Required, together with Ssk and Tsp2A, for the proper organization of smooth septate junctions (sSJs), probably by mediating cell adhesion via its homophilic interaction (PubMed:22854041, PubMed:26848177). Also required for the correct subcellular localization of several sSJ components, such as Ssk, cora and l(2)gl, and for the barrier function of the midgut epithelium (PubMed:22854041). Required for maintaining the three-layered structure of the proventriculus (PubMed:22854041). Forms a complex with Ssk and Tsp2A (PubMed:26848177). Interacts with Ssk; the interaction may be necessary for the localization of both proteins to the cell apicolateral region (PubMed:22854041). Colocalizes with Ssk, cora, dlg1, l(2)gl and Fas3 at the apicolateral region of epithelial cells in the midgut. Also localizes to the apicolateral region of epithelial cells in the outer epithelial layer of the proventriculus and Malpighian tubules. At embryonic stage 12, expressed in endoderm-derived tissues (at protein level) (PubMed:22854041). In late-stage embryos, third-instar larvae and adult flies, expressed in the midgut, outer epithelial layer of the proventriculus and Malpighian tubules, but not expressed in the foregut and hindgut (at protein level) (PubMed:22854041, PubMed:26848177). Expressed throughout embryonic, larval and adult stages (at protein level). Homozygous larval lethal. Mutant embryos hatch into the first instar larvae, but die within 1 day. In mutant larvae midgut, a few septa are present at the cell contacts but large gaps between the lateral membranes of adjacent epithelial cells are frequently observed. Mutant larvae lack the three-layered structure of the proventriculus.