Fig. 7. Models for the role of the Na⁺/K⁺ ATPase in tube-size control via the septate junction. See text for detailed discussions. (A) One possible formulation of the genetic pathways controlling tracheal tube size and septate junction barrier function. This model is based on the barrier phenotypes of the shown mutants and on genetic interactions between these mutants and nrv2-null mutants. Comparison of these phenotypes and interactions divides the mutants into three classes. nrv2 appears to act in a linear genetic pathway with gliotactin and coracle (shown in blue) but in a parallel, partially redundant, or branching pathway to varicose and cystic (shown in red). convoluted may act either downstream of a varicose/cystic pathway (indicated in red) and/or in parallel pathways (indicated in green). (B) A molecular representation of the pathways shown in A to illustrate how nrv2/coracle (blue) and varicose (red) could both be required for septate junction barrier function, but act in genetically distinguishable pathways for tube-size control. Question marks indicate uncertainty in identity or subcellular localization. Only a subset of known septate junction proteins are shown.