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
665
thylakoid precursor
protein
thylakoid
signal
sequence
chloroplast
signal
sequence
receptor
protein
in TOC
complex
(A)
thylakoid
membrane
thylakoid
space
GTP- OR ATP-
DEPENDENT
TRANSLOCATION
INTO STROMA
TOC complex
TIC complex
FOUR ROUTES TO TRANSLOCATE
PROTEIN INTO THYLAKOID SPACE
chloroplast outer membrane
chloroplast inner membrane
CLEAVAGE OF
CHLOROPLAST SIGNAL SEQUENCE
thylakoid
membrane
CYTOSOL
STROMA
exposed thylakoid
signal sequence
mature protein
in thylakoid space
1 2 3
4
STROMA
THYLAKOID
THYLAKOID
Figure 12–26 Translocation of
chloroplast precursor proteins into
the thylakoid space. (A) The precursor
protein contains an N-terminal chloroplast
signal sequence (red), followed immediately
by a thylakoid signal sequence (brown).
The chloroplast signal sequence initiates
translocation into the stroma by a
mechanism similar to that used for the
translocation of mitochondrial precursor
proteins into the matrix space, although
the translocator complexes, TOC and TIC,
are different. The signal sequence is then
cleaved off, unmasking the thylakoid signal
sequence, which initiates translocation
across the thylakoid membrane.
(B) Translocation into the thylakoid space or
thylakoid membrane can occur by any one
of at least four routes: (1) a Sec pathway,
so called because it uses components
that are homologs of Sec proteins, which
mediate protein translocation across the
bacterial plasma membrane (discussed
later); (2) an SRP-like pathway, so called
because it uses a chloroplast homolog
of the signal-recognition particle, or
SRP (discussed later); (3) a TAT (twin
arginine translocation) pathway, so called
because two arginines are critical in the
signal sequences that direct proteins into
this pathway, which depends on the H +
gradient across the thylakoid membrane;
and (4) a spontaneous insertion pathway
that seems not to require any protein
translocator.
energy
requirements
(B)
ATP +
H + electrochemical
gradient
ATP +
H + electrochemical
gradient
H + electrochemical
gradient
none
Sec pathway SRP-like pathway TAT pathway spontaneous
insertion
double membrane. The functional similarities may thus result from convergent
evolution, reflecting the common requirements for translocation across a double
membrane.
Although the signal sequences for import into chloroplasts superficially
resemble those for import into MBoC6 mitochondria, m12.29/12.29the same plant cells have both
mitochondria and chloroplasts, so proteins must partition appropriately between
the two organelles. In plants, for example, a bacterial enzyme can be directed
specifically to mitochondria if it is experimentally joined to an N-terminal signal
sequence of a mitochondrial protein; the same enzyme joined to an N-terminal
signal sequence of a chloroplast protein ends up in chloroplasts. Thus, the import
receptors on each organelle distinguish between the different signal sequences.
Chloroplasts have an extra membrane-enclosed compartment, the thylakoid.
Many chloroplast proteins, including the protein subunits of the photosynthetic
system and of the ATP synthase (discussed in Chapter 14), are located in the thylakoid
membrane. Like the precursors of some mitochondrial proteins, the precursors
of these proteins are translocated from the cytosol to their final destination
in two steps. First, they pass across the double membrane into the matrix space
(called the stroma in chloroplasts), and then they either integrate into the thylakoid
membrane or translocate into the thylakoid space (Figure 12–26A). The precursors
of these proteins have a hydrophobic thylakoid signal sequence following
the N-terminal chloroplast signal sequence. After the N-terminal signal sequence
has been used to import the protein into the stroma, a stromal signal peptidase
removes it, unmasking the thylakoid signal sequence that initiates transport