engleski

An adaptable aminoacyl-tRNA synthetase complex formation: gathering to improve decoding accuracy

Accurate reproduction of its finely adjusted proteinaceous machinery presents a major challenge for every cell. In every organism this task is imposed on a group of enzymes known as aminoacyl- tRNA synthetases (aaRS), ancient catalysts responsible for the esterification of a specific transfer RNA (tRNA) with a cognate amino acid. In evolutionary terms, the systematic pairing of the (anti)codon and the corresponding amino acid means the actual establishment of the genetic code and in that aspect, aaRSs stand at a crucial point associated with the origin of biology. The universality of the genetic code implicates that aaRSs developed from a common ancestral enzyme, presumably of minimal architecture. Modern aaRSs generally exert a modular topology, with additional domains found more often in the more evolved species. For some of these enzymes, the added domains are required for association with other cellular aaRSs thereby enhancing the efficiency of protein synthesis by a channel-ing mechanism. Many of the added domains also bestow the aaRSs with novel functions. Multisynthetase complexes (MSCs) are implicated in a wide variety of processes, such as transcriptional regulation, splicing and apoptosis. We have previously reported the existence of such complex in a methanogenic archaeon Methanothermobacter thermoautothophicus. This transient complex encompasses two aaRSs, atypical seryl- and arginyl-tRNA synthetase (ArgRS and SerRS, respectively). Due to the housekeeping nature of these enzymes, we wanted to explore the complex’ stability upon addition of the cognate substrates. While the serylation reaction precursors do not influence the complex formation, the assembly directly responds to the addition of fully modified tRNAArg, forcing ArgRS to dissociate from its protein partner, SerRS. Mutated ArgRS variants lacking N- terminal domain were shown to be incapable for SerRS binding. Interestingly, SerRS binding propensity of ArgRS decreases gradually with N- terminus’ shortening, revealing that this tRNA binding element serves also as a SerRS docking platform. Implication that SerRS and tRNAArg recognize not only the same region, but also the same surface residues of ArgRS was further demonstrated by competitive behaviour of tRNAArg and SerRS toward ArgRS binding. Since cellular homeostasis directly depends on aaRS’s decoding activity, several partially modified tRNAArg were tested for the arginylation reaction in the presence of SerRS. Although ArgRS itself barely distinguishes the various states of modification of its cognate tRNA, SerRS is able to inhibit arginylation of poorer substrates. Since near anticodon modifications in arginine system are well known to restrict the binding to the specific mRNA codon we show here how a second aaRS can act in trans as an additional safeguarding switch improving decoding accuracy.

aminoacyl-tRNA synthetases; multisynthetase complexes

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