Competition between kinesin-1 and myosin-V defines <i>Drosophila</i> posterior determination.

Abstract

Local accumulation of <i>oskar (osk</i>) mRNA in the <i>Drosophila</i> oocyte determines the posterior pole of the future embryo. Two major cytoskeletal components, microtubules and actin filaments, together with a microtubule motor, kinesin-1, and an actin motor, myosin-V, are essential for <i>osk</i> mRNA posterior localization. In this study, we use Staufen, an RNA-binding protein that colocalizes with <i>osk</i> mRNA, as a proxy for <i>osk</i> mRNA. We demonstrate that posterior localization of <i>osk</i>/Staufen is determined by competition between kinesin-1 and myosin-V. While kinesin-1 removes <i>osk</i>/Staufen from the cortex along microtubules, myosin-V anchors <i>osk</i>/Staufen at the cortex. Myosin-V wins over kinesin-1 at the posterior pole due to low microtubule density at this site, while kinesin-1 wins at anterior and lateral positions because they have high density of cortically-anchored microtubules. As a result, posterior determinants are removed from the anterior and lateral cortex but retained at the posterior pole. Thus, posterior determination of <i>Drosophila</i> oocytes is defined by kinesin-myosin competition, whose outcome is primarily determined by cortical microtubule density.