Molecular Chaperones

1
Molecular Chaperones

• The Hsp70 Super-family

2
Pathways For Protein Folding
3
Protein Folding in Cells Occurs in a Crowded
Environment where Misfolding Aggregation are
Favored
4
Hsp 70 Family Proteins Assist Protein Folding in
an Energy Dependent Process

1. Hsp70 domain structure Co-chaperone
   sites
2. Hsp70 chaperoning Cycle

5
The Hsp 70 Super-Family Contains Diverse Members
With Related Domain Structures
6
Protein Folding in Eukaryotes Involves
Multi-Chaperone Systems
Hsp60 assisted
Hsp90 assisted
7
The Hsp70 Super Family Contains Distinct
Sub-Families
8
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

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Hsp 70 Family in Drosophila

87B12-87B12
87B14-87B14
87B14-87B14
87B14-87B1587A3-87A3
70 kDa Hsps Locus
Heat-shock-protein-70Ba
Heat-shock-protein-70Bb
Hsp70Bbb
Heat-shock-protein-70Bc
Hsc70-3 Heat shock protein cognate (Grp78)
87A3
Large MW Hsp70s Hsc70 Cb (hsp110)
70C-b
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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

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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 - Hsps
• Misfolded proteins in the endoplasmic reticulum
  induce the unfolded protein response, synthesis
  of additional ER chaperones - Grps

12
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.

13
Cellular Stresses Promote Misfolding and
Aggregation of Proteins

• High temperatures
• Heavy metals
• Alcohol
• Anoxia
• Osmotic stress
• Energy starvation
• Reducing agents

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The Structure of Yeast Hsp110
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Hsp110 is a Nucleotide Exchange Factor for Hsp70
16
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. -