An architectural role of oskar mRNA in granule assembly

Ribonucleoprotein (RNP) granules are membraneless condensates that organize the intracellular space by compartmentalization of specific RNAs and proteins1. Studies have shown that RNA tunes the phase behavior of RNA binding proteins (RBPs)2-4, but the role of intermolecular RNA-RNA interactions in assembly of RNP granules in vivo remains less explored5-7. Here, we determine the role of a sequence-specific RNA-RNA kissing-loop interaction in assembly of mesoscale oskar RNP granules in the female Drosophila germline. A two-nucleotide mutation that disrupts kissing-loop-mediated oskar mRNA dimerization impairs condensate formation in vitro, oskar granule assembly in the developing oocyte - leading to defective posterior localization of the RNA, and abrogation of oskar-associated processing bodies (P-bodies) upon nutritional stress. This specific trans RNA-RNA interaction acts synergistically with the scaffold RBP, Bruno8, in driving condensate assembly. Our study highlights the architectural contribution of an mRNA and its specific secondary structure and tertiary interactions in formation of an RNP granule essential for embryonic development.