Identification of ubiquitin binding protein using TurboID analysis

Purines serve as the building blocks for DNA and RNA, confer cellular energy and signaling. Purines are generated by de novo synthesis pathway and salvage pathway. Under purine-depleted or other cellular stresses, enzymes in the purine de novo synthesis pathway form a dynamic and reversible condensate called purinosome, but the underlying mechanism of purinosome formation is unknown. In this thesis, we found that ASB11-based Cul5 E3 ligase promotes ubiquitination of PAICS, a purine de novo synthesis enzyme. This ubiquitination does not lead to PAICS degradation, but drives purinosome assembly. We provide evidence that purinosome assembly involves a liquid-liquid phase separation (LLPS) process and identify several ubiquitin binding proteins that may bind ubiquitinated PAICS through a multivalent mode to drive LLPS and purinosome assembly. Importantly, ASB11 is upregulated under the stresses that promote purinosome assembly, thus increasing the formation of ASB11/PAICS complex. Finally, we demonstrated that melanoma cells express a high level of ASB11 to confer a constitutive purinosome formation, which support their viability. In summary, our study identifies ASB11-mediated PAICS ubiquitination as a driving mechanism for purinosome assembly, the regulation of this mechanism under stressed conditions, and the importance of this regulation in cell viability.