The cholinesterase-like molecule gliotactin is required for the formation of the blood–nerve barrier in flies, and Schulte et al. (page 991) examined its localization. Gliotactin is found in the same tissues as other septate junction components, and appears to be required for septate junction formation. At the subcellular level, though, gliotactin specifically localizes to tricellular junctions, points where three epithelial cells meet, rather than being distributed among all septate junctions.
In both vertebrates and invertebrates, each tricellular junction appears to contain a pore, but little was known about this structure. Gliotactin is the first molecular marker identified for tricellular junctions. The authors propose that after septate junctions begin forming, gliotactin links them to the tricellular junction to tighten the structure, generating a mature, impermeable network of septate junctions. An analogous process may occur in vertebrates.
On page 979, Genova and Fehon identify four new components of septate junctions, including the α and β subunits of the Drosophila Na+/K+ ATPase and neuroglian, a homologue of vertebrate neurofascin. It is puzzling to find a pump in a structure that acts as a diffusion barrier, but the ATPase may help form a scaffold on which the septate junction complex assembles. The presence of neuroglian underscores the molecular and functional homology between invertebrate septate junctions and the vertebrate paranodal septate junctions that form between glial cells and neurons. The authors hope to use the genetically malleable fly system to identify additional components of septate junctions. ▪