8.A.40.7.2
Uroplakin 1b, UPK1B of 260 aas and 4 TMSs in a 3 + 1 arrangement.  Like UPK1A, it may be a component of the asymmetric unit membrane (AUM), a highly specialized biomembrane elaborated by terminally differentiated urothelial cells. May play an important role in normal bladder epithelial physiology, possibly in regulating membrane permeability of superficial umbrella cells (Chicote et al. 2017). Proper formation of helical bundles consisting of the 4 tetraspanin TMSs seems to be a prerequisite for UPIb to exit from the ER into the plasma membrane (Tu et al. 2006).