• Publié par : Christian DELAMARCHE
  • Imprimer cette page

Translation mechanisms

Protein synthesis is an efficient and vital mechanism mediated by the ribosomes. In all living organisms, it allows an accurate correspondence between the genetic information and the newly synthesized polypeptides. The process of translation needs accurate quality-¬control systems to ensure the correct read¬out of the genetic data at the cellular level. Among them, bacteria did develop a specific mechanism referred to as “trans¬-translation”, ensuring the recycling of stalled translating ribosomes and the degradation of incomplete nascent proteins when incomplete messenger RNAs (mRNAs) are translated. tmRNA (transfer messenger RNA) and SmpB (small protein B) are the main components of that process. Recent biochemical, genetic and structural data provide insights on how the tmRNA-¬SmpB complex accomplishes its duty. However, few structural studies of ribosomes working actively during trans-translation have been achieved at the moment. Cryo-electron microscopy represents a method of choice in getting sharp snapshots of macromolecular complexes such as ribosomes during the process of translation. It allowed us to previously characterize the early events leading to the recruitment of tmRNA on stalled ribosomes. However no clear information of the following steps has been yet characterized by this way. This is why we propose during this project to get a deeper understanding of the structural links existing between tmRNA, SmpB and stalled ribosomes during the process, which should help to a better knowledge of the intricate links between trans-¬translation, bacterial survival and virulence.

By employing cryo-electron microscopy techniques and single particle reconstruction we will detail the transitions of the ribonucleoprotein complex into a stalled ribosome. By this way, we expect to visualize the topological rearrangements required to move a complex approximately six times larger than a tRNA.
This study will be combined to a collaborative project started with Dr V. Ramakrishnan (Joint winner of the 2009 chemistry Nobel Prize MRC-LMB, Cambridge, UK) on the crystal structures of similar ribosomal complexes at atomic resolution. This should lead to the presentation, in structural terms, of a complete mechanistic model of trans-translation. In some bacteria, this process is known to be required for virulence and the acquisition of such data are likely to make molecular actors of trans-translation possible therapeutic targets for future medicines.