Component of the asymmetric unit membrane (AUM); a highly specialized biomembrane elaborated by terminally differentiated urothelial cells. Heterodimer with uroplakin-3A (UPK3A) or uroplakin-3B (UPK3B). Down-regulated by TGF-beta and serum in quiescent cells. Rapidly and transiently induced by TGF-beta in actively growing cells. Belongs to the tetraspanin (TM4SF) family.