This family contains uroplakin II, which is approximately 180 residues long and seems to be restricted to mammals. Uroplakin II is an integral membrane protein, and is one of the components of the apical plaques of mammalian urothelium formed by the asymmetric unit membrane - this is believed to play a role in strengthening the urothelial apical surface to prevent the cells from rupturing during bladder distension [1].

1: Mammalian uroplakins. A group of highly conserved urothelial differentiation-related membrane proteins. Wu XR, Lin JH, Walz T, Haner M, Yu J, Aebi U, Sun TT; J Biol Chem 1994;269:13716-13724. PMID:8175808