Essential nontranslational functions of tRNA synthetases

Abstract

Nontranslational functions of vertebrate aminoacyl tRNA synthetases (aaRSs), which catalyze the production of aminoacyl-tRNAs for protein synthesis, have recently been discovered. Although these new functions were thought to be 'moonlighting activities', many are as critical for cellular homeostasis as their activity in translation. New roles have been associated with their cytoplasmic forms as well as with nuclear and secreted extracellular forms that affect pathways for cardiovascular development and the immune response and mTOR, IFN-γ and p53 signaling. The associations of aaRSs with autoimmune disorders, cancers and neurological disorders further highlight nontranslational functions of these proteins. New architecture elaborations of the aaRSs accompany their functional expansion in higher organisms and have been associated with the nontranslational functions for several aaRSs. Although a general understanding of how these functions developed is limited, the expropriation of aaRSs for essential nontranslational functions may have been initiated by co-opting the amino acid-binding site for another purpose.

Figure 1. The non-translational functions of aaRSs

To a first approximation, aaRSs are organized in two groups in the cytoplasm of higher eukaryotes. Some are in a free form, while others are part of a high molecular weight multi-tRNA synthetase complex (MSC), which includes 3 scaffold proteins designated as MSC^p43, MSC^p38, and MSC^p18. These proteins not only are an essential part of the translation apparatus, but also have a myriad of cytoplasmic, nuclear and extra-cellular functions. For simplicity, the various synthetases are designated by single letters, using the standard abbreviations for amino acids.

Figure 2. Amino acid binding pocket has a vital role in some of the non-translational functions

Higher eukaryote tRNA synthetases are comprised of a catalytic domain (which is highly conserved through evolution), an RNA binding domain (for some synthetases, this domain recognizes the tRNA anticodon), and a new domain that is absent in lower organisms. The ancient amino acid-binding pocket of several aaRSs is essential for some of the non-translational functions. Ap₄A: diadenosine tetraphosphate; VE-cad: vascular endothelial cadherin; RagD: small guanosine triphosphatase D; ASK1: apoptosis signal-regulating kinase 1.

Figure 3. New domains that introduce and regulate non-translational functions

aaRSs appear to be unique in the degree to which they acquired new domains that are not essential for catalytic activity. In addition to the common leucine zipper and GST domains, most of the novel domains are restricted to the aaRS family and are not found in other proteins. Shown here are two of the domains that are unique to vertebrate aaRSs and their related functions in inflammation, development and angiogenesis. DNA-PKcs: DNA-dependent protein kinase, catalytic subunit; PARP-1: poly [ADP-ribose] polymerase 1; GAIT complex: gamma-IFN-activated inhibitor of translation (GAIT) complex.

Figure 4. Multifaceted aaRSs scaffold proteins

The three scaffold proteins that are required for the assembly of the MSC have diverse functions outside of the MSC. TNFα: Tumor necrosis factor-alpha; IL-6: interleukin-6; GP96: 96-kDa glycoprotein; NF: neurofilament; FBP: fuse-binding protein; TRAF2: TNF receptor-associated factor 2; USP29: ubiquitin specific peptidase 29; ATM/ATR: ataxia telangiectasia mutated (ATM) and ATR (ATM and Rad3-related) protein kinases.

Figure 5. New functions of aaRS fragments

Several aaRSs and MSC scaffold proteins are resected by proteolysis, alternative splicing, or alternative polyadenylation. Depicted here are examples where fragmentation activates a novel non-translational function (TyrRS, TrpRS), and a case where multiple fragments are generated from a single protein, with each fragment having a distinct activity (MSC^p43). In two examples, to achieve a homeostatic balance of downstream signaling, a novel activity associated with the full-length protein is inhibited by a fragment (EPRS^N2, MSC^p38-DX2) of the same protein (EPRS, MSC^p38). CXCR1,2: C-X-C chemokine receptor type1, 2.

Figure 6. Potential therapeutic interventions derived from aaRS non-translational functions

Animal models have established the potential therapeutic value of some of the aaRS -associated activities. (a) Administration of native full-length GlyRS or of MSC^p43 triggers a measurable response in tumor environments, where the proteins were initially discovered to be secreted as anti-tumor cytokines. (b) Fragments of aaRS and MSC scaffold proteins that display non-translational functions are potent in various disease model settings. The parts deleted in fragmentation are shown as light shadows and outlined in dashes. MSC^p43 has multiple fragments that each has a distinct activity. (c) Small molecules that inhibit or mimic the non-translational functions of aaRSs and associated proteins may have clinical utility. Compounds displaying such efficacy are now being reported, namely mimetics of MSC^p43, and MSC^p38-DX2 shRNA.