Abstract
Apical-basal polarity plays critical roles in the functions of epithelial tissues. However, the mechanisms of epithelial polarity establishment and maintenance remain to be fully elucidated. Here we show that the membrane-associated guanylate kinase (MAGUK) family protein Dlg5 is required for the maintenance of apical polarity of follicle epithelium during Drosophila oogenesis. Dlg5 localizes at the apical membrane and adherens junction (AJ) of follicle epithelium in early stage egg chambers. Specifically, we demonstrate that the major function of Dlg5 is to promote apical membrane localization of Crumbs, since overexpression of Crumbs but not other major apical or AJ components could rescue epithelial polarity defects resulted from loss of Dlg5. Furthermore, we performed a structure-function analysis of Dlg5 and found that the C-terminal PDZ3 and PDZ4 domains are required for all Dlg5’s functions as well as its ability to localize to apical membrane. The N-terminal coiled-coil motif could be individually targeted to the apical membrane, while the central linker region could be targeted to AJ. Lastly, the MAGUK core domains of PDZ4-SH3-GUK could be individually targeted to apical, AJ and basolateral membranes.
Highlights
Most of these functions were obtained from cell culture studies
It was previously reported that the adherens junction (AJ) of the follicle epithelium contain both E-cad and N-cadherin (N-cad) up to stage 9 of oogenesis[20,21], and we found that N-cad was more strongly reduced than E-cad and Arm in the mutant clones (Fig. 2K–L’)
These results indicate that Dlg[5] is required for the maintenance of apical polarity and AJ, since only 12 hours of Dlg[5] deficiency would not have affected the establishment of apical polarity and AJ for the stage 6/7 or stage 9 follicle epithelia
Summary
Our study demonstrates that Dlg[5] is required for the maintenance of apical polarity and AJ of follicle epithelia during early stages of oogenesis, since both loss-of-function mutation and RNAi knockdown of dlg[5] affected only apical polarity regulators and the sub-apical AJ components but not the basolateral regulators. A previous study found that loss of Crb, aPKC and Par[6] did not affect the lateral localization of Dlg, whereas loss of Arm caused Dlg spreading to the apical membrane of follicle cells[31]. One interesting difference between the localization patterns of the other deletion mutants (that still possessed rescue abilities; Δ1–3, Δ6–8) and the patterns of Δ4 and Δ5 is that other deletions all retained some degree of apical localization, in contrast to the lack of localization in the apical membrane for Δ4 and Δ5 (Fig. 8B) Together, these results suggest that MAGUK core and PDZ3’s requirement for Dlg5’s membrane localization in general and PDZ3-PDZ4’s requirement for Dlg5’s apical membrane localization may be critical for Dlg5’s functions in the follicle cells. We found that the N-terminal coiled coil domain, the middle linker region and the MAGUK core could be individually membrane-targeted to apical, AJ and all (apical, AJ and basolateral) regions respectively
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