Principles of ribosome-associated protein quality control during the synthesis of CFTR.

Prolonged translational arrests caused by defective mRNAs activate the ribosome-associated protein quality control (RQC) pathway, which marks harmful incomplete proteins for degradation. Multipass transmembrane proteins have increased propensity to be targeted by the RQC, raising the question of whether problems in transmembrane domain insertion and assembly can also cause RQC-eliciting translational arrests. Here, we investigated RQC-mediated quality control of CFTR, a large transmembrane protein mutated in cystic fibrosis. Reporter assays showed that although a fraction of nascent CFTR expressed in HEK293 cells arrested during translation and activated the RQC, multiple interventions compromising CFTR folding and membrane insertion did not exacerbate this response. CFTR translation abortion was also largely unaffected by regulators of translation kinetics such as codon usage, the ribosome collision sensor GCN1, and the SRP ER targeting complex. We propose that the RQC can be triggered by the inherent difficulties in synthesizing transmembrane segments, resulting from their inappropriate interaction with the protein synthesis machinery. Our study uncovers and characterizes a novel physiological role for the RQC in dealing with elongation-arrested transmembrane proteins.