Hsp40 co-chaperone Droj2 temporally regulates EcRA dynamics during collective cell migration.

Steroid hormone signaling coordinates developmental transitions and tissue morphogenesis, yet how its activity is temporally tuned during collective cell migration remains poorly understood. Here, we identify the Hsp40 co-chaperone Droj2 as a novel regulator of ecdysone signaling during Drosophila border cell migration. Through a forward genetic modifier screen, droj2 emerged as a potent enhancer of Taiman-mediated ecdysone signaling activity. Functional analyses revealed that loss of droj2 disrupted border cell detachment and directional migration, with earlier depletion causing progressively more severe defects, indicating a critical temporal requirement during migration. Mechanistically, Droj2 depletion selectively triggered precocious downregulation of the nuclear hormone receptor isoform EcRA and premature activation of the ecdysone signaling reporter, EcRE-lacZ, during oogenesis, whereas EcRB1 and Taiman expression remained unchanged. In addition to the ovary, Droj2 was also required for proper EcRA nuclear accumulation in larval salivary glands, suggesting a conserved role in coordinating developmental steroid hormone responses across tissues. Together, our findings identify Droj2 as a spatiotemporal regulator of EcRA dynamics that couples chaperone activity to steroid hormone responsiveness during collective migration. This study uncovers a previously unrecognized mechanism linking molecular chaperones with nuclear receptor regulation and developmental timing during tissue morphogenesis.