An ASCL1 C-Terminal Phospho-Degron Control ASCL1 Protein Stability in Small Cell Lung Cancer

ASCL1 is a lineage transcription factor dependency in small cell lung cancer (SCLC) and other high-grade neuroendocrine cancers, but remains undruggable. To uncover mechanisms that control ASCL1 abundance, we combined genome-wide CRISPR knockout and base-editor tiling screens with an endogenous ASCL1 reporter, quantitative proteomics, and structural modeling. We identify HUWE1 as the dominant E3 ligase for ASCL1 in SCLC and define a conserved C-terminal phospho-degron centered on Ser207 and terminal Trp/Phe residues that is necessary for HUWE1 binding and ASCL1 protein degradation. Disrupting this degron prevents HUWE1 binding and hyperstabilizes ASCL1, locks it on chromatin, and paradoxically impairs SCLC proliferation. These findings reveal a more generalizable phospho-degron recognition module in HUWE1 as a targetable regulatory switch for ASCL1 protein degradation.