Introduction to Molecular Chaperones and Their Functions

Introduction to Molecular Chaperones

  • Overview presented by Dr. Mohinder Pal.

Learning Objectives

  • To explain molecular chaperones and their actions.

  • To describe the chaperone network at a basic level and how they assist in protein homeostasis, particularly focusing on protein folding and assembly.

  • To specify the molecular actions of key chaperones:

    • Heat shock proteins (HSP70)

    • Heat shock proteins (HSP90)

    • GroEL/GroES

    • HSP100

Learning Outcomes

  • Understanding of molecular chaperones.

  • Knowledge of types of chaperones.

  • Functions of molecular chaperones:

    • Key role in protein folding and assembly.

    • Specific focus on heat shock proteins (HSP100, HSP70, HSP90, GroEL).

What are Molecular Chaperones?

  • Definition: Helper proteins that assist in the folding and assembly of other proteins.

  • Characteristics:

    • Do not become permanent components of the final folded protein structure.

    • Key types include:

    • Heat shock proteins (HSP40, HSP70, HSP90).

Importance of Chaperones in Protein Folding

  • How does a protein fold?

    • Hydrophobic residues (depicted in green) must be packed into the core.

    • Hydrophilic residues (depicted in blue) remain on the exterior.

    • Proteins may have complex folding pathways.

  • Role of Chaperones: Prevents incorrect hydrophobic amino acid interactions, which can cause folding to stall (described as a complex folding pathway).

Energy Traps in Protein Folding

  • Incorrect interactions lead to non-reversible stalls in folding, resulting in energy traps characterized by free energy changes ($ \Delta G$).

    • Chaperones help avoid or facilitate escape from these energy traps.

Types of Chaperone Functions

  • Categories of Chaperones:

    • Holder Proteins: Prevent irreversible aggregation during denaturation or synthesis.

    • Examples: sHsp, Trigger factor, Prefoldin, Hsp70.

    • Unfolders: Facilitate the unfolding process of non-native interactions.

    • Examples: GroEL and Hsp100.

    • Folders: Assist in the final correct folding of proteins, such as Calnexin and Calreticulin.

Mechanistic Action of Chaperones

  • Folder Functions:

    • Calnexin and Calreticulin are ATP-independent and involve disulfide bond rearrangements requiring enzymatic catalysis.

General Functions of Chaperones

  • General Protein Folding: Assist newly synthesized proteins.

  • Stress Response: Bind to proteins to prevent unfolding. Provide protection against thermal aggregation and inactivation.

  • Recovery Mechanisms: Involve protein folding and degradation of thermally inactivated proteins.

  • Quality Control: General and specialized chaperone functions underpin protein quality control, assembly, and disassembly in cells.

Classes of Chaperones

  1. Intramolecular Chaperones: Pro-sequences that assist protein folding internally.

  2. Ribosome Associated Chaperones: E.g., Trigger factor, NAC (nascent polypeptide-associated complex), RAC (ribosome-associated complex).

  3. Cytoplasmic Chaperones:

    • Hsp70 (ATP-dependent) with Hsp40/NEF (Newly Exposed Factor).

    • DnaK (bacterial Hsp70) alongside GrpE.

  4. Hsp60:

    • GroEL/ES complex, found in bacteria and eukaryotes (TriC or CCT).

    • Thermosome (in archaea).

  5. Prefoldin (PFD): Assists in protein folding.

  6. Small Heat Shock Proteins: Act as hold for misfolded proteins.

  7. HSP100: ATP-dependent and involved in disaggregation.

    • Clp proteins in bacteria and yeast.

  8. HSP90: ATP-dependent and crucial for stabilizing certain proteins.

  9. Calnexin/Calreticulin/ Protein Disulfide Isomerase: Assist in proper folding during synthesis.

Organization of Cytoplasmic Chaperone Systems

  • Protein Folding Levels:

    • Bacterial: Trigger factor processes simpler folding requirements.

    • Archaeal: More complex with additional protein needs like DnaK.

    • Eukaryotic: Involves Hsp70 and Hsp60 systems with specialized Chaperonins.

The Hsp70/80/90 System

  • ATPase Cycle in GroEL: GroEL undergoes conformational changes upon ATP binding that regulate region interactions for folding.

    • Folded/unfolded protein state dynamics involve ATP hydrolysis ($ ext{ATP}
      ightarrow ext{ADP}$) which triggers release.

Prefoldin Structure and Function

  • Prefoldin Overview: Present in Archaea and Eukaryotes, consisting of a hexameric structure characterized by tentacle-like coiled-coil regions. Provides nucleotide-independent activity as a chaperone substitute.

Understanding HSP100:

  • Hexameric Structure: HSP100 (e.g., ClpB, Hsp104) organizes into a central cavity facilitating unfolding processes via ATP hydrolysis.

Mechanisms of Dismantling Protein Aggregates

  • ClpB Mechanism: Involves cycles of binding and release to effectively breakdown aggregated proteins, facilitated by interactions with other chaperones like DnaK.

HSP90 Function and Impact on Client Proteins

  • Client Binding Dynamics: HSP90 binds to clients and utilizes co-chaperones to regulate activity.

  • Effects of Inhibition: Chemical inhibitors targeting HSP90 can disrupt essential processes like eye development in certain species, highlighting its role as a therapeutic target.

Summary

  • Covered molecular chaperones, their actions, classifications, and the specific mechanisms by which various chaperones, including holders, unfolders, and folders, contribute to protein homeostasis and quality control within the cellular environment.