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

THE TRANSPORT OF PROTEINS INTO MITOCHONDRIA AND CHLOROPLASTS
661
precursor
protein
signal
sequence
BINDING TO
IMPORT RECEPTORS
receptor
protein in
TOM complex
INSERTION INTO
MEMBRANE BY
TOM COMPLEX
TIM23
complex
TOM
complex
TRANSLOCATION
INTO MATRIX BY
TIM23 COMPLEX
outer mitochondrial membrane
inner mitochondrial membrane
CLEAVAGE
BY SIGNAL
PEPTIDASE
CYTOSOL
MATRIX
SPACE
mature
mitochondrial
protein
cleaved
signal peptide
Figure 12–22 Protein import by
mitochondria. The N-terminal signal
sequence of the mitochondrial precursor
protein is recognized by receptors
of the TOM complex. The protein is
then translocated through the TIM23
complex so that it transiently spans both
mitochondrial membranes (Movie 12.3).
The signal sequence is cleaved off by a
signal peptidase in the matrix space to
form the mature protein. The free signal
sequence is then rapidly degraded (not
shown).
Although the TOM and TIM complexes usually work together to translocate
precursor proteins across both membranes at the same time, they can work independently.
In isolated outer membranes, for example, the TOM complex can
translocate the signal sequence MBOC6 of precursor m12.25/12.25 proteins across the membrane. Similarly,
if the outer membrane is experimentally disrupted in isolated mitochondria,
the exposed TIM23 complex can efficiently import precursor proteins into
the matrix space.
ATP Hydrolysis and a Membrane Potential Drive Protein Import
Into the Matrix Space
Directional transport requires energy, which in most biological systems is supplied
by ATP hydrolysis. ATP hydrolysis fuels mitochondrial protein import at
two discrete sites, one outside the mitochondria and one in the matrix space. In
addition, protein import requires another energy source, which is the membrane
potential across the inner mitochondrial membrane (Figure 12–23).
The first requirement for energy occurs at the initial stage of the translocation
process, when the unfolded precursor protein, associated with chaperone proteins,
interacts with the import receptors of the TOM complex. As discussed in
Chapter 6, the binding and release of newly synthesized polypeptides from the
chaperone proteins requires ATP hydrolysis.
cytosolic hsp70
chaperones
CYTOSOL
receptor protein
in TOM complex
1
ADP
ATP
+
outer mitochondrial
membrane
P i
+
+ + +
MATRIX
SPACE
inner mitochondrial
membrane
TIM23 complex
membrane
potential
–
– – –
mitochondrial hsp70
2 (part of import ATPase)
3
ATP
ADP
+ P i
energy-dependent
conformational
change in import ATPase
Figure 12–23 The role of energy in protein import into the mitochondrial matrix space. (1) Bound cytosolic hsp70
chaperone is released from the precursor protein in a step that depends on ATP hydrolysis. After initial insertion of the signal
sequence and of adjacent portions of the polypeptide chain into the TOM complex translocation channel, the signal sequence
interacts with a TIM complex. (2) The signal sequence is then translocated into the matrix space in a process that requires the
energy in the membrane potential across the inner membrane. (3) Mitochondrial hsp70, which is part of an import ATPase
complex, binds to regions of the polypeptide chain as they become exposed in the matrix space, pulling the protein through the
translocation channel, using the energy of ATP hydrolysis.