Blasticidin S inhibits mammalian translation and enhances production of protein encoded by nonsense mRNA.

Deciphering translation is of paramount importance for the understanding of many diseases, and antibiotics played a pivotal role in this endeavour. Blasticidin S (BlaS) targets translation by binding to the peptidyl transferase center of the large ribosomal subunit. Using biochemical, structural and cellular approaches, we show here that BlaS inhibits both translation elongation and termination in Mammalia. Bound to mammalian terminating ribosomes, BlaS distorts the 3'CCA tail of the P-site tRNA to a larger extent than previously reported for bacterial ribosomes, thus delaying both, peptide bond formation and peptidyl-tRNA hydrolysis. While BlaS does not inhibit stop codon recognition by the eukaryotic release factor 1 (eRF1), it interferes with eRF1's accommodation into the peptidyl transferase center and subsequent peptide release. In human cells, BlaS inhibits nonsense-mediated mRNA decay and, at subinhibitory concentrations, modulates translation dynamics at premature termination codons leading to enhanced protein production.

Thallium(I) treatment induces nucleolar stress to stop protein synthesis and cell growth.

Thallium is considered as an emergent contaminant owing to its potential use in the superconductor alloys. The monovalent thallium, Tl(I), is highly toxic to the animals as it can affect numerous metabolic processes. Here we observed that Tl(I) decreased protein synthesis and phosphorylated eukaryotic initiation factor 2α. Although Tl(I) has been shown to interact with the sulfhydryl groups of proteins and cause the accumulation of reactive oxygen species, it did not activate endoplasmic reticulum stress. Notably, the level of 60S ribosomal subunit showed significant under-accumulation after the Tl(I) treatment. Given that Tl(I) shares similarities with potassium in terms of the ionic charge and atomic radius, we proposed that Tl(I) occupies certain K+-binding sites and inactivates the ribosomal function. However, we observed neither activation of ribophagy nor acceleration of the proteasomal degradation of 60S subunits. On the contrary, the ribosome synthesis pathway was severely blocked, i.e., the impairment of rRNA processing, deformed nucleoli, and accumulation of 60S subunits in the nucleus were observed. Although p53 remained inactivated, the decreased c-Myc and increased p21 levels indicated the activation of nucleolar stress. Therefore, we proposed that Tl(I) interfered the ribosome synthesis, thus resulting in cell growth inhibition and lethality.

Potent, Reversible, and Specific Chemical Inhibitors of Eukaryotic Ribosome Biogenesis.

All cellular proteins are synthesized by ribosomes, whose biogenesis in eukaryotes is a complex multi-step process completed within minutes. Several chemical inhibitors of ribosome function are available and used as tools or drugs. By contrast, we lack potent validated chemical probes to analyze the dynamics of eukaryotic ribosome assembly. Here, we combine chemical and genetic approaches to discover ribozinoindoles (or Rbins), potent and reversible triazinoindole-based inhibitors of eukaryotic ribosome biogenesis. Analyses of Rbin sensitivity and resistance conferring mutations in fission yeast, along with biochemical assays with recombinant proteins, provide evidence that Rbins' physiological target is Midasin, an essential ∼540-kDa AAA+ (ATPases associated with diverse cellular activities) protein. Using Rbins to acutely inhibit or activate Midasin function, in parallel experiments with inhibitor-sensitive or inhibitor-resistant cells, we uncover Midasin's role in assembling Nsa1 particles, nucleolar precursors of the 60S subunit. Together, our findings demonstrate that Rbins are powerful probes for eukaryotic ribosome assembly.

Structural basis for the inhibition of the eukaryotic ribosome.

The ribosome is a molecular machine responsible for protein synthesis and a major target for small-molecule inhibitors. Compared to the wealth of structural information available on ribosome-targeting antibiotics in bacteria, our understanding of the binding mode of ribosome inhibitors in eukaryotes is currently limited. Here we used X-ray crystallography to determine 16 high-resolution structures of 80S ribosomes from Saccharomyces cerevisiae in complexes with 12 eukaryote-specific and 4 broad-spectrum inhibitors. All inhibitors were found associated with messenger RNA and transfer RNA binding sites. In combination with kinetic experiments, the structures suggest a model for the action of cycloheximide and lactimidomycin, which explains why lactimidomycin, the larger compound, specifically targets the first elongation cycle. The study defines common principles of targeting and resistance, provides insights into translation inhibitor mode of action and reveals the structural determinants responsible for species selectivity which could guide future drug development.