Figure 9.

Model of MDCK lumen formation during cell division. Single cells embedded in extracellular matrix internalize Crb3-complex proteins into apical endosomal compartments. On formation of the spindle apparatus, the Crb3-complex members get transported to the spindle poles, whereas members of the Par–aPKC complex show a diffuse cytoplasmic localization. Crb3-complex proteins become apparent near newly nucleating microtubules that eventually form the midzone microtubules. The apical endosomes appear to get transported along the midzone bundle to the site of cytokinesis, where they are exocytosed. This lays the foundation for the formation of an apical membrane. Crb3a recruits aPKCζ through its PDZ-binding motif, most likely through interaction with Par6, to the forming apical membrane, where aPKCζ contributes to apical membrane identity through phosphorylation-dependent exclusion of basolateral proteins. Ultimately the separation of two polarized membrane domains gives rise to the formation of a tight junction. Pals1, Patj, and Par3 staining is restricted to the tight junction and excludes the apical membrane domain.