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Molecular Chaperones
The Hsp70 Super-family
Pathways For Protein Folding
Protein Folding in Cells Occurs in a Crowded
Environment where Misfolding & Aggregation
are Favored
Hsp 70 Family Proteins Assist Protein Folding
in an Energy Dependent Process
A. Hsp70 domain
structure & Cochaperone sites
B. Hsp70
chaperoning
Cycle
The Hsp 70 Super-Family Contains Diverse Members With
Related Domain Structures
Protein Folding in Eukaryotes Involves MultiChaperone Systems
Hsp90 assisted
Hsp60
assisted
The Hsp70 Super Family Contains Distinct Sub-Families
The C. elegans Hsp70 Chaperones
• Cytoplasmic & Nuclear
– Hsp70
– Hsp-1
– Hsp110 homologue
• Endoplasmic Reticulum
– Hsp-3
– Hsp-4
– Grp170 homologue
• Mitochondria
– Hsp-6
• Hsp-5 is a pseudogene
Hsp 70 Family in Drosophila
70 kDa Hsps
Locus
Heat-shock-protein70Ba
87B12-87B12
Heat-shock-protein70Bb
87B14-87B14
Hsp70Bbb
Heat-shock-protein70Bc
87B14-87B14
87B14-87B1587A3-87A3
Hsc70-3 Heat shock protein cognate (Grp78)
Large MW Hsp70s
Hsc70 Cb (hsp110)
70C-b
87A3
Hsp70 Family Members are Found in All
Major Cellular Compartments
• Cytoplasm & Nucleus
– Hsp70 (multiple paralogs)
– Hsp110 (Multiple Paralogs)
• Mitochondria & Chloroplasts
– Hsp70
• Endoplasmic Reticulum
– Grp78 (hsp 70 multiple paralogs)
– Grp170
Cells Respond to Stress by Increased
Synthesis of Molecular Chaperones
• Misfolded proteins in the cytoplasm &
nucleus induce the heat shock response,
synthesis of additional heat shock proteins
- Hsp’s
• Misfolded proteins in the endoplasmic
reticulum induce the unfolded protein
response, synthesis of additional ER
chaperones - Grp’s
The Unfolded Protein Response (UPR)
Hsp4 (grp78)
grp170
XBP-1
IRE-1
• The UPR occurs when proteins are misfolded in the endoplasmic reticulum
(ER).
• Reducing agents, such as DTT, interfere with disulfide bond formation
while drugs can interfere with glycosylation; both agents cause proteins to
misfold in the ER thus triggering the UPR.
• The product of the ire-1 gene is the sensor of misfolded proteins and when
activated removes an intron from the pre mRNA from the xbp-1 gene.
• Active xbp-1 protein (from spliced mRNA) activates the genes that code for
ER chaperones, such as hsp-4.
Cellular Stresses Promote Misfolding
and Aggregation of Proteins
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High temperatures
Heavy metals
Alcohol
Anoxia
Osmotic stress
Energy starvation
Reducing agents
The Structure of Yeast Hsp110
Hsp110 is a Nucleotide
Exchange Factor for Hsp70
Summary
Efficient folding of many newly synthesized proteins depends on assistance from
molecular chaperones, which serve to prevent protein misfolding and aggregation
in the crowded environment of the cell. Nascent chains binding chaperones,
including trigger factor, Hsp70, and prefoldin, stabilize elongating chains on
ribosomes in a Non-aggregated state. Folding in the cytosol is achieved either on
controlled chain release from these factors or after transfer of newly synthesized
proteins to downstream chaperones, such as the chaperonins. These are large,
cylindrical complexes that provide a central compartment for a single protein
chain to fold unimpaired by aggregation. Understanding how the thousands of
different proteins synthesized in a cell use this chaperone machinery has
profound implications for biotechnology and medicine. -