons, discovered in 1986, were the first identified degradation signals in short-lived intracellular proteins. These N-degrons are recognized by a ubiquitin-dependent proteolytic system called the Arg/N-degron pathway. Although bacteria lack the ubiquitin system, they also have N-degron pathways. Studies after 1986 have shown that all 20 amino acids of the genetic code can act, in specific sequence contexts, as destabilizing N-terminal residues. Eukaryotic proteins are targeted for the conditional or constitutive degradation by at least five N-degron systems that differ both functionally and mechanistically: the Arg/N-degron pathway, the Ac/N-degron pathway, the Pro/N-degron pathway, the fMet/N-degron pathway, and the newly named, in this perspective, GASTC/N-degron pathway (GASTC = Gly, Ala, Ser, Thr, Cys). I discuss these systems and the expanded terminology that now encompasses the entire gamut of known N-degron pathways.

A chemical probe to modulate human GID4 Pro/N-degron interactions

The C-terminal to LisH (CTLH) complex is a ubiquitin ligase complex that recognizes substrates with Pro/N-degrons via its substrate receptor Glucose-Induced Degradation 4 (GID4), but its function and substrates in humans remain unclear. Here, we report PFI-7, a potent, selective and cell-active chemical probe that antagonizes Pro/N-degron binding to human GID4. Use of PFI-7 in proximity-dependent biotinylation and quantitative proteomics enabled the identification of GID4 interactors and GID4-regulated proteins. GID4 interactors are enriched for nucleolar proteins, including the Pro/N-degron-containing RNA helicases DDX21 and DDX50. We also identified a distinct subset of proteins whose cellular levels are regulated by GID4 including HMGCS1, a Pro/N-degron-containing metabolic enzyme. These data reveal human GID4 Pro/N-degron targets regulated through a combination of degradative and nondegradative functions. Going forward, PFI-7 will be a valuable research tool for investigating CTLH complex biology and facilitating development of targeted protein degradation strategies that highjack CTLH E3 ligase activity.

Delineation of the substrate recognition domain of MARCHF6 E3 ubiquitin ligase in the Ac/N-degron pathway and its regulatory role in ferroptosis

Nα-terminal acetylation in eukaryotic proteins creates specific degradation signals (Ac/N-degrons) targeted for ubiquitin-mediated proteolysis via the Ac/N-degron pathway. Despite the identification of key components of the Ac/N-degron pathway over the past 15 years, the precise recognition domain (Ac/N domain) remains unclear. Here, we defined the Ac/N domain of the endoplasmic reticulum MARCHF6 E3 ubiquitin ligase through a systematic analysis of its cytosol-facing regions using alanine-stretch mutagenesis, chemical crosslinking-based co-immunoprecipitation-immunoblotting, and split-ubiquitin assays in human and yeast cells. The Ac/N domain of MARCHF6 exhibits preferential binding specificity to Nα-terminally acetylated proteins and peptides over their unacetylated counterparts, mediating the degradation of Ac/N-degron-bearing proteins, such as the G-protein regulator RGS2 and the lipid droplet protein PLIN2. Furthermore, abolishing the recognition of Ac/N-degrons by MARCHF6 stabilized RGS2 and PLIN2, thereby increasing the resistance to ferroptosis, an iron-dependent lipid peroxidation-mediated cell death. These findings provide mechanistic and functional insights into how MARCHF6 serves as a rheostatic modulator of ferroptosis by recognizing Ac/N-degron substrates via its Ac/N domain and non-Ac/N-degron substrates via distinct recognition sites.

Dipeptidyl peptidases and E3 ligases of N-degron pathways cooperate to regulate protein stability

N-degrons are short sequences located at protein N-terminus that mediate the interaction of E3 ligases (E3s) with substrates to promote their proteolysis. It is well established that N-degrons can be exposed following protease cleavage to allow recognition by E3s. However, our knowledge regarding how proteases and E3s cooperate in protein quality control mechanisms remains minimal. Using a systematic approach to monitor the protein stability of an N-terminome library, we found that proline residue at the third N-terminal position (hereafter "P+3") promotes instability. Genetic perturbations identified the dipeptidyl peptidases DPP8 and DPP9 and the primary E3s of N-degron pathways, UBR proteins, as regulators of P+3 bearing substrate turnover. Interestingly, P+3 UBR substrates are significantly enriched for secretory proteins. We found that secretory proteins relying on a signal peptide (SP) for their targeting contain a "built-in" N-degron within their SP. This degron becomes exposed by DPP8/9 upon translocation failure to the designated compartments, thus enabling clearance of mislocalized proteins by UBRs to maintain proteostasis.

Differential gene expression of salt-tolerant alfalfa in response to salinity and inoculation by Ensifer meliloti

BACKGROUND

Alfalfa (Medicago sativa L.) experiences many negative effects under salinity stress, which may be mediated by recurrent selection. Salt-tolerant alfalfa may display unique adaptations in association with rhizobium under salt stress.

RESULTS

To elucidate inoculation effects on salt-tolerant alfalfa under salt stress, this study leveraged a salt-tolerant alfalfa population selected through two cycles of recurrent selection under high salt stress. After experiencing 120-day salt stress, mRNA was extracted from 8 random genotypes either grown in 0 or 8 dS/m salt stress with or without inoculation by Ensifer meliloti. Results showed 320 and 176 differentially expressed genes (DEGs) modulated in response to salinity stress or inoculation x salinity stress, respectively. Notable results in plants under 8 dS/m stress included upregulation of a key gene involved in the Target of Rapamycin (TOR) signaling pathway with a concomitant decrease in expression of the SNrK pathway. Inoculation of salt-stressed plants stimulated increased transcription of a sulfate-uptake gene as well as upregulation of the Lysine-27-trimethyltransferase (EZH2), Histone 3 (H3), and argonaute (AGO, a component of miRISC silencing complexes) genes related to epigenetic and post-transcriptional gene control.

CONCLUSIONS

Salt-tolerant alfalfa may benefit from improved activity of TOR and decreased activity of SNrK1 in salt stress, while inoculation by rhizobiumstimulates production of sulfate uptake- and other unique genes.

An anti-biofilm cyclic peptide targets a secreted aminopeptidase from P. aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious illness, especially in immunocompromised individuals. P. aeruginosa forms biofilms that contribute to growth and persistence in a wide range of environments. Here we investigated the aminopeptidase, P. aeruginosa aminopeptidase (PaAP) from P. aeruginosa, which is highly abundant in the biofilm matrix. PaAP is associated with biofilm development and contributes to nutrient recycling. We confirmed that post-translational processing was required for activation and PaAP is a promiscuous aminopeptidase acting on unstructured regions of peptides and proteins. Crystal structures of wild-type enzymes and variants revealed the mechanism of autoinhibition, whereby the C-terminal propeptide locks the protease-associated domain and the catalytic peptidase domain into a self-inhibited conformation. Inspired by this, we designed a highly potent small cyclic-peptide inhibitor that recapitulates the deleterious phenotype observed with a PaAP deletion variant in biofilm assays and present a path toward targeting secreted proteins in a biofilm context.

Unlocking the Mysteries of Alpha-N-Terminal Methylation and its Diverse Regulatory Functions

Protein post-translation modifications (PTMs) are a critical regulatory mechanism of protein function. Protein α-N-terminal (Nα) methylation is a conserved PTM across prokaryotes and eukaryotes. Studies of the Nα methyltransferases responsible for Να methylation and their substrate proteins have shown that the PTM involves diverse biological processes, including protein synthesis and degradation, cell division, DNA damage response, and transcription regulation. This review provides an overview of the progress toward the regulatory function of Να methyltransferases and their substrate landscape. More than 200 proteins in humans and 45 in yeast are potential substrates for protein Nα methylation based on the canonical recognition motif, XP[KR]. Based on recent evidence for a less stringent motif requirement, the number of substrates might be increased, but further validation is needed to solidify this concept. A comparison of the motif in substrate orthologs in selected eukaryotic species indicates intriguing gain and loss of the motif across the evolutionary landscape. We discuss the state of knowledge in the field that has provided insights into the regulation of protein Να methyltransferases and their role in cellular physiology and disease. We also outline the current research tools that are key to understanding Να methylation. Finally, challenges are identified and discussed that would aid in unlocking a system-level view of the roles of Να methylation in diverse cellular pathways.

N-Terminally arginylated ubiquitin is attached to histone H2A by RING1B E3 ligase in human cells

Ubiquitin (Ub) is highly conserved in all eukaryotic organisms and begins at the N-terminus with Met and Gln. Our recent research demonstrates that N-terminally (Nt-) arginylated Ub can be produced in the yeast Saccharomyces cerevisiae. However, the existence of Nt-arginylated Ub in multicellular organisms remains unknown. Here we explore the mechanism for creating Nt-arginylated Ub using human embryonic kidney HEK293 cells that express various Nt-modified Ubs. We found that Gln-starting Q-Ub was converted into Glu-starting E-Ub by NTAQ1 Nt-deamidase and subsequently Nt-arginylated by ATE1 arginyltransferase in HEK293 cells. We also found that the resulting Arg-Glu-starting RE-Ub was mainly deposited on the Lys119 residue of histone H2A. Furthermore, RING1B E3 Ub ligase mediated the attachment of RE-Ub to H2A. These findings reveal a previously unknown type of histone ubiquitylation which greatly increases the combinatorial complexity of histone and ubiquitin codes.

The metabolite-controlled ubiquitin conjugase Ubc8 promotes mitochondrial protein import

Mitochondria play a key role in cellular energy metabolism. Transitions between glycolytic and respiratory conditions induce considerable adaptations of the cellular proteome. These metabolism-dependent changes are particularly pronounced for the protein composition of mitochondria. Here, we show that the yeast cytosolic ubiquitin conjugase Ubc8 plays a crucial role in the remodeling process when cells transition from respiratory to fermentative conditions. Ubc8 is a conserved and well-studied component of the catabolite control system that is known to regulate the stability of gluconeogenic enzymes. Unexpectedly, we found that Ubc8 also promotes the assembly of the translocase of the outer membrane of mitochondria (TOM) and increases the levels of its cytosol-exposed receptor subunit Tom22. Ubc8 deficiency results in compromised protein import into mitochondria and reduced steady-state levels of mitochondrial proteins. Our observations show that Ubc8, which is controlled by the prevailing metabolic conditions, promotes the switch from glucose synthesis to glucose usage in the cytosol and induces the biogenesis of the mitochondrial TOM machinery to improve mitochondrial protein import during phases of metabolic transition.

Crystal structure of the Ate1 arginyl-tRNA-protein transferase and arginylation of N-degron substrates

N-degron pathways are proteolytic systems that target proteins bearing N-terminal (Nt) degradation signals (degrons) called N-degrons. Nt-Arg of a protein is among Nt-residues that can be recognized as destabilizing ones by the Arg/N-degron pathway. A proteolytic cleavage of a protein can generate Arg at the N terminus of a resulting C-terminal (Ct) fragment either directly or after Nt-arginylation of that Ct-fragment by the Ate1 arginyl-tRNA-protein transferase (R-transferase), which uses Arg-tRNA^Arg as a cosubstrate. Ate1 can Nt-arginylate Nt-Asp, Nt-Glu, and oxidized Nt-Cys* (Cys-sulfinate or Cys-sulfonate) of proteins or short peptides. Ate1 genes of fungi, animals, and plants have been cloned decades ago, but a three-dimensional structure of Ate1 remained unknown. A detailed mechanism of arginylation is unknown as well. We describe here the crystal structure of the Ate1 R-transferase from the budding yeast Kluyveromyces lactis. The 58-kDa R-transferase comprises two domains that recognize, together, an acidic Nt-residue of an acceptor substrate, the Arg residue of Arg-tRNA^Arg, and a 3'-proximal segment of the tRNA^Arg moiety. The enzyme's active site is located, at least in part, between the two domains. In vitro and in vivo arginylation assays with site-directed Ate1 mutants that were suggested by structural results yielded inferences about specific binding sites of Ate1. We also analyzed the inhibition of Nt-arginylation activity of Ate1 by hemin (Fe³⁺-heme), and found that hemin induced the previously undescribed disulfide-mediated oligomerization of Ate1. Together, these results advance the understanding of R-transferase and the Arg/N-degron pathway.

Cryo-EM structures of Gid12-bound GID E3 reveal steric blockade as a mechanism inhibiting substrate ubiquitylation

Protein degradation, a major eukaryotic response to cellular signals, is subject to numerous layers of regulation. In yeast, the evolutionarily conserved GID E3 ligase mediates glucose-induced degradation of fructose-1,6-bisphosphatase (Fbp1), malate dehydrogenase (Mdh2), and other gluconeogenic enzymes. "GID" is a collection of E3 ligase complexes; a core scaffold, RING-type catalytic core, and a supramolecular assembly module together with interchangeable substrate receptors select targets for ubiquitylation. However, knowledge of additional cellular factors directly regulating GID-type E3s remains rudimentary. Here, we structurally and biochemically characterize Gid12 as a modulator of the GID E3 ligase complex. Our collection of cryo-EM reconstructions shows that Gid12 forms an extensive interface sealing the substrate receptor Gid4 onto the scaffold, and remodeling the degron binding site. Gid12 also sterically blocks a recruited Fbp1 or Mdh2 from the ubiquitylation active sites. Our analysis of the role of Gid12 establishes principles that may more generally underlie E3 ligase regulation.

Structural and Functional Insights into GID/CTLH E3 Ligase Complexes

Multi-subunit E3 ligases facilitate ubiquitin transfer by coordinating various substrate receptor subunits with a single catalytic center. Small molecules inducing targeted protein degradation have exploited such complexes, proving successful as therapeutics against previously undruggable targets. The C-terminal to LisH (CTLH) complex, also called the glucose-induced degradation deficient (GID) complex, is a multi-subunit E3 ligase complex highly conserved from Saccharomyces cerevisiae to humans, with roles in fundamental pathways controlling homeostasis and development in several species. However, we are only beginning to understand its mechanistic basis. Here, we review the literature of the CTLH complex from all organisms and place previous findings on individual subunits into context with recent breakthroughs on its structure and function.

N-Terminal Modifications of Ubiquitin via Methionine Excision, Deamination, and Arginylation Expand the Ubiquitin Code

Ubiquitin (Ub) is post-translationally modified by Ub itself or Ub-like proteins, phosphorylation, and acetylation, among others, which elicits a variety of Ub topologies and cellular functions. However, N-terminal (Nt) modifications of Ub remain unknown, except the linear head-to-tail ubiquitylation via Nt-Met. Here, using the yeast Saccharomyces cerevisiae and an Nt-arginylated Ub-specific antibody, we found that the detectable level of Ub undergoes Nt-Met excision, Nt-deamination, and Nt-arginylation. The resulting Nt-arginylated Ub and its conjugated proteins are upregulated in the stationary-growth phase or by oxidative stress. We further proved the existence of Nt-arginylated Ub in vivo and identified Nt-arginylated Ub-protein conjugates using stable isotope labeling by amino acids in cell culture (SILAC)-based tandem mass spectrometry. In silico structural modeling of Nt-arginylated Ub predicted that Nt-Arg flexibly protrudes from the surface of the Ub, thereby most likely providing a docking site for the factors that recognize it. Collectively, these results reveal unprecedented Nt-arginylated Ub and the pathway by which it is produced, which greatly expands the known complexity of the Ub code.

How the ends signal the end: Regulation by E3 ubiquitin ligases recognizing protein termini

Specificity of eukaryotic protein degradation is determined by E3 ubiquitin ligases and their selective binding to protein motifs, termed "degrons," in substrates for ubiquitin-mediated proteolysis. From the discovery of the first substrate degron and the corresponding E3 to a flurry of recent studies enabled by modern systems and structural methods, it is clear that many regulatory pathways depend on E3s recognizing protein termini. Here, we review the structural basis for recognition of protein termini by E3s and how this recognition underlies biological regulation. Diverse E3s evolved to harness a substrate's N and/or C terminus (and often adjacent residues as well) in a sequence-specific manner. Regulation is achieved through selective activation of E3s and also through generation of degrons at ribosomes or by posttranslational means. Collectively, many E3 interactions with protein N and C termini enable intricate control of protein quality and responses to cellular signals.

Crystal structure of yeast Gid10 in complex with Pro/N-degron

The cellular glucose level has to be tightly regulated by a variety of cellular processes. One of them is the degradation of gluconeogenic enzymes such as Fbp1, Icl1, Mdh2, and Pck1 by GID (glucose-induced degradation deficient) E3 ubiquitin ligase. The Gid4 component of the GID ligase complex is responsible for recognizing the N-terminal proline residue of the target substrates under normal conditions. However, an alternative N-recognin Gid10 controls the degradation process under stressed conditions. Although Gid10 shares a high sequence similarity with Gid4, their substrate specificities are quite different. Here, we report the structure of Gid10 from Saccharomyces cerevisiae in complex with Pro/N-degron, Pro-Tyr-Ile-Thr, which is almost identical to the sequence of the natural substrate Art2. Although Gid10 shares many structural features with the Gid4 protein from yeast and humans, the current structure explains the unique structural difference for the preference of bulky hydrophobic residue at the second position of Pro/N-degron. Therefore, this study provides a fundamental basis for understanding of the structural diversity and substrate specificity of recognition components in the GID E3 ligase complex involved in the Pro/N-degron pathway.

Multifaceted N-Degron Recognition and Ubiquitylation by GID/CTLH E3 Ligases

N-degron E3 ubiquitin ligases recognize specific residues at the N-termini of substrates. Although molecular details of N-degron recognition are known for several E3 ligases, the range of N-terminal motifs that can bind a given E3 substrate binding domain remains unclear. Here, we discovered capacity of Gid4 and Gid10 substrate receptor subunits of yeast "GID"/human "CTLH" multiprotein E3 ligases to tightly bind a wide range of N-terminal residues whose recognition is determined in part by the downstream sequence context. Screening of phage displaying peptide libraries with exposed N-termini identified novel consensus motifs with non-Pro N-terminal residues binding Gid4 or Gid10 with high affinity. Structural data reveal that conformations of flexible loops in Gid4 and Gid10 complement sequences and folds of interacting peptides. Together with analysis of endogenous substrate degrons, the data show that degron identity, substrate domains harboring targeted lysines, and varying E3 ligase higher-order assemblies combinatorially determine efficiency of ubiquitylation and degradation.