Localized translation of cell junction mRNAs is required for epithelial cell polarity

Epithelial cells exhibit a highly polarized organization along their apico-basal axis, a feature that is critical to their function and frequently perturbed in cancer. One less explored process modulating epithelial cell polarity is the subcellular localization of mRNA molecules. In the present study, we report that several mRNAs encoding evolutionarily conserved epithelial polarity regulatory proteins, including Zo-1 , Afdn and Scrib , are localized to cell junction regions in Drosophila epithelial tissues and human epithelial cells. Targeting of these mRNAs is coincident with the robust junctional distribution of their encoded proteins, and we demonstrate that they are locally translated at cell junction regions. To identify RNA binding proteins (RBPs) potentially implicated in junctional mRNA regulation, we performed systematic immuno-labeling with a collection of validated RBP antibodies, identifying a dozen RBPs with consistent junctional distribution patterns, several of which directly bind junctional transcripts. Strikingly, depletion of these RBP candidates, including MAGOH, a core component of the exon-junction complex (EJC), perturbed the junctional distribution and localized translation of Zo-1 and Scrib mRNAs, as well as the junctional accumulation of their protein products. Functional disruption of MAGO, or its interaction partner Y14, in Drosophila follicular epithelial cells perturbs the distribution of junctional transcripts and proteins. Finally, tissue microarray analysis of ovarian cancer tumor specimens revealed that expression of MAGOH and ZO-1 is positively correlated and that both proteins are potential biomarkers of good prognosis. Altogether, this work reveals that localized mRNA translation at cell junction regions is important for modulating epithelial cell polarity.