2. Jens Martensson
Uptake of Proteins into Peroxisomes
• Peroxisomes are very simple organelles having only two
subcompartments in which an imported protein can be placed: The
boundary membrane and the internal matrix.
• Proteins destined for a peroxisome possess a peroxisomal targeting
signal, either a PTS for a peroxisomal matrix protein or an mPTS for a
peroxisomal membrane protein. Several different PTSs, mPTSs, and
PTS receptors have been identified. PTS receptors bind to
peroxisome-destined proteins in the cytosol and shuttle them to the
surface of the peroxisome, where they can enter the organelle.
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3. Jens Martensson
Uptake of Proteins into Peroxisomes
• Unlike mitochondria and chloroplasts, whose imported proteins
must assume an unfolded state, peroxisomes are somehow able to
import peroxisomal matrix proteins in their native, folded
conformation, even those that consist of several subunits. The
mechanism by which peroxisomes are able to accomplish this
daunting assignment remains a matter of speculation.
• Some evidence suggests that at least one of the PTS receptors
(PEX5) moves from the cytosol into the peroxisomal membrane
where it forms a transient pore that facilitates the movement of
proteins into the peroxisome.
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4. Jens Martensson
Uptake of Proteins into Peroxisomes
• Peroxisomes are a type of microbody. Microbodies are cell organelles
bounded by a cell membrane and are used for a variety of different
processes. For example, peroxisomes contain enzymes which produce
hydrogen peroxide (and have the means for destroying it). In addition,
plants have glyoxisomes which contain the enzymes of the glyoxylate
cycle and yeasts have a variety of microbodies including ones involved
in methanol oxidation.
• In the peroxisome, the signal sequences are removed and PEX5 is
recycled with the help of PEX2, 10, and 12.
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5. Jens Martensson
Uptake of Proteins into Mitochondria
Mitochondria have four subcompartments into which proteins
can be delivered:
1. an outer mitochondrial membrane (OMM)
2. inner mitochondrial membrane (IMM)
3. intermembrane space
4. matrix
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6. Jens Martensson
Uptake of Proteins into Mitochondria
• This involves both terminal signal and internal signal patches
• In some cases, proteins for import are packaged with
chaperones, espescially Hsp70, but in other cases it would
appear that the fully-folded protein is imported intact.
• There are two major recognition sites on the outer membrane,
one recognizing fully folded proteins, the other proteins
associated with chaperones
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7. Jens Martensson
Uptake of Proteins into Mitochondria
Figure 8.47 Importing proteins into a mitochondrion.
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9. Jens Martensson
Uptake of Proteins into Mitochondria
• Before a protein can enter a mitochondrion, several events are thought
to take place. First, the protein must be presented to the mitochondrion
in a relatively extended, or unfolded, state (steps 1 and A, Figure
8.47a). Several different molecular chaperones (e.g., Hsp70 and
Hsp90) have been implicated in preparing polypeptides for uptake into
mitochondria, including ones that specifically direct mitochondrial
proteins to the cytosolic surface of the OMM (Figure 8.47a).
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10. Jens Martensson
Uptake of Proteins into Mitochondria
• The OMM contains a protein-import complex, the TOM complex, which
includes
(1) receptors that recognize and bind mitochondrial proteins
(2) protein-lined channels through which unfolded polypeptides are
translocated across the outer membrane
• Proteins that are destined for the IMM or matrix must pass through the
intermembrane space and engage a second protein-import complex
located in the IMM, called a TIM (translocase of outer membrane)
complex.
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11. Jens Martensson
Uptake of Proteins into Chloroplasts
• Chloroplasts have six subcompartments into which proteins can be
delivered:
1.inner envelope membrane
2.outer envelope membrane
3.intervening intermembrane space
4.the stroma
5.thylakoid membrane
6.thylakoid lumen
• Transfer in chloroplasts is very similar to transfer in mitochondria.
Thykaloid proteins carry two sequences, one specifies matrix, the
second thykaloid.
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12. Jens Martensson
Uptake of Proteins into Chloroplasts
• Chloroplast and mitochondrial import mechanisms exhibit many
similarities, although their translocation machineries have evolved
independently. As in the mitochondria,
1. the vast majority of chloroplast proteins (approximately 3000 in higher plants) are
imported from the cytosol,
2. the outer and inner envelope membranes contain distinct translocation complexes
(Toc and Tic complexes, respectively) that work together during import,
3. chaperones aid in the unfolding of the polypeptides in the cytosol and folding of
the proteins in the chloroplast, and
4. most proteins destined for the chloroplast are synthesized with a removable N-
terminal sequence (termed the transit peptide) that is highly variable in length and
sequence.
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References:
• Karp, Gerald. Cell and Molecular Biology: Concepts and
Experiments. 7th Edition. John Wiley & James G. Patton, Inc.
p. 316-318.
• Karp, Gerald. Cell and Molecular Biology: Concepts and
Experiments. 8th Edition. WileyPLUS Learning Space. Janet
Iwasa & Wallace Marshall, Inc. p. 304-307.
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Peroxisomes are very simple organelles having only two subcompartments in which an imported protein can be placed: the boundary membrane and the internal matrix (Section 5.10). Proteins
destined for a peroxisome possess a peroxisomal targeting signal , either a PTS for a peroxisomal matrix protein or an mPTS for a peroxisomal membrane protein. Several different PTSs, mPTSs, and PTS receptors have been identified. PTS receptors bind to peroxisome‐ destined proteins in the cytosol and shuttle them to the surface of the peroxisome, where they can enter the organelle. Recent studies have shown that peroxisomes form from the fusion of two distinct types of ER‐derived vesicles, each of which carries half of a peroxisomal translocon (also called the importomer). Fusion of the vesicles allows for the formation of a complete peroxisomal translocon, which then facilitates the movement of proteins into the peroxisome. Unlike mitochondria and chloroplasts, whose imported proteins must assume an unfolded state, peroxisomes are somehow able to import peroxisomal matrix proteins in their native, folded conformation, even those that consist of several subunits. The mechanism by which peroxisomes are able to accomplish this daunting assignment remains a matter of speculation.
