Mechanism of orphan subunit recognition during assembly quality control

Cells contain numerous abundant molecular machines assembled from multiple subunits. Imbalances in subunit production and failed assembly generate orphan subunits that are eliminated by poorly defined pathways. Here, we determined how orphan subunits of the cytosolic chaperonin CCT are recognized. Several unassembled CCT subunits recruited the E3 ubiquitin ligase HERC2 using ZNRD2 as an adaptor. Both factors were necessary for orphan CCT subunit degradation in cells, sufficient for CCT subunit ubiquitination with purified factors, and necessary for optimal cell fitness. Domain mapping and structure prediction defined the molecular features of a minimal HERC2-ZNRD2-CCT module. The structural model, whose key elements were validated in cells using point mutants, shows why ZNRD2 selectively recognizes multiple orphaned CCT subunits without engaging assembled CCT. Our findings reveal how failures during CCT assembly are monitored and provide a paradigm for the molecular recognition of orphan subunits, the largest source of quality control substrates in cells.

Automated summary

Cells have multiple molecular machines formed from subunits, and imbalance in subunit production or assembly failure leads to orphan subunits that are eliminated. This study investigates how orphan subunits of CCT are recognized and degraded. It was found that unassembled CCT subunits recruit HERC2 and ZNRD2 to facilitate degradation, and this process is necessary for optimal cell fitness. The study provides insights into the molecular recognition and quality control of orphan subunits.