Stress response and regulation of translation
Translation termination releases ribosomal subunits. Under normal conditions, they are reused for the initiation of new rounds of translation. Under stress conditions, these subunits associate to form inactive ribosomes, stabilized by the presence of the protein Stm1. When conditions become favorable, the Dom34/Hbs1 complex and the Rli1 protein dissociate inactive ribosomes to regenerate ribosomal subunits that are then used to resume translation.
Jan. 14, 2014
When exposed to stress, cells concentrate their resources to respond to this new constraint. They notably block their protein production. In a study published on January 14th in the EMBO Journal, the team of Bertrand Séraphin, assisted by the team of Rachel Green in the Johns Hopkins University (USA), identified for the first time a molecular mechanism that remobilizes ribosomes after stress. They also showed that, in normal growth conditions, this same mechanism enables the control of protein production by regulating the initiation of translation.
Stress response and blocking protein synthesis
When exposed to stress, cells must mobilize their resources to fight against these harmful conditions. In many cases (lack of nutrients, osmotic shock, chemical attack...), stressed cells block the process of protein translation, which consumes much of the available energy, in order to reallocate their resources on critical processes for their survival. Translation is catalyzed by the ribosomes. Both subunits of these large ribonucleoprotein complexes are released when translation is completed, allowing their reassembly during initiation of new translation events. In eukaryotic cells, under conditions of stress, ribosomal subunits are locked together and form inactive ribosomes. A few years ago, studies by the group of Marat Yusupov at IGBMC demonstrated that the interaction of subunits present in inactive ribosomes is enhanced by a factor called Stm1. When conditions become favorable, ribosomal subunits stored in inactive ribosomes have to be separated in order to allow translation recovery and cell growth.
Dissociation mechanism of the ribosome and translation recovery
The team of Bertrand Séraphin focused on the process allowing cells to dissociate inactive ribosomes when the stress conditions disappear. This group had previously characterized a protein complex formed by Dom34 and Hbs1 factors involved in the separation and removal of ribosomes blocked during translation. In their new study published in the EMBO Journal and carried out in collaboration with the team of Rachel Green of Johns Hopkins University (USA), Bertrand Séraphin’s team reveals that Dom34/Hbs1 complex also contributes to the mobilization of the inactive ribosomes accumulated because of a stress. Biochemical and in vivo studies show that in the presence of the Rli1 factor, Dom34 and Hbs1 are able to dissociate the ribosomes bound to Stm1, participating in the recovery of protein synthesis and thus cell growth.
Interestingly, the process of controlling the availability of ribosomal subunits also operates in normal cell growth conditions. Beyond the stress response, this study reveals an unexpected mechanism by which cells can control the initiation of translation by modulating the availability of ribosomal subunits.