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Model for quality control (QC) during CCT biogenesis. Depending on the step at which biogenesis fails, qualitatively different types of QC act on the substrate. This includes ribosome-associated quality control (RQC) if CCT translation fails, protein QC if a CCT subunit fails to fold after translation, and assembly QC for several CCT subunits that fold but do not assemble into the CCT. Unassembled CCT subunits can be recognized by ZNRD2, which engages the RLD3 domain of the E3 ligase HERC2, and ubiquitinates the subunit(s) to target it for degradation. ZNRD2 can also recognize partially assembled complexes, but the triage of these complexes between degradation and assembly remains to be investigated. ZNRD2 cannot engage assembled CCT because its interaction surfaces are buried.

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