A conserved motif in Crumbs is required for E-cadherin localisation and zonula adherens formation in Drosophila

Background: Specialised cell junctions in epithelia serve as cell-cell adhe sion sites and thus contribute to the maintenance of tissue integrity. The Drosophila gene crumbs encodes a transmembrane protein that is required for the biogenesis of the zonula adherens, a belt-like structure encircling th e apex of epithelial cells. As previously shown, expression of just the sho rt membrane-bound cytoplasmic domain is sufficient to rescue major defects associated with the loss of crumbs function. Results: The cytoplasmic domain of Crumbs is highly conserved in two putati ve crumbs homologues in Caenorhabditis elegans. To assess the significance of conserved residues, Various point mutations and deletions were introduce d into this region. Two functional domains were revealed, an amino-terminal region and the carboxy-terminal amino acids EERLI. Both are necessary for rescue of the crumbs phenotype. The EERLI motif interacts with Discs Lost, a cytoplasmic protein containing PDZ domains. Overexpression of the Crumbs cytoplasmic domain induces a transition from the single-layered epithelium to a multilayered tissue. This transition is associated with redistribution of the Drosophila homologue of the cell adhesion molecule E-cadherin, and depends on the presence of the EERLI motif. Conclusions: We propose a model in which the interaction of the Crumbs carb oxyl terminus with Discs Lost organises a membrane-associated protein compl ex in the apical cytocortex of epithelial cells. This scaffold mediates the localisation and stabilisation of the zonula adherens component DE-cadheri n, a crucial component for the maintenance of epithelial cell polarity and tissue integrity.