Human trans-editing enzyme displays tRNA acceptor-stem specificity and relaxed amino acid selectivity

Accurate translation of genetic information into proteins is vital for cell sustainability. ProXp-ala prevents proteome-wide Pro-to-Ala mutations by hydrolyzing misacylated Ala-tRNAPro, which is synthesized by prolyl-tRNA synthetase. Bacterial ProXp-ala was previously shown to combine a size-based exclusion mechanism with conformational and chemical selection for the recognition of the alanyl moiety, whereas tRNAPro is selected via recognition of tRNA acceptor-stem elements G72 and A73. The identity of these critical bases changed during evolution with eukaryotic cytosolic tRNAPro possessing a cytosine at the corresponding positions. The mechanism by which eukaryotic ProXp-ala adapted to these changes remains unknown. In this work, recognition of the aminoacyl moiety and tRNA acceptor stem by human (Homo sapiens, or Hs) ProXp-ala was examined. Enzymatic assays revealed that Hs ProXp-ala requires C72 and C73 in the context of Hs cytosolic tRNAPro for efficient deacylation of mischarged Ala-tRNAPro. The strong dependence on these bases prevents cross-species deacylation of bacterial Ala-tRNAPro or of Hs mitochondrial Ala-tRNAPro by the human enzyme. Similar to the bacterial enzyme, Hs ProXp-ala showed strong tRNA acceptor-stem recognition but differed in its amino acid specificity profile relative to bacterial ProXp-ala. Changes at conserved residues in both the Hs and bacterial ProXp-ala substrate-binding pockets modulated this specificity. These results illustrate how the mechanism of substrate selection diverged during the evolution of the ProXp-ala family, providing the first example of a trans-editing domain whose specificity evolved to adapt to changes in its tRNA substrate.

Human trans-editing enzyme displays tRNA acceptor stem specificity and relaxed amino acid selectivity

Accurate translation of genetic information into proteins is vital for cell sustainability. ProXp-ala prevents proteome-wide Pro-to-Ala mutations by hydrolyzing misacylated Ala-tRNAPro, which is synthesized by prolyl-tRNA synthetase (ProRS). Bacterial ProXp-ala was previously shown to combine a size-based exclusion mechanism with conformational and chemical selection for the recognition of the alanyl moiety, while tRNAPro is selected via recognition of tRNA acceptor stem elements G72 and A73. The identity of these critical bases changed during evolution with eukaryotic cytosolic tRNAPro possessing a cytosine at the corresponding positions. The mechanism by which eukaryotic ProXp-ala adapted to these changes remains unknown. In this work, recognition of the aminoacyl moiety and tRNA acceptor stem by human (Hs) ProXp-ala was examined. Enzymatic assays revealed that Hs ProXp-ala requires C72 and C73 in the context of Hs cytosolic tRNAPro for efficient deacylation of mischarged Ala-tRNAPro. The strong dependence on these bases prevents cross-species deacylation of bacterial Ala-tRNAPro or of Hs mitochondrial Ala-tRNAPro by the human enzyme. Similar to the bacterial enzyme, Hs ProXp-ala showed strong tRNA acceptor-stem recognition but differed in its amino acid specificity profile relative to bacterial ProXp-ala. Changes at conserved residues in both the Hs and bacterial ProXp-ala substrate binding pockets modulated this specificity. These results illustrate how the mechanism of substrate selection diverged during the evolution of the ProXp-ala family and provides the first example of a trans-editing domain whose specificity evolved to adapt to changes in its tRNA substrate.

Coding triplets in the tRNA acceptor-TΨC arm and their role in present and past tRNA recognition

The mechanism and evolution of the recognition scheme between key components of the translation system, i.e., tRNAs, synthetases and elongation factors, are fundamental issues in understanding the translation of genetic information into proteins. Statistical analysis of bacterial tRNA sequences reveals that for six amino acids, i.e. for Ala, Asp, Gly, His, Pro and Ser, a string of 10 nucleotides preceding the tRNA 3’end, carries cognate coding triplets to nearly full extent. The triplets conserved in positions 63-67 are implicated in the recognition by EF-Tu, and those conserved in positions 68-72, in the identification of cognate tRNAs and their derived minihelices, by class IIa synthetases. These coding triplets are suggested to have primordial origin, being engaged in aminoacylation of prebiotic tRNAs and in the establishment of the canonical codon set.

D-amino acid-mediated translation arrest is modulated by the identity of the incoming aminoacyl-tRNA

A complete understanding of the determinants that restrict D-amino acid incorporation by the ribosome, which is of interest to both basic biologists as well as the protein engineering community, remains elusive. Previously, we demonstrated that D-amino acids are successfully incorporated into the C-terminus of the nascent polypeptide chain. Ribosomes carrying the resulting peptidyl-D-aminoacyl-tRNA (peptidyl-D-aa-tRNA) donor substrate, however, partition into subpopulations that either undergo translation arrest through inactivation of the ribosomal peptidyl-transferase center (PTC) or remain translationally competent. The proportion of each subpopulation is determined by the identity of the D-amino acid sidechain. Here, we demonstrate that the identity of the aminoacyl-tRNA (aa-tRNA) acceptor substrate that is delivered to ribosomes carrying a peptidyl-D-aa-tRNA donor further modulates this partitioning. Our discovery demonstrates that it is the pairing of the peptidyl-D-aa-tRNA donor and the aa-tRNA acceptor that determines the activity of the PTC. Moreover, we provide evidence that both the amino acid and tRNA components of the aa-tRNA donor contribute synergistically to the extent of arrest. The results of this work deepen our understanding of the mechanism of D-amino acid-mediated translation arrest and how cells avoid this precarious obstacle, reveal similarities to other translation arrest mechanisms involving the PTC, and provide a new route for improving the yields of engineered proteins containing D-amino acids.