The KASH domain protein MSP-300 plays an essential role in nuclear anchoring during Drosophila oogenesis.

During late stages of Drosophila oogenesis, the cytoplasm of nurse cells in the egg chamber is rapidly transferred ("dumped") to oocytes, while the nurse cell nuclei are anchored by a mechanism that involves the actin cytoskeleton. The factors that mediate this interaction between nuclei and actin cytoskeleton are unknown. MSP-300 is the likely Drosophila ortholog of the mammalian Syne-1 and -2 and C. elegans ANC-1 proteins, contained both actin-binding and nuclear envelope localization domains. By using an antibody against C-terminus of MSP-300, we find that MSP-300 is distributed throughout the cytoplasm and accumulates at the nuclear envelope of nurse cells and the oocyte. A GFP fusion protein containing the C-terminal region of MSP-300 is also sufficient to localize protein on the nuclear envelope in oocytes. To eliminate the maternal gene activity during oogenesis, we generated homozygous germ-line clones of a loss-of-function mutation in msp-300 in otherwise heterozygous mothers. In the mutant egg chambers that develop from such clones, cytoplasmic dumping of nurse cells is severely disturbed. The nuclei of nurse cells and the oocyte are mislocalized and the usually well-organized actin structures are severely disrupted. These results indicate that maternal MSP-300 plays an important role in actin-dependent nuclear anchorage during cytoplasmic transport.