Quality Control in Ribosome Assembly

Seminar Details

Host: Climate Committee

Time: 4:00 pm- 5:00 pm

Location: BICH 108

Seminar Abstract

Defects in ribosome assembly can result in defective subunit formation, and if these subunits enter translation, they can lead to aberrant translation. How cells detect defective ribosomes and whether detection occurs during assembly or translation are largely unanswered questions. We recently reported that Reh1, which occupies the ribosome exit tunnel, is the last assembly factor to be released from the nascent large subunit. We suggested that its release is dependent on the growing polypeptide chain and proposed that Reh1 monitors the function of nascent subunits. To test this, we developed a system for monitoring ribosome surveillance during biogenesis or translation. In yeast, the insertion of uL16 (Rpl10) completes the catalytic center of the large subunit. Deletion of the P-site loop of uL16, which interacts with A and P site ligands, is highly toxic and leads to arrest of assembly. However, simultaneous mutations in the assembly factors Nmd3 and Tif6, relieved the toxicity and allowed the mutant ribosomes to engage in translation. These mutant ribosomes are strongly defective in translation and predominantly occupy the first few codons of open reading frames. We found that the defective ribosomes are predominantly monitored during assembly; once they enter translation, they are stabilized. Furthermore, deletion of REH1 significantly stabilized nascent ribosomes in the last steps of assembly. Thus, Reh1 is a quality control factor for nascent large subunits but contrary to our expectations it appears to sense defective ribosomes during the last step of assembly and not during translation.