Klar ensures thermal robustness of oskar localization by restraining RNP motility.

Communication usually applies feedback loop-based filters and amplifiers to ensure undistorted delivery of messages. Such an amplifier acts during Drosophila melanogaster midoogenesis, when oskar messenger ribonucleic acid (mRNA) anchoring depends on its own locally translated protein product. We find that the motor regulator Klar β mediates a gain-control process that prevents saturation-based distortions in this positive feedback loop. We demonstrate that, like oskar mRNA, Klar β localizes to the posterior pole of oocytes in a kinesin-1-dependent manner. By live imaging and semiquantitative fluorescent in situ hybridization, we show that Klar β restrains oskar ribonucleoprotein motility and decreases the posterior-ward translocation of oskar mRNA, thereby adapting the rate of oskar delivery to the output of the anchoring machinery. This negative regulatory effect of Klar is particularly important for overriding temperature-induced changes in motility. We conclude that by preventing defects in oskar anchoring, this mechanism contributes to the developmental robustness of a poikilothermic organism living in a variable temperature environment.