Mitochondria, chloroplasts, and peroxisome import'

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Chapter 12
Lecture 31, pages 678-688.

• Mitochondria, chloroplasts, and peroxisome import.

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• Organelles specialized for ATP synthesis.
• Most, but not all, proteins are encoded by the
  nuclear genome and synthesized in the cytosol.
• Proteins must be transported to one of multiple
  membranes or compartments.

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• Import of the protein into the matrix is directed
  by an N-terminal signal sequence.
• For polypeptides encoded by the nuclear genome,
  synthesis of the polypeptide is first completed
  in the cytosol. Transport occurs by a
  posttranslational mechanism.
• Signal sequence at the N-terminus associates with
  the TOM complex located in the outer
  mitochondrial membrane. TOM is both a receptor
  for the signal sequence and a protein
  translocator.
• The polypeptide is passed from TOM to TIM in the
  innermembrane. During the transport process, the
  polypeptide traverses both inner and outer
  membranes via the two translocators at a point
  known as a contact site.
• The polypeptide is imported and the signal
  peptide is removed by a signal peptidase.

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• The energetics of import - import requires ATP
  hydrolysis and an electrochemical proton
  gradient.
• ATP hydrolysis regulates the association of
  chaperone proteins in the cytosol that serve to
  keep the polypeptide in an unfolded state prior
  to association with TOM.
• Electrochemical proton gradient in the inner
  membrane draws the signal sequence through TIM
  into the matrix.
• Chaperone proteins in the matrix use the energy
  of ATP hydrolysis to pull the polypeptide into
  the matrix and guide proper folding.

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The N-terminal signal sequence for mitochondrial
import is a positively charged, amphipathic alpha
helix. Hence, the membrane potential across the
inner mitochondrial membrane may electrophorese
the signal sequence through the TIM complex.
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A second signal sequence in the polypeptide can
be involved in directing a protein to a
destination in the mitochondria other than the
matrix.
All of the signal sequences are contained within
amino acid sequence of the imported protein, and
they function subsequent to the functioning of
the N-terminal signal sequence.
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Comparison of mitochondrial and nuclear import.
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Transport into chloroplasts is similar to
mitochondria except there is a 3rd membrane that
can be targeted. Targeting the thylakoid membrane
involves a second signal sequence.
In the case of chloroplasts, the electrochemical
proton gradient is at the thylakoid membrane
where this gradient participates in transport.
Transport across the chloroplast inner membrane
is powered by GTP and ATP hydrolysis.
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Peroxisomes are organelles that perform a variety
of oxidation reactions including ones that
breakdown fatty acids and toxic molecules that
enter the cell from the blood stream. Genetic
defects in peroxisomes often cause neurological
problems because a particular lipid found in
myelin is produced in the peroxisomes. Import of
proteins involves short signal sequences. The
most unusual aspect of the transport process is
that oligomeric proteins dont have to unfold.
Very little is known about the transport process.