Hierarchical assembly and functional resilience of the mammalian RNA exosome

Most eukaryotic proteins assemble into multisubunit complexes that coordinate essential cellular functions, yet the principles governing their assembly and proteostatic control remain largely undefined. Here, we systematically dissect the cellular assembly and functional organization of the RNA exosome, an essential ribonucleolytic complex, using an inducible dual-guide CRISPR/Cas9 system in mouse embryonic stem cells. We reveal a sequential assembly pathway where Exosc2, Exosc4, and Exosc7 initiate complex formation, facilitating the incorporation of barrel and cap subunits in a defined hierarchy. Unlike other structural subunits, the terminally incorporated cap subunit Exosc1 is dispensable for cell viability, revealing a modular, functionally resilient architecture. We demonstrate that orphan subunits are selectively degraded via the ubiquitin-proteasome system, enforcing stringent quality control over RNA exosome biogenesis. These findings establish a framework for decoding the assembly logic of essential macromolecular machines and uncover previously unrecognized plasticity in the composition and function of the RNA exosome.