THE 95F UNCONVENTIONAL MYOSIN IS REQUIRED FOR PROPER ORGANIZATION OF THE DROSOPHILA SYNCYTIAL BLASTODERM

The 95F myosin, a class VI unconventional myosin, associates with particles in the cytoplasm of the Drosophila syncytial blastoderm and is required for the ATP- and F-actin-dependent translocation of these particles. The particles undergo a cell cycle-dependent redistribution from domains that surround each nucleus in interphase to transient membrane invaginations that provide a barrier between adjacent spindles during mitosis. When 95F myosin function is inhibited by antibody injection, profound defects in syncytial blastoderm organization occur. This disorganization is seen as aberrant nuclear morphology and position and is suggestive of failures in cytoskeletal function. Nuclear defects correlate with gross defects in the actin cytoskeleton, including indistinct actin caps and furrows, missing actin structures, abnormal spacing of caps, and abnormally spaced furrows. Three-dimensional examination of embryos injected with anti-95F myosin antibody reveals that actin furrows do not invaginate as deeply into the embryo as do normal furrows. These furrows do not separate adjacent mitoses, since microtubules cross over them. These inappropriate microtubule interactions lead to aberrant nuclear divisions and to the nuclear defects observed. We propose that 95F myosin function is required to generate normal actin-based transient membrane furrows. The motor activity of 95F myosin itself and/or components within the particles transported to the furrows by 95F myosin may be required for normal furrows to form.