Component of the asymmetric unit membrane (AUM); a highly specialized biomembrane elaborated by terminally differentiated urothelial cells. May play an important role in normal bladder epithelial physiology, possibly in regulating membrane permeability of superficial umbrella cells or in stabilizing the apical membrane through AUM/cytoskeletal interactions (By similarity). Homodimer; disulfide-linked. Interacts with uroplakin-2 (UPK2) (By similarity). Binds to uropathogenic E.coli fimH. Belongs to the tetraspanin (TM4SF) family.