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Sinuous (green) localizes to septate junctions.

The growth of a network of branching tubes during organogenesis and the formation of a watertight epithelial barrier may seem like very different processes, but on page 313, Wu et al. uncover a surprising connection between the two. The work also reveals a common molecular basis for the formation of invertebrate septate junctions and vertebrate tight junctions.

In the new work, the authors cloned the sinuous gene of Drosophila, which was previously found in a screen for mutations that affected tracheal tube formation. The product of sinuous shares homology with claudins, the family of proteins responsible for forming the seals of vertebrate tight junctions. Sinuous localizes to the fly septate junction, and is essential for the formation of normal barriers.

The molecular and functional similarities between tight junctions and septate junctions—structures that were previously considered analogous rather than homologous—suggest that different types of barrier junctions arose from a single, claudin-containing ancestral structure. The nonbarrier functions of junction components could have driven their divergence. For example, the activity of sinuous in tracheal development seems to be distinct from its function in septate junction barrrier formation. The authors are now trying to determine how this novel claudin works in both processes. ▪