Endocytosis and potocytosis

Endocytosis and potocytosis
Endocytosis and potocytosis are mechanisms that
cells use to import nutritious molecules from the
extracellular environment. Both mechanisms are energy-
dependent and tightly controlled. The principal
function of endocytosis is to deliver macromolecules
to lysosomes for hydrolytic processing or to transport
them across polarized cells by transcytosis (Rodman
et al., 1990). Pinocytosis refers to the trapping of a
portion of the extracellular fl uid during endocytosis.
For the internalization of certain vital macromolecules
in a minimum of extracellular fl uid, receptormediated
endocytosis provides much greater rates
than ordinary endocytosis. Potocytosis describes the
sequestration and transport of molecules and macromolecules
by caveolae. Caveolae seal off from the
plasma membrane, but remain separate from other
endocytic compartments.
Receptor-mediated endocytosis
The various pathways of receptor-mediated endocytosis
share one common feature: in each case receptor–
ligand complexes are conveyed in coated pits and
coated vesicles. However, there are differences in the
routes that ligands and receptors follow after entering
the cell. The model depicted in Fig. 3.9 is based on the
endocytosis of plasma low-density lipoprotein (LDL)
and other macromolecules in which the receptor recycles
and the ligand is degraded. In this model, the
macromolecular ligands bind to specifi c cell-surface
receptors, which are transmembrane proteins. This
binding triggers clustering of receptor–ligand complexes
at specialized internalization sites known as
coated pits. The ‘coat’ is formed entirely of the protein,
clathrin, whose heavy and light chains are organized
to form a network on the cytoplasmic face of the plasma
membrane. The coated pits invaginate and pinch
off to form intracellular coated vesicles enclosing the
receptor–ligand complexes. This fusion of the ends of
the coated pit membranes and pinching off of vesicles
is ATP-dependent. Within the cytosol, the vesicles are
uncoated by an ATP-dependent enzyme, after which
they fuse with one another to form endosomes, whose
contents are acidifi ed by a proton pump. Within the
endosomes the ligands dissociate from their receptors,
and the ligands are carried further to lysosomes,
where they undergo proteolysis. The receptors leave
the endosomes, apparently via incorporation into the
membrane of vesicles that bud off from the endosome
surface, and are recycled to the cell surface to bind new
ligand.