Modeling liquid-liquid phase separation and its impact on proteasomal substrate degradation

The proteasome, central to ubiquitin-proteasome mediated degradation of substrate proteins, undergoes liquid-liquid phase separation (LLPS) during substrate processing. However, the mechanisms governing proteasomal LLPS and its functional interplay with degradation dynamics remain elusive. Here, we developed a reaction-diffusion model with thermodynamic constraints to quantitatively characterize proteasome-associated LLPS mediated by the substrate-shuttling factor Radiation-sensitive 23 homolog B (RAD23B). The model successfully captures experimental LLPS dynamics, revealing a concentration-dependent phase transition boundary influenced by proteasome activity and molecular interactions. Significantly, LLPS was found to have bidirectional (enhancing or suppressing), context-dependent effects on degradation efficiency. This work elucidates regulatory mechanisms and functional roles of LLPS in proteasome-mediated degradation, offering a predictive quantitative model and testable hypotheses for future investigation.