Trailer Hitch tunes condensate phase behavior and mRNA partitioning to regulate P-body homeostasis during Drosophila melanogaster oogenesis

Processing bodies (P-bodies) are cytoplasmic granules that regulate mRNA storage, repression, and decay, yet how their internal organization supports selective mRNA regulation remains poorly understood. Here, we show that the conserved LSm protein Trailer Hitch (Tral) is a key organizer of P-body architecture and function in the Drosophila melanogaster female germline. Using quantitative confocal imaging, super-resolution microscopy, and chemical perturbation of intermolecular interactions, we demonstrate that Tral coordinates the incorporation and spatial organization of the core P-body proteins Me31B and Cup. Loss of Tral alters their partitioning into P-bodies, promotes demixing into distinct subdomains, and shifts condensates toward a less dynamic, structurally heterogeneous state. These organizational changes have functional consequences for mRNA storage: Tral depletion selectively releases maternal mRNA bicoid, while nanos mRNA remains P-body associated and stable. We further identify twinstar mRNA, encoding the actin regulator Cofilin, as a Tral-dependent P-body client whose localization and organization within P-bodies requires Tral:RNA interactions and electrostatic forces. Reduced twinstar mRNA levels in the absence of Tral are associated with decreased nuclear G-actin and altered transcription of me31B and cup, revealing a potential feedback mechanism that links cytoplasmic P-body organization to nuclear gene expression. Together, these findings establish Tral as a central regulator of P-body architecture that couples condensate organization to selective mRNA regulation and transcriptional homeostasis.