The role of the Ribosome Quality Control Complex (RQC) in Maintaining Protein Homeostasis in Yeast and Higher Eukaryotes

Protein biosynthesis is the most energy-consuming process during cellular proliferation and any event that interferes with protein production jeopardizes cell viability. It has been recently appreciated that cells have a number of surveillance mechanisms to counter the threat posed by translation failure. One such mechanism is provided by the ribosome quality control complex (RQC), which engages stalled ribosomes and facilitates the degradation of the partially synthesized nascent polypeptide. Mechanistic characterization of the RQC in yeast had demonstrated that the RQC tags partially synthesized polypeptides with C-terminal alanine and threonine (CAT) tails in a remarkable untemplated elongation reaction. However, the biological significance of the addition of CAT-tails had remained poorly understood. For my thesis work, I elucidated the role of CAT-tails in yeast and explored the mechanisms of translation quality control in higher eukaryotes.I discovered a surprising property of the core RQC ubiquitin ligase, Ltn1p, that marks the stalled nascent polypeptide for degradation. In contrast to most ubiquitin ligases, Ltn1p is extremely rigid and can only access a small number of amino acids right outside the ribosome exit tunnel. If this narrow window is depleted of lysine residues, the sites of ubiquitin addition, CAT-tailing becomes critical for polypeptide degradation. In particular, the addition of CAT-tails to the C-terminus of the stalled polypeptide can push out lysines sequestered in the ribosome exit tunnel, making those residues accessible to Ltn1p. My work has established that CAT-tailing provides a fail-safe mechanism for nascent polypeptide degradation that greatly expands the range of RQC-degradable substrates.Although the RQC has been well characterized in yeast, the composition and function of the mammalian complex remains poorly understood. I engineered an RQC reporter and used it to perform a genome-wide CRISPR screen. This unbiased approach allowed me to gain a comprehensive view of the mammalian RQC pathway, and to identify novel RQC components. I identified a mammalian-specific complex that translationally silences faulty mRNAs by blocking ribosome initiation. This novel quality control mechanism could prevent the formation of ribosome “traffic jams” on defective messages, alleviating the stalling burden in mammalian cells