Systematic genetic identification of functional domains on collided di-ribosomes responsible for rescue pathways upon translation arrest in Saccharomyces cerevisiae.

Translation elongation becomes arrested when various obstacles arise, such as a series of inefficient rare codons or stable RNA secondary structures, thus, causing ribosomal stalling along the mRNA. Certain wasteful and persistent stalling states are resolved by ribosome rescue pathways. For instance, collisions between stalled and subsequent ribosomes are thought to induce ubiquitination of ribosomal S20 protein by the E3 ubiquitin ligase Hel2, which triggers subsequent rescue reactions. Although structural studies have revealed specific contact sites between collided ribosomes, the ribosomal regions crucial for the rescue reaction remain uncharacterized. In this study, we performed a systematic genetic analysis to identify the molecular regions required for ribosome rescue in Saccharomyces cerevisiae. A series of dominant negative mutations capable of abolishing the rescue reaction were isolated in ribosomal proteins S20 and Asc1. Moreover, mutations in both proteins clustered on the surface of ribosomes between the collided ribosome interfaces, aligned in such a way that they seemingly faced each other. Further analysis via application of the split-TRP1 protein assay, revealed that mutation of either protein distinctively affected the functional interaction between Hel2 and Asc1, suggesting the development of differential functionality at the interface between collided ribosomes. Our results provide novel and complementary insights into the detailed molecular mechanisms of ribosomal rescue pathways.