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Summary Molecular Biology of the Cell 2 (book) (WBFA007-04)
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MBOC 2 NOTESChapter 12
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The Transport of proteins into mitochondria and Chloroplasts
Mitochondria and chloroplasts (a specialized form of plastids in green algae and plant cells)
- double-membrane-enclosed organelles.
- They specialize in ATP synthesis, using energy derived from electron transport and oxidative
phos- phorylation in mitochondria and from photosynthesis in chloroplasts.
- contain their own DNA
There are different sub compartments 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 mito- chondrial membranes:
▪ the inner membrane, which encloses the matrix space and forms
extensive invaginations called cristae
▪ the outer membrane, which is in contact with the cytosol.
Chloroplasts:
▪ outer membrane
▪ inner membrane, which enclose an intermembrane space, and the
stroma, which is the chloroplast equivalent of the mitochondrial
matrix space (Figure 12–19B).
▪ an additional subcompartment, the thylakoid space, which is surrounded by the
thylakoid membrane. The thylakoid membrane derives from the inner mem- brane
during plastid development and is pinched off to become discontinuous with it.
,Translocation into mitochondria Depends on signal sequences and protein Translocators
Mitochondrial proteins are first fully synthesized as mitochondrial precursor proteins in the
cytosol and then translocated into mitochondria by a post-translational mechanism.
signal sequences:
▪ direct all mitochondrial precursor proteins to their appropriate mitochondrial
subcompartment.
▪ signal sequence at the N-terminus that a signal peptidase rapidly removes after import.
▪ The signal sequences are both necessary and sufficient for the import and correct
localization of the proteins.
▪ Form an amphiphilic α helix, in which positively
charged residues cluster on one side of the helix,
while uncharged hydrophobic residues cluster on
the opposite side. Specific receptor proteins that
initiate protein translocation recognize this
configuration rather than the precise amino acid
sequence of the signal sequence
Multisubunit protein complexes that function as protein
translocators mediate protein movement across
mitochondrial membranes.
▪ The TOM complex transfers proteins across the outer membrane
- Required for the import of all nucleus-encoded mitochondrial proteins. It initially
transports their signal sequences into the intermembrane space and helps to
insert transmembrane proteins into the outer membrane.
▪ Two TIM complexes (TIM23 and TIM22) transfer proteins across the inner membrane.
- TIM23 complex transports some soluble proteins into the matrix space and helps
to insert transmembrane proteins into the inner membrane
- The TIM22 complex mediates the insertion of a subclass of inner membrane
proteins, including the transporter that moves ADP, ATP, and phosphate in and
out of mitochondria.
▪ the OXA complex (protein translocator in the inner mitochondrial membrane),
- mediates the insertion of those
inner membrane proteins that are
synthesized within mitochondria.
- insert some imported inner
membrane proteins that are
initially transported into the matrix
space by the other complexes.
▪ The SAM complex
- β-barrel proteins, which are particularly
abundant in the outer membrane, are
then passed on to an additional
translocator which helps them to fold
properly in the outer membrane
,Mitochondrial precursor proteins are imported as Unfolded polypeptide
Chains
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.
Interacting protein:
▪ general chaperone proteins of the hsp70 family.
▪ 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
▪ A protein could reach the mitochondrial matrix space by either crossing the two
membranes all at once or crossing one at a time.
▪ The TOM and TIM complexes usually work together to translocate precursor proteins
across both membranes at the same time, they can work independently
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. 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). Protein import by mitochondria.
, ATP hydrolysis and a membrane potential Drive protein import into the matrix space
- ATP hydrolysis fuels mitochondrial protein import at two discrete sites, one outside
the mitochondria and one in the matrix space
- 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.
- The binding and release of newly synthesized polypeptides from the chaperone
proteins requires ATP hydrolysis.
- The mitochondrial hsp70 is part of a multisubunit protein assembly that is bound to
the matrix side of the TIM23 complex and acts as a motor to pull the precursor
protein into the matrix space
- HSP70 high affinity for unfolded polypeptide chains, and it binds tightly to an
imported protein chain as soon as the chain emerges from the TIM translocator in
the matrix space undergoes a conformational change and releases the protein chain
in an ATP-dependent step, exerting a ratcheting/pulling force on the protein being
imported. After the initial interaction with mitochondrial hsp70, many imported
matrix proteins are passed on to another chaperone protein, mitochondrial hsp60.
hsp60 helps the unfolded polypeptide chain to fold by binding and releasing it
through cycles of ATP hydrolysis.
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.
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