PITHD1: An Endogenous Inhibitor of the 26S Proteasome During Cellular Dormancy

Cellular dormancy represents a state of regulated growth arrest essential for diverse biological processes, from reproduction to cancer progression. While mechanisms controlling protein synthesis in dormant cells have been identified, how cells regulate protein degradation during dormancy remains unclear. Using zebrafish oocytes, eggs and embryos as a model system, we discovered PITHD1 as an endogenous inhibitor of the 26S proteasome. Our high-resolution cryoEM structure reveals that PITHD1 simultaneously blocks three crucial functional sites on the 19S regulatory particle which are required for ubiquitin recognition, processing, and substrate translocation. This triple-lock mechanism effectively prevents protein degradation in dormant cells. Given PITHD1s evolutionary conservation across species, this mechanism likely represents a general strategy for reversible proteasome regulation during cellular dormancy. Our findings establish a new paradigm for controlling proteostasis in quiescent states.