Component of the asymmetric unit membrane (AUM); a highly specialized biomembrane elaborated by terminally differentiated urothelial cells. May play an important role in AUM-cytoskeleton interaction in terminally differentiated urothelial cells. It also contributes to the formation of urothelial glycocalyx which may play an important role in preventing bacterial adherence. Heterodimer with uroplakin-1B (UPK1B). Heterodimer formation with UPK1B is a prerequisite to exit out of the endoplasmic reticulum (ER). Bladder epithelium. Belongs to the uroplakin-3 family.