Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

730 Chapter 13: Intracellular Membrane Traffic
M6P group cause the lysosomal hydrolases to dissociate from these receptors, making
the transport of the hydrolases unidirectional. A separate transport system uses
clathrin-coated vesicles to deliver resident lysosomal membrane proteins from the
trans Golgi network to endosomes.
Transport into the Cell from the Plasma
Membrane: Endocytosis
The routes that lead inward from the cell surface start with the process of endocytosis,
by which cells take up plasma membrane components, fluid, solutes, macromolecules,
and particulate substances. Endocytosed cargo includes receptor–
ligand complexes, a spectrum of nutrients and their carriers, extracellular matrix
components, cell debris, bacteria, viruses, and, in specialized cases, even other
cells. Through endocytosis, the cell regulates the composition of its plasma membrane
in response to changing extracellular conditions.
In endocytosis, the material to be ingested is progressively enclosed by a small
portion of the plasma membrane, which first invaginates and then pinches off
to form an endocytic vesicle containing the ingested substance or particle. Most
eukaryotic cells constantly form endocytic vesicles, a process called pinocytosis
(“cell drinking”); in addition, some specialized cells contain dedicated pathways
that take up large particles on demand, a process called phagocytosis (“cell eating”).
Endocytic vesicles form at the plasma membrane by multiple mechanisms
that differ in both the molecular machinery used and how that machinery is regulated.
Once generated at the plasma membrane, most endocytic vesicles fuse with a
common receiving compartment, the early endosome, where internalized cargo
is sorted: some cargo molecules are returned to the plasma membrane, either
directly or via a recycling endosome, and others are designated for degradation
by inclusion in a late endosome. Late endosomes form from a bulbous,
vacuolar portion of early endosomes by a process called endosome maturation.
This conversion process changes the protein composition of the endosome
membrane, patches of which invaginate and become incorporated within the
organelles as intralumenal vesicles, while the endosome itself moves from the
cell periphery to a location close to the nucleus. As an endosome matures, it
ceases to recycle material to the plasma membrane and irreversibly commits
its remaining contents to degradation: late endosomes fuse with one another
and with lysosomes to form endolysosomes, which degrade their contents, as
discussed earlier (Figure 13–47).
Each of the stages of endosome maturation—from the early endosome to the
endolysosome—is connected through bidirectional vesicle transport pathways to
plasma membrane
early
endosome
CYTOSOL
multivesicular
body
recycling
endosome
intralumenal vesicle
FUSION
MICROTUBULE-MEDIATED
TRANSPORT
trans Golgi network
FUSION
late
endosome
lysosome
endolysosome
LYSOSOME
ENDOPLASMIC RETICULUM
LATE ENDOSOME
EARLY ENDOSOME
GOLGI
RECYCLING
ENDOSOME
CELL EXTERIOR
MBoC6 p.787
SECRETORY
VESICLES
Figure 13–47 Endosome maturation:
the endocytic pathway from the plasma
membrane to lysosomes. Endocytic
vesicles fuse near the cell periphery with
an early endosome, which is the primary
sorting station. Tubular portions of the early
endosome bud off vesicles that recycle
endocytosed cargo back to the plasma
membrane—either directly, or indirectly
via recycling endosomes. Recycling
endosomes can store proteins until they are
needed. Conversion of early endosomes to
late endosomes is accompanied by loss of
the tubular projections. Membrane proteins
destined for degradation are internalized
in intralumenal vesicles. The developing
late endosome, or multivesicular body,
moves on microtubules to the cell interior.
Fully matured late endosomes no longer
send vesicles to the plasma membrane,
and they fuse with one another and
with endolysosomes and lysosomes to
degrade their contents. Each stage of
endosome maturation is connected via
transport vesicles with the TGN, providing
a continuous supply of newly synthesized
lysosomal proteins.