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 to the apical membrane of epithelial cells. Loss or mislocalisation of Crb is often associated with disruption of apicobasal 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 localisation element (LE) of mRNA to 47 nucleotides, which form a putative stem-loop structure that may 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 for the 3'-UTR in regulating translation in a tissue-specific manner.This article has an associated First Person interview with the first author of the paper.