2.3 Sorting for the basolateral and apical zones of the plasma membrane
Many cells are permanently polarised, and this means that surface proteins are selectively localised to different areas of the plasma membrane, depending on their function. For example, endothelial cells have adhesion molecules on the surface that contacts the basal lamina, but receptors that take up molecules from the blood (e.g. the transferrin receptor – see below) are located on the surface of the cell that is in contact with the blood. Cell surface molecules can normally diffuse laterally within the plane of the membrane, although they may be excluded from, or concentrated in, particular areas such as lipid rafts. However, for some cells, such as the epithelial cells in the gut, the plasma membrane is divided into two distinct zones termed the basolateral and apical domains, which are separated from each other by a belt of continuous tight junctions around the cell. This structure severely restricts the free diffusion of molecules through the extracellular spaces between neighbouring cells, and is seen in a number of other tissues.
Basal membranes are now more often called basal laminae, to emphasise the fact that they are not phospholipid bilayers, but sheets of extracellular matrix.
For example, the endothelial cells in the blood vessels in the brain have a ring of continuous tight junctions, which forms a barrier betweeen the blood and the brain tissue that contributes to the so-called blood–brain barrier. The tight junctions also prevent the lateral diffusion of proteins and lipids within the plasma membrane from one zone to another.
Predict what requirement this places on the transport systems within cells that have such differentiated zones of the plasma membrane.
Proteins and lipids must be directed to either the basolateral or the apical zones by different routes.
Proteins destined for these zones are initially sorted in the Golgi apparatus and packaged in different vesicles before being directed to the appropriate zone of the membrane (Figure 10). Sorting signals have been identified in the C-terminus of proteins destined for the basolateral membrane, which ensure that the proteins reach the correct zone and are returned there should they be internalised by endocytosis.