Myosin-V regulates oskar mRNA localization in the Drosophila oocyte.

Intracellular mRNA localization is an effective mechanism for protein targeting leading to functional polarization of the cell. The mechanisms controlling mRNA localization and specifically how the actin and microtubule (MT) cytoskeletons cooperate in this process are not well understood. In Drosophila, Oskar protein accumulation at the posterior pole of the oocyte is required for embryonic development and is achieved by the transport of oskar mRNA and its exclusive translation at the posterior pole. oskar mRNA localization requires the activity of the MT-based motor Kinesin, as well as the formation of a transport-competent ribonucleoprotein (RNP) complex. Here, we show that didum, encoding the Drosophila actin-based motor Myosin-V, is a new posterior group gene that promotes posterior accumulation of Oskar. Myosin-V associates with the oskar mRNA transport complex preferentially at the oocyte cortex, revealing a short-range actomyosin-based mechanism that mediates the local entrapment of oskar at the posterior pole. Our results also show that Myosin-V interacts with Kinesin heavy chain and counterbalances Kinesin function, preventing ectopic accumulation of oskar in the cytoplasm. Our findings reveal that a balance of microtubule- and actin-based motor activities regulates oskar mRNA localization in the Drosophila oocyte.