UCSF

Ribosome-associated Quality Control (RQC) pathways protect cells from toxicity caused by incomplete protein products resulting from translation of damaged or problematic mRNAs.

One component of the RQC pathway, Ltn1p, is an E3 ubiquitin ligase that targets partially synthesized polypeptides for ubiquitin-proteasome mediated degradation. A second core RQC component, Rqc2p, not only nucleates the complex formation but also modifies the nascent polypeptide by adding a carboxyl-terminal alanine and threonine (CAT) tail through a noncanonical elongation reaction. We found that Ltn1p can efficiently access only nascent-chain lysines immediately proximal to the ribosome exit tunnel. For substrates without Ltn1p-accessible lysines, CAT-tailing by Rqc2p expanded the range of RQC-degradable substrates by exposing lysines sequestered in the ribosome exit tunnel.

Although the core RQC factors are conserved from yeast to human, much less is known about the mammalian counterparts. Interestingly, the phenotypes observed with loss of RQC in mammals are more severe and include cytotoxicity at a cellular level, and neurodegeneration at an organismal level. Due to the increased requirement for RQC in mammals, we used CRISPR-Cas9-based screening to search for additional RQC strategies in mammals. We found that failed translation leads to specific inhibition of translation initiation on that message. This negative feedback loop is mediated by two translation inhibitors, GIGYF2 and 4EHP. Both model substrates and growth-based assays established that inhibition of additional rounds of translation acts in concert with known RQC pathways to prevent buildup of toxic proteins. Inability to block translation of faulty mRNAs, and subsequent accumulation of partially synthesized polypeptides, could explain the neurodevelopmental and neuropsychiatric disorders observed in mice and humans with compromised GIGYF2 function.