Competition between kinesin-1 and myosin-V defines posterior determination.

Local accumulation of ) mRNA in the 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 mRNA posterior localization. In this study, we use Staufen, an RNA-binding protein that colocalizes with mRNA, as a proxy for mRNA. We demonstrate that posterior localization of /Staufen is determined by competition between kinesin-1 and myosin-V. While kinesin-1 removes /Staufen from the cortex along microtubules, myosin-V anchors /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 oocytes is defined by kinesin-myosin competition, whose outcome is primarily determined by cortical microtubule density.