The E protein of the Severe Acute Respiratory Syndrome Coronavirus binds the tight junction protein PALS1 through a novel PDZ domain-binding motif/PDZ domain interaction. Expression of E delays formation of tight junctions and alters cyst morphogenesis of MDCKII epithelial cells.

Although lumen generation has been extensively studied through so-called cyst-formation assays in Madin-Darby canine kidney (MDCK) cells, an underlying mechanism that leads to the initial appearance of a solitary lumen remains elusive. Lumen formation is thought to take place at early stages in aggregates containing only a few cells. Evolutionarily conserved polarity protein complexes, namely the Crumbs, Par, and Scribble complexes, establish apicobasal polarity in epithelial cells, and interference with their function impairs the regulated formation of solitary epithelial lumina. Here, we demonstrate that MDCK cells form solitary lumina during their first cell division. Before mitosis, Crumbs3a becomes internalized and concentrated in Rab11-positive recycling endosomes. These compartments become partitioned in both daughter cells and are delivered to the site of cytokinesis, thus forming the first apical membrane, which will eventually form a lumen. Endosome trafficking in this context appears to depend on the mitotic spindle apparatus and midzone microtubules. Furthermore, we show that this early lumen formation is regulated by the apical polarity complexes because Crumbs3 assists in the recruitment of aPKC to the forming apical membrane and interference with their function can lead to the formation of a no-lumen or multiple-lumen phenotype at the two-cell stage.

Asymmetric delivery and distribution of macromolecules are essential for cell polarity and for cellular functions such as differentiation, division, and signaling. Injury of podocytes, which are polarized epithelial cells, changes the dynamics of the actin meshwork, resulting in foot process retraction and proteinuria. Although the spatiotemporal control of specific protein-protein interactions is crucial for the establishment of cell polarity, the mechanisms controlling polarity-dependent differentiation and division are incompletely understood. In this study, yeast two-hybrid screens were performed using a podocyte cDNA library and the polarity protein PATJ as bait. The protein KIBRA was identified as an interaction partner of PATJ and was localized to podocytes, tubular structures, and collecting ducts. The last four amino acids of KIBRA mediated binding to the eighth PDZ domain of PATJ. In addition, KIBRA directly bound to synaptopodin, an essential organizer of the podocyte cytoskeleton. Stable knockdown of KIBRA in immortalized podocytes impaired directed cell migration, suggesting that KIBRA modulates the motility of podocytes by linking polarity proteins and cytoskeleton-associated protein complexes.

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