CDK2-Mediated Phosphorylation of Bombyx mori FADD Regulates S-Phase Progression and DNA Replication

As a fundamental life process, the cell cycle supports organismal development and genetic material stability. Phosphorylation is the principal molecular mechanism whereby the cyclin-dependent kinase (CDK)-cyclin complex triggers and regulates key events of the cell cycle, ensuring the temporally ordered progression of the cycle. Herein, Bombyx mori FAS-associated death domain (FADD) (BmFADD) was identified as a phosphorylation-dependent cell cycle regulator. We demonstrated that BmCDK2 phosphorylates BmFADD at serine 130 (Ser130), preventing its lysosomal-autophagic degradation and promoting a specific interaction (amino acids 69 to 135). This interaction drives the nuclear translocation of BmFADD and acts as a competitive regulator of BmCyclinA-BmCDK2 binding to prevent premature S-phase exit. Furthermore, we generated a transgenic silkworm strain with silk-gland-specific knockout of the BmFADD gene and found that loss of BmFADD activated mTOR, promoting endoreplication in silk gland cells and enhancing silk yield. Collectively, our results revealed a phosphorylation-dependent regulatory axis in which BmCDK2-mediated BmFADD phosphorylation at Ser130 competes against BmCyclinA that orchestrates S-phase progression. We confirmed that BmFADD modulates endoreplication through the mTOR signaling pathway. These findings expand the role of FADD in invertebrates, provide novel insights into posttranslational cell cycle regulation, and identify molecular targets for genetically engineering silkworm silk glands.