- The sorting of the receptor proteins into specific domains within the endosome for
either subsequent recycling to the membrane for reuse or for their degradation in
lysosomes.
In the case of G-protein-coupled receptors, after binding the agonist and G-protein,
the receptors undergo agonist-dependent phosphorylation by a GRK and this stimu-
lates interaction with one of theb-arrestin family of protein adaptor complexes. These
cytosolic adaptors facilitate the disruption of the interaction of the receptors with
G-proteins and the recruitment of the receptors intocoated pitsin the membrane
where they link the receptor to a protein called clathrin via a second adaptor protein.
A large number of such second adaptors have been identified and shown to belong to
one of two classes: multimeric adaptor proteins of which AP-2 is best known, and
monomeric adaptor proteins (also known as CLASPs). This group is numerically the
largest and includes Epsin1, AP180, Dab2 and CALM as well asb-arrestin. The adaptor
protein interacts with phosphoinositides and promotes both the assembly of the
coated pits and the recruitment of the activated receptors to them. There is evidence
that scaffold proteins such as Eps15 are also involved in the assembly of the clathrin
cage. Coated pits are regions in the membrane that are rich in clathrin that is located
on the cytoplasmic side of the membrane. Clathrin consists of three heavy and three
light chains which can polymerise to form a polymeric cage-like structure or lattice
that links to the C-terminal end of the receptor. The polymeric clathrin network drives
membrane deformation and the ‘budding’ of the coated pit to create anendocytic
vesicle. This budding process involves the actin cytoskeleton that is a dynamic
network of over 100 structural and regulatory proteins, and the GTPase dynamin.
With the expenditure of GTP and in collaboration with a number of BAR proteins that
are inserted into the membrane, the actin and dynamin promote the removal of the
budding vesicle from the membrane. Once the free vesicle has formed, the clathrin
coat is lost as a result of the action of one of the proteins of the heat shock protein
70 family. The vesicles then fuse to form an early endosome in which the bound
agonist is removed by the prevailing mildly acidic (pH 6.2) conditions (Fig. 17.13).
Recent evidence indicates that RTKs also undergo endocytosis from special localised
regions of the membrane termeddorsal wavesthat are large, circular protrusions in the
cell membrane. This endocytic process does not involve clathrin but it does require
specific adaptors such as Grb2, dynamin and components of the actin cytoskeleton that
combine to release the early endosome by a mechanism similar to that for GPCRs.
There are two possible fates for the receptors within the early endosome:- recycling to the membrane thereby restoring a functional receptor;
- proteolysis resulting in a net decrease in membrane receptor function, a process
known asdownregulation.
The pathways for endocytic sorting are determined by the operation ofsorting
signals. The main sorting signal appears to reside in the cytoplasmic region of
the receptor itself. Thus for theb 2 -adrenergic receptor, a PDZ-binding domain in
the C-terminal region interacts with a protein called EBP50 with the result that the
receptor undergoes recycling, the process also involvingb-arrestin and a protein705 17.5 Receptor trafficking