Discontinuous actin hexagon, a protein essential for cortical furrow formation

discontinuous actin hexagon (dah) is a maternal-effect gene essential for the formation of cortical furrows during Drosophila embryogenesis, and DAH protein colocalizes with actin in these furrows. Biochemical fractionation experiments presented here demonstrate that DAH is highly enriched in the membrane fraction and that its membrane association is resistant to high-salt and alkaline washes. Furthermore, it partitions into the detergent phase of the Triton X-114 solution, indicating its tight binding to the membranes. DAH can also interact with the actin cytoskeleton, because a fraction of DAH remains insoluble to nonionic detergent along with actin. These biochemical characterizations suggest that DAH may play a role in the linkage of the actin cytoskeleton to membranes. Using phosphatase inhibitors, we detected multiple phosphorylated forms of DAH in embryonic extracts. The DAH phosphorylation peaks during cellularization, a stage at which DAH function is critical. A kinase activity is coimmunoprecipitated with the DAH complex and hyperphosphorylates DAH in vitro. Purified casein kinase I can also hyperphos-phorylate DAH in the immune complex. Both DAH localization and phosphorylation are dis-rupted in another maternal-effect mutant, nuclear-fallout. It is possible that nuclear-fallout collabo-rates with dah and directs DAH protein localization to the cortical furrows