Klar modulates oskar RNP transport in the Drosophila oocyte. Michael A. Welte¹, Imre Gáspár², Yanxun V. Yu^1,3, Anne Ephrussi². 1) Dept Biol, University of Rochester, Rochester, NY; 2) Developmental Biology Unit, EMBL, Heidelberg, Germany; 3) Dept Biology, Brandeis University, Waltham, MA.

   In Drosophila oocytes, the plus-end motor kinesin-1 delivers osk RNP particles to the posterior pole, where they initiate germ-plasm assembly. Although many components important for osk delivery have been identified, it remains unclear how this process is temporally controlled. The Klarsicht protein regulates kinesin-1-driven lipid-droplet motion in early embryos and has been proposed to modulate transport efficiency. Here we find that the isoform of Klar is expressed during oogenesis and accumulates in distinct puncta at the posterior pole of mid-late stage oocytes, mimicking osk RNP accumulation. Klar co-immunoprecipitates with kinesin-1, and its accumulation at the posterior depends on kinesin-1 and microtubules, but not on osk or germ-plasm assembly. We conclude that Klar travels to the posterior pole with some kinesin-dependent cargoes. Klar puncta do not correspond to the previously known Klar cargoes nuclei and lipid droplets. Rather, we believe that one of these cargoes is osk RNPs: live imaging and particle tracking reveal that lack of Klar alters specific parameters of osk RNP motility; in particular, RNP run lengths are significantly increased. Using FISH analysis, we find that - compared to similarly staged wild-type oocytes - the osk RNP population is shifted towards the posterior pole in klar mutants. Based on this enhanced transport, we conclude that, in wild-type oocytes, Klar restricts osk RNP motion. In klar mutants, posterior osk RNA and Osk protein are not as tightly cortical as in the wild type, but spread out over a larger area, suggesting that prematurely delivered osk RNPs are inefficiently anchored at the cortex. Accumulation of germ-plasm components in ectopic dots suggests that this delocalization impairs germ-plasm assembly. We conclude that Klar synchronizes osk RNP arrival with the establishment of new anchoring sites. Thus, Klar plays a role in temporally integrating multiple processes during oocyte maturation.