Rqc1 and Ltn1 Prevent C-terminal Alanine-Threonine Tail (CAT-tail)-induced Protein Aggregation by Efficient Recruitment of Cdc48 on Stalled 60S Subunits.

Protein homeostasis is maintained by quality control mechanisms that detect and eliminate deficient translation products. Cytosolic defective proteins can arise from translation of aberrant mRNAs lacking a termination codon (NonStop) or containing a sequence that blocks translation elongation (No-Go), which results in translational arrest. Stalled ribosomes are dissociated, aberrant mRNAs are degraded by the cytoplasmic exosome, and the nascent peptides remaining in stalled 60S exit tunnels are detected by the ribosome-bound quality control complex (RQC) composed of Ltn1, Rqc1, Rqc2, and Cdc48. Whereas Ltn1 polyubiquitylates these nascent peptides, Rqc2 directs the addition of C-terminal alanine-threonine tails (CAT-tails), and a Cdc48 hexamer is recruited to extract the nascent peptides, which are addressed to the proteasome for degradation. Although the functions of most RQC components have been described, the role of Rqc1 in this quality control process remains undetermined. In this article we show that the absence of Rqc1 or Ltn1 results in the aggregation of aberrant proteins, a phenomenon that requires CAT-tail addition to the nascent peptides by Rqc2. Our results suggest that aberrant CAT-tailed protein aggregation results from a defect in Cdc48 recruitment to stalled 60S particles, a process that requires both Rqc1 and Ltn1. These protein aggregates contain Ltn1-dependent polyubiquitin chains and are degraded by the proteasome. Finally, aggregate characterization by proteomics revealed that they contain specific chaperones including Sis1, Sgt2, Ssa1/2, and Hsp82, suggesting that these protein aggregates may be addressed to aggresome-like structures when the RQC complex fails to deliver aberrant nascent peptides to the proteasome for degradation.