Discontinuous actin hexagon, a protein essential for cortical furrow formation in Drosophila, Is membrane associated and hyperphosphorylated

discontinuous actin hexagon (dah) is a maternal-effect gene essential for t he formation of cortical furrows during Drosophilla embryogenesis, and DAH protein colocalizes with actin in these furrows. Biochemical fractionation experiments presented here demonstrate that DAH is highly enriched in the m embrane fraction and that its membrane association is resistant to high-sal t 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 biochem ical characterizations suggest that DAH may play a role in the linkage of t he actin cytoskeleton to membranes. Using phosphatase inhibitors, we detect ed multiple phosphorylated forms of DAH in embryonic extracts. The DAH phos phorylation peaks during cellularization, a stage at which DAH function is critical. A kinase activity is coimmunoprecipitated with the DAH complex an d hyperphosphorylates DAH in vitro. Purified casein kinase I can also hyper phosphorylate DAH in the immune complex. Both DAH localization and phosphor ylation are disrupted in another maternal-effect mutant, nuclear-fallout, I t is possible that nuclear-fallout collaborates with dah and directs DAH pr otein localization to the cortical furrows.