TRRAP enables MMB-dependent mitotic transcription and determines sensitivity to CHK1 inhibition.

Precise temporal activation of mitotic genes is essential for proper cell-cycle progression, yet the chromatin-based mechanisms coordinating this process remain incompletely understood. The B-MYB-MuvB (MMB) complex is a key regulator of mitotic gene expression and becomes hyperactivated following inhibition of the ATR-CHK1 pathway, resulting in MMB-dependent premature mitotic entry, replication catastrophe, and cell death. Here, we combine proximity-dependent proteomics with a targeted siRNA screen of chromatin regulators using CHK1 inhibitor-induced premature mitosis and DNA damage as functional readouts. We identify TRRAP as a critical regulator of MMB-dependent mitotic transcription. TRRAP physically associates with B-MYB, occupies MMB target promoters, and is required for expression of mitotic genes during both normal G2/M progression and CHK1 inhibitor-induced premature mitosis. Although activation of YAP signaling bypasses the G1 requirement for TRRAP, TRRAP remains essential for G2/M gene expression and mitotic progression. Together, these findings link TRRAP-dependent chromatin regulation to MMB activity and checkpoint sensitivity.