The MAGUK Family Protein Dlg5 Regulates Apical Complexes And Follicular Epithelial Polarity In Drosophila

Cell polarity and cell-cell junctions have important roles in organizing cells into tissues during organism development. Epithelial tissues are fundamental for the construction of complex animal bodies. Apicobasal polarity is a feature of epithelial cells that is essential for their function as selective barriers between the outside world and the inside environment of an organism. Understanding the mechanisms of the establishment and maintenance of epithelial polarity may lead us to develop novel therapeutic strategies for treating human disease. Follicular epithelium in the ovary of Drosophila melanogaster has emerged as a well-established model for studying epithelial polarity in vivo. In a screen to uncover novel genes and mechanisms essential for the establishment and maintenance of epithelial polarity, we found that the gene dlg5 that encodes a member of membrane-associated guanylate kinase family is required for follicular epithelial morphogenesis.In dlg5 mutant clones, follicular epithelia display flattening phenotype. We found that the adherens junctional proteins are reduced in dlg5 mutant follicle cells and the components of apical complexes are all reduced. While, the septate junctional proteins and the cytoskeleton are not affected. These results indicate that Dlg5 stabilizes adherens junctions and apical proteins at cell-cell junctions. In expression and subcellular localization analysis, we found Dlg5 colocalized with both adherens junctional proteins and apical proteins in early stage egg chambers. This result suggests that Dlg5 probably directly interacts with adherens junctional proteins and apical proteins. When Crb is overexpressed, the apical localization of Dlg5 extends to the basolateral membrane and the cytoplasmic Dlg5 vesicles partially overlaps with Crb vesicles, suggesting that Dlg5 interacts with Crb in both cell junctions and cytoplasmic vesicles, and Dlg5 is probaly involved in Crb vesicle trafficking.Moreover, we performed an in vivo structure-function analysis of Drosophila Dlg5 which contains a coiled-coil domain, four PDZ domains, a large Linker region, an SH3 domain and a GUK domain. The result shows that the coiled-coil domain contributes to the apical localization, the PDZ domains promotes basolateral membrane targeting, the PDZ4-SH3-GUK domains mediate membrane targeting, the Linker region contributes to adherens junctional localization. The lethality rescue experiment shows that the PDZ3-PDZ4 domains are required for the lethality rescue, suggesting that PDZ3-PDZ4 domains play an important role in Dlg5 function. While, SH3-GUK domains are dispensable for Dlg5 function in individual survival.Taken together, I conclude that Dlg5 is an apical-basal cell polarity protein that functions at apical membrane domains and stabilizes the apical complexes probably though the interactions with Crb. This study represents the first genetic and functional analysis of dlg5 in Drosophila.