A putative stem-loop structure in is required for mRNA localisation in epithelia and germline cells

Crumbs (Crb) is an evolutionarily conserved transmembrane protein localised in the apical membrane of epithelial cells. Loss or mis-localisation of Crb is often associated with disruption of apico-basal cell polarity. mRNA is also apically enriched in epithelial cells, and, as shown here, accumulates in the oocyte of developing egg chambers. We narrowed down the Localization Element (LE) of mRNA to 47 nucleotides forming a putative stem-loop structure, suggesting to be recognised by Egalitarian (Egl). Mutations in conserved nucleotides abrogate apical transport. mRNA enrichment in the oocyte is affected in mutant egg chambers. A CRISPR based genomic deletion of the locus that includes the LE disrupts asymmetric mRNA localisation in epithelia and prevents its accumulation in the oocyte during early stages of oogenesis, but does not affect Crb protein localisation in embryonic and follicular epithelia. However, flies lacking the LE show ectopic Crb protein expression in the nurse cells. These data suggest an additional role of the 3'-UTR in regulating translation in a tissue specific manner.