Apicomplexa-specific tRip facilitates import of exogenous tRNAs into malaria parasites.

The malaria-causing Plasmodium parasites are transmitted to vertebrates by mosquitoes. To support their growth and replication, these intracellular parasites, which belong to the phylum Apicomplexa, have developed mechanisms to exploit their hosts. These mechanisms include expropriation of small metabolites from infected host cells, such as purine nucleotides and amino acids. Heretofore, no evidence suggested that transfer RNAs (tRNAs) could also be exploited. We identified an unusual gene in Apicomplexa with a coding sequence for membrane-docking and structure-specific tRNA binding. This Apicomplexa protein-designated tRip (tRNA import protein)-is anchored to the parasite plasma membrane and directs import of exogenous tRNAs. In the absence of tRip, the fitness of the parasite stage that multiplies in the blood is significantly reduced, indicating that the parasite may need host tRNAs to sustain its own translation and/or as regulatory RNAs. Plasmodium is thus the first example, to our knowledge, of a cell importing exogenous tRNAs, suggesting a remarkable adaptation of this parasite to extend its reach into host cell biology.

Differential effects of antofine N-oxide on solid tumor and leukemia cells.

We have studied the anti-cancer activities of antofine N-oxide isolated and purified from the medicinal plant Cynanchum vincetoxicum. Antofine N-oxide displayed a strong inhibitory effect on several solid tumor cell lines (glioblastoma, breast carcinoma and lung carcinoma) and on a T-cell leukemia cell line. Remarkably, its cytotoxic effect was considerably weaker in non-cancer cells. Antofine N-oxide was found to inhibit proliferation of the solid tumor cells whereas it caused apoptotic cell death in the leukemia cells. A microarray analysis after a short treatment revealed that the number of differentially expressed genes was considerably higher in solid tumor than in leukemia cells. Up-regulated genes in the three solid tumor cell lines include genes related to TNFα signaling, of which TNFα was among the most significantly induced. A functional analysis revealed that TNFR1 signaling was most likely activated in the solid tumor cells. The increased mRNA levels of several genes of this pathway (namely TNFα, TNFAIP3 and BIRC3) were confirmed by real-time quantitative PCR after different treatment durations. Finally a slight inhibition of NFκB-mediated transcription was observed in the same cells. Together our results suggest that inhibition of cell proliferation in solid tumor cells essentially occurs through TNFα signaling whereas this pathway is not activated in leukemia cells. Apoptotic cell death in the latter is induced by a distinct yet unknown pathway.

Low Complexity Regions behave as tRNA sponges to help co-translational folding of plasmodial proteins.

In most organisms, the information necessary to specify the native 3D-structures of proteins is encoded in the corresponding mRNA sequences. Translational accuracy and efficiency are coupled and sequences that are slowly translated play an essential role in the concomitant folding of protein domains. Here, we suggest that the well-known mechanisms for the regulation of translational efficiency, which involves mRNA structure and/or asymmetric tRNA abundance, do not apply to all organisms. We propose that Plasmodium, the parasite responsible for malaria, uses an alternative strategy to slow down ribosomal speed and avoid multidomain protein misfolding during translation. In our model, the abundant Low Complexity Regions present in Plasmodium proteins replace the codon preferences, which influence the assembly of protein secondary structures.

Plasmodial aspartyl-tRNA synthetases and peculiarities in Plasmodium falciparum.

Distinctive features of aspartyl-transfer RNA (tRNA) synthetases (AspRS) from the protozoan Plasmodium genus are described. These apicomplexan AspRSs contain 29-31 amino acid insertions in their anticodon binding domains, a remarkably long N-terminal appendix that varies in size from 110 to 165 amino acids and two potential initiation codons. This article focuses on the atypical functional and structural properties of Plasmodium falciparum cytosolic AspRS, the causative parasite of human malaria. This species encodes a 626 or 577 amino acids AspRS depending on whether initiation starts on the first or second in-frame initiation codon. The longer protein has poor solubility and a propensity to aggregate. Production of the short version was favored as shown by the comparison of the recombinant protein with endogenous AspRS. Comparison of the tRNA aminoacylation activity of wild-type and mutant parasite AspRSs with those of yeast and human AspRSs revealed unique properties. The N-terminal extension contains a motif that provides unexpectedly strong RNA binding to plasmodial AspRS. Furthermore, experiments demonstrated the requirement of the plasmodial insertion for AspRS dimerization and, therefore, tRNA aminoacylation and other putative functions. Implications for the parasite biology are proposed. These data provide a robust background for unraveling the precise functional properties of the parasite AspRS and for developing novel lead compounds against malaria, targeting its idiosyncratic domains.