Spatially distinct FRL and Ena dependent actin networks coordinate nuclear positioning in Drosophila nurse cells

Position of the nucleus is dynamically controlled to ensure a variety of cellular functions in a broad range of organisms form yeast to human. Nuclear positioning in Drosophila nurse cells is crucial during dumping when cells transfer their entire cytoplasmic content into the oocyte. An important prerequisite of effective dumping is the formation of an array of actin cables which holds the nucleus in a central position, thereby allowing transmission of the cytoplasmic cargo. Here we report the identification of FRL, a formin type of actin assembly factor, as a novel determinant of cytoplasmic actin bundle formation. We found that FRL and the formerly described Ena protein display a differential requirement. Comparison of the frl and ena loss of function situations revealed that FRL is mainly required for creation of the cytoplasmic actin subpopulation at stage 10B, while Ena mostly promotes formation of a ring canal attached actin array, already present at stage 7 and persists till dumping. Upon the concurrent absence of FRL and Ena the nuclear positioning actin cables are completely missing, strongly suggesting that nuclear positioning in the nurse cells requires the coordinated action of two spatially distinct actin networks.