Adenosine-Dependent Activation Mechanism of Prodrugs Targeting an Aminoacyl-tRNA Synthetase.

Prodrugs have little or no pharmacological activity and are converted to active drugs in the body by enzymes, metabolic reactions, or through human-controlled actions. However, prodrugs promoting their chemical bioconversion without any of these processes have not been reported before. Here, we present an enzyme-independent prodrug activation mechanism by boron-based compounds (benzoxaboroles) targeting leucyl-tRNA synthetase (LeuRS), including an antibiotic that recently has completed phase II clinical trials to cure tuberculosis. We combine nuclear magnetic resonance spectroscopy and X-ray crystallography with isothermal titration calorimetry to show that these benzoxaboroles do not bind directly to their drug target LeuRS, instead they are prodrugs that activate their bioconversion by forming a highly specific and reversible LeuRS inhibition adduct with ATP, AMP, or the terminal adenosine of the tRNALeu. We demonstrate how the oxaborole group of the prodrugs cyclizes with the adenosine ribose at physiological concentrations to form the active molecule. This bioconversion mechanism explains the remarkably good druglike properties of benzoxaboroles showing efficacy against radically different human pathogens and fully explains the mechanism of action of these compounds. Thus, this adenosine-dependent activation mechanism represents a novel concept in prodrug chemistry that can be applied to improve the solubility, permeability and metabolic stability of challenging drugs.

Analysis of relationships between peptide/MHC structural features and naive T cell frequency in humans.

The structural rules governing peptide/MHC (pMHC) recognition by T cells remain unclear. To address this question, we performed a structural characterization of several HLA-A2/peptide complexes and assessed in parallel their antigenicity, by analyzing the frequency of the corresponding Ag-specific naive T cells in A2(+) and A2(-) individuals, as well as within CD4(+) and CD8(+) subsets. We were able to find a correlation between specific naive T cell frequency and peptide solvent accessibility and/or mobility for a subset of moderately prominent peptides. However, one single structural parameter of the pMHC complexes could not be identified to explain each peptide antigenicity. Enhanced pMHC antigenicity was associated with both highly biased TRAV usage, possibly reflecting favored interaction between particular pMHC complexes and germline TRAV loops, and peptide structural features allowing interactions with a broad range of permissive CDR3 loops. In this context of constrained TCR docking mode, an optimal peptide solvent exposed surface leading to an optimal complementarity with TCR interface may constitute one of the key features leading to high frequency of specific T cells. Altogether our results suggest that frequency of specific T cells depends on the fine-tuning of several parameters, the structural determinants governing TCR-pMHC interaction being just one of them.

Steric hindrance and fast dissociation explain the lack of immunogenicity of the minor histocompatibility HA-1Arg Null allele.

The di-allelic HLA-A2 restricted minor histocompatibility Ag HA-1 locus codes for the highly immunogenic HA-1(His) and the nonimmunogenic HA-1(Arg) nonapeptides, differing in one amino acid. The HA-1(His) peptide is currently used for boosting the graft-vs-tumor responses after HLA matched HA-1 mismatched stem cell transplantation; usage of the HA-1(Arg) peptide would significantly enlarge the applicability for this therapy. Our studies on mechanisms causing the HA-1 unidirectional immunogenicity revealed marginal differences in proteasomal digestion, TAP translocation, and binding affinity, whereas both dissociation rates and structural analyses clearly showed marked differences in the stability of these two HLA-A2 bound alleles. These data provide a rationale for the lack of HA-1(Arg) peptide immunogenicity essential for the choice of tumor peptides for stem cell-based immunotherapeutic application.

Structural bases for the affinity-driven selection of a public TCR against a dominant human cytomegalovirus epitope.

Protective T cell responses elicited along chronic human CMV (HCMV) infections are sometimes dominated by CD8 T cell clones bearing highly related or identical public TCR in unrelated individuals. To understand the principles that guide emergence of these public T cell responses, we have performed structural, biophysical, and functional analyses of an immunodominant public TCR (RA14) directed against a major HLA-A*0201-restricted HCMV Ag (pp65(495-503)) and selected in vivo from a diverse repertoire after chronic stimulations. Unlike the two immunodominant public TCRs crystallized so far, which focused on one peptide hotspot, the HCMV-specific RA14 TCR interacts with the full array of available peptide residues. The conservation of some peptide-MHC complex-contacting amino acids by lower-affinity TCRs suggests a shared TCR-peptide-MHC complex docking mode and supports an Ag-driven selection of optimal TCRs. Therefore, the emergence of a public TCR of an oligoclonal Ag-specific response after repeated viral stimulations is based on a receptor displaying a high structural complementarity with the entire peptide and focusing on three peptide hotspots. This highlights key parameters underlying the selection of a protective T cell response against HCMV infection, which remains a major health issue in patients undergoing bone marrow transplantation.

Common themes in siRNA-mediated epigenetic silencing pathways.

Small interfering RNAs (siRNAs) act through RNA interference (RNAi) pathways to silence gene expression either at the transcriptional or post-transcriptional level. Here, we review mechanisms and functions of siRNA-mediated silencing pathways that promote chromatin modifications in the fission yeast Schizosaccharomyces pombe, plants and animals. In fission yeast, siRNAs are involved in heterochromatin formation and key aspects of the underlying siRNA-dependent pathway have been uncovered. Two RNAi complexes, the RNA-Induced Transcriptional Silencing complex (RITS), which contains a siRNA bound to an Argonaute protein, and the RNA-Directed RNA polymerase Complex (RDRC) are critical components of the pathway. In addition, this pathway implicates non-coding nascent transcripts synthesized by RNA polymerase II (RNApII) and the RNApII itself. In Arabidopsis thaliana, the RNA-directed DNA methylation (RdDM) pathway appears to share a similar set of proteins and enzymatic activities, suggesting that, beyond certain aspects that are specific to each pathway, part of the siRNA-mediated epigenetic silencing mechanisms are conserved between fission yeast and plants. Moreover, in both organisms the pathways target repetitive DNA sequences. This conservation of mechanisms and genomic targets might actually extend to animals as recent investigations revealed the existence of endogenous siRNA-based pathways directed against repetitive DNA sequences in flies and mammals.

Crystal structure of the IrrE protein, a central regulator of DNA damage repair in deinococcaceae.

Deinococcaceae are famous for their extreme radioresistance. Transcriptome analysis in Deinococcus radiodurans revealed a group of genes up-regulated in response to desiccation and ionizing radiation. IrrE, a novel protein initially found in D. radiodurans, was shown to be a positive regulator of some of these genes. Deinococcus deserti irrE is able to restore radioresistance in a D. radiodurans DeltairrE mutant. The D. deserti IrrE crystal structure reveals a unique combination of three domains: one zinc peptidase-like domain, one helix-turn-helix motif and one GAF-like domain. Mutant analysis indicates that the first and third domains are critical regions for radiotolerance. In particular, mutants affected in the putative zinc-binding site are as sensitive to gamma and UV irradiation as the DeltairrE bacteria, and radioresistance is strongly decreased with the H217L mutation present in the C-terminal domain. In addition, modeling of IrrE-DNA interaction suggests that the observed IrrE structure may not bind double-stranded DNA through its central helix-turn-helix motif and that IrrE is not a classic transcriptional factor that activates gene expression by its direct binding to DNA. We propose that the putative protease activity of IrrE could be a key element of transcription enhancement and that a more classic transcription factor, possibly an IrrE substrate, would link IrrE to transcription of genes specifically involved in radioresistance.