The Spectraplakin Short Stop (Shot) Organizes an Acentrosomal Microtubule Network in Early Oogenesis, Essential for Nuclear Positioning.

Abstract

Nuclear positioning relies on the coordinated organization of cytoskeletal networks to generate and balance intracellular forces. In the Drosophila oocyte, the nucleus undergoes a centering phase during mid-oogenesis before migrating asymmetrically to the cortex in a microtubule (MT)-dependent manner, a process essential for dorso-ventral axis specification. However, the origin and organization of the MT network responsible for nuclear positioning remain to be fully understood. Here, we identify a previously uncharacterized non-centrosomal Microtubule-Organizing Center (ncMTOC) at the posterior cortex of the oocyte. This ncMTOC is defined by the polarized accumulation of the spectraplakin Short Stop (Shot) together with the MT minus- end-binding protein Patronin prior to nuclear migration. Loss of Shot or Patronin results in posterior displacement of the nucleus, demonstrating that this cortical MT array generates pushing forces that maintain the nucleus at the oocyte center before directional migration. Within this ncMTOC, Shot plays a key role in recruiting both the MT severing enzyme Katanin and the MT polymerase of the XMAP215 family protein Mini-spindle (Msps), ensuring the formation of MTs that emanate from the vicinity of the posterior plasma membrane towards the oocyte cytoplasm. In addition, this ncMTOCs is also associated with {gamma}-TuRC which function independently of centrosomes. Together, these activities establish a dynamic posterior acentrosomal MT network essential for nuclear centering during early oogenesis.