Overexpressed Crumbs (green) links up with βH spectrin (red).

Forming an epithelium, one of the most basic requirements of multicellular life, requires sophisticated coordination of cellular polarity with morphogenesis. Using a Drosophila model system, Medina et al. (page 941) have now characterized an important link between polarity cues and the spectrin-based membrane skeleton.

In fly epithelia, the apical transmembrane protein Crumbs is essential for both polarization and proper cellular morphogenesis. The new work shows that Crumbs can recruit both βH spectrin and DMoesin to its cytoplasmic tail, which was already known to interact with the polarity-determining proteins Stardust and Discs-lost. The interactions suggest that Crumbs may recruit the spectrin-based membrane skeleton in response to polarity cues, stabilizing the zonula adherens and helping to define the structure of the apical end of the cell.It is interesting to note that the interaction of Crumbs with βH spectrin requires a charged amino acid at position 7 in the cytoplasmic tail. Since this feature is conserved among all known homologues of Crumbs, the interaction may be a common feature of epithelia. In humans, a homologue of βH spectrin is strongly expressed in photoreceptor cells in the eye, and mutations in the human homologue of Crumbs can cause retinitis pigmentosa. The authors are now studying whether the proteins interact the same way in humans as they do in flies. ▪