The structural basis of tRNA recognition by arginyl-tRNA-protein transferase

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.

Copper catalyzed cycloaddition for the synthesis of non isomerisable 2' and 3'-regioisomers of arg-tRNAarg

In this report, non-isomerisable analogs of arginine tRNA (Arg-triazole-tRNA) have been synthesized as tools to study tRNA-dependent aminoacyl-transferases. The synthesis involves the incorporation of 1,4 substituted-1,2,3 triazole ring to mimic the ester bond that connects the amino acid to the terminal adenosine in the natural substrate. The synthetic procedure includes (i) a coupling between 2'- or 3'-azido-adenosine derivatives and a cytidine phosphoramidite to access dinucleotide molecules, (ii) Cu-catalyzed cycloaddition reactions between 2'- or 3'-azido dinucleotide in the presence of an alkyne molecule mimicking the arginine, providing the corresponding Arg-triazole-dinucleotides, (iii) enzymatic phosphorylation of the 5'-end extremity of the Arg-triazole-dinucleotides with a polynucleotide kinase, and (iv) enzymatic ligation of the 5'-phosphorylated dinucleotides with a 23-nt RNA micro helix that mimics the acceptor arm of arg-tRNA or with a full tRNAarg. Characterization of nucleoside and nucleotide compounds involved MS spectrometry, 1H, 13C and 31P NMR analysis. This strategy allows to obtain the pair of the two stable regioisomers of arg-tRNA analogs (2' and 3') which are instrumental to explore the regiospecificity of arginyl transferases enzyme. In our study, a first binding assay of the arg-tRNA micro helix with the Arginyl-tRNA-protein transferase 1 (ATE1) was performed by gel shift assays.

Identification of an intrinsically disordered region (IDR) in arginyltransferase 1 (ATE1)

Arginyltransferase 1 (ATE1) catalyzes arginylation, an important post-translational modification (PTM) in eukaryotes that plays a critical role in cellular homeostasis. The disruption of ATE1 function is implicated in mammalian neurodegenerative disorders and cardiovascular maldevelopment, while post-translational arginylation has also been linked to the activities of several important human viruses such as SARS-CoV-2 and HIV. Despite the known significance of ATE1 in mammalian cellular function, past biophysical studies of this enzyme have mainly focused on yeast ATE1, leaving the mechanism of arginylation in mammalian cells unclear. In this study, we sought to structurally and biophysically characterize mouse (Mus musculus) ATE1. Using size-exclusion chromatography (SEC), small angle X-ray scattering (SAXS), and hydrogen deuterium exchange mass spectrometry (HDX-MS), assisted by AlphaFold modeling, we found that mouse ATE1 is structurally more complex than yeast ATE1. Importantly, our data indicate the existence of an intrinsically disordered region (IDR) in all mouse ATE1 splice variants. However, comparative HDX-MS analyses show that yeast ATE1 does not have such an IDR, consistent with prior X-ray, cryo-EM, and SAXS analyses. Furthermore, bioinformatics approaches reveal that mammalian ATE1 sequences, as well as in a large majority of other eukaryotes, contain an IDR-like sequence positioned in proximity to the ATE1 GNAT active-site fold. Computational analysis suggests that the IDR likely facilitates the formation of the complex between ATE1 and tRNAArg, adding a new complexity to ATE1 structure and providing new insights for future studies of ATE1 functions.

Evolution of the Genetic Code in the Ascoideales (CUG-Ser2) Yeast Clade: The Ancestral tRNA-Leu(CAG) Gene Is Retained in Most Saccharomycopsis Species but Is Nonessential and Not Used for Translation

In the yeast genera Saccharomycopsis and Ascoidea, which comprise the taxonomic order Ascoideales, nuclear genes use a nonstandard genetic code in which CUG codons are translated as serine instead of leucine, due to a tRNA-Ser with the unusual anticodon CAG. However, some species in this clade also retain an ancestral tRNA-Leu gene with the same anticodon. One of these species, Ascoidea asiatica, has been shown to have a stochastic proteome in which proteins contain ∼50% Ser and 50% Leu at CUG codon sites, whereas previously examined Saccharomycopsis species translate CUG only as Ser. Here, we investigated the presence, conservation, and possible functionality of the tRNA-Leu(CAG) gene in the genus Saccharomycopsis. We sequenced the genomes of 23 strains that, together with previously available data, include almost every known species of this genus. We found that most Saccharomycopsis species have genes for both tRNA-Leu(CAG) and tRNA-Ser(CAG). However, tRNA-Leu(CAG) has been lost in Saccharomycopsis synnaedendra and Saccharomycopsis microspora, and its predicted cloverleaf structure is aberrant in all the other Saccharomycopsis species. We deleted the tRNA-Leu(CAG) gene of Saccharomycopsis capsularis and found that it is not essential. Proteomic analyses in vegetative and sporulating cultures of S. capsularis and Saccharomycopsis fermentans showed only translation of CUG as Ser. Despite its unusual structure, the tRNA-Leu(CAG) gene shows evidence of sequence conservation among Saccharomycopsis species, particularly in its acceptor stem and leucine identity elements, which suggests that it may have been retained in order to carry out an unknown nontranslational function.

Structural basis of tRNA recognition by the widespread OB fold

The widespread oligonucleotide/oligosaccharide-binding (OB)-fold recognizes diverse substrates from sugars to nucleic acids and proteins, and plays key roles in genome maintenance, transcription, translation, and tRNA metabolism. OB-containing bacterial Trbp and yeast Arc1p proteins are thought to recognize the tRNA elbow or anticodon regions. Here we report a 2.6 Å co-crystal structure of Aquifex aeolicus Trbp111 bound to tRNAIle, which reveals that Trbp recognizes tRNAs solely by capturing their 3' ends. Structural, mutational, and biophysical analyses show that the Trbp/EMAPII-like OB fold precisely recognizes the single-stranded structure, 3' terminal location, and specific sequence of the 3' CA dinucleotide - a universal feature of mature tRNAs. Arc1p supplements its OB - tRNA 3' end interaction with additional contacts that involve an adjacent basic region and the tRNA body. This study uncovers a previously unrecognized mode of tRNA recognition by an ancient protein fold, and provides insights into protein-mediated tRNA aminoacylation, folding, localization, trafficking, and piracy.

Synergistic inhibition mechanism of quinazolinone and piperacillin on penicillin-binding protein 2a: a promising approach for combating methicillin-resistant Staphylococcus aureus

The production of penicillin-binding protein 2a (PBP2a), a cell wall synthesis protein, is primarily responsible for the high-level resistance observed in methicillin-resistant Staphylococcus aureus (MRSA). PBP2a exhibits a significantly reduced affinity for most β-lactam antibiotics owing to its tightly closed active site. Quinazolinones (QNE), a novel class of non-β-lactam antibiotics, could initiate the allosteric regulation of PBP2a, resulting in the opening of the initially closed active pocket. Based on our previous study, we have a basic understanding of the dual-site inhibitor ceftaroline (CFT) induced allosteric regulation of PBP2a. However, there are still limitations in the knowledge of how combining medicines, QNE and piperacillin (PIP), induce the allosteric response of PBP2a and inhibit its function. Herein, molecular dynamics (MD) simulations were performed to elucidate the intricate mechanisms underlying the combination mode of QNE and PIP. Our study successfully captured the opening process of the active pocket upon the binding of the QNE at the allosteric site, which alters the signaling pathways with a favorable transmission to the active site. Subsequent docking experiments with different conformational states of the active pocket indicated that all three inhibitors, PIP, QNE, and CFT, exhibited higher docking scores and more favorable docking poses to the open active pocket. These findings reveal the implied mechanism of QNE-mediated allostery underlying combination therapy and provide novel insights into developing innovative therapeutic modalities against MRSA.Communicated by Ramaswamy H. Sarma.

Recognition of the tRNA structure: Everything everywhere but not all at once

tRNAs are among the most abundant and essential biomolecules in cells. These spontaneously folding, extensively structured yet conformationally flexible anionic polymers literally bridge the worlds of RNAs and proteins, and serve as Rosetta stones that decipher and interpret the genetic code. Their ubiquitous presence, functional irreplaceability, and privileged access to cellular compartments and ribosomes render them prime targets for both endogenous regulation and exogenous manipulation. There is essentially no part of the tRNA that is not touched by another interaction partner, either as programmed or imposed by an external adversary. Recent progresses in genetic, biochemical, and structural analyses of the tRNA interactome produced a wealth of new knowledge into their interaction networks, regulatory functions, and molecular interfaces. In this review, I describe and illustrate the general principles of tRNA recognition by proteins and other RNAs, and discuss the underlying molecular mechanisms that deliver affinity, specificity, and functional competency.