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

660 Chapter 12: Intracellular Compartments and Protein Sorting
outer mitochondrial
membrane
TIM22
COMPLEX
inner
mitochondrial
membrane
translocation
channel
import
ATPase
TOM COMPLEX
receptors
TIM23 COMPLEX
SAM
COMPLEX
OXA
COMPLEX
CYTOSOL
INTERMEMBRANE
SPACE
MATRIX
SPACE
those inner membrane proteins that are synthesized within mitochondria. It also
helps to insert some imported inner membrane proteins that are initially transported
into the matrix space by the other complexes.
Figure 12–21 The protein translocators
in the mitochondrial membranes. The
TOM, TIM, SAM, and OXA complexes are
multimeric membrane protein assemblies
that catalyze protein translocation across
mitochondrial membranes. The protein
components of the TIM22 and TIM23
complexes that line the import channel are
structurally related, suggesting a common
evolutionary origin of both TIM complexes.
On the matrix side, the TIM23 complex
is bound to a multimeric protein complex
containing mitochondrial hsp70, which
acts as an import ATPase, using ATP
hydrolysis to pull proteins through the pore.
In animal cells, subtle variations exist in the
subunit composition of the translocator
complexes to adapt the mitochondrial
import machinery to the particular needs of
specialized cell types. SAM = Sorting and
Assembly Machinery; OXA = cytochrome
OXidase Activity; TIM = Translocator of the
Inner Mitochondrial membrane;
TOM = Translocator of the Outer
Membrane.
Mitochondrial Precursor Proteins Are Imported as Unfolded
Polypeptide Chains
We have learned almost everything we know about the molecular mechanism of
protein import into mitochondria from analyses of cell-free, reconstituted translocation
systems, in which purified mitochondria in a test tube import radiolabeled
mitochondrial precursor proteins. By changing the conditions in the test
tube, it is possible to establish the biochemical requirements for the import.
Mitochondrial precursor proteins do not fold into their native structures after
they are synthesized; instead, they remain unfolded in the cytosol through interactions
with other proteins. Some of these interacting proteins are general chaperone
proteins of the hsp70 family (discussed in Chapter 6), whereas others are
MBoC6 m12.23/12.23
dedicated to mitochondrial precursor proteins and bind directly to their signal
sequences. All the interacting proteins help to prevent the precursor proteins
from aggregating or folding up spontaneously before they engage with the TOM
complex in the outer mitochondrial membrane. As a first step in the import process,
the import receptors of the TOM complex bind the signal sequence of the
mitochondrial precursor protein. The interacting proteins are then stripped off,
and the unfolded polypeptide chain is fed—signal sequence first—into the translocation
channel.
In principle, a protein could reach the mitochondrial matrix space by either
crossing the two membranes all at once or crossing one at a time. One can distinguish
between these possibilities by cooling a cell-free mitochondrial import
system to arrest the proteins at an intermediate step in the translocation process.
The result is that the arrested proteins no longer contain their N-terminal signal
sequence, indicating that the N-terminus must be in the matrix space where the
signal peptidase is located, but the bulk of the protein can still be attacked from
outside the mitochondria by externally added proteolytic enzymes. Clearly, the
precursor proteins can pass through both mitochondrial membranes at once to
enter the matrix space (Figure 12–22). The TOM complex first transports the signal
sequence across the outer membrane to the intermembrane space, where it
binds to a TIM complex, opening the channel in the complex. The polypeptide
chain is then either translocated into the matrix space or inserted into the inner
membrane.