Chaperone Proteins - RGUHS MD Pathology (10 Marks)

Definition

Why Are Chaperones Needed?

1. Aggregation - unfolded or partially folded polypeptides expose hydrophobic regions that stick to each other, forming insoluble, cytotoxic aggregates.
2. Kinetic trapping - proteins may fold into incorrect local energy minima rather than their native conformation.

• Harper's Illustrated Biochemistry, 32nd Ed

Classification and Major Families

1. HSP70 Family (Hsp70, BiP/GRP78)

• Bind short hydrophobic sequences that emerge from the ribosome during synthesis (co-translational folding).
• Prevent premature aggregation by shielding hydrophobic segments.
• Operate via an ATP-dependent cycle: ADP-chaperone complex has high affinity for unfolded protein; ATP binding causes release of folded segments.
• Found in cytosol, mitochondria, and ER lumen (where it is called BiP/GRP78 - the master regulator of the Unfolded Protein Response).
• Work with co-chaperones of the DNAJ/HSP40 family which stimulate ATPase activity.

2. HSP60 Family / Chaperonins (GroEL-GroES in bacteria; HSP60 in eukaryotes)

• Act later in the folding process, often together with HSP70.
• Are oligomeric structures (MW ~800 kDa) - much larger than HSP70 monomers (70-100 kDa).
• The bacterial prototype GroEL is composed of two rings, each of 7 identical subunits, forming a barrel-like structure.
• GroES acts as a lid that encapsulates the unfolded protein.
• The central cavity provides a sheltered, nonpolar environment in which a misfolded polypeptide can unfold and re-fold correctly away from other proteins.
• HSP60 is the eukaryotic equivalent of GroEL.

3. HSP90 Family

• Specialized for folding of client proteins such as steroid hormone receptors, transcription factors, kinases, phosphatases, and signaling molecules.
• Hold client proteins in a partially folded, primed state ready for regulated activation.
• Mutant or oncogenic kinases (e.g., in cancer) depend on HSP90 - this is exploited therapeutically with HSP90 inhibitors.

4. HSP33

• A redox-regulated chaperone activated under oxidative stress conditions.

5. Small HSPs (sHSPs)

• Act as a first line of defense by binding misfolded proteins and presenting them to ATP-dependent chaperones for refolding or degradation.

6. ER-Resident Chaperones

• Calnexin and calreticulin - lectin-like chaperones that bind to incompletely glycosylated proteins in the ER.
• Protein Disulfide Isomerase (PDI) - catalyzes correct disulfide bond formation by reshuffling S-S bonds.
• Peptidyl Prolyl Isomerase (PPI) - isomerizes X-Pro bonds from trans to cis where required.
• BiP (GRP78) - the key HSP70-family chaperone of the ER lumen.
• Harrison's Principles of Internal Medicine, 22nd Ed | Harper's Illustrated Biochemistry, 32nd Ed

Key Properties of Chaperone Proteins (Table)

• Harper's Illustrated Biochemistry, 32nd Ed

Role in ER Quality Control and the Unfolded Protein Response (UPR)

1. Retained in the ER by chaperones.
2. Degraded via ER-associated degradation (ERAD) - retrotranslocated to the cytoplasm and targeted to the ubiquitin-proteasome system (UPS).

• Reduction in global protein synthesis (to reduce load).
• Upregulation of chaperone proteins (BiP/GRP78 is a hallmark).
• If the overload cannot be corrected - apoptosis is triggered.

• Robbins, Cotran & Kumar Pathologic Basis of Disease

Role in Protein Translocation to Organelles

• Proteins must be maintained in an unfolded state to pass through translocon pores (TOM/TIM complexes in mitochondria).
• HSP70 binds the unfolded preprotein and "pulls" it through the membrane using ATP hydrolysis.
• After import, organelle-specific chaperones (mitochondrial HSP70, HSP60) facilitate refolding inside the organelle.

Chaperones in Disease Pathology

Disaggregases - A Special Chaperone Function

• Bacteria: ClpB + DnaK/HSP70
• Yeast/plants: HSP104
• Mammals: HSP70 + HSP110 + J-domain proteins

• Harrison's Principles of Internal Medicine, 22nd Ed

Summary Diagram (Chaperone Cycle)

Nascent polypeptide (ribosome)
        ↓
   HSP70/DNAJ binding (prevents aggregation)
        ↓
   ATP hydrolysis → partial folding → release
        ↓
  Correctly folded? → YES → Native protein (functional)
        ↓ NO
   HSP60 chaperonin barrel (isolated, re-fold attempt)
        ↓
  Correctly folded? → YES → Native protein
        ↓ NO (persistent misfolding)
   Ubiquitination → Proteasomal degradation
   OR Autophagy-lysosomal pathway

Exam Points to Emphasize

1. Definition: proteins that assist folding without being part of the final structure.
2. HSP70 vs HSP60: co-translational (ribosome) vs. post-translational (barrel), monomers vs. oligomers.
3. ATP dependence: both major families use ATP-ADP cycling.
4. ER chaperones: BiP, calnexin, calreticulin, PDI - and their link to UPR/ER stress.
5. Disease relevance: cystic fibrosis (CFTR), neurodegeneration (aggregation), cancer (HSP90), autoimmune (HSP as autoantigens).
6. Ubiquitin-proteasome link: irreparably misfolded proteins tagged with ubiquitin and degraded.
7. Induction: heat, oxidative stress, toxins - hence "heat shock proteins."

• Harper's Illustrated Biochemistry, 32nd Ed - Chapters on protein folding, chaperones, and protein sorting
• Robbins, Cotran & Kumar Pathologic Basis of Disease - ER quality control, UPR, and cell injury
• Harrison's Principles of Internal Medicine, 22nd Ed - Protein quality control mechanisms and proteostasis