The asymmetric unit membrane (AUM) structure in the lumenal plasma membrane of urothelium: The effect of trypsin on the integrity of the plaques

Abstract

The lumenal plasma membrane of most mammalian urothelia is distinguished morphologically both in situ and in vitro by regions (plaque) of thickened membrane structure. Within plaque regions are regular arrays of hexagonally shaped (11.4 nm) particles which project from the free surface into the lumen, imparting an asymmetry to the unit membrane structure. When the urothelial lumenal surface is exposed to the action of trypsin, a symmetric unit membrane structure is observed along the entire length of the lumenal plasma membrane. Thickened plaque regions can no longer be distinguished from interplaque regions. Negatively stained preparations reveal that after trypsin treatment many pieces of lumenal plasma membrane are stripped of particulate structures. Hexagonally shaped particles, freed from the membrane continuum by trypsin, can be recovered and visualized in the supernatant fraction. Freeze-fractured membranes after trypsin treatment suggest that only the portion of the plaque particle protruding from the surface is removed, and a hydrophobic core of the plaque particle remains within the membrane interior. In correlation with the action of trypsin on the asymmetric unit membrane (AUM), there is a loss of the scalloped profile normally characterizing the lumenal plasma membrane so that membrane pieces after trypsinization take on a more linear ( in situ ) or more rounded ( in vitro ) shape. In essence, the lumenal plasma membrane as a whole loses its distinctive morphology and reverts after exposure to trypsin to a membrane system indistinguishable in form from many cell types.