a | Three signalling complexes (Crumbs, partitioning defective (PAR) and Scribble) associate with the cytoplasmic surface of the plasma membrane around sites of cell adhesion, which demarcates different plasma membrane domains — in this example, the apical membrane (top) and the basolateral membrane (bottom). Crumbs protein (CRB) is a transmembrane protein, but mechanisms of binding of the PAR and Scribble complexes are poorly understood (see the main text). Proteins within each of these three complexes physically interact, as do PATJ (PALS1 (protein associated with LIN-7)-1-associated tight-junction protein) and PAR6 in the Crumbs and PAR complexes. Atypical protein kinase C (aPKC; in the PAR complex) genetically interacts with, and stabilizes, CRB (broken line, +), and phosphorylates and negatively regulates (broken line, −) lethal giant larvae (LGL) in the Scribble complex. Overall, the PAR complexes reinforce the localization and activity of the Crumbs complex (thick arrow), and the PAR and Scribble complexes mutually antagonize each other (inhibition lines). b | PAR3 phosphorylation by PAR1 results in the binding of phosphorylated PAR3 and PAR5, and dislocation of PAR3 from the membrane into the cytoplasm. Similarly, PAR1 phosphorylation by aPKC or PAR4 results in the binding of phosphorylated PAR1 and PAR5, and dislocation of PAR1 into the cytoplasm (see the main text for details). SCRB, Scribble protein; STD, stardust.