Cell junctions provide important adhesion, diffusion barrier, polarity and signalling functions during epithelial development. In Drosophila,claudin-containing septate junctions (SJs) provide a diffusion barrier and control the size of tracheal tubes. The formation of SJs requires the heterodimeric Na,K-ATPase ion transporter but not, as Paul and co-workers now report, its ion-pump activity (see p. 147). By investigating the localization and function of wild-type and chimeric isoforms of the single-transmembrane Na,K-ATPase β-subunit, the researchers show that the extracellular domain of the Nrv2 isoform of this subunit is specifically required for both SJ functions. Similarly, only some isoforms of the ten-transmembrane α subunit, which contains the ATPase activity,support SJ function. Unexpectedly, mutations that inactivate the ATPase do not compromise SJ function, which indicates a pump-independent role for Na,K-ATPase in SJ formation and activity. Finally, because the rat α1 isoform completely rescues Atpα-null Drosophila mutants, the authors suggest that the Na,K-ATPase has an evolutionarily conserved role in epithelial junctions.