Regulated expression of nullo is required for the formation of distinct apical and basal adherens junctions in the Drosophila blastoderm

During cellularization, the Drosophila embryo undergoes a large-scale cytok inetic event that packages thousands of syncytial nuclei into individual ce lls, resulting in the de novo formation of an epithelial monolayer in the c ortex of the embryo. The formation of adherens junctions is one of the many aspects of epithelial polarity that is established during cellularization: at the onset of cellularization, the Drosophila beta-catenin homologue Arm adillo (Arm) accumulates at the leading edge of the cleavage furrow, and la ter to the apicolateral region where the zonula adherens precursors are for med. In this paper, we show that the basal accumulation of Arm colocalizes with DE-cadherin and D alpha-catenin, and corresponds to a region of tight membrane association, which we refer to as the basal junction. Although the two junctions are similar in components and function, they differ in their response to the novel cellularization protein Nullo. Nullo is present in t he basal junction and is required for its formation at the onset of cellula rization. In contrast, Nullo is degraded before apical junction formation, and prolonged expression of Nullo blocks the apical clustering of junctiona l components, leading to morphological defects in the developing embryo. Th ese observations reveal differences in the formation of the apical and basa l junctions, and offer insight into the role of Nullo in basal junction for mation.