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8 Protein Localization (Full Edition)
6 The Hsp70 family is ubiquitous
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Hsp70 is a chaperone that functions on target proteins in conjunction with DnaJ and GrpE.
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Members of the Hsp70 family are found in the cytosol, in the ER, and in mitochondria and chloroplasts.
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The Hsp70 family is found in bacteria,
eukaryotic cytosol, in the endoplasmic reticulum, and in chloroplasts
and mitochondria (for review see 2284). A typical Hsp70 has
two domains: the N-terminal domain is an ATPase; and the C-terminal
domain binds the substrate polypeptide (2337; 2338).
When bound to ATP, Hsp70 binds and releases substrates rapidly; when
bound to ADP, the reactions are slow. Recycling between these states is
regulated by two other proteins, Hsp40 (DnaJ) and GrpE.
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Figure 8.13
Hsp40 binds the substrate and then Hsp70. ATP
hydrolysis drives conformational change. GrpE displaces the ADP; this
causes the chaperones to be released. Multiple cycles of association
and dissociation may occur during the folding of a substrate protein.
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Figure 8.13 shows that Hsp40 (DnaJ) binds first to a nascent protein as it emerges
from the ribosome. Hsp40 contains a region called the J domain (named
for DnaJ), which interacts with Hsp70. Hsp70 (DnaK) binds to both Hsp40
and to the unfolded protein. In effect, two interacting chaperones bind
to the protein. The J domain accounts for the specificity of the
pairwise interaction, and drives a particular Hsp40 to select the
appropriate partner from the Hsp70 family.
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The interaction of Hsp70 (DnaK) with
Hsp40 (DnaJ) stimulates the ATPase activity of Hsp70. The ADP-bound
form of the complex remains associated with the protein substrate until
GrpE displaces the ADP. This causes loss of Hsp40 followed by
dissociation of Hsp70. The Hsp70 binds another ATP and the cycle can be
repeated. GrpE (or its equivalent) is found only in bacteria,
mitochondria, and chloroplasts; in other locations, the dissociation
reaction is coupled to ATP hydrolysis in a more complex way.
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Protein folding is accomplished by
multiple cycles of association and dissociation. As the protein chain
lengthens, Hsp70 (DnaK) may dissociate from one binding site and then
reassociate with another, thus releasing parts of the substrate protein
to fold correctly in an ordered manner. Finally, the intact protein is
released from the ribosome, folded into its mature conformation (for
review see 54; 63; 66).
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Different members of the Hsp70 class
function on various types of target proteins. Cytosolic proteins (the
eponymous Hsp70 and a related protein called Hsc70) act on nascent
proteins on ribosomes. Variants in the ER (called BiP or Grp78 in
higher eukaryotes, called Kar2 in S. cerevisiae), or in
mitochondria or chloroplasts, function in a rather similar manner on
proteins as they emerge into the interior of the organelle on passing
through the membrane.
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What feature does Hsp70 recognize in a
target protein? It binds to a linear stretch of amino acids embedded in
a hydrophobic context (2339; 2340). This is precisely
the sort of motif that is buried in the hydrophobic core of a properly
folded, mature protein. Its exposure therefore indicates that the
protein is nascent or denatured. Motifs of this nature occur about
every 40 amino acids. Binding to the motif prevents it from
misaggregating with another one.
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This mode of action explains how the
Hsp70 protein Bip can fulfill two functions: to assist in
oligomerization and/or folding of newly translocated proteins in the
ER; and to remove misfolded proteins. Suppose that BiP recognizes
certain peptide sequences that are inaccessible within the conformation
of a mature, properly folded protein. These sequences are exposed and
attract BiP when the protein enters the ER lumen in an essentially
one-dimensional form. And if a protein is misfolded or denatured, they
may become exposed on its surface instead of being properly buried.
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Last Revised on 2-12-2002
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54 Georgopoulos, C. and Welch, W. J.
(1993).
Role of the major heat shock proteins as molecular chaperones.
Annu. Rev. Cell Biol. 9, 601-634.
PubMed Journal
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63 Hartl, F.-U.
(1966).
Molecular chaperones in cellular protein folding.
Nature 381, 571-580.
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66 Bukau, B. and Horwich, A. L.
(1998).
The Hsp70 and Hsp60 chaperone machines.
Cell 92, 351-366.
PubMed Journal
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2284 Frydman, J.
(2001).
Folding of newly translated proteins in vitro: the role of molecular chaperones.
Annu. Rev. Biochem. 70, 603-647.
PubMed Journal
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2337 Flaherty, K. M., DeLuca-Flaherty, C., and McKay, D. B.
(1990).
Three-dimensional structure of the ATPase fragment of a 70K heat-shock cognate protein.
Nature 346, 623-628.
PubMed Journal
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2338 Zhu, X., Zhao, X., Burkholder, W. F., Gragerov, A., Ogata, C. M., Gottesman, M. E., Hendrickson, and, Gottesman, M. E.
(1996).
Structural analysis of substrate binding by the molecular chaperone DnaK.
Science 272, 1606-1614.
PubMed
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2339 Flynn, G. C., Pohl, J., Flocco, M. T., and Rothman, J. E.
(1991).
Peptide-binding specificity of the molecular chaperone BiP.
Nature 353, 726-730.
PubMed Journal
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2340 Blond-Elguindi, S., Cwirla, S. E., Dower, W. J., Lipshutz, R. J., Sprang, S. R., Sambrook, J. F., and Gething, M. J.
(1993).
Affinity panning of a library of peptides displayed on bacteriophages reveals the binding specificity of BiP.
Cell 75, 717-728.
PubMed Journal
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© Jones and Bartlett Publishers (2007)
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