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). High expression restricted to ureteric urothelium (most superficial cells); low expression in prostate. Expression in normal urothelial cells is lost in culture. Some expression in tumor cell lines derived from urothelial malignancies. Belongs to the tetraspanin (TM4SF) family.