The Bicaudal-C repression domain couples translational silencing to maternal embryonic patterning

The maternal period of embryonic development occurs before zygotic transcription is active and therefore relies heavily on post-transcriptional mechanisms such as regulated mRNA translation. Central to this regulation are RNA-binding proteins (RBPs), yet how individual RBPs orchestrate maternal gene regulation remains poorly understood. Bicaudal-C (Bicc1) is a conserved RBP and translational repressor that selects mRNAs through N-terminal KH domains and represses their translation through C-terminal activities whose mechanisms remain unclear. In the maternal Xenopus embryo, Bicc1 represses mRNAs encoding cell-fate regulators to establish the anterior/posterior axis. Here, we define the activities of the Bicc1 repression domain (Bicc1-RD). We show that the RD is sufficient for poly(A)-independent translational repression and mediates interactions with diverse RNA regulatory factors, including Bicc1 itself, the CCR4-NOT complex, and the translational regulators Lsm14, 4E-T, and Ddx6. Functional analyses reveal that RD activities are essential for maternal Bicc1-dependent patterning but dispensable for its zygotic role in left/right patterning, demonstrating mechanistically distinct Bicc1 repression strategies across developmental stages. These findings reveal how a modular RBP can coordinate stage-specific translational control during embryogenesis.