Notes on Transport to the Endoplasmic Reticulum

Transport to the Endoplasmic Reticulum

  • Overview of Cellular Transport
    • Vesicular transport involves the transfer of materials between compartments through vesicles that bud off donor compartments and fuse with recipient compartments.
    • Soluble molecules are transferred from lumen to lumen, maintaining asymmetric membrane orientation.

Structural and Functional Diversity of the Endoplasmic Reticulum (ER)

  • Components
    • The ER is continuously connected to the nuclear envelope, which consists of an inner and outer nuclear membrane.
    • Different types of ER:
    • Rough ER (RER): studded with ribosomes, involved in protein synthesis.
    • Smooth ER (SER): lacks ribosomes, involved in lipid synthesis and detoxification.

Main Functions of the Endoplasmic Reticulum

  • Three Major Functions
    1. Protein Synthesis
    • For the endomembrane system, plasma membrane, and secreted proteins.
    1. Lipid Synthesis
    • For organelles like mitochondria and chloroplasts, which also synthesize their own lipids.
    1. Calcium Ion (Ca²⁺) Storage

Protein Targeting and Transport into the ER

  • Mechanisms of Targeting to the ER
    1. Signal Sequence
    • A short peptide sequence that directs the nascent protein to the ER.
    1. Signal Recognition Particle (SRP)
    • A riboprotein complex that binds to the signal sequence.
    1. SRP Receptor
    • A protein located in the ER membrane that recognizes the SRP.
    1. Translocator Channel
    • Allows the passage of proteins into the ER.

Co-translational vs. Post-translational Import

  • Co-translational Import

    • Most common mechanism in mammals; involves direct translation and insertion into the ER.
    • Requirements:
    1. Signal sequence (16-50 amino acids, with a hydrophobic core).
    2. SRP (soluble complex that pauses translation).
    3. Energy source (GTP for mammals).

  • Post-translational Import

    • More commonly found in fungi; proteins are synthesized entirely in the cytosol before being transported to the ER.

Steps in Co-translational Import

  1. Translation Initiates
    • Signal sequence is created and binds to SRP.
  2. SRP Binding
    • SRP pauses translation and binds to the SRP receptor on the ER membrane.
  3. Translocation
    • SRP and receptor release; translation continues as the polypeptide moves through the translocator (Sec61 complex).
  4. Signal Cleavage
    • Signal peptidase cleaves the signal requiring the retention in the pore.
  5. Release
    • Mature proteins are released into the ER lumen after completing their synthesis.

Types of ER Proteins

  • Classes of Protein Translocation into the ER
    1. Soluble luminal proteins: Have N-terminal signal sequences.
    2. Single-pass transmembrane proteins: Have start-transfer and stop-transfer sequences.
    3. Multi-pass transmembrane proteins: Multiple hydrophobic regions ensuring passage through the membrane.

Determining Protein Orientation

  • A protein's orientation (N-terminus or C-terminus facing the ER lumen) depends on the position of positively charged amino acids relative to the signal peptide.
    • Positive charge before signal peptide: N-terminus in ER lumen.
    • Positive charge after signal peptide: C-terminus in ER lumen.

Functions Inside the ER

  1. Packaging of Proteins
    • Some proteins remain in the ER while others move to the Golgi apparatus or are secreted.
  2. Protein Folding
    • Proteins need to be properly folded with any misfolded proteins being targeted for degradation.

Retention Signals

  • Proteins destined to remain in the ER bear a specific sequence, such as KDEL (Lysine, Aspartic Acid, Glutamic Acid, Leucine).

Role of Glycosylation

  1. Protein Stability
    • Glycosylation makes protein regions less hydrophobic.
  2. Binding Sites
    • It creates binding sites for carbohydrate-binding proteins (lectins).
  3. Facilitation of Folding
    • Modifications help in the proper folding of proteins.

Chaperone Proteins and Folding

  • Chaperone proteins such as BiP and protein disulfide isomerase (PDI) are critical for ensuring the proper folding of proteins in the ER.
  • Glycosyl transferases add carbohydrate modifications to proteins.

Misfolded Proteins and the Unfolded Protein Response (UPR)

  • Misfolded proteins initiate the UPR, which leads to:
    • Increased translation of folding and degrading proteins.
    • A slowdown in general translation to prevent overload of misfolded proteins.

Summary of Glycosylation Functions

  • Importance of Glycosylation
    1. Increases hydrophilicity of proteins.
    2. Creates functional sites for molecular interactions.
    3. Enhances protein folding efficiency.

Conclusion on the ER’s Role in Lipid Bilayer Assembly

  • The ER is essential for assembling most lipid bilayers, highlighting its dual function in protein and lipid homeostasis.
    • Rough ER is involved in protein synthesis, while Smooth ER focuses on lipid synthesis.