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 (By similarity). Heterodimer with uroplakin-1B (UPK1B). Heterodimer formation with UPK1B is a prerequisite to exit out of the endoplasmic reticulum (ER). Expression is urothelium-specific. Belongs to the uroplakin-3 family.