Epithelial cells have an apical zonula adherens (ZA) to delineate the border between apical and basolateral membranes. The ZA forms by a poorly characterized process of cadherin and catenin clustering. On page 391, Hunter and Wieschaus find that a protein called Nullo is necessary for formation of a second set of adherens junctions, which appear transiently during fly embryo cellularization at the basal corners of the forming cells. Surprisingly, Nullo appears to act to prevent the clustering of junctional components.
Without Nullo, which is normally located at basal junctions, some cleavage furrows fail. In other furrows the apical ZA forms as usual, but basal Armadillo (Drosophila β-catenin) spreads apically. In contrast, excess Nullo has no effect on basal catenin localization, but selectively disrupts apical junctions: the catenin in lateral membranes stays fixed in place and no longer clusters...
