Uroplakins are a group of urothelial differentiation-related membrane proteins. They are components of the asymmetric unit membrane (AUM), which forms the apical plaques of mammalian urothelium and is believed to play a role in strengthening the urothelial apical surface thus preventing the cells from rupturing during bladder distention (PMID: 8175808). Uroplakin-3b (UPK3B), also named as p35, is a single-pass type I membrane protein. This protein dimerizes with uroplakin-1b (UPK1B) to exit out of endoplasmic reticulum, which is is an important early step in urothelial plaque assembly (PMID: 12446744).