Organization of adhesion complexes at epithelial cell contacts in the wild-type and mutant embryos. (A) Molecular organization of septate junctions in wild-type embryos. NrxIV forms a tripartite complex with Cont and Nrg. A highly ordered complex may be generated due to the binding of Nrx IV with the scaffolding molecules Vari and Cora. Several additional components including Gliotactin, Sinuous and Dlg are required for the formation of septate junctions, but the network of interactions mediating bridging with the Nrx IV complex is largely unknown. Vari, which displays a multimodular structure, may homo-multimerize and mediate linkage with Cora, Dlg or Sinuous. FRAP analysis indicates that the mobile fraction of Nrg and Nrx IV is low. (B) In nrx IV mutant embryos, the tripartite complex is missing, Cont is not targeted to the cell membrane and Nrg may bind homophilically. The mobile fraction of Nrg is increased to 45 % indicating rapid exchange between bound and unbound molecules. However, the presence of Vari and Cora may limit for a part the lateral mobility of Nrg by cross-linking with other components of septate junctions. (C) In cora or vari null mutant embryos, the tripartite complex between Nrx IV, Cont and Nrg may still occur, but the network of interactions mediated by the scaffolding molecules is disrupted. The mobile fractions of Nrx IV and Nrg are increased up to 64–70 %.