Essential Cell Biology 5th edition

Protein Sorting
507
the first six months of life—a grim reminder of the crucial importance of
peroxisomes, and peroxisomal protein transport, for proper cell function
and for the health of the organism.
Proteins Enter the Endoplasmic Reticulum While Being
Synthesized
The endoplasmic reticulum is the most extensive membrane system in a
eukaryotic cell (Figure 15–12A). Unlike the organelles discussed so far, it
serves as an entry point for proteins destined for other organelles, as well
as for the ER itself. Proteins destined for the Golgi apparatus, endosomes,
and lysosomes, as well as proteins destined for the cell surface, all first
enter the ER from the cytosol. Once inside the ER lumen, or embedded in
the ER membrane, individual proteins will not re-enter the cytosol during
their onward journey. They will instead be ferried by transport vesicles
from organelle to organelle within the endomembrane system, or to the
plasma membrane (see Figure 15−5).
Two kinds of proteins are transferred from the cytosol to the ER: (1) watersoluble
proteins are completely translocated across the ER membrane
and are released into the ER lumen; (2) prospective transmembrane proteins
are only partly translocated across the ER membrane and become
embedded in it. The water-soluble proteins are destined either for secretion
(by release at the cell surface) or for the lumen of an organelle of
the endomembrane system. The transmembrane proteins are destined
to reside in the membrane of one of these organelles or in the plasma
membrane. All of these proteins are initially directed to the ER by an ER
signal sequence, a segment of eight or more hydrophobic amino acids
(see Table 15–3, p. 502), which is also involved in the process of translocation
across the membrane.
Unlike the proteins that enter the nucleus, mitochondria, chloroplasts,
or peroxisomes, most of the proteins that enter the ER begin to be
threaded across the ER membrane before the polypeptide chain has been
completely synthesized. This requires that the ribosome synthesizing
the protein be attached to the ER membrane. These membrane-bound
ribosomes coat the surface of the ER, creating regions termed rough
endoplasmic reticulum because of its characteristic beaded appearance
when viewed in an electron microscope (Figure 15–12B).
(A)
(B)
10 µm nucleus
200 nm
Figure 15–12 The endoplasmic reticulum
is the most extensive membrane network
in eukaryotic cells. (A) Fluorescence
micrograph of a living plant cell showing
the ER as a complex network of tubes.
The cell shown here has been genetically
engineered so that it contains a fluorescent
protein in the ER lumen. Only part of the
ER network in the cell is shown. (B) An
electron micrograph showing the rough
ER in a cell from a dog’s pancreas, which
makes and secretes large amounts of
digestive enzymes. The cytosol is filled with
closely packed sheets of ER, studded with
ribosomes. A portion of the nucleus and its
nuclear envelope can be seen at the bottom
left; note that the outer nuclear membrane,
which is continuous with the ER, is also
studded with ribosomes. For a dynamic view
of the ER network, watch Movie 15.3.
(A, from P. Boevink et al., The Plant Journal
15:441–447, 1998. With permission from
John Wiley & Sons; B, courtesy of Lelio Orci.)