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

658 Chapter 12: Intracellular Compartments and Protein Sorting
import and export receptors, continually shuttle between the cytosol and nucleus.
The monomeric GTPase Ran provides both the free energy and the directionality for
nuclear transport. Cells regulate the transport of nuclear proteins and RNA molecules
through the NPCs by controlling the access of these molecules to the transport
machinery. Newly transcribed messenger RNA and ribosomal RNA are exported
from the nucleus as parts of large ribonucleoprotein complexes. Because nuclear
localization signals are not removed, nuclear proteins can be imported repeatedly,
as is required each time that the nucleus reassembles after mitosis.
The Transport of Proteins into
Mitochondria and Chloroplasts
Mitochondria and chloroplasts (a specialized form of plastids in green algae
and plant cells) are double-membrane-enclosed organelles. They specialize in
ATP synthesis, using energy derived from electron transport and oxidative phosphorylation
in mitochondria and from photosynthesis in chloroplasts (discussed
in Chapter 14). Although both organelles contain their own DNA, ribosomes,
and other components required for protein synthesis, most of their proteins are
encoded in the cell nucleus and imported from the cytosol. Each imported protein
must reach the particular organelle subcompartment in which it functions.
There are different subcompartments in mitochondria (Figure 12–19A): the
internal matrix space and the intermembrane space, which is continuous with
the cristae space. These compartments are formed by the two concentric mitochondrial
membranes: the inner membrane, which encloses the matrix space
and forms extensive invaginations called cristae, and the outer membrane,
which is in contact with the cytosol. Protein complexes provide boundaries at the
junctions where the cristae invaginate and divide the inner membrane into two
domains: one inner membrane domain surrounds the cristae space, and the other
domain abuts the outer membrane. Chloroplasts also have an outer and inner
membrane, which enclose an intermembrane space, and the stroma, which is the
chloroplast equivalent of the mitochondrial matrix space (Figure 12–19B). They
have an additional subcompartment, the thylakoid space, which is surrounded by
the thylakoid membrane. The thylakoid membrane derives from the inner membrane
during plastid development and is pinched off to become discontinuous
with it. Each of the subcompartments in mitochondria and chloroplasts contains
a distinct set of proteins.
New mitochondria and chloroplasts are produced by the growth of preexisting
organelles, followed by fission (discussed in Chapter 14). The growth depends
mainly on the import of proteins from the cytosol. The imported proteins must
be transported across a number of membranes in succession and end up in the
appropriate place. The process of protein movement across membranes is called
protein translocation. This section explains how it occurs.
NUCLEUS
PLASTIDS
MITOCHONDRIA
PEROXISOMES
ENDOPLASMIC RETICULUM
LATE ENDOSOME
LYSOSOME
EARLY ENDOSOME
CYTOSOL
GOLGI
CELL EXTERIOR
SECRETORY
VESICLES
MBoC6 mp678/p713
(A) MITOCHONDRION
(B) CHLOROPLAST
outer
membrane
cristae
space
inner
membrane
matrix space
intermembrane
space
outer
membrane
inner
membrane
intermembrane
space
thylakoid
membrane
stroma
(matrix space)
thylakoid
space
Figure 12–19 The subcompartments
of mitochondria and chloroplasts. In
contrast to the cristae of mitochondria (A),
the thylakoids of chloroplasts (B) are not
connected to the inner membrane and
therefore form a sealed compartment with
a separate internal space.