Essential Cell Biology 5th edition

500 CHAPTER 15 Intracellular Compartments and Protein Transport
Figure 15–4 Mitochondria are thought
to have originated when an aerobic
bacterium was engulfed by a larger
anaerobic eukaryotic cell. Chloroplasts
are thought to have originated later in a
similar way, when a eukaryotic cell with
mitochondria engulfed a photosynthetic
bacterium. This theory would explain why
these organelles have two membranes,
possess their own genomes, and do
not participate in the vesicular traffic
that connects the compartments of the
endomembrane system.
anaerobic
eukaryotic cell
aerobic
bacterium
nucleus
plasma
membrane
bacterial plasma membrane
bacterial outer membrane
internal
membranes
DEGRADATION OF
PLASMA MEMBRANE
DERIVED FROM
EUKARYOTIC CELL
early aerobic
eukaryotic cell
mitochondria with
double membrane
traffic that connects the interiors of most of the other membrane-enclosed
organelles to one another and to the outside of the cell.
PROTEIN SORTING
Before a eukaryotic cell divides, it must duplicate its membrane-enclosed
organelles. As cells grow, membrane-enclosed organelles enlarge by
incorporation of new molecules; the organelles then divide and, during
cell division, are distributed between the two daughter cells. Organelle
growth requires a supply of new lipids to make more membrane and
a supply of the appropriate ECB5 proteins—both e15.04/15.04 membrane proteins and the
soluble proteins that will occupy the interior of the organelle. Even in
cells that are not dividing, proteins are being produced continually. These
newly synthesized proteins must be accurately delivered to their appropriate
organelle—some for eventual secretion from the cell and some
to replace organelle proteins that have been degraded. Directing newly
made proteins to their correct organelle is therefore necessary for any
cell to grow and divide, or just to function properly.
For some organelles, including mitochondria, chloroplasts, and the interior
of the nucleus, proteins are delivered directly from the cytosol. For
others, including the Golgi apparatus, lysosomes, endosomes, and the
inner nuclear membrane, proteins and lipids are delivered indirectly via
the ER, which is itself a major site of lipid and protein synthesis. Proteins
enter the ER directly from the cytosol: some are retained there, but most
are transported by vesicles to the Golgi apparatus and then onward to the
plasma membrane or to other organelles. Peroxisomes make use of both
pathways. Although these organelles acquire some of their membrane
proteins from the ER, the bulk of their digestive enzymes enter directly
from the cytosol.
In this section, we discuss the mechanisms by which proteins enter
membrane-enclosed organelles from the cytosol. Proteins made in the
cytosol are dispatched to different locations in the cell according to specific
address labels contained in their amino acid sequence. Once at the
correct address, the protein enters either the membrane or the interior
lumen of its designated organelle.
Proteins Are Transported into Organelles by Three
Mechanisms
The synthesis of virtually all proteins in the cell begins on ribosomes in
the cytosol. The exceptions are the few mitochondrial and chloroplast