Trailer Hitch coordinates P-body organization and facilitates transcript-specific mRNA regulation through nuclear actin-mediated feedback loop

Processing bodies (P-bodies) are dynamic, membraneless organelles that mediate mRNA storage, translational repression, and decay. While the roles of individual P-body proteins in transcript recruitment are well characterized, how the emergent biophysical properties of P-bodies contribute to selective mRNA regulation remains poorly understood. Here, we identify the RNA-binding protein Trailer Hitch (Tral) as a key regulator of P-body composition and physical state during Drosophila melanogaster oogenesis. Loss of Tral disrupts P-body structure, leading to elevated levels of Cup and reduced levels of Me31B. This compositional shift is driven by degradation of twinstar mRNA, which encodes an actin regulator, resulting in reduced nuclear actin levels and altered transcription of P-body components. Using super-resolution microscopy, RNAi-mediated knockdowns, and chemical treatments, we show that Tral is also essential for transcript-specific mRNA partitioning into P-bodies. We find that in Tral-depleted egg chambers, twinstar mRNA exhibits reduced and mislocalized P-body association, bicoid mRNA dissociates from P-bodies and is degraded, and nanos mRNA remains stably localized. This suggests that selective mRNA retention within P-bodies is governed by a network of molecular interactions, including electrostatic forces, hydrophobic contacts, and direct protein:RNA binding, which are tuned by Tral. Together, our findings position Tral as a central coordinator of P-body autoregulation, integrating transcript stability, nuclear actin dynamics, and condensate organization to govern selective mRNA partitioning.