Collision of expanding actin caps with actomyosin borders for cortical bending and mitotic rounding in a syncytium.

The early Drosophila embryo is a large, single, syncytial cell. Its cortex is re-modelled to form furrows that compartmentalize dividing nuclei. Before furrow ingression, Rac-induced Arp2/3 networks generate an apical actin cap and Rho-induced actomyosin networks encircle each cap. How these two networks transform a flat cortex into a honeycomb-like, compartmental array remains unclear. With centrifugal growth, each apical cap meets its actomyosin border and furrow ingression begins. Through genetics and live molecular imaging, the cap guanine nucleotide exchange factor Sponge, a Rac-GTP sensor, and Arp3 coated the forming cap and then spread down the lateral furrows, as part of a continuous actin network. The adjacent actomyosin zone maintained its segregation from the growing cap network, forming a thin circumferential border at the base of furrows. By increasing or decreasing myosin activity genetically, the actomyosin borders were found to restrict and organize cap growth. Inversely, actin cap perturbation revealed that each growing cap seems to push physically against the actomyosin border and compress its components as the furrow ingression occurs. Overall, the growing actin caps appear to collide physically with actomyosin borders, bend, and continue growing for furrow ingression. This mesoscale mechanism seems to bud small compartments from a large cell for mitotic cell rounding in a syncytium.