Receptor-mediated endocytosis and lysosome formation

Extracellular molecules to be degraded are
taken into the cell by endocytosis. First, the
molecules are bound to specific cell surface receptors
(receptor-mediated endocytosis). The
loaded receptors are concentrated in an invagination
of the plasma membrane (coated pit).
This separates from the plasma membrane and
forms a membrane-enclosed cytoplasmic compartment
(coated vesicle). Hormones, growth
factors, energy-delivering proteins, and numerous
viruses and toxins also enter cells by receptor-
mediated endocytosis (see p. 360). The cytoplasmic
lining of the vesicle consists of a network
of a trimeric protein, clathrin. The clathrin
coat is quickly lost within the cell, and an endosome
forms, which fuses with membrane vesicles
fromthe Golgi apparatus to form larger endosomal
compartments. Here, the receptors are
separated from the ligands and are returned to
the cell surface in membrane vesicles (receptor
recycling). Parts of the membrane are also reused.
The ligands are nowwithin amultivesicular
body (endolysosomes). Hydrolases (lysosomal
enzymes) are transported from the Golgi
apparatus to an endolysosome in clathrin-enclosed
vesicles after they become equipped
with a recognition signal (mannose-6-
phosphate receptor), required for uptake into
the endolysosome and for normal functioning
of the lysosome.