In *obe* mutant epithelia, centrosomes (red) flank adherens junction clusters (blue), giving the appearance of a division symbol, or obelus.Vichas et al. reveal that an RNA helicase controls epithelial cell polarity by regulating alternative splicing of the apical membrane protein Crumbs.

In a genetic screen, Vichas et al. found that mutations in a gene called *obelus* (*obe*) perturbed epithelial cell polarity and intercellular junctions in early *Drosophila* embryos. *obe* mutant cells displayed an expanded apical membrane domain, and their intercellular adherens junctions clustered into single large aggregates instead of becoming evenly distributed across cell--cell interfaces. In addition, centrosomes were in aberrantly close proximity to the clustered junctions.

*obe* encodes an RNA helicase homologous to human and yeast proteins that are core components of the spliceosome, which catalyzes pre-mRNA splicing. Surprisingly, however, only a few transcripts were differentially spliced in *obe* mutant embryos. One of these encoded the apical membrane determinant Crumbs. In *obe* mutants, *crumbs* transcripts frequently retained an additional exon, increasing the expression of a Crumbs isoform with an extra EGF-like repeat in its extracellular domain. Overexpressing this isoform induced apical expansion, junction aggregation, and centrosome mispositioning similar to *obe* mutants. In contrast, overexpressing the shorter Crumbs isoform that usually predominates in early embryos had only mild effects on centrosome location and a distinct effect on junction organization.

Obelus therefore controls epithelial polarity and adherens junction organization by regulating *crumbs* splicing. It remains to be determined how the inclusion of an additional EGF-like repeat has such a marked affect on Crumbs activity.

[^1]: Text by Ben Short
