Chaperoning Protein Folding and the Endoplasmic Reticulum

Learning Goals

  • Understand the structure and function of the endoplasmic reticulum (ER).

  • Identify which proteins are synthesized by free ribosomes and which by membrane-bound ribosomes.

  • Comprehend the mechanisms cells employ to handle misfolded proteins, including the unfolded protein response and chaperone proteins.

Exam Question Example

  • Question: Identify the FALSE statement.

    • A. Peptides adopt the conformation requiring the least effort.

    • B. In a folded protein, non-polar amino acids appear in the core of the protein.

    • C. All amino acids possess different side chains.

    • D. Side chains are used in the formation of peptide bonds.

Synthesis and Folding of Proteins

  • Translation and Folding:

    • Translation is not the end process; it is coupled with folding.

    • Proteins begin to fold as they are synthesized (secondary structure forms during translation).

    • Ribosomes involved in protein synthesis can be classified as:

    • Membrane Bound:

      • Synthesize proteins for organelles (ER, Golgi, lysosomes), membranes, or secretion.

    • Free Ribosomes:

      • Synthesize all other intracellular proteins.

Understanding the Endoplasmic Reticulum (ER)

  • Structure:

    • Comprised of a network of tubules and flattened sacs, interconnected with the outer nuclear membrane.

    • Can occupy up to 10% of cell volume.

  • Functions:

    • Central to lipid and protein biosynthesis.

  • Types of ER:

    • Rough ER:

    • Contains membrane-bound ribosomes; involved in synthesis, folding, and post-translational modification of proteins.

    • Smooth ER:

    • Lacks bound ribosomes; functions in lipid synthesis and detoxification; stores Ca2+; transports proteins from ER to Golgi.

    • Specialization:

    • Some cells (e.g., hepatocytes) have abundant smooth ER for specific functions like detoxification.

Chaperone Proteins and Folding Assistance

  • Need for Chaperones:

    • Most proteins require assistance for proper folding after emerging from ribosomes (co-translational protein folding).

  • Types of Chaperones:

    • Molecular Chaperones (e.g. Hsp70):

    • Bind to and stabilize unfolded or partially folded proteins, preventing aggregation.

    • Work with a small set of associated proteins and recognize hydrophobic areas on incomplete folds.

    • Chaperonins (e.g. Hsp60):

    • Create an isolated environment that helps facilitate proper folding of proteins.

  • Process of Folding:

    • Both chaperoines and molecular chaperones often need multiple cycles to achieve correct folding.

Recognition of Misfolded Proteins

  • Mechanism:

    • Identifying misfolded proteins relies on specific markers; e.g., number of glucoses on precursor oligosaccharides.

    • Properly folded proteins exit the ER quickly; misfolded ones are retained.

    • Misfolded proteins are tagged for retention via lectins and glucosyl transferase, which adds glucose to the oligosaccharides, indicating incomplete folding.

Response to Improperly Folded Proteins

  • Degradation:

    • Improperly folded proteins are retrotranslocated back to the cytoplasm for degradation.

    • This process activates the unfolded protein response (UPR).

  • UPR Pathways:

    • The UPR involves multiple pathways that together enhance protein chaperone levels and may activate programmed cell death if misfolded proteins accumulate excessively.

Summary

  • The ER comprises rough and smooth types, each with unique roles in protein and lipid management.

  • Chaperones play a critical role in assisting with protein folding and preventing aggregation of misfolded proteins.

  • Differentiation between newly synthesized and misfolded proteins utilizes oligosaccharide markings.

  • The UPR mobilizes resources to address the accumulation of misfolded proteins in the ER.

Understanding of the structure and function of the endoplasmic reticulum

Understanding of which proteins are synthesised by free ribosomes and which proteins are synthesised by membrane bound ribosomes

Understanding of the mechanisms by which cells identify and deal with misfolded proteins (including the unfolded protein response and chaperone proteins)