The Double-Length Tyrosyl-tRNA Synthetase from the Eukaryote Leishmania major Forms an Intrinsically Asymmetric Pseudo-Dimer

Abstract

The single tyrosyl tRNA-synthetase (TyrRS) gene in trypanosomatid genomes codes for a protein that is twice the length of TyrRS from virtually all other organisms. Each half of the double-length TyrRS contains a catalytic domain and an anticodon-binding domain, however the two halves retain only 17% sequence identity to each other. The structural and functional consequences of this duplication and divergence are unclear. TyrRS normally forms a homodimer in which the active site of one monomer pairs with the anticodon-binding domain from the other. However, crystal structures of Leishmania major TyrRS show that instead the two halves of a single molecule form a pseudo-dimer resembling the canonical TyrRS dimer. Curiously, the C-terminal copy of the catalytic domain has lost the catalytically important HIGH and KMSKS motifs characteristic of Class I aminoacyl-tRNA synthetases. Thus the pseudo-dimer contains only one functional active site, contributed by the N-terminal half, and only one functional anticodon recognition site, contributed by the C-terminal half. Despite biochemical evidence for negative cooperativity between the two active sites of the usual TyrRS homodimer, previous structures have captured a crystallographically-imposed symmetric state. As the L. major TyrRS pseudo-dimer is inherently asymmetric, conformational variations observed near the active site may be relevant to understanding how the state of a single active site is communicated across the dimer interface. Furthermore, substantial differences between trypanosomal TyrRS and human homologs are promising for the design of inhibitors that selectively target the parasite enzyme.

Twin Attributes of Tyrosyl-tRNA Synthetase of Leishmania donovani

“…Fisetin was found to inhibit the parasite growth by inhibiting the LdTyrRS aminoacylation activity. This data are in agreement with a previous report that suggests binding of fisetin to the active site of trypanosomal TyrRS (13). Several subtle yet crucial differences between Leishmania and human TyrRS (13) could potentially be exploited for the design of structurebased inhibitors for LdTyrRS.…”

“…A recent comprehensive study highlighted the structural and functional aspect of TyrRS from Plasmodium falciparum (24). The structural analysis of the TyrRS orthologue from L. major reveals several crucial differences between the host and pathogen tyrosyl-tRNA synthetase active sites (13). These differences could potentially be exploited for the design of structure-based inhibitors of parasite TyrRSs.…”

“…The crystal structure of Leishmania major TyrRS has been recently solved, and it is known to exist as an asymmetric pseudodimer (13). In the present study, we report the catalytic promiscuity and moonlighting function of L. donovani TyrRS (LdTyrRS).…”

“…A co-crystal structure of resveratrol bound to the active site of human TyrRS has been reported (19). Another flavanoid compound fisetin has also been identified to bind TyrRS of L. major (13). To test the effi- cacy of these compounds on L. donovani, log phase promastigotes were cultured in the presence of increasing concentrations of fisetin and resveratrol.…”

Highlights on Trypanosomatid Aminoacyl-tRNA Synthesis

“…The crystal structures of the L. major TyrRS show that it is an intrinsically asymmetric pseudo-dimer that contains a functional active site only in the N-terminal "monomer" (Larson et al 2011a ) (Fig. 12.4 ).…”