During their final maturation in the cytoplasm, pre-60S ribosomal particles are converted to translation-competent large ribosomal subunits. Here, we present the mechanism of peptidyltransferase centre (PTC) completion that explains how integration of the last ribosomal proteins is coupled to release of the nuclear export adaptor Nmd3. Single-particle cryo-EM reveals that eL40 recruitment stabilises helix 89 to form the uL16 binding site. The loading of uL16 unhooks helix 38 from Nmd3 to adopt its mature conformation. In turn, partial retraction of the L1 stalk is coupled to a conformational switch in Nmd3 that allows the uL16 P-site loop to fully accommodate into the PTC where it competes with Nmd3 for an overlapping binding site (base A2971). Our data reveal how the central functional site of the ribosome is sculpted and suggest how the formation of translation-competent 60S subunits is disrupted in leukaemia-associated ribosomopathies.
The lissoclimides are unusual succinimide-containing labdane diterpenoids that were reported to be potent cytotoxins. Our short semi-synthesis and analogue-oriented synthesis approaches have provided a series of lissoclimide natural products and analogues that expanded the structure-activity relationships in this family. The semi-synthesis approach yielded significant quantities of chlorolissoclimide that permitted evaluation against the NCI’s 60 cell line panel and allowed us to obtain an X-ray co-crystal structure of the synthetic secondary metabolite with the eukaryotic 80S ribosome. While it shares a binding site with other imide-based natural product translation inhibitors, chlorolissoclimide engages in a particularly interesting and novel face-on halogen-π interaction between the ligand’s alkyl chloride and a guanine residue. Our analogue-oriented synthesis provided many more lissoclimide compounds, which were tested against aggressive human cancer cell lines, and for protein synthesis inhibitory activity. Finally, computational modeling was used to explain the SAR of certain key compounds, setting the stage for structure-guided design of better translation inhibitors.
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