Aminoacyl-tRNA synthetases are essential enzymes that transmit information from the genetic code to proteins in cells and are targets for antipathogen drug development. Elucidation of the crystal structure of cytoplasmic lysyl-tRNA synthetase from the malaria parasite Plasmodium falciparum (PfLysRS) has allowed direct comparison with human LysRS. The authors' data suggest that PfLysRS is dimeric in solution, whereas the human counterpart can also adopt tetrameric forms. It is shown for the first time that PfLysRS is capable of synthesizing the signalling molecule Ap4a (diadenosine tetraphosphate) using ATP as a substrate. The PfLysRS crystal structure is in the apo form, such that binding to ATP will require rotameric changes in four conserved residues. Differences in the active-site regions of parasite and human LysRSs suggest the possibility of exploiting PfLysRS for selective inhibition. These investigations on PfLysRS further validate malarial LysRSs as attractive antimalarial targets and provide new structural space for the development of inhibitors that target pathogen LysRSs selectively.

Summary microRNAs were recently found to be regulators of the host response to infection by apicomplexan parasites. In this study, we identified two immunomodulatory microRNAs, miR-146a and miR-155, that were co-induced in the brains of mice challenged with Toxoplasma in a strain-specific manner. These microRNAs define a characteristic fingerprint for infection by type II strains, which are the most prevalent cause of human toxoplasmosis in Europe and North America. Using forward genetics, we showed that strain-specific differences in miR-146a modulation was in part mediated by the rhoptry kinase, ROP16. Remarkably, we found that miR-146a deficiency led to better control of parasite burden in the gut, and likely of early parasite dissemination in the brain tissue, resulting in the long-term survival of mice.

Glutamine synthetases are ubiquitous, homo-oligomeric enzymes essential for nitrogen metabolism. Unlike types I and II, which are well described both structurally and functionally, the larger, type IIIs are poorly characterized despite their widespread occurrence. An understanding of the structural basis for this divergence and the implications for design of type-specific inhibitors has, therefore, been impossible. The first crystal structure of a GSIII enzyme, presented here, reveals a conservation of the GS catalytic fold but subtle differences in protein-ligand interactions suggest possible avenues for the design GSIII inhibitors. Despite these similarities, the divergence of the GSIII enzymes can be explained by differences in quaternary structure. Unexpectedly, the two hexameric rings of the GSIII dodecamer associate on the opposite surface relative to types I and II. The diversity of GS quaternary structures revealed here suggests a nonallosteric role for the evolution of the double-ringed architecture seen in all GS enzymes.