Fig. 1.

Scrib module proteins define the basolateral domain of epithelial cells. (A) Scheme of a Drosophila ovariole. The germline cells of each egg chamber are surrounded by an epithelium of follicle cells (gray) with their apical sides pointing to the nurse cells (green) and oocyte (dark green). (B) Scheme illustrating how wild-type polarity is defined by mutual antagonism between apical (red) and basolateral (blue) proteins. (B, Left) Wild type. aPKC phosphorylates and restricts Baz/Par3 from moving apically from adherens junctions, prevents apical localization of Lgl via phosphorylation, and recruits the apical determinant Patj. Basolateral Lgl inhibits aPKC activity. Cortical localization of the Scrib module involves a linear hierarchy with Dlg recruiting Scrib. Dlg and Scrib work together to protect basolateral Lgl from phosphorylation by aPKC, by inhibiting aPKC movement into the basolateral domain. Dlg and Scrib are also responsible for the recruitment of unknown basolateral effectors which help to define the basolateral domain. (B, Right) scrib or dlg mutants. Cells which are mutant for dlg and/or scrib lack the protection of Lgl. Apical aPKC spreads into the basolateral domain, phosphorylating basolateral Lgl and displacing it into the cytoplasm. Basolateral aPKC can then recruit other apical determinants like Patj, leading to a respecification of the basolateral domain into an apical domain.