Required for assembly of smooth septate junctions (sSJs), together with Ssk and mesh. Important for barrier function of the midgut epithelium. Forms a complex with Ssk and mesh. Detected in embryos from stage 15 onwards (at protein level). In first instar larvae, detected in midgut, Malpighian tubules and outer epithelial layer of the proventriculus (OELP) (at protein level). Belongs to the tetraspanin (TM4SF) family.